HR: 1330h
AN: NS23A-04    [Abstracts]
TI: Large-scale 3D inversion of frequency domain controlled-source electromagnetic data
AU: * Miller, C R
EM: carlylemiller@mail.boisestate.edu
AF: Department of Geosciences, Boise State University 1910 University Drive, Boise, ID 83725 United States
AU: Routh, P S
EM: routh@cgiss.boisestate.edu
AF: Department of Geosciences, Boise State University 1910 University Drive, Boise, ID 83725 United States
AU: Donaldson, P
EM: pdonalds@boisestate.edu
AF: Department of Geosciences, Boise State University 1910 University Drive, Boise, ID 83725 United States
AU: Oldenburg, D W
EM: doug@eos.ubc.ca
AF: Geophysical Inversion Facility, Department of Earth and Ocean Sciences University of British Columbia, Vancouver, BC V6T 1Z4 Canada
AB: Controlled Source Audio-Frequency Magnetotellurics (CSAMT) is a frequency domain EM sounding technique. The CSAMT source is a grounded horizontal electric dipole approximately one to two kilometers in length. This dipole source generates both inductive and galvanic currents so that the observed electric field arises due to both the static the vector potentials. At low frequencies, the behavior of the fields is similar to that observed in a geometric sounding such as a direct current experiment. At higher frequencies, the inductive character of the source modifies the behavior of the fields so that the experiment becomes more like a frequency sounding. Higher frequency data are useful for imaging near-surface features and lower frequency data are sensitive to deeper structure. Inversion of controlled source EM data provides a means to image the subsurface electrical conductivity structure. We consider a 3D CSAMT data set acquired over a known geothermal resource area in Western Idaho. The data are amplitudes and phases of the electric and magnetic fields acquired at 25 frequencies. The conductivity contrast between the geothermal fluid conduits and the resistive host material allows us to relate the inverted conductivity image to the distribution of fluid flow pathways in the geothermal system. Our 1D CSAMT inversion of the 3D data set indicates regions of conductive fluid pathways in the subsurface. Our next step is to invert these data using the full Maxwell's equations in 3D. Inversion of a single frequency data set at 2 Hz using the 3D frequency domain inversion algorithm (Haber et. al, 2004) shows regions of fluid circulation indicated by zones of higher conductivity. Comparing the images from different single frequency inversions allows us to identify persistent features in the conductivity image that adequately satisfy the data. With the aid of synthetic modeling we are investigating what frequencies? and what geometries? are appropriate to better resolve these targets. In this paper we will present a strategy to invert a large-scale EM data set. Haber, E., Ascher, U. and Oldenburg, D., 2004, Inversion of 3D electromagnetic data in frequency and time domain using an inexact all-at-once approach: Geophysics, Soc. of Expl. Geophys., 69, 1216-1228.
DE: 0699 General or miscellaneous
DE: 3210 Modeling
DE: 3260 Inverse theory
DE: 8424 Hydrothermal systems (8135)
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly