Volume 128, Issue 9 e2023JC019980
Research Article

Fiber-Optic Observations of Internal Waves and Tides

E. F. Williams

Corresponding Author

E. F. Williams

Seismological Laboratory, California Institute of Technology, Pasadena, CA, USA

Now at Department of Earth and Space Sciences, University of Washington, Seattle, WA, USA

Correspondence to:

E. F. Williams,

[email protected]

Contribution: Formal analysis, Data curation, Writing - original draft

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A. Ugalde

A. Ugalde

Institute of Marine Sciences, ICM-CSIC, Barcelona, Spain

Contribution: Formal analysis, Data curation, Writing - review & editing, Supervision

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H. F. Martins

H. F. Martins

Instituto de Optica, CSIC, Madrid, Spain

Contribution: Methodology, Formal analysis, Data curation, Writing - review & editing

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C. E. Becerril

C. E. Becerril

Department of Electronics, Polytechnic School, University of Alcalá, Alcalá de Henares, Spain

Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, IRD, Géoazur, France

Contribution: Data curation, Writing - review & editing

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J. Callies

J. Callies

Environmental Science and Engineering, California Institute of Technology, Pasadena, CA, USA

Contribution: Writing - review & editing, Supervision

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M. Claret

M. Claret

Institute of Marine Sciences, ICM-CSIC, Barcelona, Spain

Contribution: Formal analysis, Writing - review & editing

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M. R. Fernandez-Ruiz

M. R. Fernandez-Ruiz

Department of Electronics, Polytechnic School, University of Alcalá, Alcalá de Henares, Spain

Contribution: Methodology, Writing - review & editing

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M. Gonzalez-Herraez

M. Gonzalez-Herraez

Department of Electronics, Polytechnic School, University of Alcalá, Alcalá de Henares, Spain

Contribution: Methodology, Writing - review & editing, Supervision

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S. Martin-Lopez

S. Martin-Lopez

Department of Electronics, Polytechnic School, University of Alcalá, Alcalá de Henares, Spain

Contribution: Methodology, Writing - review & editing, Supervision

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J. L. Pelegri

J. L. Pelegri

Institute of Marine Sciences, ICM-CSIC, Barcelona, Spain

Contribution: Writing - review & editing, Supervision

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K. B. Winters

K. B. Winters

Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA

Contribution: Software, Writing - review & editing, Supervision

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Z. Zhan

Z. Zhan

Seismological Laboratory, California Institute of Technology, Pasadena, CA, USA

Contribution: Writing - review & editing, Supervision

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First published: 09 September 2023

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.

Key Points

  • Distributed acoustic sensing on seafloor cables can resolve temperature changes associated with internal wave and boundary layer dynamics

  • We show temperature transients from solitons in the Strait of Gibraltar and from the propagation of internal tidal fronts at Gran Canaria

  • We also recover a signal proportional to barotropic tidal pressure including the fortnightly variation

Plain Language Summary

Distributed acoustic sensing (DAS) measures changes in the propagation time of light along finite segments of an optical fiber, which can be caused by both elastic deformations and temperature variations. We present two case studies of long-period temperature signals recorded with DAS on submarine cables offshore southern Spain and in the Canary Islands. These temperature signals are associated with internal waves, gravity waves that propagate on the ocean's density stratification. We also recover a signal matching the tidal pressure, which likely represents elastic strain, suggesting potential value of ocean-bottom DAS for seafloor geodesy and tsunami monitoring.

Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

Data Availability Statement

All 4.5 days of DAS data from the Strait of Gibraltar necessary to reproduce Figure 2 and the 3 days of DAS data from Gran Canaria necessary to reproduce Figures 3 and 4 are available through the CaltechDATA repository (Williams et al., 2023). Figures were produced using GMT6 (Wessel et al., 2019).