Passive Deep Ocean Thermometry using Hydroacoustic Signals from Submarine Earthquakes

Abstract

Measuring temperature variations of the deep ocean is necessary to evaluate the heat flux between the atmosphere and the hydrosphere and thus calibrate climate change models. In the last two decades a passive alternative to profiling oceanographic floats has emerged: hydroacoustic thermometry. This method consists of using the oceanic ambient noise field as a source of acoustic waves and hydrophone arrays as receivers. These sensors are part of the International Monitoring System which is in place for the verification of the Comprehensive Nuclear-Test-Ban Treaty. They are positioned at a water depth of approximately 1 km, in the Sound Fixing and Ranging channel. This channel is a low-velocity layer that functions as an acoustic wave guide, thus facilitating very long range propagation with little attenuation. This study analyses transient signals between 2005 and 2018 triggered by submarine earthquakes and detected at station H10, situated near Ascension Island in the South Atlantic Ocean. This station consists of two three-element (triplet) arrays with an aperture of approximately 2 km each. The triplets are 126 km apart. Array processing techniques applied to individual triplets are prone to be biased by local conditions of the array surroundings. We demonstrate that this bias is largely suppressed when jointly processing both triplets as one six-elements array. Due to the malfunction of element S1, data quality decreased after October 2013 and our results are less robust. For the 2005-2013 period, we retrieve a temperature of 4.3 to 4.8 °C.