Passive probing of the sound fixing and ranging channel with hydro-acoustic observations from ridge earthquakes

Abstract

The International Monitoring System includes a hydro-acoustic part to verify the Comprehensive Nuclear-Test-Ban Treaty. Besides explosive signals, monitoring stations also detect acoustic waves from earthquakes that travel through the SOund Fixing And Ranging (SOFAR) channel. The travel times of such detections are listed in the Reviewed Event Bulletin, which is statistically evaluated for the stations located in the Pacific, Indian, and Atlantic Oceans. The celerities of ridge earthquakes are calculated to build up a homogeneous data-set, based on similar source mechanisms. The celerity is defined as the epicentral distance divided by the travel time. The global characteristics of these celerities can be well understood in terms of temperature variations in the SOFAR channel. A detailed velocity profile has been retrieved for the Atlantic Ocean where large differences (14 m/s) are found between the southern and northern parts of the basin. Propagation modeling with normal modes supports these findings, which shows that the celerity gives an estimate of the sound speed in the SOFAR channel. These results compare remarkably well with those from active experiments, showing the ability of passively probing the SOFAR channel with hydro-acoustic waves from earthquake sources.

Tracking Submarine Volcanic Activity at Monowai: Constraints From Long‐Range Hydroacoustic Measurements

“…Derived sound speed values average at 1,474 m/s and are well within two standard deviations (±14 m/s) of the nominal estimate of 1,481 m/s (Figure d). The offset could be explained by the movement of the hydrophone moorings due to deep ocean currents as well as local variations in ocean temperature, and hence, sound speed across the array (e.g., Evers & Snellen, ).…”

“…Long‐range propagation of low‐frequency underwater sound phases is a key feature of the hydroacoustic waveform component of the International Monitoring System (IMS). As part of the verification regime for the Comprehensive Nuclear‐Test‐Ban Treaty (CTBT) of 1996, the objective of the IMS hydrophone network is to globally detect underwater nuclear explosions, but the comprehensive installation also enables the study of natural phenomena, including among others, earthquake rupture propagation (Guilbert et al, ; Tolstoy & Bohnenstiehl, ), tsunami signals (Matsumoto et al, ), ocean acoustic propagation (Evers & Snellen, ), and marine mammal vocalization (Le Bras et al, ; Ward et al, ). A total of 11 hydroacoustic receiver sites are in operation worldwide, six of which are hydrophone triplet arrays, typically deployed at remote ocean islands and near the SOFAR channel axis.…”

Deep ocean sound speed characteristics passively derived from the ambient acoustic noise field

“…A study with deterministic transient signals from earthquakes has already shown the ability to probe the deep ocean's temperature with hydroacoustic recordings from the IMS (Evers & Snellen 2015). In this study, it is aimed to passively retrieve the deep ocean temperature from the ambient acoustic noise field.…”

A study on the ambient noise field at a hydroacoustic array near Robinson Crusoe Island

“…The aforementioned sources confine the dominant sources of the hydroacoustic network's ambient noise field. Recent studies have shown the potential of passive hydroacoustic thermometry in the SOFAR channel using traveltime variations of ridge earthquakes (Evers & Snellen 2015) and by crosscorrelating ambient signals recorded at two distinct IMS arrays, H10N and H10S, located near Ascension Island in the South Atlantic Ocean (Woolfe et al 2015;Evers et al 2017).…”