Resistivity Surveys of the Truckee Basin
Applied Geophysics -492-692
By Amie Lamb, Katie Ryan, Justin Skord and Nicole Spencer
Introduction
This paper discusses methods, results and interpretations of resistivity data collected at two
locations in the Truckee basin. Necessary background on the theory of electrical resistivity is provided
followed by discussion of methods, results and interpretations pertaining to this study.
Background/Introduction
Electrical resistivity surveys are used to determine subsurface geology at shallow depths,
including fault zones, mineral and fluid content, porosity and degree of saturation. A subsurface
resistivity distribution is created by measuring the flow of current in the ground. This is quantified by
Ohms Law:
J=σE
Current density (J) is the product of conductivity of the medium (σ) and electric field intensity (E).
Electric field potential is generally measured as a current point source (I) surrounded by an elemental
volume (ΔV) located at (xs, ys, zs):
-·[σ(x,y,z)ΔV)ζ(x – xs)ζ(y – ys)ζ(z – zs)
Giving the subsurface potential distribution at a location. This equation is used to determine observed
potentials of 2-D and 3-D geologic structures by numerical techniques and modeling. Resistivity
changes are measured in the field in both the vertical and horizontal directions by the Wenner array
which measures to a moderate depth but with a strong signal that is important for measuring in areas
with high background noise (Loke, 1996).
The primary structures we looked for were faults that would be delineated by changes in
resistivity through bedrock and sediments. Igneous and metamorphic rocks generally have high
resistivites that can range from 100 to 1 million ohm m, and can vary depending on fracturing and
saturation. Sedimentary rocks will generally have low resistivities, ranging from 10 to 5000 ohm m.
A fault zone could reveal a region of lowered or heightened resistivity depending on degree of
fracturing, rock type, and water content (Loke, 1996).
In the past, resistivity surveys have been conducted to find groundwater and geothermal
resources. In 2007, a resistivity study was conducted on the Redfield Campus at UNR in looking for a
possible geothermal energy resource. Vertical electrical soundings were used to find changes in resistivity
with depth. Electrical surveys helped discover the presence of an aquifer containing hot, saline
geothermal fluids (Huebner et. Al, 2007). In 1984, a series of resistivity studies were conducted in the
Spanish Springs area, northeast of Reno, to determine groundwater quality. Ringstad and Bugenig were
specifically looking for total dissolved solids in excess of 2,000 mg/L within a layered andesitic rock
aquifer. Exploration wells were drilled to confirm the electrical sounding results.
Methods
Introduction
Resistivity sounding and profiling has been conducted to observe possible changes in
electrical properties in two locations. Surveys were done along the Truckee River to detect
changes in resistivity due to fault zones. The second survey was done near Manzanital lane in
Reno to see the possible resistivity is effected by a known hydrothermal system in that area.
The following section introduces our survey locations and various methods utilized. There is
also a description of our process for modeling our resistivity soundings.
Survey locations
During the week of March 13, 2010, Justin Skord, Nicole Shivers, Katie Ryan, Amie Lamb and
others conducted electrical surveys along the Truckee river in downtown Reno and along Manzanita
lane in southern Reno. Many measurements were made but not all were included in our
interpretations.
General Setup
There are many different kinds of resistivity surveys but in general the arrangement does not
change much. The two outer electrodes, ‘C’, input current into the ground; the inner electrodes ‘P’
measure the voltage. The figure below shows the basic arrangement of a resistivity survey.
30m
C1
P1
A-spacing=10m
P2
C1
Fig. 2 The diagram above shows a generlized configuration our resistivity surveys.
The arrows labeled “C” are electrodes which are placed on the outside of the survey
line. They generate the electric current in the ground. The arrows labeled “P”
measure the voltage created by the two outer electrodes, “C.” The total length of
this survey divided by 3 is known as the A-spacing. In with a total length of 30m, we
have an A spacing of 10m.
Survey Types
Two types of resistivity surveys were conducted: soundings and profiles. Resistivity soundings
detect variation in resistivity with depth. Resistivity profiles detect variation in resistivity with lateral
changes in location.
Resistivity Sounding
In a sounding the general location of the survey remains in place but the electrode spacing is
varied. The survey measures deeper with increasing spacing between electrodes. This spacing between
electrodes is known as the A-spacing. This type of survey is frequently used for determination of water
table depth. For our purposes determination of water table depth is important.
There are two types of sounding methods: the Wenner and Schlumberger. In the Wenner type
both the current and potential electrodes are evenly spaced. The a-spacing begins small, at one meter
and is increased along defined intervals to a maximum a-spacing which we chose to be 6.81 meters.
The apparent resistivity is calculated at each a-spacing according to the following equation.
ρa= 2πAR,
where R is the resistance reading on the MiniRes, and A is the A-spacing.
