Hydrates Cruise in the Gulf of Mexico

In June 1998, the U.S. Geological Survey and the University of Mississippi Marine Minerals Technology Center studied the upper- and middle-continental slope of the Mississippi Canyon. The DEM image (442 kb) of study area shows the ship tracklines.

They investigated the distribution of potential gas-hydrate deposits and possible links between hydrate occurrence and sea-floor failures. More than 900 km of very-high-resolution seismic reflection data were recorded utilizing the R/V Tommy Munro (187 kb).

Four seismic sources were used (35/35 and 105/105 cu.in. GI-airguns; 15 cu.in. water gun; deep-tow Huntec boomer). Arrivals were recorded either by a 24-channel, 250-m-long ITI streamer or by the Huntec's two receivers, giving penetrations up to 1.5 s (~1300 m) for airguns and 0.3 s (~240 m) for the Huntec. Ocean bottom seismometers were used to acquire wide-offset seismic data over a known hydrate mound.

Gas hydrates are commonly recognized in seismic profiles from areas other than the Gulf of Mexico by a bottom simulating reflector (BSR) at the base of the gas hydrate stability zone (GHSZ). In the Gulf, however, reports of well-defined BSRs are rare. The shallow stratigraphy and structure are complex in the study area due largely to salt tectonism and mass movements. Swath bathymetry data (226 kb) show many active, broad slump bodies on the sea floor, bordered by high fault scarps and pierced locally by diapirs. The numerous faults, diapirs, buried slump bodies, and otherwise deformed stratigraphic units characterize the shallow subsurface.

There is also great variability in seismic amplitudes in the shallow sedimentary section above the base of GHSZ. Many deep seated faults and diapirs have anomalous high-amplitude reflections (129 kb) near the sea floor like those seen over confirmed gas-hydrate mounds (241 kb). BSRs may occur, but are subdued and difficult to identify. Strong "shingled" reflections are seen in places directly below the inferred base of GHSZ, within a regional anomalous stratigraphic unit up to 150 m thick at sub-bottom depths of about 250-400 m. The unit may hold free gas that feeds sea-floor hydrate mounds and hydrate deposits we infer along faults and stratigraphic pathways in the overlying sedimentary section. Gas hydrates may in turn facilitate some shallow subsurface failures by providing added gas and water to underconsolidated overpressured sequences. The local occurrences of high-amplitude reflections and acoustically opaque zones near some local bathymetric features (e.g., mounds, scarps) suggest that hydrates are most likely to be found around shallow-subsurface diapirs and faults.

References

Cooper, A.K., Hart, P., and I. Pecher, 1998, Cruise Report for a Seismic Investigation of Gas Hydrates in the Mississippi Canyon Region, Northern Gulf of Mexico -- Cruise M1-98-GM, USGS Open-File Report OF 98-506, 21 p.

Cooper, A.K., Hart, P., McGee, T. and I. Pecher, 1999, Shallow Subsurface Structure Of Potential Gas Hydrate Zones In The Mississippi Canyon Region From High-Resolution Seismic Data, Engineering Foundation Meeting on Gas Hydrates, Park City, Utah, July 1999.

Cooper, A.K., McGee, T., Hart, P. and I. Pecher, 1999, High-Resolution Seismic Study Of Gas Hydrate Areas In The Mississippi Canyon, Northern Gulf Of Mexico, AGU Annual Meeting, San Francisco, CA, December, 1998.

Cooper, A.K, Pecher, I., McGee, T. and P. Hart, 1999, High-Resolution Seismic Study Of Gas Hydrate Areas In The Mississippi Canyon, Northern Gulf Of Mexico, AAPG Annual Meeting, San Antonio, Texas, April 1999.

McGee, T., Cooper, A.K., Pecher, I., Grace, C., and R. Buchannon, 1998, Operations Report for Cruise M1-98-GM, 8-19 June 1998, Marine Minerals Technology Center internal report, 21 p.