HR: 14:55h
AN: GP13A-05 [Abstracts]
TI: A Pilot Marine EM Study of Hydrate Ridge, Oregon.
AU: * Weitemeyer, K
EM: kweiteme@ucsd.edu
AF: Scripps Institution of Oceanography, IGPP/MC 0225, La Jolla, CA 92093-0225
United States
AU: Constable, S
EM: sconstable@ucsd.edu
AF: Scripps Institution of Oceanography, IGPP/MC 0225, La Jolla, CA 92093-0225
United States
AU: Key, K
EM: sconstable@ucsd.edu
AF: Scripps Institution of Oceanography, IGPP/MC 0225, La Jolla, CA 92093-0225
United States
AU: Behrens, J
EM: jbehrens@ucsd.edu
AF: Scripps Institution of Oceanography, IGPP/MC 0225, La Jolla, CA 92093-0225
United States
AB:
It has long been proposed (e.g. Nigel Edwards, U.\ Toronto) that EM methods may be able to detect and map gas hydrate, which
is more resistive than host sediments and thus provides an electrical target. While the base of hydrates often produces a
distinctive seismic signature (the bottom-simulating reflector, or BSR), the gradational upper surface is less well imaged
using seismology, and some hydrates are known to exist without a prominent BSR. In August of 2004 the Scripps Institution of
Oceanography Marine EM Lab collected magnetotelluric (MT), dipole-dipole controlled source electromagnetic (CSEM), and
controlled source magnetotelluric (CSMT) data across Hydrate Ridge, about 70 km offshore Newport, Oregon. Three component
electric field data and two component magnetic field data were recorded at 25 evenly spaced sites along a 14.4 km east-west
line that coincides with 2D and 3D seismic transects and ODP Leg 204 well-log data. Forward calculations of models with
conservative resistivity contrasts suggest that radial mode CSEM electric fields at frequencies of 5 Hz (and up to the 7th
square wave harmonic of 35 Hz) will give a measurable electric field response to shallow hydrates at source-receiver ranges
between 500 m to 2500 m. An important part of this experiment is to compare well log data with electrical conductivity
estimates of hydrate {\it in situ}, unmodified by either drilling or sample collection. We will present the CSEM and (CS)MT
data from this new experiment along with preliminary interpretations.
UR: http://www.marineemlab.ucsd.edu
DE: 8105 Continental margins and sedimentary basins
DE: 0925 Magnetic and electrical methods
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting