On the possibility of using acoustic reverberation for remote sensing of ocean dynamics

Abstract

The two point correlation function of diffuse noise fields produced by distributed random sound sources carries useful information on the medium of sound propagation. Such information can be used for performing passive acoustic tomography of the ocean. In a number of cases that are important for practice, the noise field in the ocean is predominated by contributions of individual point sources. Here, a theoretical study is presented on the possibility of determining the sound speed and current velocity in the water column by the correlation processing of reverberation signals measured by two vertical receiving arrays. In other words, we study the possibility of replacing the diffuse noise produced by a great number of delta correlated sources by waves generated by a localized source and scattered at the rough surface and bottom of the ocean for sensing the medium. The correlation function of scattered waves is calculated by using the method of small perturbations. It is shown that the correlation processing of the scattered waves offers an opportunity of measuring the acoustic nonreciprocity and reconstructing the field of sound speed in the fluid, without using any acoustiLc transceivers.

Acoustic Green's function extraction from ambient noise in a coastal ocean environment

“…The theoretical foundation of noise interferometry is now well established [Rytov, 1956;Lobkis and Weaver, 2001;Lobkis, 2004, 2005;Wapenaar, 2004;Roux et al, 2004;Snieder, 2004Snieder, , 2007Sabra et al, 2005aSabra et al, , 2005bWapenaar et al, 2006;Godin, 2006Godin, , 2009aGodin, , 2009bGodin, , 2009cGodin, , 2010Godin, , 2012Garnier and Papanicolaou, 2009;Froment et al, 2010;Brown, 2011]. Experimental successes of noise interferometry have been reported in many fields [Duvall et al, 1993;Rickett and Claerbout, 1999;Duroux et al, 2010;Haney, 2009;Sabra et al, 2007aSabra et al, , 2007bWeaver and Lobkis, 2001;Davy et al, 2013;Godin et al, 2014], most notably seismology [Campillo and Paul, 2003;Sabra et al, 2005c;Shapiro et al, 2005;Yang et al, 2007;Stankiewicz et al, 2010;Poli et al, 2012].…”

Application of time reversal to passive acoustic remote sensing of the ocean

“…It should be emphasized that the success of a passive TRM does not rely on the noise field being perfectly diffuse; underwater noise never is perfectly diffuse. Under realistic assumptions about noise sources in the ocean, NCCF consists of the same ray or normal arrivals as the superposition of the Green functions G AB (t) and G BA (-t); the ray travel times and phases of individual normal modes are the same in the NCCF and in the Green functions, while the amplitudes of the rays or normal mode components of NCCF are determined by noise directionality and are generally different from those generated by a point source [4,24,27]. However, an accurate reproduction of the kinematic structure of the Green functions (i.e., the ray travel times and normal mode phases) is exactly what is required for the timereversed field to focus at the location of the virtual source [13,14].…”

Green’s function retrieval in a field of random water waves

“…Simulations were shown to be in very good agreement with theoretical predictions, but a caveat should be noted: Green's functions computed using Eq. (28) are causal only in the nondispersive limit. Note, however, that non-causal pre- More significant than our simulations, wavetank measurements were shown to be in good agreement (recall Fig.…”

“…In recent years it has become appreciated that deterministic wave propagation information can be extracted from an apparently random wave field via a process that can be described as random wave interferometry; the cross-correlation of measurements at two locations of a random wave field yields an approximation to the Green's function that describes propagation between those locations [1,2,3,4,5,6,7,8,9,10,11,12,13]. Random wave interferometry has been widely investigated in the context of elastic waves in solids [14,15,16], including seismic [17,18,19,20,21,22] and helioseismic [23,24] applications, and sound waves in fluids, including applications to ocean acoustics [25,26,27,28,29,30,31,32,33,34,35] and atmospheric acoustics [36,37,38]. The underlying theory is widely applicable, as it can be applied to any type of linear wave propagation.…”

“…Solid curve: cross-correlation function C AB (t) of η(x A , t) and η(x B , t) in an open dispersive water wave system (assuming ω 2 = gk), computed by coherently averaging approximately 5000 realizations of the correlation function based on 160 s blocks of simulated surface elevation. Dashed curve:D(t) * [Gη(x B |x A , t) + Gη(x A |x B , −t)], where Gη(x B |x A , t) was computed using Eq (28)…”