Seismic Oceanography of the Gulf of Mexico

Near-surface circulation within the Gulf of Mexico is dominated by the Loop Current and its generation of anticyclonic eddies, which periodically detach and migrate westward. The interaction of these eddies with the continental slope may play a significant role in transporting near-surface shelf waters into the central Gulf. Here, a 180 km long seismic profile acquired by ION Geophysical in July 2002 is analyzed. This profile crosses the Sigsbee Escarpment, starting in water depths of ~ 80 m and culminating in water depths of ~ 3000 m. It traverses a prominent anticyclonic eddy visible on altimetric surveys. The source consisted of a tuned airgun array (volume = 4800 cubic inches, pressure = 2000 psi). Reflections were recorded on a 9 km streamer towed at 9 m depth with 360 hydrophones spaced every 25 m. Shots were fired every 50 m yielding a fold of cover of 90. These data were processed using a standard workflow (e.g. band-pass filtering, direct wave removal, accurate velocity picking, stacking, migration). By changing the length of the streamer during processing, the `imaging time' of a single location can be varied between 3 and 30 minutes. The stacked image compares well with legacy hydrographic measurements. The top kilometer is characterized by bright, undulatory reflectivity associated with Subtropical Undercurrent and Antarctic Intermediate Water layers. At greater depths, the profile is acoustically transparent due to the nearly constant temperature and salinity of deeper Gulf waters. This hydrographic structure suggests that double diffusion may occur. However, thermohaline staircases are not observed, suggesting the presence of turbulent mixing. Diapycnal diffusivity will be calculated using spectral analysis of tracked seismic reflections. Temporal evolution of thermohaline structure will also be investigated.