In the Schlumberger array, only the current electrodes are moved. The potential electrodes
remain in place. The equation for apparent resistivity in the Schlumberger array is:
π A2R/4B
where A is the a-spacing, B is the distance between the potential electrodes, and R is the resistance
reading on the MiniRes. Fewer measurements with the Schlumberger array were possible because of
length limitations, and in this report none of our schlumberger soundings were modeled.
The figures below show the general setups of both the Wenner and Schlumberger sounding
methods.
Fig. 2. The Wenner sounding array. In the
Wenner array all of the electrodes are
evenly spaced. Apparent resistivity is
calculated rather simply according to the
equation shown below the diagram.
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Fig. 3. The Slumberger sounding array.
The potential electrodes are spaced much
more closely than the current electrodes.
The potential electrodes are not moved
while the current electrodes are moved for
each measurement. The apparent
resistivity is calculated differently from the
wenner sounding method
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Resistivity profile
This type of survey moves location for each measurement while the electrode a-spacing
remains constant. A resistivity sounding is often performed to find the optimal a-spacing for a resistivity
profile. Then only one a spacing measurement is taken at each location keeping the depth of
measurement constant while the location changes. In this study we conducted three resistivity profiles.
Two were done at an a-spacing of 6.81 meters and one was done at an a-spacing of 3.81 meters. We
utilized Resistivity profiling to detect possible changes in water saturation or water level due to
suspected fault zones along the Truckee River.
Modeling techniques
Modeling of Wenner soundings was done with the Resis computer program which comes
attached to the Berger text: Introduction to Applied Geophysics. This simple program allows modelers
model several layers in the subsurfase at several unique apparent resistivities.
The first step is selecting either the Wenner or Schlumberger sounding method. Our soundings
utilize the Wenner sounding method. Values of apparent resistivity are entered into the model and
recalculated by the modeling program. The data is then plotted on a graph of a-spacing vs. apparent
resistivity. Once the graph of a-spacing vs. apparent resistivity is generated the number of layers in your
model must be selected. For this study we used a three layer model mainly because our soundings only
had a maximum a-spacing of 6.81 meters meaning they only suvey resistivity to a depth of that amount.
Thus it is unlikely that more than three unique layers of resistivity are were observed in our soundings.
The final step in the modeling process is to adjust thicknesses and resistivities of each of the
layers so that the model matches the data as closely as possible. Once this is done, interpretations of
how resistivity changes with depth, can be made.
Results
Wenner Soundings
Truckee River Sounding
TRK24 easting: 258681
Fig. 1 Map location of TRK 24 resistivity wenner
sounding. A sounding measures changes in
resistivity with depth. The sounding was done at
an easting of 258681.
Fig 2 shows a model of the TRK24 sounding. Six apparent resistivities were found at six different a
spacings defined by the Wenner method. The table on top consists of three layers. The top-most
layer is 2m thick and has a resistivity of 600 ohm-m. The middle layer is 3m thick and has a
resistivity of 100 ohm-m. The bottom layer has an infinite thickness and a resistivity of 450 ohmm.
The graph at the bottom shows how well the model described above matches with our sounding
measurements.
Manzanita
MNZ1 easting: 258850
Fig. 3 Map above shows location of MNZ1 Wenner
sounding. Note that the location shows the survey in
the middle of the street. This is obviously an error in
plotting of the northing UTM. The Easting is verified
to be accurate.
Figure 4. Model of MNZ1 wenner sounding. The table at the top consists of three
layers. The top most layer is 1m thick with a resistivity of 39 ohm-m. The middle
layer is 5m thick with a resistivity of 23 ohm-m. And the bottom layer is infinitely
thick with a resistivity of 38 ohm-m. The graph at the bottom shows that the
model fits reasonably well with the data collected.
Wenner Profiles
Truckee River
Figure 5. Provile along truckee river. The profile was done at a constant a-spacing of
3.16m. This allows for lateral changes in resistivity to be observed. The graph at the
bottom shows the apparent resistivity according to its location along the profile.
There is one very high reading at station 24 which has an apparent resistivity of
greater than 700 ohm-m. The other five stations hover between 100-200 ohm-m.
Manzanita Profiles
Figure 6. Above shows a map of the locations of the MNZ4 survey. The graph
below plots the apparent resistivity vs. location. All measurements were done
at a constant a-spacing of 6.81 meters.
Figure 7. Map at the top shows the location of the MNZ6 profile. This profile consists of
nine measurements at a constant a-spacing of 6.81 meters. The apparent resistivity
values range from 20-60 ohm-meters. Station 10 shows the highest apparent resistivity
value at 56 ohm-m and station 14 shows the lowest value at just of 20 ohm-meters.
