Listening to the seafloor with optical fibers

Abstract

Measurements of seismic ground motion at the seafloor provide critical insights into earthquake and tsunami hazards, deep-Earth structure, plate tectonics, submarine volcanism, and interactions between the ocean and solid Earth. Yet ocean-bottom seismometers (OBSs) and observatories are exceedingly scarce: Although about 70% of Earth’s surface is covered by water, less than 1% of the global network of permanent broadband seismic stations is installed at the seafloor (see “Deploying seismometers where they’re needed most: Underwater,” Physics Today online, 24 May 2019). The emerging field of fiber-optic seismology offers a promising new paradigm for ocean-bottom instrumentation: distributed sensing rather than point sensing. Submarine fiber-optic cables for intercontinental telecommunications and power transmission traverse the global oceans and can be harnessed for distributed sensor networks. As seismic and ocean waves stretch and compress optical fibers at the seafloor, the light traveling through them encodes valuable information. With distributed acoustic sensing (DAS) and other new fiber-seismic methods, that information can be exploited for geophysical monitoring (see Physics Today, March 2018, page 24). This Quick Study explains how.