HR: 17:15h
AN: H14C-06    [Abstracts]
TI: Mapping hydrogeophysical structures with time--domain electromagnetic methods: Resolving small-scale details with large loops and three--component measurements
AU: * Weiss, C J
EM: cjweiss@sandia.gov
AF: Sandia National Laboratories, Geophysical Technology Department PO 5800 MS-0750, Albuquerque, NM 87185 United States
AU: Li, Y
EM: ygli@mines.edu
AF: Colorado School of Mines, Department of Geophysics, Golden, CO 80401 United States
AU: Nabighian, M
EM: mnabighi@mines.edu
AF: Colorado School of Mines, Department of Geophysics, Golden, CO 80401 United States
AB: One of the outstanding problems in managing water resources in geologically complex aquifers is to develop improved techniques for mapping compartmentalization due to faulting. And although the role of faults in aquifer dynamics can vary considerably, knowledge of their location is key to understanding aquifer recharge and developing a sensible model for predicting aquifer response due to anthropogenic loads. We have explored the application of time--domain electromagnetic methods for mapping shallow aquifer faults on the western flanks of the Estancia Basin, central New Mexico. The field site is underlain by massive Pennsylvanian limestones (Madera Group) subsequently faulted by Laramide tectonics of the Ancestral Rockies and Neogene extension of the Rio Grande Rift. Two experimental configurations were deployed: a large $50 \times 40$ m transmitter loop with receiver stations located on a 5 m grid over the loop's interior; and an azimuthal survey consisting of a smaller fixed transmitter with receiver stations at $\sim$2 m intervals along a 30 m radius circle centered on the transmitter. Three--component transients of magnetic field due to a fast linear ramp--off in the transmitter were recorded at each station. As a rapid reconnaisance tool, the azimuthal experiment is well--suited for identification of subsurface fault planes since symmetry constraints require a vanishing azimuthal $\hat\phi$ component of magnetic field when the electrical strike, or fault plane, lies in the $\hat\phi$ direction. However, each of the experimental configurations revealed that the site's electrical structure is far more three--dimensional than previously believed and is not dominated by the response of a previously identified fault plane. Instead, we have observed spatially coherent transient signals which may indicate compartmentalization over length scales as small as a few tens of meters. Sections of this work were performed at Sandia National Laboratories. Sandia is a multi--program laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE--AC04--94AL85000.
DE: 8010 Fractures and faults
DE: 1832 Groundwater transport
DE: 1894 Instruments and techniques
DE: 0694 Instrumentation and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting