HR: 14:40h
AN: GP13A-04 INVITED     [Abstracts]
TI: Broadband Marine Magnetotelluric Exploration of the Crust at a Petroleum Prospect and a Mid-Ocean RIdge
AU: * Key, K W
EM: kkey@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, MC 0225, La Jolla, CA 92093-0225 United States
AU: Constable, S C
EM: sconstable@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, MC 0225, La Jolla, CA 92093-0225 United States
AB: Broadband marine magnetotelluric (MT) instrumentation developed at Scripps Institution of Oceanography enables resolution of electrical resistivity structure at much shallower depths than previously attainable. While marine seismic reflection surveys have routinely surveyed crustal structure on the continental shelves and mid-ocean ridges, traditional marine MT sensors were only capable of measuring long period fields and so MT experiments were limited to studying mantle structure. The introduction of low-noise sensors allows the broadband MT instrument to now measure the shorter period fields that contain information about crustal resistivity structure. We present two case studies of using the broadband MT system at areas previously surveyed with seismic methods. The joint interpretation of both seismic and MT models for these case studies leads to an improved geological interpretation. At Gemini Prospect in the northern Gulf of Mexico we have collected 42 MT sites in a grid over a three-dimensional (3D) resistive salt structure associated with the petroleum prospect. Depth migrated seismic reflection profiles from a 3D seismic survey at Gemini allow for the verification of two-dimensional (2D) MT inversion models. Combined images of the MT resistivity and seismic reflection profiles show that 2D MT can recover that salt body despite its 3D shape. A steeply dipping and overhanging resistive feature correlates with a previously uninterpreted strong reflection and illustrates how MT can constrain structure in regions where the seismic method performs poorly. A thin and shallow resistive feature shown outside the seismic salt volume may indicate a change in porosity or pore fluids associated with a natural trap in the sediments. At the East Pacific Rise near 9{$^\circ50'$}N, a pilot survey using the broadband instruments shows sensitivity to structure at shallower depths than previous ridge MT experiments. Two-dimensional inversion of data from 4 MT sites shows a high conductivity zone located in the crust and shallow mantle that is associated with the ridge magmatic system and that agrees well with seismic tomography studies of a nearby section of the ridge. Resistivities beneath the ridge imply a crustal partial melt fraction of about 1-20%, in accordance with the seismic results. Nearly vertical isotherms calculated from the seismic model imply deep hydrothermal circulation acts to convectively cool the flanks of the partial melt region, however, the MT model suggests that this region is probably limited to within a few kilometers of the ridge axis. We augment this study with a preliminary look at data from our experiment this past February, where we collected 69 additional MT sites along two transects at the EPR near 9{$^\circ$} N.
UR: http://marineemlab.ucsd.edu
DE: 5109 Magnetic and electrical properties
DE: 3035 Midocean ridge processes
DE: 1515 Geomagnetic induction
DE: 0994 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting