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Abstract Although typically used to measure dynamic strain from seismic and acoustic waves, Rayleigh‐based distributed acoustic sensing (DAS) is also sensitive to temperature, offering longer range and higher sensitivity to small temperature perturbations than conventional Raman‐based distributed temperature sensing. Here, we demonstrate that ocean‐bottom DAS can be employed to study internal wave and tide dynamics in the bottom boundary layer, a region of enhanced ocean mixing but scarce observations. First, we show temperature transients up to about 4 K from a power cable in the Strait of Gibraltar south of Spain, associated with passing trains of internal solitary waves in water depth <200 m. Second, we show the propagation of thermal fronts associated with the nonlinear internal tide on the near‐critical slope of the island of Gran Canaria, off the coast of West Africa, with perturbations up to about 2 K at 1‐km depth and 0.2 K at 2.5‐km depth. With spatial averaging, we also recover a signal proportional to the barotropic tidal pressure, including the lunar fortnightly variation. In addition to applications in observational physical oceanography, our results suggest that contemporary chirped‐pulse DAS possesses sufficient long‐period sensitivity for seafloor geodesy and tsunami monitoring if ocean temperature variations can be separated.
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Towards tsunami early-warning with Distributed Acoustic Sensing: Expected seafloor strains induced by tsunamis
Tsunami wave observations far from the coast remain challenging due tothe logistics and cost of deploying and operating offshoreinstrumentation on a long-term basis with sufficient spatial coverageand density. Distributed Acoustic Sensing (DAS) on submarine fiber opticcables now enables real-time seafloor strain observations over distancesexceeding 100 km at a relatively low cost. Here, we evaluate thepotential contribution of DAS to tsunami warning by assessingtheoretically the sensitivity required by a DAS instrument to recordtsunami waves. Our analysis includes signals due to two effects induced by thehydrostatic pressure perturbations arising from tsunami waves: thePoisson’s effect of the submarine cable and the compliance effect of theseafloor. It also includes the effect of seafloor shear stresses andtemperature transients induced by the horizontal fluid flow associatedwith tsunami waves. The analysis is supported by fully coupled 3-Dphysics-based simulations of earthquake rupture, seismo-acoustic wavesand tsunami wave propagation. The strains from seismo-acoustic waves andstatic deformation near the earthquake source are orders of magnitudelarger than the tsunami strain signal. We illustrate a data processingprocedure to discern the tsunami signal. With enhanced low-frequencysensitivity on DAS interrogators (strain sensitivity ≈2×10 at mHz frequencies), we find that, on seafloorcables located above or near the earthquake source area, tsunamis areexpected to be observable with a sufficient signal-to-noise ratio withina few minutes of the earthquake onset. These encouraging results pavethe way towards faster tsunami warning enabled by seafloor DAS.
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- Award ID(s):
- 2311206
- PAR ID:
- 10536419
- Publisher / Repository:
- ESS Open Archive
- Date Published:
- Format(s):
- Medium: X
- Institution:
- ESS Open Archive
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Posted Content:
- https://doi.org/10.22541/essoar.171052484.46251426/v1
