Measuring hydrodynamics and exploring nearshore processes using distributed sensing of fiber-optic cable strain

Distributed Acoustic Sensing (DAS) is a new method for recording oceanographic processes using seafloor fiber-optic cables, such as telecommunication cables. DAS returns spatially distributed measurements of cable strain, which can be related to hydrodynamic pressure, turning a submarine cable into a dense sampling array. A reinforced fiber-optic cable was installed in the cross-shore from the dune toe to similar to 15-m-water depth at the USACE Field Research Facility in Duck, NC to quantitatively compare DAS strain to co-located pressure sensors. We develop a methodology for transferring DAS strain to dynamic pressure and evaluate the performance of DAS to measure shallow- and intermediate-water waves in the incident band (0.04-0.4 Hz). A frequency-dependent empirical transfer function from DAS strain to dynamic pressure at the seabed is derived from the ratio of strain and pressure power spectra. DAS-derived significant wave heights and peak periods were consistent with pressure sensors (typical rmse = 0.2 m and 1 s) over a wide range of dynamic conditions (0.24-4 m wave heights and 3-18 s periods). DAS data were input into the cBathy algorithm to calculate cross-shore bathymetric profiles and were used to calculate cross-shore wave reflection. Preliminary applications of DAS to record wave splitting and shoreline position were explored. With this field data, we demonstrate new applications of DAS for measuring nearshore processes and encourage further exploration. The promising potential of DAS lies in the near-bed data volume provided, real-time capability, and ability to sample in any weather or light.