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Murray, Paul E.1, Milo Backus1, Michael DeAngelo1, Sergey Fomel1, Robert Graebner1, Bob Hardage1, Lesli Wood2 
(1) Bureau of Economic Geology, Austin, TX 
(2) The University of Texas at Austin, Austin, TX

ABSTRACT: Evaluating Marine Gas Hydrates with 4-C OBC Seismic Data

We have analyzed multicomponent seismic data acquired with 4-component ocean-bottom-cable (4-C OBC) technology in the Gulf of Mexico (GOM) to determine if 4-C OBC data offer advantages over conventional P-wave seismic data for evaluating gas-hydrate systems. We found the following applications make 4-C OBC seismic technology an appealing option for studying marine gas-hydrates. 
1. P-SV wavefields image structures and stratigraphy inside P-wave “wipeout zones” that are associated with many gas-hydrate deposits in the northern GOM. The inability to image geologic detail internal to these zones with conventional P-wave seismic data has frustrated efforts to build models of thermogenic gas-hydrate systems in the GOM. 
2. In some shallow seafloor strata, P-SV data image different depositional sequences than do P-P data. An improved understanding of the stratal architecture associated with gas-hydrate deposits is achieved by combining P-P and P-SV interpretations. 
3. P-SV data provide a better spatial resolution of shallow seafloor strata than do P-wave data. This improved resolution is important when analyzing subtle depositional features associated with gas-hydrate deposits. 
4. The Vp/Vs velocity ratio determined from 4-C OBC data allows robust mapping of lithofacies distributions in shallow seafloor strata. Combining this new marine-seismic attribute with published studies of Vp/Vs measurements on core samples indicates areas where seafloor strata have increased sand content (increased grain size) and increased shale content (decreased grain size). 
5. The Vp and Vs velocities obtained by processing and interpreting 4-C OBC data allow bulk and shear moduli of seafloor sediments to be estimated. Maps of these elastic moduli are critical for evaluating seafloor stability across gas-hydrate locations. 
These research findings will be illustrated using examples of 4-C OBC seismic data from a selected study area in the northern GOM.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.