Discussion
http://www.moombarriga.com.au/userfiles/image/image009.jpg
Figure 8. The figure above shows general values of resistivity for various rock
types and other materials. The resistivity found in our modeling and profiles
range between 20-600 ohm-meters. These values fall close to values for clay
and gravel deposits. These deposits would be expected at these kinds of
geologic environments.
Wenner Soundings
The two sounding models differ greatly from each other. The sounding along the Truckee, in
general, has much higher apparent resistivity values. Resistivity here is at least 100 ohm-meters at ever
a-spacing. The sounding located at Manzanita has much lower values which do not exceed 60 ohmmeters. The top layer in the model has a thickness of two meters and apparent resistivity of 600 ohmmeters. This value corresponds to a gravel and sand saturated in fresh water (see figure eight). Gravel
and sand saturated in fresh water is very typical for an environment near a river. The middle layer of
the model has a much lower value of apparent resistivity compared to the top layer. This layer, which is
three meters thick, can be interpreted as some type of clay deposit (see figure eight). The third and
lowest layer in the model jumps back up to a resistivity of 450 ohm-meters likely representing a return
to sand and gravel deposits (see figure 8). The Truckee river sounding model depicts a low resistivity
layer bounded on the top and bottom by higher resistivity layers. This has been interpreted as a layer of
clay in between sand and gravel deposits.
The Manzanita sounding, in general, has much lower apparent resistivity’s than the Truckee
sounding. The difference between these two locations is not surprising. The Truckee River is a river
environment likely with a shallow water table which consists largely of fresh water. The Manzanita area
is a known geothermal resource area. Thus lower resistivity would be expected due to the salt content
in the ground water. The top layer of the Manzanita model is one meter thick with a resistivity of 39
ohm-meters. This very thin top layer could be dry clay (see figure 8). The middle layer is five meters
thick with a resistivity of 29 ohm-meters. This thin middle layer may be clay similar to the top layer but
may be more saturated in the brinish water related to the geothermal activity in this area. The bottom
layer begins at a depth of six meters. The layer shows an increase in resistivity up to 39 ohm-meters.
This higher resistivity layer could be a shift in lithology from clay to sand and gravel deposits saturated in
brinish groundwater. While it is clear that these two soundings are very different in their electrical
properties there is similarity in that each sounding shows a lower resistivity middle layer bounded at the
top and bottom by layers of higher resistivity.
Wenner Profiles
Truckee river, 13-27 (3.16 a-spacing)
The Truckee river profile consists of 6 measurements each taken at an a-spacing of 3.16 meters.
There is one anomalously high resistivity reading at station 24(see figure five). This station shows an
apparent resistivity of close to 700 ohm-meters. This value is higher than the other five stations by
factor of seven. There are several possibilities for the anomalously high reading at this station. Station
24 could be resting over a fault zone. Fault zones could lead to both resistivity high and low anomalies.
A high resistivity anomaly may indicate the presence of fractured rock material lacking fluid to fill in pore
spaces; thus a high resistivity anomaly is created. Another possibility for this high could be the presence
of a large boulder at this depth. The boulder may be igneous in composition which is a rock type known
to have very high resistivity properties.
Manzanita 4 (6.81 a-spacing)
The Manzanita four profile consists of six measurements each taken at a-spacing 6.81 meters.
There is an anomalously low reading at station three along this profile (see figure six). Stations one, two
and three show a declining trend in apparent resistivity; and station three shows the lowest value of this
declining trend with an apparent resistivity of nearly 150 ohm-meters. This low may be due to the
presence of a fault zone near this station. A fault zone can produce a low resistivity anomaly if there is
fractured rock that is saturated with fluid in the pore spaces of the broken material.
Manzanita 6 profile (6.81 a-spacing)
The Manzanita-six profile consists of nine measurements taken at a-spacing of 6.81 meters.
There are two anomalous sections of this profile. Station 10 is anomalously high and stations 14 and 16
are anomalously low. These anomalies could be fault related. Station 10 may be resting over a dry
fractured fault zone while stations 13 and 16 further to the west could be over a fluid saturated portion
of a fault-zone. Further comparison with other geophysical techniques utilized in this study is needed to
make more constrained interpretations of this resistivity data.
References
L. Huebner, G. Oppliger, T. Van Gundy, N. Mankhemthong, J. McDonald, W. Robertson, J. Shoffner, G.
Johnson, and A. Murtagh. 2007. Shallow Geophysical Investigations on the Redfield Campus, Steamboat
Hills, Nevada—Phase One Results. University of Nevada, Reno.
Ringstad, Clyde A., Bugenig, Dale C. 1984. Electrical Resistivity Studies to Delimit Zones of Acceptable
Ground Water Qulaity.
Loke, M.H. 1996. Tutorial : 2-D and 3-D electrical imaging surveys.
Boyd, Thomas. Introducion to applied geophysics.
http://www.earthsci.unimelb.edu.au/ES304/index.html. 1999