Methods
Brief on each method
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.
Sources of error
Gravity
Potential Sources of error in the field which will affect the precision of the instrument are
elevation inaccuracies, disequilibrium of the zero length spring due to heat, terrain corrections
which reached a maximum of 0.3 mGal, locations of the survey (e.g. busy streets and climbing
relatively steep hills).
Results
Brief on each method
Gravity
Two gravity profiles are presented in this section. The first is located in 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 on the Prater line there is an anomaly
in the plot that occurs at a distance of 1700 m, and has been interpreted to be a fault dipping to
the west where the sediments meet the bedrock. Using GM-SYS the Hidden Valley line gave a
more complicated profile and had a more difficult interpretation. The line was interpreted to have
two faults making a small grabben on the eastern side of the line just before the eastern range
front. There was no fault(s) found going across the 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 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 interpritation, but it is still not a 100% correct geological interpretation of the
area, but the calculated gravity is not matching the observed gravity. This example shows that
gravity anomalies do not have one correct interpretation and there has to be more data to give a
good interpretation of the studied area. (Fugure. Grav 3)
Describe 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 a large contributing factor to the precision of the
instrument. Three data points (HVE9, HV9, and HVW10) were also removed because of
instrument inaccuracies.
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.