Distributed sensing of earthquakes and ocean-solid Earth interactions using two orthogonal fiber optic telecom seafloor cables, offshore Methoni, Greece.
Abstract
A transformative step in earth science could be achieved by densifying the geophysical instrumentation of the oceans, which cover the majority of the surface of our planet. Turning existing fiber optic seafloor telecom cables into dense arrays of seismo-acoustic sensors is, in principle, possible now via Distributed Acoustic Sensing (DAS) technology, but the potential remains to be evaluated in practice. Here, we present results obtained from one week of measurements on two offshore cables laid approximately orthogonally, offshore Methoni, south-west of Peloponnese, Greece. The north-west oriented cable is 13 km long and reaches a depth of 1800 m, while the south-west cable is 26 km long and reaches a depth of 3700 m. Both cables cover part of the Hellenic subduction zone. The optical acquisition systems were set to provide longitudinal strain-rate measurements at least every 20 m. The shortest cable is connected to a multi-instrumental underwater station with an ocean bottom broadband seismic sensor, while two temporary broadband seismic sensors were installed on land. This seismic station setting allows us to evaluate strain for comparison with the DAS data. This calibration is based on earthquakes recorded at local and teleseismic distances, as well as ambient seismic noise. At all times, a clear background signal is recorded. We show that this signal is originating from the ocean wave pressure signal interacting with the sloping sea floor at shallow depth, and from the non-linear surface gravity wave interaction at greater depths. For each of these two wavefields, the quasi-orthogonal setting of the two cables allows to discriminate their azimuths and/or different components. These processes, at the origin of microseismic noise, are related to changes in the meteorological conditions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMOS13B1523R
- Keywords:
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- 1222 Ocean monitoring with geodetic techniques;
- GEODESY AND GRAVITY;
- 4564 Tsunamis and storm surges;
- OCEANOGRAPHY: PHYSICAL;
- 4594 Instruments and techniques;
- OCEANOGRAPHY: PHYSICAL;
- 7299 General or miscellaneous;
- SEISMOLOGY