Optical remote sensing of sound in the ocean

Abstract

Abstract. We propose a remote sensing technique to measure sound in the upper ocean. The objective is a system that can be flown on an aircraft. Conventional acoustic sensors are ineffective in this application, because almost none (∼0.1%) of the sound in the ocean is transmitted through the water/air interface. The technique is based on the acoustic modulation of naturally occurring bubbles near the sea surface. It is clear from the ideal gas law that the volume of a bubble will decrease if the pressure is increased, as long as the number of gas molecules and temperature remain constant. The pressure variations associated with the acoustic field will therefore induce proportional volume fluctuations of the insonified bubbles. The lidar return from a collection of bubbles is proportional to the total void fraction, independent of the bubble size distribution. This implies that the lidar return from a collection of insonified bubbles will be modulated at the acoustic frequencies, independent of the bubble size distribution. Moreover, that modulation is linearly related to the sound pressure. A laboratory experiment confirmed the basic principles, and estimates of signal-to-noise ratio suggest that the technique will work in the open ocean. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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“…This pressure can be amplified in shallow water, such as lagoons [4,177]. Churnside et al, 2014 [176] suggested theoretical approaches that use ARS images (i.e., LiDAR) to measure sound in the upper ocean by modelling natural bubbles near the sea surface. Soares et al, 2019 [177] used the RS approach based on SS hydrophone (called digitalHyd SR-1) to record the noise between summer and winter in the Ria Formosa lagoon in Portugal.…”

Inland and Near-Shore Water Profiles Derived from the High-Altitude Multiple Altimeter Beam Experimental Lidar (MABEL)

“…The analysis of lidar returns from photon counting systems is, in many ways, similar to analysis using analog systems (E.g. Churnside, Naugolnykh, and Marchbanks, 2014;Guenther, 1985;Guenther, LaRocque, and Lillycrop, 1994). A principal difference is that, instead of analyzing a full waveform return from a single pixel illuminated by a high energy analog pulse, an equivalent, but not identical, histogram must first be generated from along track returns.…”