HR: 1340h
AN: B13A-0168    [Abstracts]
TI: A look at the data from ``Constraining the Magmatic Budget of the EPR at 9 N Using Broadband Marine MT''
AU: * Key, K
EM: kkey@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Constable, S
EM: sconstable@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Behrens, J
EM: jbehrens@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Heinson, G
EM: Graham.Heinson@adelaide.edu.au
AF: University of Adelaide, University of Adelaide, Adelaide, SA 5005 Australia
AU: Weiss, C
EM: cjweiss@sandia.gov
AF: Sandia National Laboratories, PO Box 5800, MS-0750, Albuquerque, NM 87185 United States
AB: In February 2004 we collected data for the world's largest marine electromagnetic (EM) experiment at the RIDGE2000 integrated study site (ISS) on the East Pacific Rise (EPR) near 9{$^\circ$}N. Electrical conductivity is a strong function of fluid content and temperature, whether magma or seawater, and so the objectives of this RIDGE2000 funded experiment are to map the hydrothermal circulation systems removing heat from the mid-ocean ridge magmatic system, quantify the total amount of melt in the crustal magma chamber, and examine the relationship between mantle melting and crustal melt accumulation. We used two electromagnetic methods to accomplish these goals: a) Marine magnetotelluric (MT) method. A seafloor instrument records natural variations in Earth's electric and magnetic fields for 2 days to 2 weeks. When converted to frequency domain impedance functions and inverted, these data can be used to obtain images of seafloor conductivity up to hundreds of kilometers deep. b) Marine controlled-source EM (CSEM) sounding. An EM transmitter is deeptowed close to the seafloor to provide a man-made source of EM energy. The seafloor recorders monitor the transmitted electric fields, which provide similar information to the MT method except that (i) the CSEM method has better resolution at shallow depths and (ii) the CSEM method is better at measuring resistive (cf.\ conductive) rocks. Forty broadband EM receivers, developed at Scripps Institution of Oceanography with support from the petroleum exploration industry, were deployed a total of 72 times to yield 69 sites of MT/CSEM data. Twenty of the MT deployments were along a 30 km aperture transect across the ISS "bull's eye" focus area at 9{$^\circ50'$}N, in order to image the electrical conductivity structure in the crust and shallow mantle in the vicinity of the ridge axis. Further to the south, 40 MT sites were collected along a 200 km aperture transect at 9{$^\circ30'$}N to target both crustal and upper mantle conductivity structures. This larger aperture transect was positioned to avoid 3D structure associated with the Lamont seamonts and the Clipperton transform. Twelve additional MT sites were collected in a grid along the ridge axis between 9{$^\circ30'$}N and 9{$^\circ36'$}N. For the CSEM portion of the experiment, we deep-towed the newly developed Scripps Undersea EM Source Instrument (SUESI) about 100 m above the seafloor while transmitting a 2 Hz square wave of dipole moment about 20,000 Am. The MT receivers also record the transmitted CSEM fields, and so three CSEM tows were performed while the forty receivers were deployed along the southern line and grid, yielding a total of about 80 km of tows and 1,300 receiver-hours of CSEM data.
UR: http://marineemlab.ucsd.edu/Projects/EPR2004
DE: 5109 Magnetic and electrical properties
DE: 3035 Midocean ridge processes
DE: 1515 Geomagnetic induction
DE: 0994 Instruments and techniques
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting