Environmental Science and Engineering Seminar

Distributed acoustic sensing (DAS) provides a novel opportunity to turn coastal seafloor fiber optic cables into high-resolution surface wave measurement arrays. A DAS interrogator is connected to the shore end of a fiber to measure strain or strain-rate by observing the reflection of lasers off impurities in the glass. Strain or strain-rate is responsive to variations in seafloor pressure (as well as acoustic and other waveforms in the water column) allowing each channel to act like a seafloor pressure mooring. This allows a fiber up to tens of kilometers in length with channel spacing of meters to act like a series of thousands of virtual wave buoys. Thus, it provides a particularly appealing method for observing spatial and temporal changes in wave conditions in regions with high spatial gradients, such as seasonally ice-covered coastal environments.

The presence of surface wave signals have been observed in cables globally in a wide range of environments, and these signals have now been successfully used to quantify wave spectra and statistics. Here we demonstrate methods for wave retrieval from seafloor cables with DAS using observations from a variety of seafloor cable types in the Alaskan Arctic and the coastal Atlantic (Martha's Vineyard, MA). Using empirical calibrations specific to each cable, the DAS-derived wave statistics agree well with conventional in situ measurements (correlations of R2 > 0.8). We summarize these diverse calibration datasets in an effort to understand and describe the controls on strain-rate response to surface waves. This approach provides notably higher spatial resolution of wave statistics than achievable by other in situ methods, and may provide new opportunities to understand the evolution of surface gravity waves in challenging coastal environments.