On the Detection Capabilities of Underwater Distributed Acoustic Sensing

TLDR

Distributed acoustic sensing (DAS) transforms standard telecommunication cables into dense seismo‑acoustic arrays, offering great potential for underwater seismology. The authors investigate whether DAS can record transient ground deformations for seismic monitoring by analyzing ambient noise, earthquakes, and phase velocities on three Mediterranean dark fibers. They determine apparent velocities from well‑recorded earthquakes, use them to convert DAS S‑wave strain spectra to ground motion spectra, and quantify how apparent velocity affects detection by comparing predicted signal amplitudes to recorded noise levels. The study shows that recording quality along underwater fibers varies with bathymetry and apparent phase velocities, that DAS‑converted spectra agree with nearby on‑land seismometers after site correction, that slower waves generate higher strains and improve detection, and that DAS detection is comparable to broadband sensors and superior to on‑land fibers, underscoring its promise for seismic monitoring and early warning.

Abstract

Abstract The novel technique of distributed acoustic sensing (DAS) holds great potential for underwater seismology by transforming standard telecommunication cables, such as those currently traversing various regions of the world’s oceans, into dense arrays of seismo‐acoustic sensors. To harness these measurements for seismic monitoring, the ability to record transient ground deformations is investigated by analyzing ambient noise, earthquakes, and their associated phase velocities, on DAS records from three dark fibers in the Mediterranean Sea. Recording quality varies dramatically along the fibers and is strongly correlated with the bathymetry and the apparent phase velocities of recorded waves. Apparent velocities are determined for several well‐recorded earthquakes and used to convert DAS S‐wave strain spectra to ground motion spectra. Excellent agreement is found between the spectra of nearby underwater and on‐land seismometers and DAS converted spectra, when the latter are corrected for site effects. Apparent velocities greatly affect the ability to detect seismic deformations: for the same ground motions, slower waves induce higher strains and thus are more favorably detected than fast waves. The effect of apparent velocity on the ability to detect seismic phases, quantified by expected signal‐to‐noise ratios, is investigated by comparing signal amplitudes predicted by an earthquake model to recorded noise levels. DAS detection capabilities on underwater fibers are found to be similar to those of nearby broadband sensors, and superior to those of on‐land fiber segments, owing to lower velocities at the ocean‐bottom. The results demonstrate the great potential of underwater DAS for seismic monitoring and earthquake early warning.