Characterization of the Highway 95 Fault in lower Fortymile Wash using electrical and electromagnetic methods, Nye County, Nevada
The Highway 95 Fault is a buried, roughly east-west trending growth fault at the southern extent of Yucca Mountain and Southwestern Nevada Volcanic Field. Little is known about the role of this fault in the movement of groundwater from the Yucca Mountain area to downgradient groundwater users in Amargosa Valley. The U.S. Geological Survey (USGS) Arizona Water Science Center (AZWSC), in cooperation with the Nye County Nuclear Waste Repository Project Office (NWRPO), has used direct current (DC) resistivity, controlled-source audio magnetotelluric (CSAMT), and transient electromagnetics (TEM) to better understand the fault. These geophysical surveys were designed to look at structures buried beneath the alluvium, following a transect of wells for lithologic control. Results indicate that the fault is just north of U.S. Highway 95, between wells NC-EWDP-2DB and -19D, and south of Highway 95, east of well NC-EWDP-2DB. The Highway 95 Fault may inhibit shallow groundwater movement by uplifting deep Paleozoic carbonates, effectively reducing the overlying alluvial aquifer thickness and restricting the movement of water. Upward vertical hydraulic gradients in wells proximal to the fault indicate that upward movement is occurring from deeper, higher-pressure aquifers. From December 2006 to January 2007, the USGS and NWRPO collected dipole-dipole DC resistivity data to characterize the Highway 95 Fault. Modeled data from the resistivity study agreed with mapped faults from gravity anomalies and highlighted a prominent fault within 1.5 km of Highway 95, thought to be the Highway 95 Fault. Results of the dipole-dipole resistivity survey warranted further study. From March to April of 2008, the USGS and Nye County continued their geophysical investigation of the Highway 95 Fault using TEM and CSAMT geophysical techniques. TEM and CSAMT data were collected along the same profile as the dipole-dipole resistivity data. Modeled data from these additional studies yielded similar results to the dipole-dipole resistivity study. An area of distinct resistivity change was detected within 1.5 km of Highway 95, and it is thought that this change is the Highway 95 Fault. Coordinated application of electrical and electromagnetic geophysical methods provided better characterization of the Highway 95 Fault. The comparison of dipole-dipole resistivity, TEM, and CSAMT data confirm faulting of an uplifted block of resistive Paleozoic Carbonate that lies beneath a more conductive sandstone unit. A more resistive alluvial basin-fill unit is found above the sandstone unit, and it constitutes only about 150 m of the uppermost subsurface.