Volume 127, Issue 5 e2021JC018375
Research Article

Surface Gravity Wave Interferometry and Ocean Current Monitoring With Ocean-Bottom DAS

Ethan F. Williams

Corresponding Author

Ethan F. Williams

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

Correspondence to:

E. F. Williams,

[email protected]

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Zhongwen Zhan

Zhongwen Zhan

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

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Hugo F. Martins

Hugo F. Martins

Instituto de Óptica, CSIC, Madrid, Spain

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María R. Fernández-Ruiz

María R. Fernández-Ruiz

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

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Sonia Martín-López

Sonia Martín-López

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

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Miguel González-Herráez

Miguel González-Herráez

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

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Jörn Callies

Jörn Callies

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

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First published: 28 April 2022
Citations: 5

Abstract

The cross-correlation of a diffuse or random wavefield at two points has been demonstrated to recover an empirical estimate of the Green's function under a wide variety of source conditions. Over the past two decades, the practical development of this principle, termed ambient noise interferometry, has revolutionized the fields of seismology and acoustics. Yet, because of the spatial sparsity of conventional water column and seafloor instrumentation, such array-based processing approaches have not been widely utilized in oceanography. Ocean-bottom distributed acoustic sensing (OBDAS) repurposes pre-existing optical fibers laid in seafloor cables as dense arrays of broadband strain sensors, which observe both seismic waves and ocean waves. The thousands of sensors in an OBDAS array make ambient noise interferometry of ocean waves straightforward for the first time. Here, we demonstrate the application of ambient noise interferometry to surface gravity waves observed on an OBDAS array near the Strait of Gibraltar. We focus particularly on a 3-km segment of the array on the continental shelf, containing 300 channels at 10-m spacing. By cross-correlating the raw strain records, we compute empirical ocean surface gravity wave Green's functions for each pair of stations. We first apply beamforming to measure the time-averaged dispersion relation along the cable. Then, we exploit the non-reciprocity of waves propagating in a flow to recover the depth-averaged current velocity as a function of time using a waveform stretching method. The result is a spatially continuous matrix of current velocity measurements with resolution <100 m and <1 hr.

Key Points

  • Seafloor horizontal strain measured with distributed acoustic sensing is sensitive to ocean surface gravity wave (OSGW) pressure

  • Ambient noise interferometry can be used to measure the dispersion relation of OSGW and infer flow velocity

  • We resolve the spatio-temporal pattern of the tidal current along a 3-km submarine cable segment in the Strait of Gibraltar

Plain Language Summary

Ocean currents are challenging to measure because they are complex: flow varies across more than six orders of magnitude in space and time. The wavespeed of ocean surface gravity waves propagating in a current encodes information about the velocity of the current, providing an opportunity to measure current velocity from ocean wave records. In particular, waves propagating along the current move faster than waves propagating against the current, which is termed non-reciprocity. By cross-correlating ocean wave records at two locations, we can measure the non-reciprocity and thereby recover an estimate of the average current velocity. In this study, we employ distributed acoustic sensing to measure ocean surface gravity wave propagation along an ocean-bottom fiber optic cable. As waves pass over the cable, they exert a small force at the seafloor which deforms the cable and stretches the fiber within. By repeatedly probing the fiber with a laser, we can measure these minute deformations at each point along the fiber. We demonstrate this method on a power transmission cable in the Strait of Gibraltar, monitoring the spatio-temporal evolution of the tidal current over a period of 4.5 days.

Conflict of Interest

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

Data Availability Statement

The data used in this paper are proprietary and not available for public release. However, the code necessary to reproduce the methods of this paper is available at https://github.com/ethanfwilliams/OSGW_interferometry_tutorial, using the public data set of Williams et al. (2019) (http://dx.doi.org/10.22002/D1.1296).