Analyzing deep-water near seafloor geology with chirp sonar sub-bottom profiles : Green Canyon, Gulf of Mexico

Hernandez, Jaime, 1968-
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The study area is located on the continental slope in the Green Canyon deep-water area of the Gulf of Mexico. This area is being investigated by the Bureau of Economic Geology as part of several active gas hydrate studies across the area. The chirp sonar profiles used in my study were collected with an Autonomous Underwater Vehicle (AUV) utilizing a frequency-modulated seismic (sonar) source that emitted a 2 to 8 kHz sweep (chirp) frequency signal (wavelengths less than 2 meters). The recording time is limited to about 50 milliseconds, with time zero occurring at the altitude of the AUV about 50 meters above the seafloor. The signal images to about 40 meters below the seafloor, and profiles are as long as 5 km. An interpretation of deep-water, near sea-floor geology has been conducted using both chirp sonar profiles and multibeam bathymetry. Seismic reflections from within the shallow sediments are caused primarily by contrasts in density, rather than acoustic velocity. Reflections were successfully simulated using a model with a constant velocity of 1560 m/s and densities of 2.1 g/cc for sand, 1.4 g/cc for mud, and 1.7 g/cc for silty sand. The chirp sonar profiles imaged near-seafloor geology at nearly a meter scale and allowed for detailed interpretation. The interaction of soft sediment deformation, creep movement and neo-tectonic activity related to gas expulsion controlled the actual topography of the sea-floor. The geologic time represented in the 50 milliseconds of chirp data recorded with the AUV, which is about 40 meters of depth, is approximately 0.050 Ma. B.P., consistent with a depositional rate of 0.8 meters per 1000 years. Reflection patterns are interpreted to be related to fluctuations in sea level. High reflectivities (density contrasts) are interpreted to be deposited during the last sea level low stand, and low reflectivities are interpreted as sedimentation during the last high stand. Sediments deposited during the low stand are proximal while others are distal, deposited in deeper water environments. Shallow structures observed in the chirp sonar profiles are mainly related to gas mobilization mechanisms, which shaped the topography of the seafloor in conjunction with soft sediment deformation and creep movement. The geomorphic features are related in some cases to gas expulsion zones such as pockmarks and mud volcanoes. Other structures are related to soft sediment deformation and creep mobilization. Soft sediment deformation is confined to the deepest part of the minibasin, while the other features are not depth dependent. Highly deformed intervals at the bottom of the sequence seem to control subsequent sedimentation.