HR: 0800h
AN: GP11A-0823    [Abstracts]
TI: Joint Magnetotelluric-Transient Electromagnetic Imaging of Basin-Bounding Faulting in the Rio Grande Rift, New Mexico
AU: * Quesada, R
EM: quesadarm@pwcsd.navy.mil
AF: San Diego State University, Department of Geological Sciences, San Diego, CA 92182-1020 United States
AU: Jiracek, G R
EM: jiracek@moho.sdsu.edu
AF: San Diego State University, Department of Geological Sciences, San Diego, CA 92182-1020 United States
AU: Alumbaugh, D L
EM: alumbaugh@engr.wisc.edu
AF: University of Wisconsin-Madison, Department of Civil and Environmental Engineering, Madison, WI 53706 United States
AU: Hasterok, D
EM: dhasterok@mines.utah.edu
AF: University of Utah, Department of Geology and Geophysics, Salt Lake, UT 84112 United States
AU: Pellerin, L
EM: pellerin01@aol.com
AF: Green Enginnering, 8471 Foxlair Circle, Anchorage, AK 99507-3668 United States
AU: Young, G R
EM: g.russell.young@mail.utexas.edu
AF: University of Texas-Austin, Geological Science Department, Austin, TX 78712-0254 United States
AB: During the summers of 2001-2004 100-m spaced magnetotelluric (MT) recordings, supported by transient electromagnetic (TEM) soundings, were collected during the SAGE program (Summer of Applied Geophysical Experience) in the Rio Grande rift of New Mexico. The profiles of less than 10-km length were aimed at imaging the upper 4 km in the vicinity of the La Bajada fault, a major basin bounding feature. Two different stages of inversion were applied to the data. First, smoothest inversion approaches were applied to derive two-dimensional (2-D) images from the MT data and one-dimensional (1-D) models from the TEM results. The 1-D TEM results were then `stitched' together to produce pseudo-2-D images. The second stage of imaging involved combined constrained inversion and hypothesis testing. Here, a priori knowledge was used to position discrete interfaces between conductive basin fill and resistive basement and intrusives detected by the smooth inversion results. The inversions were then rerun. The results indicate a basin-ward thickening of sediments with two normal faults, the La Bajada fault and a previously unknown fault. Depth to resistive (over 1000 ohm-m), crystalline basement is approximately 2500 m on the downthrown (western) end of the lines, 1500 m between the two faults, and 700 m on the upthrown (eastern) side of the La Bajada fault. Using the joint TEM and MT images the conductive basin fill is subdivided into: 1) an unsaturated surface layer, 50 to 100-m thick, of high resistivity (90 to 150 ohm-m), 2) a 200 to 500-m thick section containing the freshest water and/or least amount of conductive clay with moderately high resistivity (about 10 to 40 ohm-m), and 3) a deeper, more conductive zone of about 5 to 10 ohm-m with salty water and/or high clay content.
UR: http://www.sage.lanl.gov
DE: 1831 Groundwater quality
DE: 1515 Geomagnetic induction
DE: 0694 Instrumentation and techniques
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0925 Magnetic and electrical methods
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting