Introduction
Gravity surveying is a tool used to measure minute variations in the Earth’s gravitational
field. A gravity survey is in essence giving definition to a subsurface feature by using a single
intrinsic property of all rocks and minerals, density. Variations in density both near and far
below the surface of the Earth yield a difference in the observed Gravity at the surface of the
Earth. If rock densities are known then estimates of depth to a subsurface feature can be
determined. Given a grid of surveys, an anomaly can be defined by both depth and aerial extent
thus rendering a three dimensional geometry of the subsurface (Telford, 1990).
Gravity surveying is valuable in finding depth to bedrock, mineral exploration, and
constraining other datasets such as: Magnetics, Resistivity, Refraction Microtremor (ReMi),
Reflection, and Refraction Surveys.
Methods
Gravity
The locations for gravity survey were chosen for two different reasons. 1) To fill survey
gaps in the northeastern portion of the Truckee Meadows Basin dataset and 2) to profile a
smaller sub-basin in the southeastern portion of the Truckee Meadows Basin. 63 gravity stations
were occupied over a 3 day period using a LaCoste and Rhomberg gravimeter. The sample
intervals varied along both lines from a minimum of 100 meters to a maximum of 400 meters
(Figures. Grav 1 and 2). All gravity measurements were corrected for tidal variation, instrument
wander, and instrument equilibration using a field base station. The field base station was
occupied consistently on a three to four hour interval. The local base stations were corrected to a
known local base station at Scrughan Engineering and Mines (SEM) on the University of Nevada
Reno Campus. A critical portion of the data collection was estimating the hammer B (2 m-16 m)
and C (16 m- 53 m) ring terrain corrections.
Data was initially culled, condensed, and processed using Grav2D and later was
reprocessed using GM-SYS. The data was corrected for drift using a linear fit to the field base
stations then again corrected to the SEM station in order to correct for instrument drift and tidal
wander. The terrain corrections, Hammer B and C ring corrections were added to the dataset to
yield the Simple Bouguer Anomaly. For future processing and interpretation of basin sediment
fill and bedrock the densities used for Kate peak is 2.61 g/cc, Hunter Creek is 1.76 g/cc, and 1.7
g/cc for water filled diatomatious sediment (Abbot and Louie, 2000).
Results
Gravity
GM-SYS modeling
Two gravity profiles are presented in this section. The first is located along an east-west
line along Prater Way in Sparks, Nevada. The second is presented for an east-west profile along
the Mira Loma Dr. in southern Reno, Nevada. Using GM-SYS the Prater line shows an anomaly
occurring at a distance of 1700 m, and has been interpreted to be a fault dipping to the west
where basin sediment meets the denser basin volcanic rocks. Using GM-SYS the Hidden Valley
line shows a more complicated profile. The line was interpreted to have two faults making a
small graben on the east-central portion of the survey just before the eastern range front. There
are no faults interpreted across a small hill between Hidden Valley and the Truckee Meadows
Basin. The data tables and map localities of both surveys have been attached. The locations of
interpreted faults are marked on the GM-SYS model profiles and also located on topographic
base maps (Figures. Grav 1 and 2).
The GM-SYS model (top) (Figure. Grav 3) calculated gravity matches the observed gravity, but
the geological interpretation of the area is not possible. The bottom GM-SYS model has a more
realistic geological interpretation, but it is still not a 100% correct geological interpretation of the
area, but the calculated gravity does not match the observed gravity. This example shows that
gravity anomalies do not have one correct interpretation therefore, be more data is required to
make a reasonable interpretation of the studied area. (Figure. Grav 3)
Talwanian Inversion using Grav2D
Prater Way
The Talwanian model produced for Prater Way shows an initial basin depth of 0.5 km
depth on the west side of the TMB and a progressive shift to zero sediment thickness on the east
side of the TMB (Figure Grav 4). The magnitude of the Simple Bouguer Anomaly along the
Prater Way survey is -13 mGal (Figure Grav 5). The talwanian inversion basin thickness is
consistent with other datasets located south of Prater Way (Widmer et al., 2007). The basin
seems to be shallowing to a zero thickness along the eastern portion of the TMB, from the
current Truckee River North.
Hidden Valley
The Talwanian model for the Hidden Valley dataset is complex and shows a smaller
basin anomaly, the inversion shows a 2.7-~4 km deep basin near the Hidden Valley dog park
(Figure Grav 6). The magnitude of the Simple Bouguer Anomaly associated with the large basin
is -7 mGal (Figure Grav 7). No other datasets are available to compare the results of the Gravity
Survey against.
Sources of error
Gravity
Potential Sources of error in the field which will affect the precision of the instrument
are: 1) elevation inaccuracies of 0.20 cm yielding a consistent error of 0.08 mGal, 2)
disequilibrium of the zero-length spring due to heat, 3) terrain corrections which reached a
maximum of 0.3 mGal at station HVW 9, 4) locations of the survey (e.g. busy streets and
climbing relatively steep hills).
Uncertainties, missed targets
Gravity
Initially three gravity lines were planned; however, only two were effectively executed.
The third line is still an open and glaring target to refine the geometry of the northeastern portion
of the Truckee Meadows Basin (D’Andrea Parkway). Uncertainties in the dataset itself are spring
equilibration issues on the first day of field work, the questionable data are HVE1-HVE9 and
remain in the dataset. Elevation error is potentially a large contributing factor to the precision of
the instrument. Three data points (HVE9, HV9, and HVW10) were also removed because of
instrument inaccuracies.
Figures
Gravity Figures
Figure Grav 1. Map showing line of transect through Hidden Valley (Black Dots). Interpreted faults are indicated
with red lines on map and black lines on GM-SYS model.
Figure Grav 2. Map showing line of transect along Prater Way. Red dots indicate locations of observed gravity. Red
line on map is location of interpreted fault from GM-SYS plot (above).
Figure Grav 3. Two contrasting interpritations of Hidden Valley line. The top interpretation is not geologically
reasonable and fits the data. The bottom set of models is geologically reasonable but does not fit the data.
W
E
Figure Grav 4. Prater Way Grav2D processing showing basin depth of 0.5 km thinning the 0.0 km to the
east (right). Processing constraints are a Tolerance of 1 mGal and a density contrast of -0.5 g/cc.
W
E
Figure Grav 5. Prater Way Grav2D processing showing basin anomaly in mGal. The maximum anomaly is
~-13 mGal in the west (left) and the minimum anomaly is ~-2 mGal along the eastern (right) side of
basin.
W
E
Figure Grav 6. Hidden Valley survey line processed using Grav2D showing a more complex basin
geometry with a presumably deep basin in the east(right)-central portion of the survey line. Processing
constraints are a tolerance of 2 mGal and a density contrast of -0.33 g/cc.
W
E
Figure Grav 7. Hidden Valley Grav2D processing showing basin anomaly in mGal. The maximum
anomaly is ~-7 mGal in the west (left) and the minimum anomaly is ~-1 mGal along the eastern (right)
side of plot.
Work Cited
Abbot, R.E., and Louie, J.N., 2000, Depth to bedrock using gravimetry in the Reno and Carson City,
Nevada, Area basins: Geophysics, V. 65, p. 430-450.
Telford, W.M., Geldhart, L.P., and Sheriff, R.E., 1990, Applied Geophysics: New York, Cambridge
University Press, 770 p.
Widmer, M.C., Cashman, P.H., Benedict, F.C., Trexler, J.H., 2007, Neogene through Quaternary
stratigraphy and structure in a portion of the Truckee Meadows Basin; a record of recent
tectonic history: Geological Society of America Abstracts with programs, v. 39, no. 4 p.9