Ocean Surface Gravity Wave Interferometry with Seafloor DAS

Abstract:

The cross-correlation of a diffuse or random wavefield at two points has been demonstrated, both theoretically and observationally, to recover an empirical estimate of the Green’s function for a wave propagating between those two points under a wide variety of source conditions. Over the past two decades, the practical development of this principle, termed ambient noise interferometry, has revolutionized the fields of seismology and acoustics. Methods based on a stochastic description of the ocean-surface gravity wave (OSGW) field have been employed by the physical oceanography community for almost a century. Yet, because of the spatial sparsity of conventional water column and seafloor instrumentation, array based processing approaches like ambient noise interferometry have not been widely utilized for ocean waves.

Ocean-bottom distributed acoustic sensing (OBDAS) repurposes pre-existing optical fibers laid in seafloor cables as dense arrays of broadband strain sensors, which observe both seismic waves and ocean waves. Effort is still required to quantify the sensitivity and instrument response of OBDAS; however, recent studies have demonstrated that the measured OBDAS strain from OSGWs is approximately proportional to the theoretical seafloor pressure. The thousands of sensors in an OBDAS array make ambient noise interferometry of OSGWs straightforward for the first time.

Here, we demonstrate the application of ambient noise interferometry to OSGWs observed on an OBDAS array near the Strait of Gibraltar. We focus particularly on a 6-km segment of the array on the continental shelf, containing 600 channels at 10-m spacing. By cross-correlating the raw strain records, we compute empirical OSGW Green’s functions for each pair of stations. We compare various tomographic approaches, including beamforming and frequency-time analysis, to measure the OSGW dispersion relation with <100-m resolution in space and 1-hr resolution in time. We observe OSGW velocities vary by up to 1 m/s over each 12-hr period due to tidally modulated currents. Potential applications of this new method include measuring the wave- and current-induced bottom shear stress in coastal geomorphology, estimating the demands of offshore structures, and even studying coastal tsunami propagation.