Imaging Fine-scale Density Structure in the Ocean with Seismic Reflection Data

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Recent studies have shown that boundaries between water masses of different density result in reflections that can be observed in seismic data originally acquired to image the structure beneath the seafloor. Because seismic reflection data have been collected throughout the world oceans over past several decades, they provide an opportunity to extend spatial and temporal constraints on a variety of ocean processes. It is proposed to examine the extensive seismic reflection database collected since the mid-1980s along the continental margin of the western US, which were originally acquired to study tectonic processes associated with the Mendocino transform fault and Cascadia subduction zone. One objective is to analyze local perturbations of the density field that are observed over the eastern end of the Mendocino Escarpment to test the hypothesis that they result from tidally induced mixing. A possible reflection from the top of a bottom mixed layer may also be associated with interaction of internal waves with the seafloor on the flank of the Escarpment. Another objective is to determine temporal and spatial variations in the strength and distribution of eddies in the California undercurrent. These eddies have been detected by drifters and are apparent in the extensive hydrographic data base acquired offshore Oregon and northern California between 1997 and 2003 as part of the NSF/NOAA GLOBEC program. Because the hydrographic and seismic data overlap in time and space, there is an opportunity to test the validity of ocean density models derived from seismic data and map the spatial extent of the eddies a times in the past when no other data to do this exist, thus extending the historical record. Reprocessing and inversion will be needed to optimize imaging of the fine-scale velocity/density structure of the ocean; most of the existing data have been processed using parameters that have distorted, and often eliminated, signals originating in the ocean rather than beneath the seafloor. Changes in ocean current regimes have broad impacts on ecology and climate. The project will also provide training for a graduate student in digital signal processing techniques of broad applicability in geophysics. Finally, the results will be used in public presentations that the PIs give in local schools and other forums designed to transmit the importance and excitement of earth science and oceanography.