Collaborative Research: Quantitative Estimates of Oceanic Turbulence and Temperature Structure from Seismic Reflection Data

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OCE-0648620Recent work by our group shows that oceanic finestructure can be imaged in great detail usinglow-frequency (10-150 Hz) seismic reflection profiling, a technique that is widely used to image the solid earth but until recently has not been systematically applied to studies of oceanic internal structure. These findings open up a new field of endeavor in physical oceanography, which we call "seismic oceanography" (SO). Development of this new technique into a tool that can produce useful (and trusted) information on dynamical properties of interest to physical oceanographers is entering a critical stage. Much progress has been made in the first three years of effort. We have achieved a basic physical understanding of the origin of low-frequency acoustic reflections in the ocean. Our group and others have produced fascinating images of finestructure in numerous settings, including fronts, Meddies, intrathermocline lenses, warm-core rings, water-mass boundaries, and thermohaline staircases, some of which raise unexpected questions about the processes controlling the distribution of oceanic finestructure. We have shown that information on internal-wave spectra and temperature contrasts can be gleaned from seismic data. Yet despite these early successes, there remains considerable uncertainty about the extent to which (and under what circumstances) useful, reliable, quantitative information can be gleaned from seismic data. Seismic oceanography has yet to find its niche. In this study funds are requested for development and application of new SO analysis techniques that will help determine that niche. We will address two fundamental questions: (1) What quantitative information about dissipation, internal waves, and temperature structure can be gleaned from seismic images of finestructure? (2) What are the limitations and uncertainties of the method? Preliminary results are promising. A new theory of horizontal wavenumber (kx) spectra of isopycnal slopes suggests that seismic reflection images may be especially well suited to estimating turbulence dissipation, as the turbulent subrange of kx spectra extends to surprisingly large horizontal scales (100 m), which are easily imaged seismically. Our calculations show that reasonable estimates of turbulence dissipation can indeed be derived from seismic images. Our preliminary tests of full-waveform inversion to SO data shows that typical temperature finestructure can be resolved by seismic data at commonly acquired frequencies, though uncertainties are currently poorly characterized. We propose to undertake continued method development and apply these techniques to about a dozen publicly available legacy seismic data sets, from a variety of oceanic environments. Fully processed images on ~20 seismic sections, estimate dissipation from kx spectra, and invert seismic waveforms to estimate temperature profiles will be produced by comparing our results to ground-truth control from coincident XBT/CTD information.Intellectual Merit: We are in the formative stages of developing what may become a very useful tool for imaging oceanic finestructure. Preliminary work shows that this tool can provide unique information on turbulence dissipation that may help improve our ability to measure oceanic mixing and map "hotspots" of mixing. The work proposed here will advance seismic oceanography to a more quantitative state and therefore is a logical next step in determining the capabilities and limitations of low-frequency acoustic imaging in study ocean structure and dynamics.Broader Impacts: This study will develop a new approach for imaging ocean structure anddynamics, which exert a major control on the Earth's climate. Our study will have implications for understanding such processes as ocean mixing and the distribution of heat and salt in the ocean's interior. Our work serves NSF's broader goals in numerous ways that go beyond the specific realm of increasing physical oceanographic knowledge. Specifically, we will (1) advance a new cross-disciplinary field to the improvement of both marine geology and geophysics and physical oceanography; (2) help train a new cadre of graduate students and postdocs in this rapidly developing field; (3) involve an undergraduate student in cutting-edge research; and (4) support gender diversity in science, by training two female students.