Electrical Resistivity and Seismic Surveys at the Nevada Test Site, Nevada, April 2007

Abstract

In April 2007, the USGS collected direct-current (DC) electrical resistivity data and shear-(S) and compressional-(P) wave seismic data to provide new detail of previously mapped, overlapping fault splays at two administrative areas in the Nevada Test Site (NTS). In NTS Area 7, we collected twodimensional DC resistivity data along a transect crossing the Yucca Fault parallel to, and between, two transects along which resistivity data were collected in a previous study in 2006. In addition, we collected three-dimensional DC resistivity data in a grid that overlies part of the 2007 transect. The DC resistivity data show that the fault has a footwall that is more conductive than the hanging wall and an along-strike progression of the fault in a location where overlapping splays are present. Co-located with the northernmost of the two 2006 DC resistivity transects, we acquired S-and P-wave seismic data for both reflection and refraction processing. The S-wave data are corrupted by large amounts of converted (P-wave) energy likely due to the abundance of fractured caliche in the shallow subsurface. The P-wave data show minimal reflected energy, but they show clear refracted first arrivals. We have inverted these first arrival times to determine P-wave seismic velocity models. The seismic model for the transect in Area 7 shows low velocities extending to the base of the model at the location of the Yucca Fault, as well as low velocities at the eastern end of the transect, in the vicinity of the adjacent crater. These new surveys provide further detail about the geometry of the Yucca Fault in this location where it shows two overlapping splays. We collected P-and S-wave seismic data along a transect in the southern part of NTS Area 2, corresponding with the location of a 2006 DC resistivity transect that targeted a set of small faults identified with field mapping. Again, the S-wave data are difficult to interpret. The P-wave data show 1 clear first arrivals that we inverted, yielding a velocity model that shows lateral heterogeneity similar to the 2006 DC resistivity models. Finally, we collected P-wave data along a second transect in Area 2, located north of the first line and in an area of a very minor fault that was targeted by another 2006 DC resistivity survey. The P-wave refraction velocity model shows generally high velocities, with a zone of somewhat lower velocities in the central part of the transect. The position of the low velocity zone corresponds with the location of a minor fault, though it is unclear whether the two are related. Together, these results demonstrate the value of geophysical data for mapping the subsurface extent of faults. The 2007 DC resistivity data complement the 2006 data and provide important new detail of the overlapping fault splays. The seismic data demonstrate the ability of P-wave refraction methods to identify the damage zones at faults, and they show the difficulties associated with S-wave methods in areas with caliche. Combining all of the geophysical data from the Area 7 stu...

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“…Waves generated by this critically-refracted wave are the first to arrive at geophones located beyond a critical distance from the seismic source. The path and velocity of the energy can be determined by the arrival time and the geophone distance, and the velocity by extension can indicate what material type is present (Haines et al, 2008). These surveys are ideally used to study shallow, gently dipping layers (Milsom and Eriksen, 2011).…”

“…Longitudinal waves move through a material the fastest, and are called primary waves or P-waves (Burger et al, 2006). They are also often called compressional waves based on the type of stress propagated (Haines et al, 2008), which depends on incompressibility and rigiditiy constants for the medium. Transverse waves move more slowly and are thus termed secondary or S-waves, and are also referred to as shear waves because they respond to the rigidity constant of the material.…”

“…Values vary widely depending on rock or sediment type, which makes them more easily distinguished (Dobecki and Romig, 1985;Haines et al, 2008).…”