Applied geophysical characterization of the
shallow subsurface: Towards quantifying recent
landscape evolution and current processes in the
Boulder Creek watershed, CO.
by
K. M. Befus
B.S., Wheaton College, 2008
A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillment
of the requirements for the degree of
Master of Science
Department of Geological Sciences
2010
This thesis entitled:
Applied geophysical characterization of the shallow subsurface: Towards quantifying
recent landscape evolution and current processes in the Boulder Creek watershed, CO.
written by K. M. Befus
has been approved for the Department of Geological Sciences
Prof. Anne F. Sheehan
Prof. Robert S. Anderson
Prof. Suzanne P. Anderson
Prof. Craig H. Jones
Date
The final copy of this thesis has been examined by the signatories, and we find that
both the content and the form meet acceptable presentation standards of scholarly
work in the above mentioned discipline.
iii
Befus, K. M. (M.S., Geological Sciences)
Applied geophysical characterization of the shallow subsurface: Towards quantifying
recent landscape evolution and current processes in the Boulder Creek watershed,
CO.
Thesis directed by Prof. Anne F. Sheehan
I use minimally invasive shallow geophysical techniques to image the structure
of the critical zone from surface to bedrock (0-25 m) throughout three small drainages
within the Boulder Creek Critical Zone Observatory (BcCZO). Shallow seismic refraction (SSR) and electrical resistivity tomography (ERT) provide complementary methods
for determining the physical characteristics of the shallow subsurface. Results of the
SSR surveys provide a pseudo-3D network of critical zone compressional wave velocity
(Vp) structure within each catchment.
The evolution of each catchment within the BcCZO contains signals of both erosion and weathering dependent upon the large-scale geomorphic processes down to the
microbial weathering of mineral grains. The geophysical approach describes the arena
for the small-scale processes while also providing a quantitative description of the critical
zone structure at an instant in time. I use these tools to establish a three-dimensional
model of critical zone architecture within three catchments with significantly different
recent and continuing geomorphic forcings: fluvial rejuvenation, long-term quiescent
erosion, and glaciation. I find bedrock Vp greater than 3500 m/s, regolith Vp generally
less than 700 m/s and weathered bedrock ranging from 700-3500 m/s if present.
This model will guide investigations of critical zone processes that include landscape and hydrologic modeling and will assist in expanding point measurements of
chemical and biological processes to the catchment scale.
iv
Acknowledgements
First, I would like to thank my advisor, Anne Sheehan, for the opportunity to
work with her on this project and for the constant encouragement in my research. I
appreciate working as a research assistant as part of the Boulder Creek Critical Zone
Observatory (BcCZO) funded by NSF grant NSF-EAR 0724960.
I would also like to thank my fantastic field assistants Katherine Anarde, Graham
McClave, Travis Kelsay, and Dave Culp for working long hours for an entire summer to
supply me with the abundance of seismic refraction lines that are the bulk of my thesis.
I acknowledge the Mentorship Program of the University of Colorados Department of
Geological Sciences and the BcCZO in funding my field assistants. Also, I thank Austin
Andrus for taking time out of working on his own related IRIS project to assist me with
my fieldwork.
Special thanks to Craig Jones, Bob Anderson, Suzanne Anderson, Anne Sheehan,
and Matthias Leopold for carefully and thoughtfully reading my rough thesis. Their
comments and corrections improved both the science and readability of this thesis.
Finally, I would like to thank Matthias Leopold for not only sharing his electrical resistivity tomography data but also for continually collaborating with me on
interpretations and cautioning me throughout my graduate work.
Contents
ABSTRACT ...............................................................................................................iii
ACKNOWLEDGEMENTS .....................................................................................iv
CONTENTS ...............................................................................................................v
LIST OF TABLES ....................................................................................................vii
LIST OF FIGURES ................................................................................................viii
CHAPTERS
1 Introduction .............................................................................................................1
2 Background .............................................................................................................3
2.1. Critical Zone .............................................................................................3
2.2. The Boulder Creek Critical Zone Observatory .........................................7
2.3. Catchment Overviews..............................................................................13
3 Geophysical Methods ............................................................................................20
3.1. Shallow Seismic Refraction (SSR) .........................................................22
3.1.1. Background ...............................................................................22
3.1.2. Field Methods ..........................................................................29
3.1.3. Analysis ....................................................................................31
3.1.4. Strengths and Limitations ........................................................36
3.2. Electrical Resistivity Tomography (ERT) ...............................................39
3.1.1. Background ...............................................................................39
3.1.2. Field Methods ..........................................................................41
3.1.3. Analysis ....................................................................................43
3.1.4. Strengths and Limitations ........................................................44
3.3. Electromagnetic Induction .......................................................................45
3.4. Ground-penetrating radar .........................................................................46
3.5. Geophysical Methods Summary ..............................................................47
4 Results ....................................................................................................................49
4.1 Betasso Catchment Results .......................................................................53
4.2. Gordon Gulch Catchment Results ...........................................................58
4.3. Upper Green Lakes Valley Results ..........................................................62
4.4. Results Summary .....................................................................................65
5 Discussion .............................................................................................................66
5.1. Geophysical Interpretations .....................................................................66
5.2 Geomorphic Analysis ...............................................................................71
5.3. Future Directions .....................................................................................74
5.4. Conclusions ..............................................................................................75
REFERENCES .........................................................................................................78
APPENDIX 1: SSR Line Descriptions ...................................................................82
APPENDIX 2: SSR GUI ..........................................................................................91
List of Tables
Table 2.1. Overview of soil types and resulting names found within the Boulder
Creek watershed. A particular soil is named for the dominant factor contributing to its
development, in this case either the moisture or temperature regime..........................10
Table 4.1. P-wave velocity and electrical resistivity assignments for subsurface
materials in the Boulder Creek watershed...................................................................52
Table 4.2. Overview of average of all individual line maximum and minimum depths
to specific velocity layers in the Betasso catchment. z1 specifies the thickness of
overburden material (Vp < 500 m/s). z2 marks the depth to 1400 m/s as a central value
for weathered material or compacted colluvium. z3 represents the depth to crystalline
bedrock velocities (Vp > 3500 m/s). Separations between the eastern and western
slopes (relative to central ravine axis) are shown as well as valley center values from
the upper portion of the catchment with locations in the catchment also shown (upper
versus lower)................................................................................................................53
Table 4.3. Overview of average of all individual line maximum and minimum depths
to specific velocity layers in the Gordon Gulch catchment. z1 specifies the thickness
of overburden material (Vp < 500 m/s). z2 marks the depth to 1400 m/s as a central
value for weathered material or compacted colluvium. z3 represents the depth to
crystalline bedrock velocities (Vp > 3500 m/s). Separations between the northern and
southern slopes are shown with locations in the catchment also shown (upper versus
lower)...........................................................................................................................59
Table 4.4. Overview of average of all individual line maximum and minimum depths
to specific velocity layers in the upper Green Lakes Valley catchment. z1 specifies the
thickness of overburden material (Vp < 500 m/s). z2 marks the depth to 1400 m/s as a
central value for weathered material or compacted colluvium. z3 represents the depth
to crystalline bedrock velocities (Vp > 3500 m/s). Slope, valley and crest represent the
unique survey areas......................................................................................................63
List of Figures
Figure 2.1. Weathering within the critical zone is a function of climatic,
anthropogenic and tectonic forcings in the past. In turn, atmospheric and geomorphic
responses from the critical zone weathering engine are recorded in the geologic
record. (Figure 3 from Brantley et al., 2007).................................................................4
Figure 2.2. Cartoon showing strain in a nonplanar fault in the crust. At the bend,
cracks form to accommodate the stress (red lines) with inactive cracks (blue) moved
towards the surface. (Figure 2 from Molnar et al., 2007)..............................................7
Figure 2.3. The Boulder Creek watershed spans from the continental divide to the
confluence with the South St. Vrain River to the east (outlined in red). Three
catchments (blue) span three erosional regimes marked by glacial ornamentation and
scouring, long-term erosion, and rejuvenated fluvial incision. Maximum glaciation in
the area is from late Pleistocene time (Madole et al., 1999)..........................................9
Figure 2.4. Slope aspect effects on soil development in the Front Range near Boulder
show north-facing slopes with more developed E and Bt horizon. Less-developed
south-facing slopes may be a result of less slope stability or soil moisture and
temperature differences. (Figure 7 from Birkeland et al., 2003).................................11
Figure 2.5. The Betasso catchment resides within the forcing of rejuvenated river
incision at its confluence with Boulder Creek and spans transient erosional signal
working upwards to the post-Laramide erosional surface. 10 m contours of
elevation.......................................................................................................................14
Figure 2.6. The Gordon Gulch catchment lies within the low relief upland (Rocky
Mountain) surface with a strong west to east flow orientation. 10 m contours of
elevation.......................................................................................................................16
Figure 2.7. The upper Green Lakes Valley catchment lies within the glacially-scoured
geomorphic regime running down from the Continental Divide. 10 m contours of
elevation.......................................................................................................................18
Figure 3.1. Figure 3.1. Examples of P wave (top) and S wave (bottom) propagation.
(adapted from L. Braile: http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm)....24
Figure 3.2. Simplified cartoon example of seismic refraction. Multiple wavefront
sources reveal the behavior explained by Huygen’s principle. Velocity increases with
depth from red to yellow to blue. Wavefronts depicted as concentric circles as drawn
do not show the true refraction of the wave energy at the intervals as an increase in
velocity would result in larger wavelengths................................................................26
Figure 3.3. Samples of seismic data recorded in the field. (a) Unnormalized shot
gather and (b) normalized shot gather of the same data. Attenuation of the seismic
energy causes wave amplitude to decrease with distance in (a). Note clear delineation
of first arrival of seismic energy in the normalized data (b) until noise confuses the
signal by the last channel (51 m). Shot location at 0 m...............................................28
Figure 3.4. Seismic traveltime plot of first arrivals of the P wave for multiple shots
(color). The dashed black line represents the calculated traveltimes from network raytracing. RMS = 0.88 ms...............................................................................................32
Figure 3.5. Apparent resistivity distribution for a single ERT line with one roll-along
segment. Initial interpretation of the apparent resistivity structure gives insight into
the subsurface structure but must be inverted to obtain accurate resistivities and
locations of structures..................................................................................................42
Figure 4.1. Overview of shallow geophysical lines collected in the Betasso
catchment.....................................................................................................................49
Figure 4.2. Overview of shallow geophysical lines collected in the Gordon Gulch
catchment.....................................................................................................................50
Figure 4.3. Overview of shallow geophysical lines collected in the upper Green Lakes
catchment.....................................................................................................................50
Figure 4.4. Three SSR transects through the Betasso catchment. Locations of the
transects are shown in red in the inset catchment map. Low velocity (red) values are
thick in the upper transect, but are thinner in the lower two transects.........................54
Figure 4.5. Three ERT transects through the Betasso catchment. Locations of the
transects are shown in red in the inset catchment map and are co-located with the SSR
transect lines.................................................................................................................55
Figure 4.6. Fence diagram model of P wave velocities geographically oriented in the
Betasso catchment........................................................................................................58
Figure 4.7. Four SSR transects across the Gordon Gulch catchment. Locations of the
transects are shown in red in the inset catchment map. North-facing slopes show
consistently deeper low velocities and likely describe deep weathering compared to
the profiles on the south-facing slope..........................................................................60
Figure 4.8. Fence diagram model of P wave velocities geographically oriented in the
upper drainage of the Gordon Gulch catchment..........................................................62
Figure 4.9. Majority of SSR lines from upper Green Lakes Valley separated by
topographic location. Note bedrock velocites at 5-7 m depth along the ridge crest.
Deep low velocities in the northern slope lines likely reflect blocky slope deposits
with a large amount of air between blocks. SSR line numbers in this figure do not
correspond to SSR lines mentioned in the text............................................................64
Figure 5.1. Interpreted subsurface structure from the uppermost northeastern set of
co-located SSR (top) and ERT (bottom) lines in Betasso. Apparent difference in
subsurface architecture is likely an effect of ERT’s sensitivity to water content,
potentially outlining water content rather than material differences as suggested in
this interpretation. Other differences in depth to bedrock between the sections are
likely due to unique electrical and seismic signals of weathering. Tree locations are
approximate..................................................................................................................67
Figure 5.2. Weathering models of crystalline bedrock for explaining the apparent
disconnect between velocity and resistivity subsurface structures. (a) Planar fractures
surrounding by a rim of weathering especially with high water content would affect
the recovered resistivity structure much more than the P wave velocity structure. (b)
Pockets of weathering would lead to a similar drop in resistivity as clays develop
while measured P wave arrival time changes would remain minimal.........................69
Figure 5.3. Comparison of a co-located ridge-top set of SSR and ERT lines shows the
effect of fractures in the development of the landscape from unique geophysical
subsurface structures. Note that low resistivity values from 140-165 m extend much
deeper than low velocities. Also, note the reversal of resistivity color scheme for ease
of comparison..............................................................................................................70
Figure A2.1. Opening screen for the SSR GUI...........................................................91
Figure A2.2. SSR line 37 in the Betasso Catchment is plotted here with a constant
depth line at 5 m below the ground surface. The blue asterisk corresponds to the blue
asterisk location in the map plot..................................................................................92
Figure A2.3. The location for SSR line 37 in the Betasso catchment is plotted in
yellow. All other SSR lines in Betasso are red. The boundary of the drainage area for
Betasso is black and filled with green. 10 m contour lines are in blue. Distances are
meters N-S or E-W from the center of the catchment.................................................93
Chapter 1
Introduction
Exploration of the shallow subsurface can provide insight into the processes
that control the geomorphic evolution of the landscape. Sensitive systems requiring
broad spatial information demand innovative methods for delineating subsurface
structure and weathered profile development. Shallow applied geophysical techniques
fulfill these requirements while also determining specific properties of the subsurface.
I explore subsurface structure and the development of weathered profiles in
three drainages within the Boulder Creek watershed in the Front Range of northern
Colorado. The field areas span three erosional regimes experiencing recent and
continuing geomorphic forcings: fluvial rejuvenation, long-term quiescent erosion,
and glaciation. I employ the seismic refraction and electrical resistivity tomography
methods towards quantifying the erosional signature of these forcings within the
shallow subsurface of the three catchments.
I hypothesize the existence of measurable signals within the drainages related
to the specific erosional regime active in the area. Thus, in Betasso where rejuvenated
river incision has dropped the outlet of the drainage, I predict thin sediment cover and
weathering profiles at the base of the catchment that thicken up-drainage. Within the
more likely steady low-relief upland surface, I hypothesize the Gordon Gulch
drainage will possess the thickest weathered profiles of the three study areas. I
hypothesize shallow depths to fresh bedrock within the glacially scoured upper Green
Lakes Valley. Finally, I hypothesize that while the erosional regime of each
catchment affects the development of the weathered profile, other site-dependent
attributes such as slope or aspect should contribute significantly to the weathered
profile.
In the following sections I develop my argument. First, I define and describe
the concept of the critical zone and then apply it to the Front Range of Colorado. I
also introduce the field areas within the Boulder Creek watershed in detail. Secondly,
I describe the geophysical methods I employed to test my hypotheses including the
background physics, field methods, analysis, strengths and limitations of both seismic
refraction and electrical resistivity. I also include quick overviews of groundpenetrating radar and electromagnetic induction as potential tools for studying the
critical zone. Next, I review my results. Finally, I discuss my findings in the context
of critical zone development in the Boulder Creek watershed, including the
challenges inherent in collecting and analyzing geophysical data in this mountainous
terrain and discuss areas ripe for further investigation.
Chapter 2
Background
2.1. Critical Zone
The interactions between weathering and transport processes sculpt terrestrial
landscapes. Geomorphic processes such as glacial quarrying, river incision and
landslides alter the Earth’s surficial carapace. This highly dynamic surface in respect
to geologic time encompasses the majority of hydrological, geochemical and
biological activities that shape the subaerial landscape.
Recently, the term “critical zone” has been applied to the shallow terrestrial
realm spanning from the lowest extent of groundwater to the top of the vegetation
canopy (Brantley, 2007; Anderson, 2007). The definition of critical zone is inherently
scale-dependent both spatially and in time, thus allowing a conceptual lower
boundary where the resident time of the groundwater fits within the time period of
interest. Within the critical zone rock evolves through various stages of decay and is
overlain by soil. Together they support the local ecosystem. The properties of the
weathered rock, soil horizons and biological activity control water flow character,
rock strength, active chemical reactions and how the subterranean ecosystems interact
with the local materials (Figure 2.1). Therefore, critical zone structure exerts a first
order control on subsurface processes that without road and riverbank cuts or cliff
edges remains poorly constrained. Studying the architecture of the critical zone
Figure 2.1. Weathering within the critical zone is a function of climatic,
anthropogenic and tectonic forcings in the past. In turn, atmospheric and geomorphic
responses from the critical zone weathering engine are recorded in the geologic
record. (Figure 3 from Brantley et al., 2007).
improves our quantitative understanding of the landscape in order to more accurately
predict the effects of climate and land use change.
Moving from the broad scale to an individual parcel of the landscape reveals
the machinery of the critical zone that may eventually exhume subsurface materials
(Anderson et al., 2007). High erosional efficiency relative to weathering rate results
in little to no soil development, with a corresponding thin critical zone (Anderson et
al., 2006). If, however, weathering rates outpace the removal of material, the critical
zone is thick, and may exhibit more weathering in the subsurface. Taking a broader
view of the landscape displays an intricately variable pattern of critical zone
development. This pattern depends upon multiple geographic properties ranging from
climate to vegetative cover to slope and many more to be discussed further below.
Returning to the small scale, a coherent block of bedrock filling the lower portion of
the aforementioned parcel through time experiences consecutively greater amounts of
biological, chemical and physical surficial processes as the ground surface lowers or
weathering agents and conduits such as fractures in overlying material move
downwards towards the fresh bedrock (Anderson et al., 2007). As the rock
approaches the surface, these processes may create a positive feedback fueling more
and faster weathering. Conversely, if the parcel represents a plot of land with low
erosional potential, the thickening of the critical zone due to poor transport processes
could sufficiently mantle the bedrock beyond the reach of surficial processes. With no
surficial processes active over the time scale of interest, the parcel of rock may
become buried below the chosen critical zone if groundwater remains absent.
At some instant, parts of the originally intact bedrock will be freed from their
neighbors. This process is weathering. Now ready for transport, the weathered rock
has become mobile regolith, and its fate is governed by gravity-driven processes.
Throughout this weathering machine that drives bedrock towards the surface,
environmental processes interact to set how quickly originally intact bedrock enters
the critical zone. Climate regulates which chemical and biological activities are
operating. Precipitation and temperature control both vegetation types that generally
stabilize hillslopes and burrowing organisms that efficiently mix soils (Birkeland et
al., 2003). Relief creates potential for loss and transportation of soil and sediments
down-slope as parent materials undergo weathering characteristic to their physical
properties and chemical compositions. However, in many environments the most
important ingredient for pedogenesis is the time over which these many factors
interact and convolve (Birkeland et al., 2003).
Bedrock mineralogy and physical properties determine which of the many
weathering processes and rates are active in a particular landscape. The mineralogy of
a rock establishes the chemical vulnerabilities that can decompose or chemically
weather a rock. Depending on the availability of water and biological agents, the
decomposition of rock occurs through oxidation and reduction reactions, solution,
hydrolysis and ion exchange governed by the mobility of the ions involved (Ritter et
al., 2002). The disintegration of rock into smaller but chemically unaltered pieces
depends upon the forces at work but also on the physical bonding of the rock matrix.
Igneous and metamorphic rocks are generally tougher to break apart, as they do not
require a secondary cement to maintain coherence. Additionally, the tectonic history
of a landscape can predispose rock to disintegration. Molnar et al. (2007) hypothesize
that rock forced to the surface through the motion of lithospheric nonplanar thrust
faults undergoes brittle deformation that fractures the rock where the fault bends
(Figure 2.2). Alternatively, releasing lithostatic and tectonic compressional stresses
associated with denudation of the bedrock also may create fractures on a landscape
scale. Conversely, top-down propagation of fractures into rock may be driven by
frost-cracking near the ground surface that is dependent upon water availability and
the time spent within the temperature-determined frost-cracking window (Anderson,
1998; Hales and Roering, 2005). These fractures, if spaced closely enough, create
more easily removed blocks of bedrock. In most cases, disintegration and
Figure 2.2. Cartoon showing strain in a nonplanar fault in the crust. At the bend,
cracks form to accommodate the stress (red lines) with inactive cracks (blue) moved
towards the surface. (Figure 2 from Molnar et al., 2007)
decomposition are inherently linked with fractures exposing larger surface areas to
more water at greater depths, which serve to promote more chemical and biological
activity.
2.2. The Boulder Creek Critical Zone Observatory
On the eastern flank of the Front Range of north-central Colorado, a broad
range of scientists work together to characterize how the physical and chemical
weathering and transport processes control the structure of the critical zone, and the
impact this architecture has on hydrological, geochemical and biological functions
within the Boulder Creek Critical Zone Observatory (BcCZO). Studying the critical
zone development over the 1160 km2 Boulder Creek watershed (Figure 2.3), the
BcCZO ranges from the mountainous headwaters to the confluence of Boulder Creek
with the South St. Vrain River in the plains, spanning 2500 m of elevation (1480–
4120 m). The study focuses on three distinct erosional regimes in the mountainous
portion of the watershed (Birkeland et al., 2003). Along the continental divide, the
watershed was partially glaciated as U-shaped valleys and multiple over-deepenings
testify. Cosmogenic radionuclide (CRN) ages for terminal moraines place the
maximum extent of the glaciers occurring ~17 ka (Figure 2.3) and retreating to near
oblivion at the divide between 14-12 ka (Ward et al., 2007). East, beyond the glacial
limit, the broad Rocky Mountain surface with low relief at high elevation (2500-2750
m) retains deeply weathered profiles up to 15 m thick (Isherwood and Street, 1976;
Dethier and Lazarus, 2006). The final erosional regime comprises renewed bedrock
channel incision into the Rocky Mountain surface from a drop in base level in the
plains from the last 5 Ma (Anderson et al., 2006). This resulted in steep slopes and
deep canyons spanning to the range front. All three erosional regimes carve into
Precambrian crystalline bedrock exhumed during the Laramide orogeny (65-45 Ma)
of mainly granodiorite (1.7 Ba) and older biotite gneisses (Lovering and Goddard,
1950).
Previous studies of soil development in and around the Boulder Creek
watershed describe first order landscape features that in part capture the influence of
weathering and erosion on critical zone formation. Birkeland et al. (2003) describe
the distribution of soil types and their characteristic profiles in the Front Range
focused in the Boulder Creek watershed. They emphasize the role of climate in the
variation of soil and vegetation over elevation spanning 15°C mean annual
temperature (-4°C in the alpine and 11°C in the plains) and 56 cm in mean annual
precipitation from plains grassland (46 cm) through the lower and upper montane
forests and into the subalpine forest and alpine tundra (102 cm). Thus, the main soils
Figure 2.3. The Boulder Creek watershed spans from the continental divide to the confluence with the South St. Vrain River to the east
(outlined in red). Three catchments (blue) span three erosional regimes marked by glacial ornamentation and scouring, long-term
erosion, and rejuvenated fluvial incision. Maximum glaciation in the area is from late Pleistocene time (Madole et al., 1999).
of the Boulder Creek watershed are within the cryic soil temperature regime (using
the prefix ‘cry’) and ustic soil moisture regime (using the prefix ‘ust’) with the
dominant regime’s prefix used to describe an individual soil. Cyric soils are
characteristic of cold environments with a mean annual temperature of 8°C or below
(Soil Survey Staff, 1998). Ustic soils contain little moisture throughout the year,
however, when the soils become moist, generally when conditions are suitable for
plant growth (Soil Survey Staff, 1998). Table 2.1 summarizes these soil types found
in the Boulder Creek watershed.
In addition, the slope aspect contributes to the soil profile development
(Figure 2.4). At lower elevations, soils are generally Alfisols (Cryalfs and some
Ustalfs) on the north-facing slopes with more developed E and clay-rich Bt horizons
Table 2.1. Overview of soil types and resulting names found within the Boulder
Creek watershed. A particular soil is named for the dominant factor contributing to its
development, in this case either the moisture or temperature regime.
Ustic Moisture
Regime
Definition
Soil Type
dry but moisture
available during
season of plant
growth
Soil formation
f(moisture)
Cryic Temperature
Regime
Mean Annual
Temperature 8°C
Soil formation
f(temperature)
Soil
Name
Mollisol
1. less developed
to no E hor.
2. little to no Bt
(clay-rich) hor.
X
Ustoll
X
Cryoll
Alfisol
1. developed E
hor.
2. has Bt hor.
X
Ustalf
X
Cryalf
X
Cryept
Inceptisol
least developed E
and Bt hor.
Figure 2.4. Slope aspect effects on soil development in the Front Range near Boulder
show north-facing slopes with more developed E and Bt horizon. Less-developed
south-facing slopes may be a result of less slope stability or soil moisture and
temperature differences. (Figure 7 from Birkeland et al., 2003)
and Mollisols (Ustolls and Cryolls) on the south-facing slopes, marking a difference
in either age due to stability of the slope or active processes caused by a difference in
soil moisture or mean annual temperature (Birkeland et al., 2003). At higher
elevations Inceptisols (Cryepts) cover the landscape with little dependence on aspect
marked by relatively high precipitation and low temperature annual means.
Furthermore, Birkeland et al. (2003) quantify the degree of weathering using a
seven-class classification of granite weathering for Front Range outcrops. In this
scheme, class 1 is unweathered rock and class 7 represents material that has the rock
fabric but has been chemically weathered enough to contain clay. Birkeland et al.
(2003) find rocks of mainly class 1 west of the Rocky Mountain surface, especially in
the glaciated valleys. Class 2 (oxidized with angular joint-plane junctions) applies for
the rest of the mountainous areas with the exception of a band of classes 4-6,
increasingly rounded joint-plane junctions that disappear at class five and
disaggregates easily in class 6, directly west of the contact of sedimentary and
igneous and metamorphic rocks along the range front. Even with these low values of
weathering degree, Birkeland et al. (2003) find the Front Range slopes to be largely
transport-limited. A transport-limited landscape weathers at a rate greater than the
active erosional processes can remove the material, resulting in a net increase of
material covering fresh bedrock.
The application of soil profile studies provides important geomorphic
constraints on landscape shaping processes. Through radiocarbon and CRN exposure
dating of soils, Schildgen and Dethier (2000) estimate the incision rate to be 150
m/Ma for the major knickpoint, or oversteepened portion of the longitudinal profile of
the river, and in this case a waterfall in Boulder Canyon separates the Rocky
Mountain surface from the rejuvenated river incision. In addition, Dethier et al.
(2000) use soil profiles of Boulder Creek river terraces to calculate an incision rate of
40 m/Ma at the range front in Boulder Canyon. Denudation rates from Birkeland et al.
(2003), based on general rates of soil profile development, indicate 100 ky of
quiescence on the low-relief surface, yielding a lowering of the landscape at less than
10 m/Ma and potentially less than 1 m/Ma. Dethier and Lazarus (2006) set the
denudation rate of the highly weathered rolling surface to be 22 ± 7 m/Ma for the past
10-40 ky. Dethier and Lazarus (2006) also require a minimum of 230 ky to greater
than 1340 ky for the weathered profiles to develop using simple weathering box
models based on 10Be CRN dates. Significant for understanding the local critical zone
structure, Dethier and Lazarus (2006) also interpret data from 1661 wells in the high
low-relief terrain and find highly weathered regolith is on average 3.3 m thick and the
mean depth to bedrock is 7 m using both the Clayton seven-class classification
scheme and from material densities. For the area west of the Rocky Mountain surface,
Birkeland et al. (2003) estimate denudation rates between 50-100 m/Ma to 5-10
m/Ma on the assumption that the Pinedale glaciation (~17 ka) removed pre-existing
soils leaving 15-20 ky for the soils to develop (~1 m). The thickness of mobile
material divided by the denudation rate gives the residence time and an estimate of
landscape stability. These denudation rates and soil profile ages constrain the age and
rate of development of the critical zone within the Boulder Creek watershed.
2.3. Catchment Overviews
In the BcCZO three small catchments were chosen to study erosion and
weathering effects on the development of the critical zone. Each erosional regime
contains one of these catchments for studying weathering from the inherently coupled
geological, biological and chemical perspectives. Additionally, the change in
elevation across the Boulder Creek watershed encompasses the strong climatic
differences in mean annual precipitation and temperatures.
The Betasso catchment is located 3.2 km from the range front and drains an
area of 0.46 km2 directly into Boulder Creek with an average elevation of 1924 m and
vertical range of 240 m (Figure 2.5). This catchment’s steep, thinly mantled slopes
Figure 2.5. The Betasso catchment resides within the forcing of rejuvenated river incision at its confluence with Boulder Creek and
spans transient erosional signal working upwards to the post-Laramide erosional surface. 10 m contours of elevation.
near the confluence with Boulder Creek Canyon show the high erosional potential
caused by the rejuvenated river incision of Boulder Creek. However, further up the
catchment, the slopes shallow, with much thicker profiles of weathered bedrock and
soil. Ponderosa pine forest covers the majority of Betasso with a few small open
grassy meadows. Immediately adjacent to the Betasso catchment are several other
catchments with grass on gentle slopes near the top that change to pine forest on steep
slopes lower topographically. This change may mark the contact between the Rocky
Mountain surface and the landscape under higher erosional potential from the
increased channel incision of Boulder Creek. Betasso is underlain by the 1.7 Ga
Boulder Creek granodiorite, which outcrops along the ridges and on the steep slopes
where the catchment joins Boulder Creek Canyon. A prominent bedrock tor on the
western ridge (Bummer’s Rock) shows the granodiorite to be highly fractured with
generally less than 1.5 m spacing between cracks (E. Dengler, 2009 KECK
presentation). Multiple ravines coalesce, and a stream drains the occasional snowmelt
out of the catchment through a central gully running northwest to southeast. Not all of
the ravines are small, with a few cutting down greater than 5 m through rock and
colluvium. Since Betasso may span the renewed river incision at its base to the less
dynamic rolling upland surface at its head, studying the development of the critical
zone in Betasso should capture the geomorphic transience of the local landscape.
The Gordon Gulch catchment lies within the Rocky Mountain surface
covering an area of 2.7 km2 (Figure 2.6). Gordon Gulch is split into upper and lower
sections that both drain west to east with a north-south bend between them with an
average elevation of 2716 m and vertical range of 325 m. A dichotomy in vegetative
Figure 2.6. The Gordon Gulch catchment lies within the low relief upland (Rocky Mountain) surface with a strong west to east flow
orientation. 10 m contours of elevation.
cover on the slopes strongly reflects this orientation, with dense lodgepole pine forest
on the north-facing slopes and mainly sparse ponderosa pine with some undergrowth
on the south-facing slopes. Aspens and shrubs occupy much of the valley floor.
Gordon Gulch therefore supplies an excellent study area for determining the effect of
aspect on critical zone development related primarily to vegetation types and
structure. The aspen groves in the valley floors accompany marshy conditions with
water flowing in the valley bottom stream year-round. Apparently inconsistent for the
rolling upland surface, Gordon Gulch contains steep topography with many bedrock
exposures of Precambrian granites, gneisses and schists especially in the lower
portion of the catchment. Scars in the vegetative cover represent recent logging
activity in the catchment and are generally coated in a layer of wood chippings that
will eventually contribute to local critical zone processes.
The upper Green Lakes Valley catchment covers an area of 2.2 km2 with a
mean elevation of 3743 m and 485 m vertical range starting from the continental
divide in the west and trending eastward (Figure 2.7). As part of the glacially-scoured
terrain, Green Lakes Valley has the characteristic U-shape with polished bedrock
surfaces and six string-of-pearl lakes with two inside the study area. Predominantly
exposed bedrock and talus, this catchment is wholly above tree line, and tundra
grasses and shrubs grow mainly in the valley floor and gentler dipping south-facing
slope (25-30°). Well-developed but generally thin Cryept soils cover much of the
catchment with a large constituent of wind-blown deposits (loess) (Birkeland et al.,
2003). Cryent and Cryept soils represent hillslope soils while Histosols are prevalent
on the wetter valley floor (Williams et al., 2001). The Arikaree glacier covers a small,
Figure 2.7. The upper Green Lakes Valley catchment lies within the glacially scoured geomorphic regime running down from the
Continental Divide. 10 m contours of elevation.
uppermost portion of the valley. The catchment is bounded on the south by near
vertical cliffs of Precambrian meta-sedimentary rocks. A small rock glacier flows into
the valley from the southern wall. The Niwot Ridge interfluve comprises the northern
boundary of the watershed and has been studied for over fifty years spanning
geomorphic, hydrologic, climatic and biogeochemical properties (cf.,
http://www.colorado.edu/mrs/mrspubs.html). Towards the lower half of the
catchment, Niwot Ridge slopes steeply into the valley and is covered by alpine shrubs
and grasses. Recent studies show potential for permafrost in Green Lakes Valley, and
permafrost has been found on Niwot Ridge outside the catchment boundary in
addition to periglacial features (Ives and Fahey, 1971; Janke, 2005).
Chapter 3
Geophysical Methods
A key to studying the critical zone is to avoid significantly altering the active
processes in the attempt to quantify or understand them. Capturing the present
architecture of the critical zone without upsetting the system therefore becomes a
challenge where riverbanks, road cuts, and cliffs fail to provide sufficient exposure to
integrate the critical zone across the landscape. Digging or augering a hole allows
restricted insight into the structure and processes at depth, but once the material is
replaced, the immediately adjacent system will have changed. Even coring the
shallow surface results in a disturbed system with altered or enhanced water flow
paths through the disturbed volume. Shallow geophysical techniques provide a
minimally invasive suite of methods for determining subsurface structure by its
physical properties. Also, geophysical surveys can provide broader spatial coverage
than exposing the subsurface. Therefore, geophysical surveys allow a characterization
of critical zone architecture in the large-scale that must be interpreted and understood
by studying the subsurface with select excavations and all available natural sections.
This cooperation can extend such point measurements to broader areas to describe the
critical zone more fully.
Shallow geophysical investigations have been applied to geomorphic
problems similar to the processes studied in the BcCZO. Geophysical techniques have
been used to classify soils and surficial deposits for over twenty-five years in
sensitive and remote environments (Olson, 1985). Schrott and Sass (2008) provide an
excellent overview of the use of ground-penetrating radar, seismic refraction, and
electrical resistivity for understanding geomorphic signals in the landscape. Included
in their overview are such applications as finding sediment mantling thickness on
bedrock, mapping the thickness of the active layer, outlining landslides, sensing
seasonal variations in soil fluid content and determining permafrost properties.
Multiple studies explain in more detail the geophysical methods effective at
delimiting permafrost presence and depth (Vonder Mühll et al., 2001, 2002; Kneisel
and Hauck, 2008; Leopold et al. 2008a). Within the Boulder Creek watershed,
previous geophysical surveys have focused on permafrost, periglacial and slope
processes. Leopold et al. (2008a) performed shallow seismic refraction and groundpenetrating radar surveys on Niwot Ridge below the BcCZO upper Green Lakes
Valley catchment boundary to obtain shallow subsurface structure and permafrost
presence. Additionally, Leopold et al. (2008b) applied the same geophysical tools
along with electrical resistivity tomography to understand the structure of periglacial
slope deposits at the same sites on Niwot Ridge. Leopold et al. (in review, 2010) also
employed a suite of seismic refraction, electrical resistivity tomography and groundpenetrating radar lines on the rock glacier in the upper Green Lakes Valley catchment
to better understand its internal structure and hydrology.
As studying the structure of the critical zone with respect to weathering and
erosion is the BcCZO’s main objective, shallow geophysical techniques offer an
excellent toolset for obtaining spatially extensive data of subsurface physical
properties with little environmental turbulence of the critical zone. Particularly,
shallow seismic refraction (SSR), electrical resistivity tomography (ERT),
electromagnetic (EM) induction and ground-penetrating radar (GPR) comprise a suite
of techniques with individual strengths that build upon results from the other methods
when focused towards understanding critical zone structure in the Boulder Creek
watershed. In the following section I provide an overview of each technique starting
with the physics exploited, followed by field methods and analysis, and finish by
discussing the strengths and limitations of the particular method. I focus on SSR and
ERT.
3.1. Shallow Seismic Refraction (SSR)
3.1.1. Background
The shallow seismic refraction (SSR) method employs the physics of wave
propagation through the ground to understand subsurface properties, namely the
speed of a particular wave through the subsurface media. The seismic community
refers to this speed as the seismic velocity of the material even though no direction is
inherent in the value. In the effort to conform to the literature I also refer to wave
speeds as wave velocities.
Seismic waves propagate through the transfer of energy between adjacent
particles in any medium. This transfer of energy at the particle level constitutes a
conveyance of stresses (a force per area) radially away from the seismic source that
are accommodated by strains (a change in shape and dimensions) dependent upon the
elastic properties of the materials involved. The wave equation represents the
movement of a wave as a disturbance () in the medium from either dilation or
rotation (Telford et al., 1990):
1 2
= 2
V 2 t 2
(3.1)
in both time and space with V, the wave velocity (speed), as a proportionality
constant that describes the medium.
At the moment seismic energy is released, particle motion initializes the wave
propagation character and determines the speed of the spreading wavefronts.
However, at later times, the interactions between the particles in the medium
eventually coalesce into particular wave types. Of main interest in shallow seismic
refraction are the body waves: compressional or primary (P) waves and the shear or
secondary (S) waves with the order referring to the arrival of the waves at a seismic
station (Figure 3.1). Rayleigh and Love waves comprise surface waves and are
generally labeled “ground roll” in seismic data. Even so, surface waves are useful in
understanding subsurface properties (Park et al., 1999; Harry et al., 2005).
The velocity of P and S waves are a function of the medium’s Lamé
constants’ values (’ and μ) and density () (Telford et al., 1990; Barton, 2006):
VP =
'+2μ
=
VS =
μ
4
μ
3
K+
(3.2)
(3.3)
with Vp and VS the P and S wave velocities respectively, K is the bulk modulus
relating the ratio of volumetric stress and strain, and is the measure of resistance of
a material to shear strain, called the shear modulus. While Equations 3.2 and 3.3 seem
Figure 3.1. Examples of P wave (top) and S wave (bottom) propagation. (adapted
from L. Braile:http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm)
to require a decrease in both P and S wave velocities with increasing density, μ and K
both increase more quickly as a function of density, so generally increasing density
results in higher P and S wave velocities (Telford et al., 1990). Since elastic constants
are always positive, the P wave of a given medium is always faster than the S wave.
Also, because is zero in fluids, S waves do not propagate through fluids. By
measuring both the P and S wave velocities in a survey, the value of Poisson’s ratio
() describing the compressibility of the medium can be computed using the
following relationship (Barton, 2006):
2
Vp 2
V = S 2 .
(3.4)
Vp 2 1
VS As a proxy for compressibility, Poisson’s ratio provides a potential method for
capturing signals of weathering in the subsurface as increasing water, air, and
unconsolidated material content in the subsurface will increase the hard rock value of
~ 0.25 to potentially > 0.45 (Barton, 2006).
Seismic waves move spherically away from a source and the location of the
wavefront as a function of time is important in studying the velocity properties of the
subsurface. Huygen’s principle assists in understanding the movement of the
wavefront from the source by stating that every point on a wavefront at one instant in
time becomes a source of new waves at the next time step (Figure 3.2). Therefore, the
movement of a wavefront through a material can be calculated by selecting points
along a wavefront and then multiplying the velocity of the material at those points by
the time step to yield the distance the individual wavefronts have traveled, which
collectively give the total wavefront from the seismic source. When a wave
encounters a contrast in the wave velocity due to a change in subsurface material
properties, the wave energy is reflected and refracted by the interface. By treating the
movement of the wavefront as raypaths that must perpendicularly intersect
wavefronts, wave reflection and refraction can be quantified geometrically. Using this
ray theory, the angle of ray incidence on a reflecting interface must equal the angle of
Figure 3.2. Simplified cartoon example of seismic refraction. Multiple wavefront
sources reveal the behavior explained by Huygen’s principle. Velocity increases with
depth from red to yellow to blue. Wavefronts depicted as concentric circles as drawn
do not show the true refraction of the wave energy at the intervals as an increase in
velocity would result in larger wavelengths.
reflection. With a refracting interface, the raypaths depend on the angle from vertical
(i) and the velocity of the layer (Vi) through (Telford et al., 1990):
sin(1 ) sin( 2 )
=
=p
(3.5)
V1
V2
where p is the raypath parameter. In a two-layer medium with a higher velocity in the
second layer, a ray will be refracted along the interface (2 = 90°) when the angle of
incidence of the raypath is the critical angle with:
V1
(3.6)
V2 .
This refraction of energy along the interface then emits new waves in agreement to
sin(C ) =
Huygen’s principle and can eventually outpace the direct wave traveling along the
ground surface (Figure 3.2).
Any distance away from the seismic source experiences P waves before S and
surface waves. Therefore, both the direct wave along the ground surface and the
refracted or head waves from velocity interfaces mark the first arrivals of energy at
any given location. The distance required for the head wave to outpace the direct
wave is the “crossover distance” (xc) and equals:
x c = 2z
(V2 + V1 )
(V2 V1 )
(3.7)
where z is the depth to the interface. To image a velocity layer, the crossover distance
of the consecutive layers must be greater than the receiver spacing.
As the suite of seismic waves propagate spherically from a point source at
their characteristic velocities, the energy created by the seismic source remains
constant while the wavefront increases in size (geometrical spreading). This results in
less energy reaching particles further away from the source. Wave energy is also
absorbed by particles, transforming the mechanical energy into heat. Energy
partitioning at interfaces in the subsurface decreases the wave energy as a function of
distance. These processes of wave energy loss are collectively called the “wave
attenuation” in a given environment and follows the exponential relationship (Telford
et al., 1990):
I = I0e x
(3.8)
where I0 is the initial intensity, or amount of energy passing through a unit area in a
unit of time, of the wave, I is the intensity at a distance x away from the source, and is the absorption coefficient with units of decibels (dimensionless unit) per
wavelength. Thus, as the seismic signal is recorded at greater distances, the amplitude
of the waves decreases (Figure 3.3a). Weakening signal strength with distance
Figure 3.3. Samples of seismic data recorded in the field. (a) Unnormalized shot
gather and (b) normalized shot gather of the same data. Attenuation of the seismic
energy causes wave amplitude to decrease with distance in (a). Note clear delineation
of first arrival of seismic energy in the normalized data (b) until noise confuses the
signal by the last channel (51 m). Shot location at 0 m.
restricts the maximum source offset as the signal to noise ratio dwindles (Figure
3.3b).
Seismographs record the wavefield or sum of the wave energy in seismic
refraction surveys with receivers called geophones. Within a geophone, a conducting
coil is suspended by dampened springs and surrounded by a magnet. Ground motion
from seismic waves causes the magnet to move with the ground motion while the coil
remains stationary. This generates a voltage within the coil and is connected to the
seismograph with a cable. Thus the intensity of ground motion at a geophone is
directly proportional to the voltage transmitted to the seismograph. Geophones are
built to have a natural frequency of oscillation determined in part by the strength of
the magnetic field and the number of turns in the coil. Higher natural frequencies are
more applicable for shallow refraction or reflection surveys.
3.1.2. Field Methods
In the BcCZO shallow seismic refraction surveys, I used a 24-channel
Geometrics Stratavisor seismograph with twenty-four 40 Hz vertical component
geophones. A ~5 kg steel sledgehammer produced the seismic source of energy when
striking a 10 x 10 x 7 cm 15 kg steel plate on the ground surface. Leopold et al.
(2008a) calculate that such a source produces about 150 Nm of torque and yields
vertical blow energy valid for a survey depth range of 2-40 m. Using a standard
geophone spacing of 2 m, the spread length covered 46 m, allowing generally a
minimum 15 m depth of investigation (although this is dependent upon the subsurface
geometry and velocity structure). Increased geophone spacing allows a greater depth
of investigation at the cost of a lower resolution survey. Multiple gathers with
different geometries (change geophone spacing) can overcome this shortcoming. Five
shots within the spread gave detailed shallow data while off-end shots 2 m and 5 m
from the end geophones provide deeper information.
GPS points marked the end geophones of each 24-geophone spread. A
carefully straightened tape measure ensured that the geophones were in-line.
Topographic relief along the line was measured using an inclinometer and the bearing
with a compass. Rock outcropping along the line and significant vegetation changes
were noted. When additional spreads were used, the final geophone of the previous
spread became the first geophone of the next spread, allowing continuity of results
across spreads. Occasionally, multiple end geophones were retained as the beginning
channels of the next survey.
At each shot and geophone location, up to 5 cm of organic surficial debris was
removed to better couple the shot plate and geophones with the ground surface. Also,
three hammer blows secured the plate and compacted additional organic matter or
loose soil to improve the plate’s coupling with the ground. When recording the
seismic signal, ten shots were stacked, or summed, at each location to reduce
uncorrelated noise and increase the amplitude of the seismic signal. Three to five
blows at each shot location are standard (Schrott and Sass, 2008; Leopold et al.,
2008a), but noise from wind and energy loss due to steep, irregular topography along
the lines made ten blows necessary to ensure quality data.
3.1.3. Analysis
After ensuring that quality data was collected in the field, I picked the first
arrival of the P wave for all the recorded data using SeisImager PickWin (OYO
Corporation). Over 220 individual spreads were recorded in the three catchments with
almost 2,000 shot locations, yielding almost 50,000 single channel records (traces). I
manually adjusted the gain of each trace to ensure proper first arrival selection.
Beyond a notch filter centered at 60 Hz to reduce noise from electrical sources, I
applied no filters when picking first arrivals. This information was saved for each line
of one to many spreads in a single traveltime file.
As both P wave arrival time and distance from the source are recorded in the
travletime plot (Figure 3.4), discrete slopes in a travltime curve equal the reciprocal
of the apparent velocity of each visible subsurface layer. The thickness of a
homogeneous overburden layer (z) with a velocity (V1) is found using (Telford et al.,
1990):
V1t i
(3.9)
2cos(C )
where ti represents the back projection to the time axis of the refracted portion of the
z=
traveltime curve.
Inversion of the traveltime data yields a two-dimensional velocity structure.
Within the SeisImager Plotrefa (OYO Corporation) software, three inversion
techniques are available: time-term inversion, delay-time reciprocal method, and
tomographic inversion.
In the time-term method, linear least squares is used to solve the best discretelayer solution to the traveltime data (Geometrics and OYO, 2006). Using the
Figure 3.4. Seismic traveltime plot of first arrivals of the P wave for multiple shots
(color). The dashed black line represents the calculated traveltimes from network raytracing. RMS = 0.88 ms.
traveltime plot, up to three distinct velocity layers are manually assigned using breaks
in the slope of the traveltime curve for each shot. The critical angle (c) is then
calculated using equation 3.6 with the velocities calculated from the traveltime plot,
and the total traveltime with a distance (x) between source and receiver then equals
(Geometrics and OYO, 2006):
t = 2 S1 cos(C ) z + x S2
(3.10)
where S1 and S2 are the reciprocal of the velocity, or slowness, of layers one and two
respectively and if the thickness (z) of layer one is known and uniform. Under more
realistic circumstances with unknown and variable overburden thickness, the total
traveltime (tm) as a matrix equation this two-layer case with “time-terms” () is
(Geometrics, personal communication, 2010):
(3.11)
where
n = S1cos(c)
with n the total number of source and receiver locations and m the number of
traveltimes. In Equation 3.12 cmn is either a zero or one with a maximum of two ones
per row. The application of the time-term linearizes the equation, but the velocity of
the first layer must still be calculated from the slope of the traveltime curve. With the
data now in the form Gm=d, a least squares solution of the form m=(GTG)-1GTd
yields the depth to and slowness of the refractor, giving the velocity structure of this
simple problem. The time-term method as coded by Geometrics allows a maximum
of three velocity layers.
The reciprocal method uses the difference between forward and reverse shots
to calculate the depth (z) to the interface using (Geometrics and OYO, 2006):
DT V1
(3.12)
cos(C )
where DT is the delay-time between the forward and reverse traveltime curves and V1
z=
the top layer’s velocity. In this way, the reciprocal method more appropriately
describes irregular layer thicknesses than the time-term method. However, this
method requires assigning of up to five layers to traveltimes based on slope-breaks
and the creation of reduced traveltime curves for each set of opposite shots.
Therefore, the reciprocal method takes much more time to apply than the simpler
time-term inversion and the more intricate tomographic inversion.
SeisImager/2D employs a nonlinear least squares approach to traveltime
tomographic inversion (Geometrics and OYO, 2006). Inverting tomographic
traveltime data is nonlinear because the recorded traveltime is a function of both the
velocity structure and pathway of the seismic energy, which are inherently linked.
Therefore, computing velocity model solutions require an iterative technique. An
initial velocity model must be supplied for the tomographic inversion either from a
time-term velocity model or can be created using a vertical pseudo-gradient function
with velocity increasing with depth and specified by the user (Sheehan et al., 2005).
The initial model is automatically discretized into cells of increasing height with
depth to account for less sensitivity with depth. The reciprocal velocity values of
these cells make up the starting slowness matrix (S0). A network ray-tracing
algorithm calculates traveltimes from each source-receiver pair using a smallest
distance approach yielding apparent first arrival times (T0) (Geometrics and OYO,
2006). Nodes assigned to the edges and corners of each cell control the potential ray
paths between each source-receiver pair. The ray-tracing step also calculates the
lengths of each raypath that make up the initial Jacobian matrix (L). The time
difference (T) between the recorded traveltimes and T0 is used to solve for the
necessary adjustments to the slowness matrix (S) using least squares (Geometrics
and OYO, 2006):
(3.13)
where l is the length of the ray within a particular cell, n is the number of rays passing
through a single cell and m is the number of cells. S is added to S0 and network raytracing restarts the process in the next iteration. This sequence of ray-tracing followed
by inverting for the slowness adjustment is repeated for a number of iterations set by
the user with T decreasing with each iteration until sequential iterations do not
significantly change the misfit. No convergence limit can be set in SeisImager/2D.
Also, separate vertical or horizontal smoothing of the resulting velocity model can be
applied based on a three-term weighted moving average filter.
Tomographic inversion is the most flexible technique available in
SeisImager/2D that is capable of both imaging structures and lateral complexities due
to its discretization into small cells. As such, I collected seismic refraction data to
employ tomographic inversion of the traveltimes, performing multiple shots within
the boundaries of line with five per spread for more source-receiver pairs. This
creates higher raypath density and results in more complete sensitivity to the
subsurface.
I created the initial models for the tomographic inversion to smoothly increase
from 300 m/s to 5000 m/s with depth. The values span a range of compiled P wave
velocities from very loose sediment to fresh granodiorite (Barton, 2006). The depth of
the 5000 m/s layer was fixed to be 10 m below the surface with the total model space
extending another 10 m to 20 m. In this way, if fresh bedrock is near the surface, high
velocities should be present after the inversion at shallow depths while thicker soils,
more sediment, or greater weathering should result in lower velocities at depth. I
applied a horizontal smoothing with the center cell of the three-cell filter weighted
twice as much as the adjacent cells (smoothing weight = 0.5). I used a standard ten
iterations for each seismic line that generally converged to a root mean square error <
2 ms (Figure 3.4). This standardized approach allowed quick and efficient analysis for
the large number of lines but nearly always over-fit the data beyond the maximum 2-4
ms uncertainty associated with manually picking P wave arrival times. Raypath
coverage provides an estimate of the depth sensitivity and resolution of individual
cells in the inverted section, but raypaths are necessarily linked to the selected initial
model.
3.1.4. Strengths and Limitations
Seismic refraction is an important component of the geophysical surveys of
the BcCZO, but does not provide a complete set of physical properties for
understanding the structure of the critical zone. Seismic refraction surveys are
especially adept at constraining the depth to interfaces with significant velocity
changes from lower to higher velocity (as required for refraction according to Snell’s
law). As the assumed subsurface of the field sites consists of soil mantling weathered
bedrock and all overlying relatively fresh crystalline bedrock, velocity profiles
inverted from seismic refraction surveys should delineate both interfaces; both
represent velocity increases with depth. However, if the interface has instead a
smooth velocity gradient, as is quite possible within a weathering profile,
constraining the thickness of overlying regolith and depth to bedrock require careful
interpretation both with geophysical and direct measurements along with consistent
definitions of when weathered material becomes regolith and what constitutes
weathered versus fresh bedrock.
In some cases, seismic refraction cannot represent subsurface structures. If a
layer of lower velocity underlies a layer of high velocity, the refracted waves from the
low velocity layer do not bend up towards nor outpace the overlying high velocity
layer’s refracted wave, leaving no first arrivals to mark the layer. This produces a
hidden layer that is difficult to resolve with only seismic refraction; however, if the
layer represents a high hydraulic conductivity water flow path, then electrical
resistivity tomography could potentially resolve the layer. Additionally, when a layer
at depth is thin relative to the geophone spacing, this layer may be undetectable if the
velocity is not significantly higher in the thin layer relative to adjacent layers
(Equation 3.7).
Even with these hidden layers, traveltime tomography can image thin layers
and heterogeneous horizontal features where the time-term and reciprocal methods
cannot by separating the model space into cells rather than solving for distinct
velocity layers. In the inversion software, an option by default prevents the velocity
from decreasing with depth. After running multiple lines from these field areas,
insignificant differences in structure between preventing and allowing velocity
decreases with depth suggested that the default setting yielded satisfactory sections.
Vertical features in the subsurface are more likely to be represented by traveltime
tomography. Vertical features are, however, smoothed by the inversion process, thus
requiring other geophysical methods to image near-vertical features.
Seismic refraction surveys can image the depth to the water table (Milsom,
2008; Telford et al., 1990). S wave velocities are identical in both saturated and
unsaturated material, whereas P wave velocities increase with increasing saturation.
Therefore, measurements of both P and S wave velocities in refraction surveys can
very accurately determine the depth to the water table by allowing calculations of
Vp/Vs as a function of depth. In the BcCZO catchments, the depth to groundwater is
broadly unknown and likely beyond the depth of investigation ( > 20 m) in Betasso,
but upper Green Lake Valley and Gordon Gulch have marshy conditions at the
surface all year. Water and weathered bedrock P-wave velocity ranges overlap and
complicate the delineation of the water table in the lower two catchments.
Additionally, the nonuniqueness associated with inverting geophysical
datasets inherently applies to traveltime tomography (Ivanov et al., 2005a, b; Palmer,
2010). As both depth (or path length) and velocity (or slowness) are dependent upon
each other in the inversion, the minima of the error function map in the parameter
space for refraction tomography takes the shape of a valley of plausible values rather
than single or multiple point minima (Ivanov et al., 2005a, b). Thus, a single inverted
seismic section is not necessarily either the most plausible or accurate solution. The
choice of the initial model steers the inversion process towards a particular range of
solution minima (Palmer, 2010). More a priori information added to the initial model
may yield a more accurate recreation of subsurface structure (Ivanov et al., 2005).
3.2. Electrical Resistivity Tomography (ERT)
3.2.1. Background
Electrical resistivity tomography (ERT) measures the resistance of the ground
to an electrical current. Based on the application of Ohm’s law, which states that the
voltage equals the current times the resistance, ERT measures the resistivity of the
subsurface incorporating the physics of current flow. Resistivity is the resistance
calculated from an applied current and measured voltage drop then multiplied by a
geometric factor dependent on the survey setup. In four-electrode surveys, depth
soundings and lateral profiling comprise the two exploratory procedures for studying
subsurface structure. In depth soundings, the distance between the electrodes is
expanded for greater and greater depths of investigation with the center fixed.
Conversely, in lateral profiling, the distance between the electrodes is kept constant
and the array is moved as a whole for a constant depth of investigation along the
survey line.
For a single vertical depth sounding reading, four metal electrodes are used.
Two electrodes provide a measured current while the other two electrodes measure
the voltage drop somewhere within the electrical field produced by the current
electrodes. The three most popular arrangements for the electrodes are the dipoledipole, Schlumberger and Wenner arrays. In the dipole-dipole setup, the current
electrodes are separated from the potential electrodes by a distance that is
proportional to the depth of investigation. The dipole-dipole array is sensitive to
vertical resistivity features but has a lower signal-to-noise ratio than the others. The
Schlumberger and Wenner arrays both have the potential electrodes within the limits
of and in-line with the current electrodes, thus measuring the voltage drop of a section
of the electric field between the current electrodes. In the Wenner array, the spacing
between each electrode with its neighbor is the same while in the Schlumberger array
the distance between the potential electrodes can stay constant while the current
electrodes are moved and vice versa to ensure a measurable potential with the current
electrodes spread much further apart than the potential electrodes. As any electrical
resistivity measurement is sampling over a most likely heterogeneous volume of
material, the potential electrodes measure an electric field distorted by threedimensional subsurface structures. Thus, the resistivity calculated from these
measurements is denoted the apparent resistivity, which only becomes true resistivity
after inverting the data for a resistivity structure. For the dipole-dipole array, the
equation relating the measured current (I), voltage drop (V) and apparent resistivity
(a) is (Telford et al., 1990):
2n 3V
(3.14)
I
where n is the separation factor of the current and potential electrodes and the
a =
separation between the individual pairs of electrodes, for the Wenner array (Telford et
al., 1990):
2aV
(3.15)
I
where a is the spacing between each electrode and the next, and for the Schlumberger
a =
array (Telford et al., 1990):
L2 V (3.16)
2 I where L is the half the spacing between the current electrodes and is half the
a =
spacing of the potential electrodes. In this way the resistance calculated by measuring
the current and potential becomes an apparent resistivity that is a function of the
spacing of the electrodes.
To keep the subsurface from becoming polarized, the current is injected in
opposite directions at a frequency low enough to ensure the electrical field time to
reverse.
Historically, these apparent resistivity-spacing values were plotted on log-log
graphs and transparencies with resistivity-depth master curves were overlaid to find
the best subsurface geometry match. First approximations of layer resistivities can be
estimated when the apparent resistivity-spacing curves approach zero slopes with
steep slopes between adjacent layers of very different resistivity. These master curves
can now be fit using forward stochastic modeling to find the resistivity values and
layer thicknesses that best fit the data.
In ERT, tens to hundreds of electrodes are used with an automatic switch
mechanism that activates four electrodes at a time, speeding up field procedures by
not requiring moving two or more electrodes after each measurement. Both depth
sounding and lateral profiling are achieved with one long line of many electrodes.
The automatic switch can be programmed using any array type or multiple allowing a
comparison of resulting apparent resistivity pseudosections with minimal additional
survey time. ERT results in apparent resistivity values from the many combinations
that are distributed across the profile and multiple depths (Figure 3.5).
3.2.2. Field Methods
In the BcCZO ERT surveys, we measured apparent resistivity values with a
lightweight 100-channel resistivity system with roll-along capability (Lippmann
Figure 3.5. Apparent resistivity distribution for a single ERT line with one roll-along
segment. Initial interpretation of the apparent resistivity structure gives insight into
the subsurface structure but must be inverted to obtain accurate resistivities and
locations of structures.
Geophysikalische Messgeräte 4-Punkt light hp). The same line description methods
were used as in SSR. An electrode spacing of 0.75 m to 1.5 m was used with depths
of investigation ranging between 10 m and 20 m depending on the subsurface
resistivity structure analyzed using RES2DINV v3.55 (Loke and Barker, 1995).
Apparent resistivities were recorded using the Geotest software v2.20m. Multiple
readings at each active quadripole averaged multiple values from alternating current
injection directions with user notification if the standard deviation was above a set
error threshold.
To perform a survey, 100 steel electrodes were pounded into the ground using
a rubber mallet to a minimum depth of 10 cm. Low contact resistance between the
ground and electrodes was achieved by performing the majority of the surveys during
the late Spring when the soils were damp. Otherwise, water poured around the base of
electrodes generally lowered contact resistance enough to continue the
data collection. Hillslope surveys ran from the hillcrest to the valley floor or vice
versa and generally required multiple roll-along sections. This was achieved by
leaving the last 25 channels of a survey in the ground and leap-frogging the other 75
channels forward, allowing much longer, continuous surveys.
3.2.3. Analysis
Beyond field data quality checks, M. Leopold used RES2DINV to analyze the
data (M. Leopold, personal communication, 2009). RES2DINV is a commercial
software package that inverts for the resistivity structure using apparent resistivity
values recorded in the field with a nonlinear smoothness-constrained, least squares
inversion technique (Loke and Barker, 1995):
(J T J + C T C) p = J T g
(3.17)
where J is the Jacobian matrix, is the damping factor, C is a smoothness matrix, p is
a vector of the chance in resistivity to fix the starting model and g is a vector of the
misfit between the measured apparent resistivities and the calculated apparent
resistivities. The Jacobian matrix is precomputed based on the electrode geometry.
Forward modeling calculates the apparent resistivity values after each iteration to find
the misfit vector. The initial model for the first iteration is a homogeneous half-space
with a resistivity value equal to the average of the recorded apparent resistivities.
RES2DINV performs multiple iterations until the convergence error limit set by the
user is reached.
The procedure for inverting the ERT lines in this study follow a standard
outline (M. Leopold, personal communication, 2009). First, outliers in the data were
removed based on the assumption that adjacent measurements cannot vary greatly as
the electric field should remain quite similar between adjacent acquisitions. The nonlinear, and thus iterative, inversion of the apparent resistivity data included
topographic modeling to take into account the strong influence of slope for accurately
calculating the electrical current density in the forward model. A moderate initial
damping factor of 0.3 applied smoothing to avoid over-fitting the data with a
minimum of 0.03. Also, increased damping with depth was applied to not over fit the
data as the resolution of the survey decreased with depth. A vertical to flatness filter
was applied to the inversion ranging from 0.7 to 1.2 with higher values allowing more
vertical features and lower values smoothing vertical features. In general, the
inversions took one to five iterations to reach the 3% root-mean-square convergence
limit.
3.2.4. Strengths and Limitations
Just as SSR describes the velocity structure, ERT provides detailed electrical
resistivity subsurface structure for each of the profiles with information on the depth
to interfaces. However, ERT’s ability to image vertical features surpasses SSR. Even
so, ERT and SSR are sensitive to very different physical properties of the subsurface
and provide complementary datasets for understanding the subsurface.
The conductivity of fluids within subsurface materials greatly changes the
overall resistivity of that layer, making ERT very sensitive to the presence of water.
Thus, inverted ERT sections accurately set the elevation of the water table or
delineate areas with higher water saturation. Conversely, ice has a very high
resistivity that makes ERT useful for determining the presence or absence of
permafrost and ice lenses in the alpine portions of the Boulder Creek watershed.
ERT is also very sensitive to clay content as clays have very low resistivities
and are very adept at holding water. As the profiles in the BcCZO generally become
more clay rich with increased weathering, ERT can be used to estimate the degree of
bedrock weathering at depth.
Similar to seismic velocities, electrical resistivity values of subsurface
materials often overlap requiring an understanding of the local geology and
subsurface properties to appropriately interpret. However, the electrical resistivity of
most materials ranges over many orders of magnitude depending not only on the
weathering degree and water saturation but also on the composition of the rock or
material further complicating the interpretation.
As electrical fields are necessarily three-dimensional, off-line structures can
greatly affect the voltage drop measured between electrodes, especially conductive
objects such as pipes or fences. For this reason, such structures are generally avoided
if possible. However, these objects when geological are often the targets of the
surveys.
3.3. Electromagnetic Induction
EM induction for finding terrain conductivity applies Faraday’s law of
induction (Telford et al., 1990). A transmitter coil creates a magnetic field that
induces small currents in the ground in the form of eddy currents that are then
detected by inducing a current in a receiver coil a certain distance away. The ratio
between the transmitted and measured fields is proportional to the integrated
conductivity structure of the subsurface over the depth sensitivity determined by the
spacing of the transmitter and receiver coils. Mutual inductance is used as the ratio of
the voltage induced in the receiver over the current transmitted by the source coil. As
the secondary field attenuates with depth, the sensitivity is greatest at shallower
depths relative to the coil spacing and samples a roughly cylindrical volume.
Horizontal, co-planar coil EM induction, termed Slingram, systems are most
common for ground conductivity surveys. Fixed-distance, mobile Slingram systems
can be operated by a single person. These EM instruments have a maximum depth of
investigation approximately twice the separation of the coils but depend upon the
ground conductivity structure (Milsom, 2008). In this way, broad areas of
volumetrically integrated ground conductivity can be measured quickly.
EM induction surveying provides a quick method for obtaining the average
shallow conductivity structure of the subsurface and is similar to ERT in its
sensitivity to clays and water content. However, EM induction integrates over a
volume with a set depth range making it more useful for deciding locations of the
more detailed ERT lines or delineating broad subsurface structures.
Only reconnaissance EM induction surveys were conducted for the BcCZO
project, but EM induction holds potential for further use in future studies.
3.4. Ground-penetrating radar (GPR)
The GPR method uses the reflection of high frequency (1 MHz – 1000 MHz)
electromagnetic waves to obtain a detailed image of subsurface interfaces (Milsom,
2008). The electrical permittivity or dielectric constant of the subsurface materials
determine the amount of energy available to reflect back to the surface (low values)
rather than dissipate through the medium (high values). Low frequency wave surveys
probe deeper with less resolution than high frequency surveys. A transmitter antenna
creates the electromagnetic wave that interacts in the subsurface and the returning
reflected waves are sensed with a receiver antenna. These electromagnetic waves
travel at speeds generally much lower than the velocity of light in air (0.30 m/ns) as
determined by the material properties of the subsurface. Fixed-offset profiling allows
the lateral surveying of the subsurface while common mid-point soundings are useful
for setting the velocity structure of the subsurface for converting two-way traveltime
to depth in reflection sections.
In materials with high electrical permittivities, the electromagnetic energy
attenuates quickly and GPR’s depth of investigation is greatly reduced. Both water
and clay have high permittivities, as it is also a function of the conductivity of the
material, reducing GPR’s effectiveness in some terrains and for imaging below the
water table. However, since the dielectric constant of ice is very low compared to
most earth materials, GPR is excellent for permafrost and ice thickness studies.
Multiple GPR lines were run in the field areas using a 1 m fixed offset of the
antennae and 0.5 m spacing between measurements. Generally one common midpoint survey per line constrained the general velocity values for the line. One set of
co-located ERT, SSR and GPR lines was run in the Betasso catchment, however the
GPR data has not been distributed beyond the primary data collector.
3.5. Geophysical Methods Summary
Shallow geophysical techniques provide a snapshot of subsurface properties
and structure that capture the current state of critical zone development. By
overlapping multiple methods, complementary datasets can be compared to
understand the processes active both in the landscape and in the subsurface. As
multiple techniques employ unique physics, distinct but inherently linked properties
provide separate confirmations of structure or demand interpretation that resolves the
differences.
Chapter 4
Results
The bulk of the geophysical data were recorded over May through August of
2009. ERT surveys in Betasso were collected at the beginning of May to ensure moist
conditions to reduce ground resistance that had previously ruined multiple surveys. In
Betasso I collected over 4.5 km of SSR lines (Figure 4.1). The majority of the 4.4 km
Figure 4.1. Overview of shallow geophysical lines collected in the Betasso catchment.
Figure 4.2. Overview of shallow geophysical lines collected in the Gordon Gulch
catchment.
Figure 4.3. Overview of shallow geophysical lines collected in the upper Green Lakes
catchment.
of SSR lines in Gordon Gulch were collected in the upper drainage with one major
cross-valley set of lines in lower Gordon Gulch (Figure 4.2). Difficult accessibility
and adverse weather conditions limited the days open for data collection in upper
Green Lakes Valley, therefore only a little over 1.2 km of SSR lines were gathered
(Figure 4.3). Across the three catchments, 2.75 km of ERT lines were collected, with
1.6 km in Betasso, 0.6 km in Gordon Gulch and 0.48 km in Green Lakes Valley. I had
access to the inversions for the ERT lines within the Betasso catchment (M. Leopold,
personal communication, 2009). All but one ERT line in Betasso were co-located
with SSR lines that make up a dense survey web.
I selected SSR line locations to capture the expected geomorphic processes in
each catchment. In Betasso the lines were distributed to capture any potential
structural differences dependent on location in the catchment (i.e. upper or lower; east
or west slope) or location on the hillslopes (i.e. valley, slope or ridge crest). Also,
lines were run through both grassy meadows and pine forests, often crossing the
transition zone between them. In Gordon Gulch cross-valley lines were run to capture
the effects of slope aspect and position on subsurface. Upper Green Lakes Valley
lines were selected based on the ability to perform a survey (i.e. sufficient cover over
bedrock to ensure geophone coupling with the ground) necessarily biasing results
towards areas with less exposed rock. Thus, to achieve a small sampling of
topographic features, SSR surveys were run in the valley bottom, on the northern
slope, and northern ridge crest (the southern slopes are dominantly cliff faces). Both
ERT and SSR lines in all three catchments avoided bedrock exposures at the surface.
Smoothed tomographic inversion scheme was used for separately inverting
the ERT and SSR data. Occasionally the inverted sections show sharp vertical
velocity or resistivity contrasts, thus marking a distinct layer. However, a weathered
profile may smoothly progress from bedrock to regolith in both electrical and seismic
properties. Characteristic velocity and resistivity values were chosen to approximately
mark major subsurface changes (Table 4.1). Regolith and unconsolidated material
generally have values of less than 700 m/s and 700 m but may vary if compacted
(Vp > 1200 m/s) or dry ( > 1000 m). Weathered or more compacted materials are
Table 4.1. P-wave velocity and electrical resistivity assignments for subsurface
materials in the Boulder Creek watershed.
P-wave velocity
Resistivity
Unconsolidated
< 700 m/s
< 700 m
material/regolith
Weathered bedrock
700-3500 m/s
700-1200 m
Crystalline bedrock
> 3500 m/s
> 1200 m
approximately 1400 m/s and 1000 m but span the property space between regolith
and rock. Crystalline bedrock has values greater than 3500 m/s and 1200 m. These
ranges fit with values reported in the literature although generally are lower in value
as the rock is weathered (Barton, 2006; Kneisel and Hauck, 2008). SSR studies within
the Boulder Creek watershed show similar velocity structures to those listed in Table
4.1 using three layer models (Leopold et al., 2008a, b).
Individual geophysical lines are used to explore the character of underlying
bedrock weathering patterns. Constructing fence diagrams allows further insight into
the three-dimensional weathering distribution including the impact of the local
topography. Additionally, referencing the geophysical results to geographic
coordinates with elevation values creates an accessible dataset that can be paired with
other measurements for more insightful interpretations.
As the BcCZO aims to be a cross-discipline research endeavor, I created
multiple methods for quickly accessing the information from inverted SSR sections.
Detailed line descriptions for every SSR line collected in this study are listed in
Appendix 1. In addition, I designed a graphical user interface (GUI) along with
multiple import and plotting functions within in MATLAB (The Mathworks) for
quickly plotting each SSR velocity profile for both visual analysis and to create
images for future publications. A detailed description of and instructions of how to
use the GUI are included in Appendix 2.
4.1. Betasso Catchment Results
Three sets of ERT and SSR lines cross the entire Betasso catchment towards
the top, middle and bottom of the drainage area (Figures 4.4 & 4.5). Table 4.2
summarizes all SSR lines in the Betasso catchment separated by line locations.
Continuous lines were not possible because of the steep gully running down the
center of the catchment, smaller ravines, and a road.
The upper drainage cross-section (#1) is composed of four lines (SSR 3-6 &
11-13, ERT N-S 1-4). The onset of bedrock velocities began at 12 m deep with some
Table 4.2. Overview of average of all individual line maximum and minimum depths
to specific velocity layers in the Betasso catchment. z1 specifies the thickness of
overburden material (Vp < 500 m/s). z2 marks the depth to 1400 m/s as a central
value for weathered material or compacted colluvium. z3 represents the depth to
crystalline bedrock velocities (Vp > 3500 m/s). Separations between the eastern and
western slopes (relative to central ravine axis) are shown as well as valley center
values from the upper portion of the catchment with locations in the catchment also
shown (upper versus lower).
Depth of Velocity Layer (m)
Max z1
Max z2
Max z3
Min z1
Min z2
Min z3
z1
Variation (m)
(Max-Min)
z2
z3
2.2
2.4
1.5
6.1
6.2
5.2
13.1
13.2
11.7
1.0
1.1
0.8
3.0
3.3
1.8
9.0
8.6
9.5
1.2
1.3
0.7
3.1
2.9
3.4
4.2
4.5
2.2
Lower (n=7)
1.5
1.7
1.4
5.3
5.5
5.0
13.4
12.3
13.9
0.8
0.9
0.9
2.7
2.5
2.6
9.6
10.0
9.4
0.7
0.8
0.6
2.6
3.0
2.4
3.8
2.3
4.4
Central (n=6)
3.3
7.0
11.7
1.8
4.5
9.8
1.5
2.5
1.8
Overall Average
(n=37)
2.1
6.0
13.0
1.1
3.1
9.3
1.1
2.8
3.6
East (n=18)
Upper (n=13)
Lower (n=5)
West (n=12)
Upper (n=5)
Figure 4.4. Three SSR transects through the Betasso catchment. Locations of the transects are shown in red in the inset catchment
map. Low velocity (red) values are thick in the upper transect, but are thinner in the lower two transects.
Figure 4.5. Three ERT transects through the Betasso catchment. Locations of the transects are shown in red in the inset catchment map
and are co-located with the SSR transect lines.
short (< 10 m) convexities bringing the 3500 m/s layer to within 8 m of the ground
surface. Bedrock resistivity values were found at the depth of investigation (~25 m)
but also reach the ground surface. Some deep high velocity layers conflict with
deeper high resistivity layers towards the center of the drainage, highlighting a
pervasive disconnect between the methods that will be explored in the discussion
section. Weathered bedrock velocities are no deeper than 7 m while the depth of
equivalent resistivities range from the surface to the depth of investigation (~25 m).
Even regolith resistivity values surpass the depth of investigation while the velocity
values range up to 5 m depth thickening towards the valley center.
In the middle elevation range of Betasso, two ERT (S-N 1 & N-S 5) and SSR
(18, 19 & 28-31) lines cross the catchment (#2), separated in the middle by the large
central gully. Both ERT and SSR lines showed less than 2 m of regolith-valued
materials for the majority of the cross-section, with ERT showing up to 10 m on the
northeastern slope. SSR showed weathered values no deeper than 5 m. The ERT lines
had bedrock values at the surface near the topographically highest ends of the lines
and deepening closer to the valley floor while SSR had greater values approaching
the surface downslope. Fast velocity (>2000 m/s) knobs in both lines approached the
surface.
At the low cross-section (#3) in Betasso, two ERT (N-S 6 & S-N 2) and SSR
(20-24, 26 & 27) lines cross the drainage area with a gap at the central gully. Less
than 2 m of slow-velocity material was present in the SSR lines. Low resistivities
surpassed 10 m thick on the eastern slope and were generally 5 m thick on the
western slope. Weathered material values ranged between 2 to 7 m for the seismic
values where resistivities surpassed 10 m especially on the east slope. Bedrock
resistivities were not inferred within depth of investigation on the east but appeared as
shallow as 3 m towards the bottom of the western slope. Bedrock velocity values are
inferred 8 m from the surface but reach depths of 17 m near the western ridge crest.
Comparing all SSR lines in Betasso, the depth to the characteristic material
velocities in Table 4.1 show some correlation with location in the catchment (Table
4.2). Lines near the center of the drainage have an average 1.8-3.3 m of slow material
compared to 1.0-2.2 m and 0.8-1.5 m on the eastern and western slopes, respectively.
The central lines lie mainly in the upper portion. Slow material was thicker (> 1 m)
higher in the catchment compared to lower on both the east (0.9 m) and west slopes
(0.3 m). Overall, slow material in Betasso was on average 1.1-2.1 m thick (n =37). In
the central lines the middle velocities ranged between mean depths of 4.5-7 m and
was shallower on the east (3.0-6.1 m) and west (2.5-5.5 m) slopes. The onset of midvelocity values averaged depths of 3.1-6.0 m in Betasso. Bedrock values were found
at 9.3-13.0 m depth. Shallower fast layers were found in both the upper central
portion (9.8-11.7) and lower east slope (9.5-11.7 m) with deeper and more variable
fast velocities on the upper east slope (8.6-13.2 m) and western slopes (upper: 10.012.3 m; lower: 9.4-13.9 m).
As part of the analysis of the Betasso lines, the coordinates of each inverted
seismic line allowed plotting of the velocity sections within a geographic framework
such as a geographic information system (GIS) or MATLAB. Thus, a pseudo threedimensional network of SSR lines plotted over digital orthophoto quad (air photo)
draped on topography is available for further analysis with other datasets (Figure 4.6).
Figure 4.6. Fence diagram model of P wave velocities geographically oriented in the
Betasso catchment.
4.2. Gordon Gulch Catchment Results
Four sets of SSR lines cross the Gordon Gulch catchment, three covering the
upper drainage and one in the lower (Figure 4.7). In addition, two ERT lines were run
in dry conditions along SSR transect lines. Low relief in the upper catchment allowed
continuous transects across the drainage area. Table 4.3 summarizes all SSR lines
recorded in the Gordon Gulch catchment.
Starting at the lower transect (#4), about 500 m from the outlet of the Gordon
Gulch watershed, are SSR lines 1-8. On the northern slope, I find 3500 m/s and
greater velocities residing at 14 m depth, with high velocities (> 2000 m/s) reaching
to within 3 m of the ground surface. Slow-velocity material reaches a maximum
thickness of 3 m near the valley center. I find abundant weathered values above 5 m
depth, but generally, these values are at 3 m depth for the majority of the line. The
Table 4.3. Overview of average of all individual line maximum and minimum depths
to specific velocity layers in the Gordon Gulch catchment. z1 specifies the thickness
of overburden material (Vp < 500 m/s). z2 marks the depth to 1400 m/s as a central
value for weathered material or compacted colluvium. z3 represents the depth to
crystalline bedrock velocities (Vp > 3500 m/s). Separations between the northern and
southern slopes are shown with locations in the catchment also shown (upper versus
lower).
Depth of Velocity Layer (m)
Max z1
Max z2
Max z3
Min z1
Min z2
Min z3
z1
Variation (m)
(Max-Min)
z2
z3
1.6
1.5
1.8
4.6
4.9
3.7
12.9
13.0
12.7
0.9
0.8
1.1
3.1
3.2
2.8
10.9
10.5
12.0
0.7
0.7
0.7
1.5
1.8
0.9
2.0
2.5
0.7
Lower (n=3)
1.9
1.6
2.5
5.6
4.9
7.3
15.3
15.0
16.0
1.0
0.9
1.3
3.7
3.1
5.0
10.9
10.7
11.2
0.9
0.7
1.2
1.9
1.7
2.3
4.5
4.3
4.8
Central (n=7)
1.9
5.3
11.9
0.9
3.6
8.4
0.9
1.6
3.4
Overall Average
(n=35)
1.7
5.0
13.4
0.9
3.4
10.4
0.8
1.7
3.0
North (n=18)
Upper (n=13)
Lower (n=5)
South (n=10)
Upper (n=7)
southern slope velocity section shows bedrock values at 19 m depth that shallow to 13
m near the valley center. Slow-velocity material was generally 3 m thick but less than
1 m towards the top of the southern slope. 1400 m/s remained at 5-7 m depth.
Therefore, the northern slope SSR lines revealed fast velocities at consistently
shallower depths than on the southern slope.
The lowest of the three transects in upper Gordon Gulch (#3) is made up of
SSR lines 30-35. The northern slope had thin slow material (< 1 m) with 1400 m/s at
4 m depth. Bedrock values were found at 10 m depth with a broad 75 m knob of fast
material that reaches to within 4 m of the surface. In the valley center bedrock values
are found within 5 m from the ground surface but quickly deepen to 16 m depth along
the southern slope. Weathered values on the southern slope remain 2 m from the
surface except for multiple pockets bringing the velocity layer to a maximum 8 m
depth. Slow velocity material is less than 1 m thick.
Figure 4.7. Four SSR transects across the Gordon Gulch catchment. Locations of the transects are shown in red in the inset catchment
map. North-facing slopes show consistently deeper low velocities and likely describe deep weathering compared to the profiles on the
south-facing slope.
The next set of SSR lines (11, 12 & 15-17) was 400 m up-drainage and
crosses through an open meadow (#2). Bedrock velocities on the southern slope
appeared at 19 m depth. In the central valley bedrock velocities lay at a depth of 13 m
and shallow to 8 m for most of the northern slope, and is interrupted by a 50 m long
deepening to 15 m depth. Weathered velocity material ranges from 4-6 m depth on
the southern slope, and is thinner (~2 m) on the northern. Slow-velocity material
reached a maximum thickness of 2 m under the central meadow but was generally < 1
m thick.
The final SSR transect (9 & 20-25) ran towards the top of Gordon Gulch (#1)
through dense aspen forest. Bedrock velocities were found at 15 m depth on the
southern slope with < 1 m of slow material. Weathered velocities mainly extended to
depths of 5 m but reached 2 m from surface above a 60 m-wide high velocity mound.
Bedrock velocities in the center of the drainage appeared at 15 m depth with 1400 m/s
at 5-9 m depth and slow material 1-2 m thick. Bedrock velocities deepen from 11 m
to 15 m from the bottom to top of the north slope. Weathered velocity material ranged
from 2-4 m deep on the northern slope capped by < 1 m of slow material.
Table 4.3 summarizes the depths to the selected velocity layers. Across the 35
SSR lines in Gordon Gulch, average depth to bedrock and weathered material
velocities was 10.4-13.4 m and 3.4-5.0 m, respectively. Regolith velocity material
varied between a mean 0.9-1.7 m thick. Bedrock velocity interfaces were deeper on
the southern slope (10.9-15.3 m) compared both the valley (8.4-11.9 m) and northern
slope (10.9-12.9 m).
Figure 4.8. Fence diagram model of P wave velocities geographically oriented in the
upper drainage of the Gordon Gulch catchment.
Again, plotting the inverted SSR sections provides a navigable pseudo-3D
network of lines for further analysis in a GIS or MATLAB (Figure 4.8).
4.3. Upper Green Lakes Valley Catchment Results
Extreme topography in the upper Green Lakes catchment restricted SSR lines
to snapshots of the subsurface structure of the valley floor, northern slope and
northern ridge (Figure 4.9; Table 4.4). A single ERT line ran up the northern slope
nearly co-located with a SSR line. We avoided bedrock and talus at the surface
biasing our lines towards areas with more bedrock cover.
The set of SSR lines (1,2 & 10-13) covering a small portion of the valley
captures a depression in the topography draped with glacial till from the most recent
glaciation extending onto exposed bedrock (Caine, personal communication).
Bedrock velocities reached to within 2.5 m of the ground surface but averaged 4.7-9.7
Table 4.4. Overview of average of all individual line maximum and minimum depths
to specific velocity layers in the upper Green Lakes Valley catchment. z1 specifies
the thickness of overburden material (Vp < 500 m/s). z2 marks the depth to 1400 m/s
as a central value for weathered material or compacted colluvium. z3 represents the
depth to crystalline bedrock velocities (Vp > 3500 m/s). Slope, valley and crest
represent the unique survey areas.
