HR: 0800h
AN: NS31B-0383    [Abstracts]
TI: Electrical Resistivity Imaging of Subterranean Void Space for Assessment of Endangered Species Habitat
AU: * Weissling, B P
EM: bweissling@swca.com
AF: SWCA, Inc., 6200 UTSA Blvd., Suite 102, San Antonio, TX 78249, United States
AU: White, K
EM: kwhite@swca.com
AF: SWCA, Inc., 6200 UTSA Blvd., Suite 102, San Antonio, TX 78249, United States
AB: The challenge of identifying and delineating subterranean habitat for endangered species in karst environments has been addressed through the application of near-surface geophysical techniques. Electrical resistivity imaging (ERI) in both galvanic DC and capacitance-coupled modes has been applied to the problem of imaging subsurface voids, potentially conducive to karst invertebrate habitat, in two distinctly different geologic, geophysical, and environmental settings. Surveys were conducted in extrusive volcanic terrain on the south shore of Kauai, Hawaii, a site known for lava tube formation, and in limestone karst terrain in central Texas. The two study sites were distinctly different in their geophysical settings in terms of surface layer and subsurface background resistivities, values at the Kauai site ranging from 1000 - 5000 ohm-meters and at the Texas site 100 - 800 ohm-meters, values reflecting differing lithology, porosity, and pore fluid content. An Advanced Geosciences Inc. (AGI) Supersting R8 DC resistivity system was the primary instrumentation utilized for both surveys, with a capacitance-coupled Geometrics Inc. OhmMapper TR-2 system utilized on the Kauai site for reconnaissance profiles. Opportunities existed for direct comparisons of Supersting and OhmMapper pseudo- section profiles. Supersting lines were acquired with a mixed array combining the horizontal resolution sensitivity of the dipole-dipole array with the vertical resolution sensitivity of the Inverse Schlumberger array. At both sites, surveys were conducted over known and mapped cave passage for validation of the techniques. Forward simulation modeling was conducted to verify resistivity anomaly signatures of known void spaces. Results were highly encouraging and serve to reinforce the karst-imaging capabilities of electrical resistivity, especially when mixed array types are utilized.
DE: 0925 Magnetic and electrical methods (5109)
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting