Integrated Hydrogeologic Framework Study in Spring Valley, Nevada Using Audiomagnetelluric, Gravity, Magnetic and Borehole Data
Abstract
Geophysical surveys were undertaken to identify carbonates, range-front faults, basin stratigraphy, and site drill holes within basins of eastern Nevada. Defining the distribution of basin-fill deposits and their association with the carbonate-rock aquifer system that underlies this region is important for assessing ground-water resources. Regional north-trending structures are a primary structural control on the hydrogeology of the valleys in eastern Nevada. In order to identify significant subsurface structures, audiomagnetotelluric (AMT) data and resulting models were analyzed together with potential field data and geophysical and geological borehole logs in Spring Valley, NV. The AMT method is a valuable tool for estimating electrical resistivity of the earth over depth ranges from a few meters to less than one kilometer. We collected AMT sounding data at 200m intervals along three profiles in southern Spring Valley using a Geometrics Stratagem EH4 system, four-channel, tensor system recording in the range of 10 to 92,000 Hz. To augment the low signal in the natural field, a transmitter of two horizontal-magnetic dipoles was used from 1,000 to 70,000 Hz. Profile A extends 12.6 km from the Fortification Range on the west across southern Spring Valley to the southern Snake Range in the east. Two shorter (~ 2 km) profiles span the margins of the Fortification Range and the Snake Range to the south of Profile A. Data were recorded with the electric field (E) parallel and perpendicular to the N-S regional geological strike direction. Ground magnetic data were collected along two of the profiles and gravity data were collected both regionally as well as in detail along the profiles at 200-400m station spacing. Borehole data, including both lithological and geophysical logs extending 350 m and 500 m below the surface, were available from two water monitoring and testing wells along our AMT profiles Two-dimensional, inverse models were computed from the E perpendicular mode data using a conjugate gradient, finite-difference method to test dimensionality of the structure, resolution and the depth of investigation. Our preferred AMT models show detailed structure within the alluvial basin. Interpretation of the AMT models, along with the other geophysical data, defines several faults, some of which may influence ground-water flow in the valley, as well as the underlying Paleozoic carbonate, and volcanic and clastic rocks within the basin. Basin thickness estimates derived from gravity data agree with the base of the volcanic rocks imaged with the AMT data. Fine-grained (clay-rich) material can be differentiated within the basin fill. Comparison of AMT models with borehole and lithology data help constrain our interpretation, yet also unveil limitations of both datasets.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMNS13A..03M
- Keywords:
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- 1515 GEOMAGNETISM AND PALEOMAGNETISM / Geomagnetic induction;
- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation;
- 1835 HYDROLOGY / Hydrogeophysics