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USGS Open-File Report 03-474, G1-03-GM, Gas Hydrates Cruise Report, R/V Gyre, 1-14 May 2003, Northern Gulf of Mexico


Cruise G1-03-GM resulted in 1033 km of high-resolution multichannel seismic reflection data collected in two regions in the northern Gulf of Mexico. A total of 779 km of data were collected in the vicinity of Keathley Canyon lease block 195. Approximately 254 km were collected to the east in the Atwater Valley lease block 14 on the floor of the Mississippi Canyon. Multichannel data quality was excellent, with the GI gun configured with 13/13 in3 chambers providing the best overall source for the cruise. All data were demultiplexed and processed through stack at sea, providing near real-time feed back on data coverage and results. A notable operational achievement was to define the geometry in geographic coordinates during the processing sequence.

Plots of the stacked data contain abundant reflections for interpreting the shallow stratigraphic and structural setting of each region. The data set contains excellent coverage and detail for understanding the geological framework and seismic characterization of the hydrate stability zone. The Keathley Canyon data define thick sediment in the basins and thinner, disrupted sediments in the structural high separating the basins. There is a rich pattern of unconformities, pinch-outs, on-laps, and faults between the basin centers and edges. The Atwater Valley data reveal a more complicated seismic stratigraphy with many unconformable shallow reflections and sub-sea-floor diffractions. The vent/mound sites show abundant sea-floor diffractions, discontinuous reflections, zones of very strong amplitudes, zones of wash outs, and possible pull-downs. Both regions have abundant diffractions at the sea floor and within reflecting units, indicating the importance of post-cruise migration of the data. There are numerous amplitude anomalies and variations that are consistent with acoustic indicators of hydrate and related gas, but additional post cruise analysis is required to interpret and map these features.

The Knudsen bathymetry data were much more variable in quality and coverage because of their dependence on the weather. During the middle portion of the cruise, when a strong southerly air flow generated a short swell and sea state, the bathymetry rarely functioned robustly. This was probably due to the location of the transducers on the bow of the Gyre where the pitch and roll of the ship created maximum cavitation and bubble interference.

Each of the objectives laid out prior to the cruise was either fully met or is expected to be met with additional post-cruise processing, analysis, and interpretation of the seismic data.

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Title Page / Abstract / Introduction / Acknowledgments / Geologic Settings / Cruise Objectives / Cruise Strategy / Instrumentation / Data Processing / Data Handling / Marine Mammal Mitigation / Operational Summary / Meeting Objectives Success / Summary / References / Appendices
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