Depth of Velocity Layer (m)
Max z1
Slope (n=3)
Valley (n=6)
Crest (n=3)
Overall Average (n=12)
Max z2
7.3
1.8
1.8
3.2
12.0
4.1
3.0
5.8
Max z3
15.7
9.7
9.3
11.1
Min z1
2.3
0.8
1.0
1.2
Min z2
4.0
1.9
3.0
2.7
Min z3
14.0
4.7
6.7
7.5
Variation (m)
(Max-Min)
z2
z3
z1
5.0
1.0
0.8
2.0
8.0
2.2
0.0
3.1
1.7
5.0
2.7
3.6
m with 1400 m/s between depths of 1.9-4.1 m. Slow-velocity material averaged 0.81.8 m thick.
Two sets of SSR lines (3, 8 & 9) separated by 500 m were run up the northern
slope. The first line had slow-velocity material > 12 m thick with knobs of faster
velocities (> 1000 m/s) reaching up to 4 m depth. Bedrock velocities found at depths
greater than the depth of investigation (~22 m). Conversely, the second line shows
bedrock velocities at 15 m depth with a long bench at 10 m depth. Slow-velocity
material ranged from 1-5 m thick. Weathered velocities varied from 2-10 m depth.
Finally a set of SSR lines (5-7) covered a portion of the northern ridge crest.
Bedrock velocities were found at 7-8 m depth with some undulations apparent in
slightly slower velocities. Weathered bedrock velocities remained at 3 m depth. Lowvelocity material was < 2 m thick.
Overall, bedrock velocities in the upper Green Lakes Valley catchment were
encountered at 7.5-11.1 m depths. Weathered velocities averaged 2.7-5.8 m depth.
Slow-velocity material averaged 1.2-3.2 m thick and was generally less than 2 m
thick.
Figure 4.9. Majority of SSR lines from upper Green Lakes Valley separated by topographic location. Note bedrock velocites at 5-7 m
depth along the ridge crest. Deep low velocities in the northern slope lines likely reflect blocky slope deposits with a large amount of
air between blocks. SSR line numbers in this figure do not correspond to SSR lines mentioned in the text.
4.4. Results Summary
Summer fieldwork in the three drainages of the Boulder Creek watershed
resulted in subsurface characterizations spanning the major features of each
catchment. Differences in subsurface structure described above will be discussed in
the next section and compared to the other locations.
Chapter 5
Discussion
5. 1. Geophysical Interpretations
Assigning geologic descriptions to the geophysical structure gives insight into
the subsurface beyond electrical or seismic properties.
Low-velocity material (< 700 m/s) represents unconsolidated material from
developing soil profiles to colluvium or slope debris deposits. Unweathered granite
and gneiss velocities range from 3500-7500 m/s and are generally above 4000 m/s
(Barton, 2006). Thus, subsurface layers with moderate velocities (700-3500 m/s)
represented various grades of weathering of crystalline bedrock (classes 4-7). Highly
compacted deposits may also show velocities > 700 m/s but < 2000 m/s. Less
weathered crystalline bedrock is shown where > 3500 m/s (classes 1-3). These values
correspond with Leopold et al. (2008a and 2008b) from major refractors present on
Niwot Ridge just outside the upper Green Lakes valley. The first layer Leopold et al.
find was interpreted as soil and unconsolidated material, ranging between 233-368
m/s. Periglacial slope deposits with more compaction than layer 1 have velocities
ranging from 567-777 m/s. In two lines, Leopold et al. find a weathered bedrock zone
or even more compacted deposits with 1650 m/s. Finally, Leopold et al. show
crystalline bedrock at 2908-3920 m/s. Despite unique bedrock lithologies within the
three catchments, I set the standard 3500 m/s bedrock value to all three field areas to
capture differences in crystalline bedrock weathering, though minor differences in the
fresh bedrock P wave velocities based on composition are likely.
The resistivity and velocity sections from the Betasso catchment using
appropriate values for the subsurface materials did not show similar structure in the
subsurface (Figure 5.1). This apparent disconnect highlights the differences in the
Figure 5.1. Interpreted subsurface structure from the uppermost northeastern set of
co-located SSR (top) and ERT (bottom) lines in Betasso. Apparent difference in
subsurface architecture is likely an effect of ERT’s sensitivity to water content,
potentially outlining water content rather than material differences as suggested in
this interpretation. Other differences in depth to bedrock between the sections are
likely due to unique electrical and seismic signals of weathering. Tree locations are
approximate.
sensitivities of ERT and SSR. ERT is more sensitive to and better at resolving vertical
features, while SSR requires increasing velocity with depth and smoothes vertical
features. This does not completely explain the differences in resulting structural
differences, only comments on resolving the complex bedrock surface topography.
Ideally, ERT and SSR would show similar features in the subsurface affected only by
their sensitivities. This was not the case in Betasso. While major features in the
subsurface did match, the extreme differences in resistivity and velocity gradients and
expected depth to bedrock required other interpretations.
ERT profiles in Betasso show bedrock values ranging both closer to and
further from the surface than their SSR equivalents. When bedrock values approached
the ground surface in the ERT lines, rock was also noted at the surface. However,
deep, low resistivity values in ERT lines likely represented deeper bedrock but not as
deep the section suggested. Rather, these values resulted from higher water saturation
of the bedrock, potentially delineating areas with denser fracture networks. Therefore,
shallower bedrock suggested by the SSR lines likely accurately describe the
subsurface architecture, as wet fracture networks will not affect the P wave velocity if
sufficient seismic pathways through solid rock persist.
A model of weathering in the ground must then explain the difference
between the resulting electrical resistivity and velocity profiles. One explanation lies
in the difference in response to water saturation, as explained above. However,
unique effects and patterns of weathering on these geophysical signals offer another
explanation. Assuming isovolumetric weathering, as in the case of saprolite, the
density decreases as silicates are weathered into clays, leading to both lower
Figure 5.2. Weathering models of crystalline bedrock for explaining the apparent
disconnect between velocity and resistivity subsurface structures. (a) Planar fractures
surrounding by a rim of weathering especially with high water content would affect
the recovered resistivity structure much more than the P wave velocity structure. (b)
Pockets of weathering would lead to a similar drop in resistivity as clays develop
while measured P wave arrival time changes would remain minimal.
velocities and resistivity values. Fractures providing conduits for water likely increase
local weathering rates with an influx of ions for both chemical and biological activity.
A curtain of vertical fractures, or a single fracture plane perpendicular to the survey
line, would slow seismic wave propagation but likely lack the influence to be imaged
in the inversion, whereas the resistivity could be lowered significantly with higher
water saturation and clay content (Figure 5.2a). Alternatively, if weathering pockets
within in the crystalline rock may also form sporadically along fractures, and the
resulting effect on the geophysical signals would be quite different (Figure 5.2b).
Seismic velocities determined from traveltimes would remain roughly the same, as
the first arrivals would be from fresh pathways. Electrical resistivities, however,
would be significantly lower. The distribution of these weathering products greatly
affects the recorded signals. Qualitatively comparing the ERT and SSR lines in
Betasso, weathering affects the signals in unique and separate ways.
Comparison of a co-located ridge-top set of SSR and ERT lines from Betasso
shows the effect of fractures in the development of the landscape from unique
geophysical subsurface structure (Figure 5.3). Between 140-165 m a low resistivity
pocket extends beyond the depth of investigation (< 15 m). A similar low velocity
pocket lies between 100-150 m but does not extend past 10 m depth. An explanation
could be derived from the possibility of subtlely different line acquisition locations,
especially in the horizontal translation of the pocket, or due to the SSR inversion
requirement of increasing velocity with depth. The SSR inversion does result in the
deepening of bedrock velocities but is correctly shown very near the surface in the
resistivity values from 48 m to the end of the line.
Figure 5.3. Comparison of a co-located ridge-top set of SSR and ERT lines shows the
effect of fractures in the development of the landscape from unique geophysical
subsurface structures. Note that low resistivity values from 140-165 m extend much
deeper than low velocities. Also, note the reversal of resistivity color scheme for ease
of comparison.
A joint inversion scheme could overcome the apparent differences in the
inverted profiles. The joint inversion method using cross-gradients constrains the
cross-gradient of the inverted data to be equal to or near zero under the assumption
that the methods are sensing the same subsurface structures (Gallardo and Meju,
2003, Gallardo and Meju, 2004, Linde et al., 2006). Applying this constraint yields
inverted sections with necessarily similar features. If the differences between the
resistivity and velocity sections in Betasso resulted from the survey setup and
inversion techniques of the two methods and not from unique sensitivities to the
distribution of weathering a cross-gradient constrained inversion of the datasets
would prove more insightful and likely more accurate than individual inversions.
However, the creation of a joint inversion scheme was beyond the scope of this
project.
5.2. Geomorphic Analysis
The SSR and available ERT lines cover a large portion of the three
catchments studied in the BcCZO. They capture first the structure of the subsurface,
focusing on critical zone development, but, secondly, should reveal snapshots of any
signals of the active weathering in the erosional regimes.
Within the Betasso catchment, the transient signal of increased erosion from a
drop in base-level or outlet location disappears between the middle (#2) and upper
(#1) line transects. Thick deposits of colluvium remain high in the catchment,
untransported down drainage, suggesting increased stability. Alternatively, the upper
transect represents a snapshot of the subsurface where the signal of increased
weathering could be active but only for a short period of time. Thus, the effect of the
rejuvenated incision of Boulder Creek extends a maximum of ~1000 m up the
Betasso drainage basin but more than 500 m. Also, the sections on the western slope
show slightly deeper weathering than those on the eastern slopes in the lower portion
of the catchment. A potential explanation comes from the aspect of the western slope
facing northeast and therefore in shadow more often than the eastern slope. This
would lead to less evaporation and more water availability for weathering processes
in the subsurface.
A very prominent aspect-driven variation in the weathering extents on the
opposite north- and south-facing slopes in Gordon Gulch. Within the low relief upper
portion of the drainage, insufficient erosion continues to allow deep weathering and
build-up of unconsolidated material to depths in excess of 20 m and 7 m, respectively.
However, lower in the catchment the north-facing slope consistently displayed much
deeper weathering. Sub-bedrock velocities (Vp < 3500 m/s) extend well beyond 10 m
on the north-facing slope but thin to 5-7 m at the valley floor and further north up the
south-facing slope. In the lower drainage of Gordon Gulch a similar situation reveals
sub-bedrock velocities beyond 15 m depth on the southern (north-facing) slope and 510 m on the northern (south-facing) slope. This significant difference in weathering
distribution is driven by aspect but likely manifest through the greater moisture
retention of the north-facing slope dependent upon the vegetation. The results brought
forth by Dethier and Lazarus (2006) of regolith thicknesses of 3.3 m and bedrock at 7
m depth in the post-Laramide erosional surface match the valley floor and southfacing slope velocity profiles. However, the data of Dethier and Lazarus (2006) fail to
capture the bimodal distribution of weathering related to aspect suggested by the
velocity models. Even so, the geophysical lines were laid out to avoid bedrock
outcrops and very rough terrain where inserting both geophones and electrodes is
difficult. This bias results in an overestimation of the overall distribution and
thickness of the weathered zone above bedrock.
Sampling the subsurface with applied geophysical lines in the upper Green
Lakes Valley catchment images interesting geomorphic features. A small till deposit
draped 5 m of material over the irregular bedrock surface. Deep (~10 m) block fields
lie on the northern slope in a semblance of stability surrounding shallowly buried torlike features, resulting in a smoothly sloping ground surface that hides the complexity
of the subsurface. Finally, the velocity sections on the ridge show an irregular
bedrock surface topography under an otherwise relatively smooth ground surface
buried below 5-7 m of less consolidated material.
In the upper Green Lakes Valley potential exists for permafrost. However,
these geophysical results do not locate permafrost, although they do not extensively
cover the catchment nor do they unequivocally mark its absence (Janke, 2004).
Unfortunately, both permafrost and crystalline bedrock have P wave velocities greater
than 2500 m/s (Kneisel and Hauck, 2008). However, the resistivity of ice is generally
much higher than the resistivity of crystalline bedrock, allowing permafrost to be
imaged by ERT (Kneisel and Hauck, 2008). In the ERT lines included in this study,
no clear permafrost values were found that could then be used to better interpret the
SSR profiles in the upper Green Lakes Valley.
5.3. Future Directions
This thesis provides both the basis for these geophysical studies and an
investigation of subsurface structure interpreted in terms of landscape evolution
processes. As limited co-located seismic refraction and electrical resistivity lines
provided valuable complementary datasets, further ERT lines will enhance the
interpretation of SSR lines, especially within the Gordon Gulch catchment. Also,
GPR can image finer structure than either ERT or SSR and will be very useful in
understanding very shallow soil development features near the ground surface. EM
induction surveys within the three catchments would provide even more laterally
extensive datasets of ground conductivity and show potential for studying the link
between dense fracture spacing and topography.
The extensive SSR datasets collected during this research hold multiple
opportunities for further analysis. First, when collecting the SSR data an increased
record length for all lines records the entire wavetrain from each shot. While not
adding any extra time to fieldwork, these long records allow the data to be applied
towards the creation of S wave profiles with surface wave techniques such as the
multi-channel analysis of surface waves (MASW) (Park et al., 1999). Select SSR
lines in the Betasso catchment underwent experimental MASW analysis and show
great promise for more extensive use. Applying the MASW technique to the SSR data
could allow the calculation of Vp/Vs and Poisson’s ratio throughout the profile.
Alternatively, S wave refraction surveys using horizontally oriented geophones would
allow the creation of S wave velocity profiles, but these would require more work in
the field. Additionally, a detailed study of the character and sources of attenuation in
the SSR data may reveal important information on the effect of weathering on seismic
wave propagation and be useful for robustly quantifying weathering across the SSR
lines.
Importantly, these and future geophysical datasets require more nongeophysical work in the field to obtain ground truth. With ground truth, the
geophysical values of P wave velocity and resistivity can be calibrated to rock types,
their amount of weathering or links to fracture density and water content. Soil pit data
exist for multiple sites in the Gordon Gulch catchment, but extend to a maximum
depth of 1.6 m, barely entering the sensitivity range for either technique. Borehole
drilling with consequent geophysical logging in Betasso is planned and will provide
critical ground truth information for reconciling ERT and SSR profiles.
Further geomorphic applications of these geophysical datasets exist as well.
The character of the weathering distribution recorded in the SSR and ERT profiles
can be used to guide numerical landscape evolution models with the application of
weight functions. The geophysical results suggest the importance of aspect in
weathering processes. More localized analysis of these geophysical profiles may be
used towards quantifying landscape processes such as hillslope stability models in the
upper Green Lakes Valley or hydrologic modeling with a better understanding of
water flow paths from ERT lines.
5.4. Conclusions
The critical zone represents the thin, life-sustaining realm of the terrestrial
surface. As such, the processes that control the development and consequent
transformation of the critical zone prove to be essential targets for ensuring the
continued health of the planet as the breadth of human influences on the system
continues to grow. A first order requirement for studying the critical zone is the
knowledge of the architecture of the subsurface. This subterranean structure reflects
both historic and current processes. Shallow applied geophysical methods detect
material physical properties in the subsurface with minimally invasive disruption of
the active processes. Additionally, these techniques can cover larger areas in less time
than point measurements from boreholes or deep soil pits. However, point
measurements provide a critical constraint for interpreting the geophysical results.
Shallow seismic refraction and electrical resistivity tomography surveys
throughout three catchments within the Boulder Creek watershed capture major
features of weathering and transport processes in the subsurface. In Betasso, where
fluvial rejuvenation at its base increasing erosional transport, SSR and ERT lines
show unconsolidated materials present at 750 m but nearly absent at 500 m from the
outlet. Conversely, deep weathering throughout Betasso remains with generally, but
not always, bedrock values near the surface further upslope from the valley center.
Deep weathering (> 15 m) occurs in Gordon Gulch, reflecting the expected signal of
low erosional potential of the post-Laramide surface. However, the majority of deep
weathering seems to be controlled by the moisture and vegetational regimes, which
strongly depend on slope aspect. Shallow slopes towards the top of Gordon Gulch
also display thick unconsolidated materials and deep weathering (> 15 m). Finally,
while the glacially scoured upper Green Lakes Valley is predominantly bedrock close
to the surface, thick (>10 m) blockfields and buried bedrock knobs persist on the
steep northern slope. Surveys on the northern ridge crest reveal bedrock consistently
at 5-7 m depth.
Differences between subsurface structures from SSR and ERT profiles result
from unique sensitivities, inversion methods and possibly from separate responses to
weathering patterns. However, these inconsistencies provide additional constraints on
the characteristics of the subsurface in a critical zone context. Low resistivity values
in areas with relatively higher seismic velocities suggest either high clay content or
more water saturation, thus outlining areas of intense weathering or higher fracture
density.
By collecting dense networks of geophysical lines, a pseudo-3D model of
subsurface properties spans each catchment and can be used to guide future research
efforts. Including other physical measurements with the geophysical data within a
geographical information system (GIS) creates both a method for checking the
correlation of unique measurements and for interpreting cooperative results. Finally, a
graphical user interface (GUI) for displaying the SSR results should allow quick
retrieval of geophysical results.
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Appendix 1 - SSR Line Descriptions
*co-located with ERT line
Betasso
Line
1*
2
3*
4
5
6*
7*
8
9
Description
Runs along NW ridge, somewhat forested. Rock exposed at surface. 1400
m/s within 1 m of surface (max 5 m), 3500 m/s at 12 m depth.
Runs up center wedge between gullies at top of catchment. 1400 m/s about 8
m from surface at bottom (near large soil profile exposure) to 2 m at top
where rock exposed at surface. 3500 m/s at 12 m on near exposure but
deepens to > 15 m under rock exposure. < 500 m/s ~ 5 m deep at bottom and
shallows uphill.
Runs down ridge near picnic area N to S. 1400 m/s fluctuates ± 2 m around 5
m deep. 3500 m/s stays about 11 m deep but deepens slightly downhill (1 m
or so). Lower velocities (2000-2500 m/s) have hummocky/undulating
appearance – looks kind of like broad buried tors. 500 m/s remains within 2.5
m of surface, some small amplitude undulations.
Cross-line for #3 (90 m, 35 m on #4) running along slope W to E. Starts near
large tor then runs through meadowy area. 500 m/s < 2 m depth. 1400 m/s ~
4 m depth. 3500 m/s about 8 m depth with almost 25 m pocket deepening
velocity to 12 m.
Cross-line for # 3 (40 m, 35 m on #5) running along slope W to E. Starts on
exposed bedrock (at 0 m, first geophone at 5 m) with bedrock again at 45-50
m and 5 m from last geophone. 3500 m/s near start of line ~ 3 m deep, at 26
m slopes down to a steady 9 m depth. 1400 m/s remains near 3 m deep. 500
m/s < 2 m thickness.
Continuation of #3 across road runs S down hill ending adjacent to steep
central gully below the confluence of 2 upper gullies. 500 m/s on upper
section ~ 2 m thick increasing to about 5 m thick towards end with pocket
from 95-100 m. 1400 m/s variable between 2-5 m near start to 5-10 m near
end. 3500 m/s begins at 4 m depth, increases to 8 m quickly then lowers to
~12 m for the meadowy second half of line.
Shares start location with #3 and runs SE along ridge. Rocky at surface
around 50 m (and 105 m). After about 90 m enters into treeless meadow.
3500 m/s 12-16 m deep, deeper at top than bottom. 1400 m/s undulates
around 5 m depth with jump to surface at 105 m. < 1000 m/s 3 m thickness.
2000-2500 m/s undulates 7 times with height of about 5 m from 3500 m/s
layer.
Cross-line for #7 (125 m, 56 m on #8) running N through meadow. 3500 m/s
12-13 m deep. 2000-2500 m/s hump apparently at ridge crest between 60100 m on line and is 6 m higher than rest of values. 1400 m/s at 7 m (and
almost 5 m thick in 10 m section) before crest and then to 3.5 m deep. Near
start of line < 500 m/s almost 4 m thick, thinning to 1 m near crest and
continuing 1 m thick beyond crest.
Continuation of #8 at slightly different bearing that crosses meadowy
10
11*
12*
13
14
15
16
17
18*
drainage NE of Betasso ending on bedrock outcrop. <500 m/s depth of 1 m
near start increasing quickly to 3 m with 4 m max at the poorly developed
gully (propagating uphill) then gently thinning to 1 m for last 50 m. 1400 m/s
~ 5 m thick across line deepening to 7 m at gully. 3500 m/s 7 m deep near
line start and end and stepping down to > 13 m especially on N end of line.
Cross-line for #7 (~80 m, ~175 m on #10) starting on N end of meadowy
drainage NE of Betasso starting near picnic area. 3500 m/s begins ~ 5 m deep
increasing to 15 m deep over 30 m and remains mostly constant with one
undulation (40-50 m, 5 m height) to step down to ~ 24 m deep at 150 m
along line. 1400 m/s generally below 5 m deep down to about 8 m.
Short line running up W slope of Betasso near confluence of the upper 2
large gullies ending just across abandoned pipe road. 3500 m/s ~ 12-14 m
constant depth. < 500 m/s above 3 m. 1400 m/s starts at 9 m depth and
undulates up to 7 m.
Continuation of #11 after jog of ~75 S running up W slope of Betasso
starting just above abandoned pipe road. Rocky at surface along last 2/3 or
line with bedrock exposures. 3500 m/s average depth of 16 m with multiple
undulations ranging up to 9 m depth between 130-150 m. < 500 m/s ~ 3 m
thick for 0-90 m and 175-end with other sections < 1 m thick. 1400 m/s ~ 7
m depth near start of line (0-80 m) then undulates following faster velocity to
within 1-2 m of ground surface.
Cross-line for #12 (127, 27 m on #13) running N-S along slope contour.
Crosses rocky section noted in #12. 3500 m/s starts at 4 m depth and steps
down to ~ 8 m within first 15 m. 1400 m/s around 3 m. < 500 m/s 1 m thick
and mostly less.
Cross-line for #2 very near start (gully confluence) and directly over large
soil profile exposure running SW-NE (used 1 m spacing). < 500 m/s 2.5 m
thick. 3500 m/s at 9 m with very strong contact with lower velocities. 1400
m/s about 6 m deep.
Cross-line for #2 ~25 m uphill from #14 running S-N (1.5 m spacing). 1400
m/s starts at 8 m depth and climbs to 5 m by end of line. 3500 m/s
consistently 11-12 m deep and quite sharp. < 500 m/s ~4 m thick near start
and shallows to ~ 2 m thick
Cross-line for #2 (~60 m, 21 m on #16) and ~25 m uphill from #15 running
S-N (1.5 m spacing). 3500 m/s sharp contact ranging smoothly from 10-12 m
from start to end of line. 1400 m/s from 6-8 m following same trend. < 500
m/s consistently 3 m thick.
Cross-line for #2 (~90 m, 19.5 m on #17) and ~25 m uphill from #16 running
S-N (1.5 m spacing). From 70-110 m in grassy but steep meadow ~ 50 m
below Betasso Rd. Bedrock outcrops 5 m past line. 3500 m/s consistently 12
m deep until step up around 110 m to 8 m depth. 1400 m/s at 5 m depth until
last 30 m of line when disappears. < 500 m/s < 2 m thick with small pockets
2 m thick.
Runs W-E down western slope halfway down Betasso starting past smooth
ridge with bedrock outcropping near the start (~10-30 m). Crosses through
meadow at start and then plunges into forest as the slope increases.
19*
20*
21
22
23*
24
25
26
27
Abandoned pipe rd crosses line at 113-115 m with active pipe trace (no trees
and suspiciously gravely/grassy). < 500 m/s < 1 m thick along entire line.
3500 m/s undulates with average depth of 15 m up to 10 m at the middle (5580 m) and end (140 m-end) of the line. 1400 m/s generally 4-5 m deep with
‘fast’ pockets having a deeper and thicker moderate velocity layer.
Runs NE-SW down eastern slope halfway down Betasso (approximately the
across-valley continuation of #18). Rocky surface along entire line with large
bedrock outcrops near line (e.g. 15 m from 21 m on line). 3500 m/s > 7 m
depth, shallower towards the top of the slope and deepens to ~12 m for the
majority of the line and ends with last 20 m near 7 m depth. > 500 m/s < 1 m
across line. 1400 m/s 2 m deep until 80-90 m where disappears and returns at
2 m but reaches short sections with depths of 5 m.
Runs SW-NE up eastern slope towards the bottom of Betasso starting next to
bike trail. 3500 m/s ~ 13 m deep with small section (130-140 m) 8 m deep.
1400 m/s < 6 m depth to sections < 1 m depth. < 500 m/s 1-2 m thick.
Cross-line for #20 (127m, 0 m on #21) running E-W along slope contour.
Sharp contact with 3500 m/s at 14-15 m depth. Knob of 2500-3000 m/s
sticking up ~10 m (10-20 m) from 3500 m/s layer potentially reaching the
ground surface ~ 20 m. 1400 m/s 3 m deep at start and deepens to 6-8 m deep
after knob. < 500 m/s 1 m thick until after knob when becomes 1-1.5 m thick.
Cross-line for #20 (~80 m, 38 m on #22) running S-N along slope contour on
steep sparsely vegetated slope. 3500 m/s 10-11 m depth from start to end. <
500 m/s < 1 m thick. 1400 m/s < 3 m depth to not present.
Runs E-W up western slope towards the bottom of Betasso (roughly opposite
#20). Starts on bedrock exposure and passes over crest of ridge. 3500 m/s
starts at 9-11 m depth (0-50 m) and then deepens to 15-17 m until near the
ridge crest (~12 m deep). Over the deeper fast layer, 2000-3000 m/s knobs
extend upward 9 m. 1400 m/s starts 2 m from surface and deepens to 5-7 m
from 60 m to the end. < 500 m/s generally 1 m but patches of 2 m thick.
Cross-line for #23 (116 m, 36 on #24) running S-N along Betasso’s western
ridge crest. Pebbly on surface with rock exposed beyond end of line. < 500
m/s < 1 m thick with pocket ~ 2 m thick (20-25 m). 3500 m/s at depths of 1314 m. 1400 m/s undulating greatly but generally > 4 m deep to max of 8 m.
Runs W-E down western slope equally spaced between #23 and #18 with no
cross-lines. Starts in meadow and becomes steeper ~ 60 m with trees and
rock around line. Near crest, 3500 m/s starts at 10 m deep and then deepens
to 17 m and then goes to 6-10 m deep after 70 m. 1400 m/s lies 3-4 m from
ground surface. < 500 m/s < 1 m for entire line.
Cross-line for #23 (~50 m, 30 m on #26) running from N-S along contour.
Rocky areas along line. 1400 m/s ~2-3 m depth. < 500 m/s 1-1.5 m thick.
3500 m/s 10-13 m deep. 2000-3000 m/s knobs almost reaching surface (e.g.
27-32 m).
Cross-line for #23 (10 m, 28 m on #27) running N-S with a rocky outcrop at
7 m. < 500 m/s < 1 m thick. 1400 m/s spotty around 1.5-2 m depth. 3500 m/s
at 9 m depth with scoop (10-25 m) down to ~11 m and knob up to 5 m depth
(25-40 m).
28
29
30
31
32
33
34
35
37
38
Cross-line for #18 (~67 m, near middle of #28) running N-S roughly along
slope contour. < 500 m/s ~ 1 m thick. 1400 m/s ranging from 4 m deep on the
ends of the line to 2 m in the center. 3500 m/s at 11-12 m depth with bench
apparent 17-35 m in slightly lower velocities.
Cross-line for #18 (~145 m, near middle of #29) running N-S roughly along
slope contour from meadows to forested area after pipe clearing (~38 m).
3500 m/s at 12-13 m depth. < 500 m/s < 1-1.5 m thick. 1400 m/s 4 m deep to
7 m deep towards end of line (40-48 m).
Cross-line for #19 (145 m, near middle of #30) running NW-SE roughly
along slope contour with major outcrop from 40-44 m. Thin soil noted along
line. < 500 m/s < 1 m thick (0-12 m) to 2 m thick in pocket (12-17 m) and
then < 1 m thick. 1400 m/s 6 m deep at start of line (0-20 m) up to 2-4 m
depth for rest of line, 2 m from 37-44 m. 3500 m/s at ~11 m depth with 3 m
tall knobs of lower velocity (22-32 m; 35-45 m).
Cross-line for #19 (~70 m, near middle of #31) running N-S roughly along
slope contour with bedrock expected near surface. 3500 m/s at 7.5-8.5 m
depth. 1400 m/s at 2-3 m depth. < 500 m/s < 1 m thick.
2nd half of #1, see #1 for description. Originally two additional spreads but
2nd spread was recorded with too low of a sample frequency for picking first
arrivals.
Cross-line for #38 (~halfway along both) running NW-SE through small
meadow on hillslope between the two major drainages in Betasso. < 500 m/s
< 1 m thick. 1400 m/s 3-5 m below ground surface. 3500 m/s 13-14 m deep
with bench at 9 m depth (50-61 m).
Cross-line for #6 (~100 m, ~15 m on #34) running E-W along slope contour.
3500 m/s at 13 m for most of line with end of line at 10 m (87 m-end). 1400
m/s at 5 m depth from 0-60 m and 3 m depth from 60 m-end. < 500 m/s 2 m
thick from 0-20 m and thins to < 1 m thick for rest of line.
Overlap of #6 with 4.5 Hz geophones (essentially last 100 m of #6, starting at
50 m) running N-S from forested area to meadow ending < 10 m from gully
below confluence of 2 upper gullies. 3500 m/s at 12 m depth. 1400 m/s starts
at 3 m depth (50-70m) and then ranges from 3-8 m depth in two pockets (85105 m, 125 m-end). < 500 m/s < 1 m thick except in pockets where reaches 3
m thick.
(no #36 because lost track of numbering in the field) Runs across meadow NS paralleling upper gully starting ~40 m up-drainage from the end of #6.
Vegetation changes from meadow to pine forest ~ 115 m. Ends in small
grassy area on edge of gully. < 500 m/s generally < 1 m thick except in first
meadow where reaches 3 m thick (0-75 m) where starts to thin. 1400 m/s
follows the same general trend ranging between 2-7 m. Below first meadow,
3500 m/s at 11-13 m depth with sharp contact to ~ 100 m. 3500 m/s remains
at 9-12 m depth for rest of line but contact not as sharp with lower velocities
reaching up to 4 m below the ground surface.
Runs roughly parallel to #6 ~100 m to the SE running N-S starting 5 m from
Betasso Rd and ending ~40 m from central gully. Upper portion runs ~10 m
from borrow pit along bike path. < 500 m/s < 1 m thick. 1400 m/s 2-4 m
depth when present. 2000-3000 m/s undulate between 5-12 m depth. 3500
m/s remains 14 m deep for majority of line but deepens slightly at end to 17
m (170-180 m).
Gordon Gulch
Line
1*
2
3
4
5
6
7
8
9
Description
Runs S-N up northern slope of lower Gordon Gulch along lowest soil
transect line with MEW flags (~400 m from parking area) starting in small
aspen grove near stream (Matthias’ slope deposit lobe area) and ending 15 m
below bedrock exposure. 3500 m/s at 14 m depth. 1400 m/s at 5 m (0-20 m)
but 3 m or less for rest of line. < 500 m/s 3 m thick for 0-20 m and < 1 thick
for rest of line. Mid-velocity values show depth variability deeper for 0-80 m
and shallow for 80-190 m.
Cross-line to #1 (190 m, near middle of #2) running W-E roughly along
contour. 3500 m/s at 14 m depth with ledge of high velocity from 5-17 m at 5
m depth deepening to 9 m. 1400 m/s generally not present but otherwise < 2
m. Small slow velocity pocket 40-44 m. < 500 m/s < 1 m thick.
Cross-line to #1 (~120 m, near middle of #3) running W-E roughly along
contour. 3500 m/s at ~13 m deep with a knob of higher velocities reaching to
1 m of surface from 0-14 m. 1400 m/s mostly < 2 m depth but reaches 2.5 m
and is not present above knob – sharp contact or bedrock on surface. < 500
m/s < 0.5 m thick.
Cross-line to #1 (~60 m, near middle of #4) running W-E along contour.
3500 m/s at 9 m depth for 0-25 m and 10.5 for rest. 1400 m/s at 3 m depth.
<500 m/s 1.5 m thick.
Runs N-S up southern slope of lower Gordon Gulch directly across valley
from #1 starting 5 m upslope from creek. Rock noted at surface 90-97 m and
172-184 m. 3500 m/s at 13 m depth 0-42 m and deepens upslope to ~19 m.
1400 m/s at 5-7 m depth. < 500 m/s mostly 3 m thick down to < 1 m.
Cross-line to #5 (~120 m, near middle of #6) running W-E along contour.
3500 m/s at 12.5-15 m depth. 1400 m/s at 7-9 m depth from 17 m –end with
~1000 m/s reaching sharper contact with >2000 m/s at 10 m depth. < 500 m/s
< 2 m thick 0-15 m and 2-3 m thick for the rest.
Cross-line to #5 (~40 m, near middle of #7) running W-E along contour
through small gully. 3500 m/s at 8-9 m deep from 0-15 m deepening to > 14
m. 1400 m/s at 3-5 m depth. < 500 m/s < 1.5 m thick.
Cross-line to #1 (near start, 41 m on #8) running W-E south of path within
aspen meadow along creek. Sharp contact with 3500 m/s at 10-12 m depth
with layer sloping steadily to the east – does not follow ground surface. 1400
m/s at 5-6 m depth. < 500 m/s 2-3 m thick.
