Magnetotelluric Data, Across Quartzite Ridge, Nevada Test Site, Nevada Page: 4 of 175
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Introduction
Nuclear weapons are integral to the defense of the United
States. The U.S. Department of Energy, as the steward of these
devices, must continue to gauge the efficacy of the individual
weapons. This could be accomplished by occasional testing at the
Nevada Test Site (NTS) in Nevada, northwest of Las Vegas. Yucca
Flat Basin is one of the testing areas at the NTS. One issue of
concern is the nature of the somewhat poorly constrained
pre-Tertiary geology and its effects on ground-water flow in the
area subsequent to a nuclear test. Ground-water modelers would
like to know more about the hydrostratigraphy and geologic
structure to support a hydrostratigraphic framework model that is
under development for the Yucca Flat Corrective Action Unit
(CAU) .
During 2003, the U.S. Geological Survey (USGS) collected and
processed Magnetotelluric (MT) and Audio-magnetotelluric (AMT)
data at the Nevada Test Site in and near Yucca Flat to help
characterize this pre-Tertiary geology. That work will help to
define the character, thickness, and lateral extent of
pre-Tertiary confining units. In particular, a major goal has
been to define the upper clastic confining unit (UCCU) in the
Yucca Flat area. Interpretation will include a three-dimensional
(3-D) character analysis and two-dimensional (2-D) resistivity
model. The purpose of this report is to release the MT soundings
across Quartzite Ridge, Profiles 5, 6a, and 6b, as shown in
Figure 1. No interpretation of the data is included here.
Magnetotelluric Method
The MT method is a passive surface geophysical technique
that uses the Earth's natural electromagnetic fields to
investigate the electrical resistivity structure of the
subsurface. The resistivity of geologic units is largely
dependent upon their fluid content, porosity, degree of
fracturing, temperature, and conductive mineral content (Keller,
1989). Saline fluids within pore spaces and fracture openings
can reduce the resistivity of a resistive rock matrix.
Resistivity also can be lowered by the presence of conductive
clay minerals, carbon, and metallic mineralization. It is common
for altered volcanic rocks to contain authigenic minerals that
have resistivities of one tenth of those of the surrounding rocks
(Nelson and Anderson, 1992). Increased temperatures cause higher
ionic mobility and mineral activation energy, reducing rock
resistivities significantly. Unaltered, unfractured igneous
rocks are moderately to highly resistive (hundreds to thousands
of ohm-m), whereas fault zones will show low resistivity (less
than 100 ohm-m) when they are composed of rocks that are
fractured enough to have hosted fluid transport and consequent
mineralogical alteration (Eberhart-Phillips and others, 1995).3
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Williams, J. M. & Rodriguez, B. D. Magnetotelluric Data, Across Quartzite Ridge, Nevada Test Site, Nevada, report, November 23, 2005; United States. (https://digital.library.unt.edu/ark:/67531/metadc782206/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.