Acoustic propagation through anisotropic internal wave fields: Transmission loss, cross-range coherence, and horizontal refraction

Abstract

Results of a computer simulation study are presented for acoustic propagation in a shallow water, anisotropic ocean environment. The water column is characterized by random volume fluctuations in the sound speed field that are induced by internal gravity waves, and this variability is superimposed on a dominant summer thermocline. Both the internal wave field and resulting sound speed perturbations are represented in three-dimensional (3D) space and evolve in time. The isopycnal displacements consist of two components: a spatially diffuse, horizontally isotropic component and a spatially localized contribution from an undular bore (i.e., a solitary wave packet or solibore) that exhibits horizontal (azimuthal) anisotropy. An acoustic field is propagated through this waveguide using a 3D parabolic equation code based on differential operators representing wide-angle coverage in elevation and narrow-angle coverage in azimuth. Transmission loss is evaluated both for fixed time snapshots of the environment and as a function of time over an ordered set of snapshots which represent the time-evolving sound speed distribution. Horizontal acoustic coherence, also known as transverse or cross-range coherence, is estimated for horizontally separated points in the direction normal to the source-receiver orientation. Both transmission loss and spatial coherence are computed at acoustic frequencies 200 and 400 Hz for ranges extending to 10 km, a cross-range of 1 km, and a water depth of 68 m. Azimuthal filtering of the propagated field occurs for this environment, with the strongest variations appearing when propagation is parallel to the solitary wave depressions of the thermocline. A large anisotropic degradation in horizontal coherence occurs under the same conditions. Horizontal refraction of the acoustic wave front is responsible for the degradation, as demonstrated by an energy gradient analysis of in-plane and out-of-plane energy transfer. The solitary wave packet is interpreted as a nonstationary oceanographic waveguide within the water column, preferentially funneling acoustic energy between the thermocline depressions.

Modeling and Forecasting Ocean Acoustic Conditions

“…In this approximation, azimuthal energy flux is zero. This is fine for many applications, but there are many instances where this approach gives notably incorrect results, as illustrated in many papers (e.g., Oba and Finette 2002;Badiey et al 2005;Lynch et al 2010).…”

“…This, in turn, can lead to strong time-dependent sound fluctuations if evolving structures in the water column strongly perturb the seabed interactions, for example, structures found in nonlinear internal waves (Zhou et al 1991;Preisig and Duda 1997;Duda and Preisig 1999;Oba and Finette 2002). (In that situation, knowing seafloor properties is as critical as knowing the nature of the refraction within the water if a fully quantitative result is desired, but here behavior of the water column is emphasized).…”

Three-dimensional acoustic propagation through shallow water internal, surface gravity and bottom sediment waves

“…Combination of ducting and antiducting events creates very significant scintillations of the acoustic intensity. These effects were first described for internal waves theoretically in [9] and numerically in [10,8]. The first experimental observations of these strong intensity fluctuations (6-10 dB) compared to the "no waves" case, with an along wave geometry for internal waves passing over a fixed source and receiver, were made during the SWARM95 experiment [8] and described in [50].…”

“…Also as part of the SWARM95 experiment, Badiey [8] measured acoustic signals for an along waves path and showed that strong intensity fluctuations are caused by horizontal ducting of the acoustic signal between internal waves. This was the first experimental evidence of an out of vertical plane acoustic interaction with internal waves, which was predicted theoretically in [9] and numerically in [10,11]. The Shallow Water 2006 Experiment (SW06) [12,13] was the first effort that concentrated on measuring the fully three-dimensional variability of the water column as well as acoustic signals coming from both the across and along internal waves directions.…”

Initial results from a cartesian three-dimensional parabolic equation acoustical propagation code

“…Note that a different 3D PE code, FOR3D, has been used for some years in ocean acoustics, perhaps most recently by Naval Research Laboratory investigators [5,6]. FOR3D uses a rangeazimuth computational grid and implements narrow-angle azimuthal coupling.…”