Runs N-S down northern slope along upper snow transect crossing central
path at ~25 m just east of larger (2-track) path. Ends at lowest part of valley
along this trend. 3500 m/s at 14-16.5 m deep. 1400 m/s at 5-9 m. < 500 m/s
1-2 m thick.
10
11*
12*
13
14
15
16
17
18
19
20
Runs W-E down center of upper Gordon Gulch through a meadowy aspen
area. Crosses #9 (~70 m, 15 m on #10). 3500 m/s at 13-15 m deep with
prominent knob reaching to 4 m depth from 40-60 m. 1400 m/s 5-9 m deep.
< 500 m/s 0.5-3 m thick. Thicker slow velocities towards end of line.
Long S-N trending line across the large meadow in upper Gordon Gulch
starting on the southern slope and ending 1 m from road (County Rd 233A).
Runs along soil transect flags (pre-digging). <500 m/s < 1 m for most of line
with ~2 m thick for 80-175 m (meadow). 1400 m/s 4-6 m on southern slope
and ~2 m on northern slope (higher distance values). 3500 m/s at 8 m deep
on northern slope and parallels ground surface with pocket down to 15 m
from 280-320 m. 3500 m/s ~13 m deep over middle of line (meadowy) from
70-160 m and ~19 m deep for 0-50 m.
Runs E-W roughly parallel to and south of path starting in the upper meadow
and continuing through aspens into pine. Ends near end of #10. 3500 m/s at 9
m depth for 0-65 m and then deepens gradually to 19 m by end of line up the
catchment. 1400 m/s at 3 m depth until end (~190 m) where deepens to 13 m
under slow pocket. < 500 m/s < 2 m thick until towards end reaches 4 m
thick.
Cross-line linking ends of #12 and #10 running S-N. < 500 m/s ~2 m thick.
1400 m/s at ~9 m depth. 3500 m/s at ~12-14 m deep.
Cross-line to #12 (~40 m, near middle of #14) running S-N on west edge of
upper meadow. 3500 m/s slopping from 9 m near start to 14 m at end. 1400
m/s 3 m deep with 5 m depth near end. < 500 m/s < 2 m thick.
Northern-most cross-line to #11 (~320 m, near middle of #15) running W-E
along contour. 3500 m/s at 10.5 m depth closely parallel to ground surface.
1400 m/s at ~2 m depth. < 500 m/s ~ 0.5 m thick.
Northern cross-line to #11 (130 m, near middle of #16) running W-E along
contour with large rock outcrop north of 0 m. 3500 m/s at 6.5 m depth
parallel to ground surface with small knobs. 1400 m/s at 2 m depth with 5-15
~1.5 m depth. < 500 m/s < 0.5 m thick for start and otherwise < 2 m thick.
Cross-line to #11 (near start ?, near start of #17) running E-W along the
southern edge of the upper meadow approximately parallel to #12. 3500 m/s
at 14-16 m depth with large knob from 120-170 m at depth of 7 m but
otherwise follows surface elevation. 1400 m/s at 4 m depth from 0-60 m and
then deepens to 8 m from 60-90 and then shallows to 3 m almost to end when
deepens again to 5 m depth (173 m - end). < 500 m/s < 1 m thick for most of
line with deep pockets to 3 m thick from 85-105 m and 2 m thick from 180 m
– end.
Semi-continuation of #17 along base of southern slope starting at end of #17
running E-W. 3500 m/s at 13-14 m depth with clean contact and roughly
parallels ground surface. 1400 m/s ~6 m depth. < 500 m/s < 1 m thick with
patches almost 2 m thick.
Cross-line to #18 (end of #18, start of #19) running S-N linking #18 with
#10. 3500 m/s at 11-13 m depth roughly parallel to ground surface with clean
contact. 1400 m/s at ~5 m depth. < 500 m/s ~ 2 m thick.
Cross-line to #10 (near start of #10 and #20) running S-N up northern slope
21
22
23
24
25
26
27
28
29
30
of upper Gordon Gulch along upper snow and soil pit transects ending 5 m
from rock shelf and ledge. 3500 m/s at ~11 m depth and parallels ground
surface. 1400 m/s at ~3 m depth. < 500 m/s < 1 m thick.
Continuation of #20 S-N up northern slope along transect line starting after
rock ledge at slightly different bearing ending 10 m from large rock ledge.
3500 m/s at 9-10 m depth from 0-22 m and deepens to 13-15 m for rest of
line. 1400 m/s at 2 m depth from 0-25 m and deepens to ~5 m for rest of line.
< 500 m/s < 1 m thick.
Cross-line to #21 running E-W to link with #23. 3500 m/s at 15 m depth
roughly parallel to ground surface. 1400 m/s at 1-2 m depth. < 500 m/s < 1 m
thick.
Semi-continuation of #21 offset by ~50 m running S-N up northern slope
crossing a two-track road and ending 5 m from 233A. 3500 m/s at 12-13 m
depth. 1400 m/s at 2-4 m depth. < 500 m/s < 1 m thick.
Runs N-S up southern slope along upper snow transect approximately
starting at start of #9 and running up to ridge crest. Rock exposure along
first 100 m on east of line. 3500 m/s at ~15 for most of line with apparent
ledge at ~10 m depth from 70 m – end. 1400 m/s at ~5 m depth reaching 1 m
above ledge and down to 9 m from 20-60 m. < 500 m/s < 1 m thick.
Cross-line to #24 (near start, near end of #25) running E-W near ridge crest.
3500 m/s at ~11 m depth parallels near flat ground surface. 1400 m/s at 5 m
depth. < 500 m/s ~ 0.5 m thick.
Runs W-E down center of flat drainage area just east of the Switzerland Trail
through open area and then into aspen grove. 3500 m/s at ~10 m for majority
of line and at 6 m from 200 m – end. 1400 m/s at 4 m depth for most of line
and at 7 m for 80-160 m. < 500 m/s generally < 1 m thick but up to 3 m thick
(110-130m).
Central cross-line for #26 running S-N through dense aspen grove and ending
near dirt path in sparse pine forest. 3500 m/s at 12 m depth for majority of
line but quickly steps up to 4 m depth from 90 m – end. Three pockets of
deep reaching low velocities from 12-28 m, 37-52 m and 58-76 m with 1400
m/s at 6 m depth and < 500 m/s reaching 3 m thick. Otherwise 1400 m/s at
~3.5 m and < 500 m/s < 1 m thick.
Eastern cross-line to #26 running S-N through dense aspen grove ending in
sparse pine forest 50 m past dirt path. 3500 m/s at 11 m depth but reaches 6
m depth in last 10 m of line. 1400 m/s at 3-4 m depth. < 500 m/s mostly < 1
m thick but ~ 2 m thick for center of line (40-80 m)
Western cross-line to #26 running S-N through upper clearing near
Switzerland Trail. 3500 m/s at 12-13 m depth but ramps up to 3 m by end of
line (160 m – end). 1400 m/s 2-3 m depth. < 500 m/s < 1 m thick.
Long line runs N-S down northern slope of upper Gordon Gulch skirting
233a near the valley floor starting on the ridge crest and ending in very
marshy area with standing water. 3500 m/s at ~10 m depth for majority of
line. Some knobs from 50-100 m and slightly shallower at very start (0-10 m,
4 m depth) and from 300 m – end (8 m deep). 1400 m/s at ~4 m depth. < 500
m/s < 1 m thick.
31
32
33
34
35
Cross-line to #30 near end running E-W south of dirt path and skirting the
marshy conditions. 3500 m/s at 8-9 m. 1400 m/s at 3 m depth. < 500 m/s ~ 1
m thick.
Cross-line to #30 halfway up northern slope running E-W paralleling and
finally crossing County Rd 233a. 3500 m/s at ~12-13 m depth and shallows
up to 8 m for last 10 m of line. 1400 m/s at 3 m depth. < 500 m/s < 1 m thick.
Northern cross-line to #30 running E-W along contour. 3500 m/s at 10-11 m
depth. 1400 m/s at 2-3 m depth. < 500 m/s < 1 m thick.
Continuation of #30 up southern slope of upper Gordon Gulch starting in
marshy area pashing multiple rock exposures and clear-cut areas. 3500 m/s at
5 m depth from 0-20 m stepping to 9 m depth from 20-50 m down to 15-16
m for the rest of the line. 1400 m/s at 2 m for 0-50 m where two pockets
bring layer down to 4 m followed by shallowing to < 2 m (130-145 m) with a
large pocket afterwards taking layer down to 8 m. < 500 m/s < 1 m thick.
Cross-line to #34 (middle-ish, near start of #35) running W-E along contour.
3500 m/s ~14 m depth mostly parallel to ground surface. 1400 m/s at 3 m
depth (very gentle gradient). < 500 m/s < 1.5 m thick.
Upper Green Lakes Valley
Line
1
2
3*
4
5
6
7
Description
Runs W-E along grassy area on the southwest of Green Lake 4 and roughly
along slope. 3500 m/s at 7-10 m. 1400 m/s < 4 m depth. < 500 m/s ~2 m
thick near start of line (0-50 m), thins to < 1 m (50-90 m), grows to ~2 m
(90-130 m) and finally thins to < 1 m for the rest of the line.
Continuation of #1 at slightly different bearing as a bedrock wall blocked
further spreads on #1. Runs W-E near balancing rock on path. < 500 m/s < 1
m thick. 1400 m/s 2 m to 4 m deep over first half of line and disappears after
27 m. 3500 m/s at 12 m depth at beginning of line and ramps up to 4 m by
the end of the line.
Runs S-N up the north slope (Niwot Ridge) starting just above a large
bulbous bedrock knoll. 3500 m/s deeper than depth of investigation for most
of survey (~22 m) but knobs at 19 m depth (50-70 m, 110 m-end). Knobs
appear of 1000-1500 m/s from 50-80 m. Low velocity material (< 1000) with
thickness of 14 m (0-50 m) to < 4 m thick over knobs and deepening to ~ 12
m afterwards.
No GPS or slope data, approaching ridge crest near end of line 3. Did not
finish line b/c very adverse weather conditions.
Extension of #3 with 50 m gap from failed #4 running S-N up and over the
crest of Niwot Ridge. 3500 m/s 7-8 m deep with some ups and downs (order
of 1 m). 1400 m/s ~ 3 m depth. < 500 m/s < 2 m thick.
Cross-line to #5 (~halfway along #5, ~50 m on #6) running W-E along the
crest of Niwot Ridge. 3500 m/s at 8 m depth with undulations 3 m in height
(e.g. 85-95 m). 1400 m/s ~3 m deep. < 500 m/s < 2 m thick. Videotaped
collecting a spread of this line.
Cross-line to #6 (106 m, 28 m on #7) and roughly parallel to #5 (east of #5)
8
9
10
11
12
13
running S-N. 3500 m/s at ~8 m depth. 1400 m/s at ~3 m depth. < 500 m/s <
1.5 m thick.
Cross-line to #3 (~135 m, 45 m on #8) running from W-E along slope
contour. 3500 m/s present for short portion of line (13-25 m) at 13 m depth
but otherwise beyond depth of investigation ~15 m. 1400 m/s at 10 m depth
with a very gentle gradient starting at 4 m depth. < 500 m/s < 2 m thick and
up to 3 m thick at start of line.
Runs SW-NE up the north slope (Niwot Ridge) starting near outlet to Green
Lake 4’s outlet pond (~60 m from waterfall) and passes near time-elapse
camera towards end of line. 3500 m/s generally ~15 m deep with broad
higher portion (100-190 m) at 10 m deep. 1400 m/s ~5 m deep and ranges
from 2-10 m. < 500 m/s mostly < 2 m thick with sections of ~5 m thick (e.g.
38-55 m)
Cross-line to #1 (~30 m, ~halfway on #10) running S-N with last 15 m on
angular boulders on the shores of Green Lake 4. Starts on edge of
bedrock/large boulders. 3500 m/s begins 4 m from surface and steps down to
~ 13 m depth (24-34 m) and then steps back up to 2.5 m. 1400 m/s ranges
from depths of 0.5-7 m mimicking the 3500 m/s layer. < 500 m/s is < 0.5 m
thick at the start and end and reaches a thickness of almost 3 m over middle
(10-33 m).
Cross-line to #1 (~80m, ~halfway on #11) running S-N with start on edge of
bedrock/large boulders east of #10. 3500 m/s at ~6 m depth at start and steps
up to 5 m by end of line (at 38 m). 1400 m/s between 1.5-3 m deep and
shallower towards the end. < 500 m/s < 1.5 m at start to < 0.5 m at end.
Cross-line to #1 (~110 m, ~halfway on #12) running S-N with start on edte
of bedrock/large boulders east of #11. Line runs within 1 m of soil
moisture/temperature recording box and crosses conduit. 3500 m/s at 6-9 m
depth closely following topography. 1400 m/s at 3 m depth for most of line
and becomes 2 m deep from 43 m-end. < 500 m/s ~ 1 m thick and up to 1.5
m.
Cross-line to #1 (end, ~end of #13) running S-N with start on bedrock with
bedrock at surface at 15 m and is east of #12. 3500 m/s at 6-8 m deep with
two knobs reaching a depth of 2 m (10-16 m) and 5 m (28-33 m). 1400 m/s
when present lies at 2.5-3.5 m depth. < 500 m/s < 1 m except for 43 m – end
where up to 1.5 m thick.
Appendix 2 – SSR GUI
The purpose of this graphical user interface (GUI) is to allow quick and efficient
viewing of the shallow seismic refraction (SSR) data collected as part of the Boulder
Creek Critical Zone Observatory.
Requires Mathworks MATLAB installed with the ability to use GUI’s.
Originally written on a Mac but will work on a PC.
Instructions for use:
1. Download compressed directory with the SSR inversion files and MATLAB
M-files (SSRgui.zip). The data will be available for download in the future
from the Boulder Creek Critical Zone Observatory data download website:
http://czo.colorado.edu/html/data_summary.shtml
2. Decompress/unzip the directory to your computer. You may rename the
folder, but do not change the names of the sub-directories or move individual
files.
3. Open MATLAB.
4. Navigate to the SSRgui or renamed directory in the MATLAB file structure.
5. To run the GUI, type “GUIssrczo” into the command window without the
quotes.
6. The GUI opens to the introduction image of the three field areas within the
broader geomorphic context (Figure A2.1). The program must be restarted to
see this image after plotting a seismic line.
Figure A2.1. Opening screen for the SSR GUI.
Figure A2.2. SSR line 37 in the Betasso Catchment is plotted here with a constant
depth line at 5 m below the ground surface. The blue asterisk corresponds to the blue
asterisk location in the map plot.
7. Automatically, the Betasso catchment is selected. To view the first SSR line in
Betasso, click the “First Line” button.
8. Navigate between lines in a catchment by using the “Previous Line” or “Next
Line” buttons. Alternatively, the desired line can be entered into the text box
labeled “Line Number” towards the right end of the window. Click “Apply” if
the plot does not change to input line number.
9. On the far left of the button window, the current line number is displayed and
updates with each new plot in the form “Line
1” (Figure A2.2).
10. To change the catchment, use the drop-down list labeled “Select Catchment”
located in the middle of the window.
11. To change the plot type, use the drop-down list labeled “Plot Type” to the
right of the select catchment drop-down menu. Options are: Faceted, Contour,
Filled Contour, or Both. Faceted is the standard for the plots that I show in my
thesis with cells outlined in a grid. Contour only plots the contour lines of
selected velocity values and can be changed in the vsplot.m file. Filled
contour plots the same contour values but fills the area between lines with its
particular color. Both plots a faceted velocity profile with its contours
overlain.
12. To plot a constant depth line, type a numeric value into the text box on the left
side of the window with “m Depth” to its immediate right. Click “Apply”
button to re-plot with the constant depth line now shown in dashed red (Figure
A2.2).
13. The “New Figure” button opens the current plot in a new figure, allowing
manipulation with other MATLAB commands. An example command is
set(gca,’xdir’,’reverse) to reverse the x-axis direction.
14. The “Map” button plots the current catchment with all the SSR lines in red
and the plotted line in yellow with the start point shown with a blue asterisk
(Figure A2.3). Topographic contour interval in these plots is 10 m.
15. Multiple image save options are available by pressing the “Save” button. This
is useful for beginners with MATLAB, but much more flexibility in saving
images is gained by using the “New Figure” button.
16. The “Help” button explains the purpose and shows which lines are available
for plotting in each catchment.
17. The “Close” button closes both the main GUI window and the Map window if
it is open.
18. For assistance selecting a particular line location rather than a line number,
see Appendix 1 – SSR Line Descriptions.
Figure A2.3. The location for SSR line 37 in the Betasso catchment is
plotted in yellow. All other SSR lines in Betasso are red. The boundary of
the drainage area for Betasso is black and filled with green. 10 m contour
lines are in blue. Distances are meters N-S or E-W from the center of the
catchment.