HR: 1330h
AN: NS23B-06    [Abstracts]
TI: Resistivity Imaging of Spring Valley, Nevada Using the Audiomagnetotelluric Method
AU: * McPhee, D K
EM: dmcphee@usgs.gov
AF: U.S. Geological Survey, MS 989, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Pellerin, L
EM: pellerin@ak.net
AF: Green Engineering, Inc., 6543 Brayton Drive, Suite B, Anchorage, AK 99507 United States
AU: Chuchel, B
EM: chuchel@usgs.gov
AF: U.S. Geological Survey, MS 989, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Dixon, G L
EM: gldixon@ida.net
AF: Southwest Geology, Inc., 323 West Zoo North, Blackfoot, ID 83221 United States
AB: Audiomagnetotelluric (AMT) sounding data collected in Spring Valley, NV show significant two-dimensional (2D) structure within the upper kilometer of the valley and help define the shallow basement surface. We collected AMT data along two profiles in the southern part of Spring Valley in the Fall 2004, using the Geometrics StrataGem EH4 system, a four channel, natural and controlled-source 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 Limestone Hills in the east with soundings recorded every 200 m. Profile B is a 2-km long, roughly E-W trending line located at the northern margin of the Fortification Range, roughly parallel to and 12 km NW of Profile A, with sounding spacings of 200-400 m. Data were recorded with the electrical field (E) parallel and perpendicular to the regional geological strike direction. We computed our preferred two-dimensional, inverse models from the E perpendicular mode data using the conjugate gradient, finite-difference method of Rodi and Mackie (2001) and a 100 ohm-m half-space, starting model. Inverse models were also computed using both modes and an equivalent model resulted, with a slightly higher RMS fit, indicating the two-dimensionality of the structure. Various starting models were used to test the depth of investigation. The model along Profile A shows detailed structure within the alluvial basin. Preliminary interpretation shows a clear transition between unsaturated (200-500 ohm-m) and saturated alluvium/volcanic rocks (20-50 ohm-m) at roughly 100 m depth. Highly-resistive (>1000 ohm-m) carbonate rocks are clearly defined at the east end of Profile A, and the locations and dips of several range-front and inter-basin faults, which lack surface expression, are delineated throughout the upper 1 km of section. In addition, our results define the shape of and the depth to the basement surface, which correlates well with depth to basement estimates derived from the inversion of gravity data. The model along Profile B shows conductive features within carbonate rocks that crop out nearby. The saturated alluvial valley fill and volcanic rocks are well defined along with the location of a range-front fault. As these results show, the AMT technique is a valuable tool for defining subsurface structure and stratigraphy within Spring Valley to roughly 1 km depth.
DE: 0600 ELECTROMAGNETICS
DE: 1219 Local gravity anomalies and crustal structure
DE: 8015 Local crustal structure
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly