Preliminary results from the Gulf of Mexico gas hydrate CSEM experiment

As a result of the successful 2004 pilot study at Hydrate Ridge, offshore Oregon, USA, to assess the use of electrical methods for seafloor gas hydrate characterization, we collected a more extensive CSEM data set in October, 2008 over four geologically distinct areas in the Gulf of Mexico with water depths that varied from 900 to 3,000 m. These sites were Green Canyon 955 and Walker Ridge 313, Mississippi Canyon 118 and Alaminos Canyon 818. Several aspects of our work differentiate it from earlier studies. We deployed more seafloor receivers -94 compared to 25 at Hydrate Ridge and recorded every EM component except the vertical magnetic field. We towed 18 lines during a cumulative period of 100 hours in various patterns at each of the sites. In addition, a fixed offset three axis electric field receiver was towed in tandem 300 m behind the transmitter at altitudes of 50--100 m above the seafloor giving better coupling with the seafloor and providing short-offset data for all transmitter positions. We transmitted 200 Amperes of current from a 50 m dipole with a binary waveform that has about two decades of frequency content, from 0.50 Hz to about 50 Hz. The overall result was an expanded set of transmitter--receiver offsets extending the depth of investigation from the seafloor to the base of the hydrate stability field, and even deeper. We present apparent resistivity pseudosections of the surveyed regions and make comparisons to the other data sets available of each of the four sites within the Gulf of Mexico. We also present frequency derived apparent resistivies from the towed 3-axis electric field receiver.