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 most 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 using telecommunication fibers is
investigated here by analyzing ambient noise, earthquake signals, and
their associated phase velocities, on DAS records from three dark fibers
in the Mediterranean Sea. The recording quality varies dramatically
along the fibers and is strongly correlated with the bathymetry and the
apparent phase velocities of the 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 ground motion
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. The results demonstrate the
great potential of underwater DAS for seismic monitoring and earthquake
early warning.