Ray-theoretic localization of an impulsive source in a stratified ocean using two hydrophones

Abstract

A method is presented for passive localization of impulsive acoustic sources in a stratified ocean by measuring relative times of direct and surface-reflected arrivals at two hydrophones. The proposed method is based on ray theory and takes into account the effects of refraction on the geometry of acoustic paths ͑ray bending͒ and travel times, generalizing previous approaches based on the homogeneous-ocean assumption ͑straight-line localization͒. If the hydrophone depths are known, then the source depth and distance from each hydrophone can be estimated from the three differential arrival times. If in addition the hydrophone separation is known, the bearing of the source can be estimated as well. Apart from the effects on ray geometry and travel times, stratification affects localization by introducing shadow zones and caustics. For source locations in the neighborhood of caustics, the localization problem accepts two solutions, one of which is the true source location and the other one which is an artifact ͑ghost solution͒. This ambiguity can be resolved in the case of tracking moving sources since as the source moves away from the caustic only the track corresponding to the true source position will be continued. The present method has been developed primarily for monitoring the dive behavior of sperm whales producing acoustic clicks, yet it is general and can be applied for the localization and tracking of any kind of impulsive acoustic source in the ocean.

Unmanned localization of sperm whales in realistic scenarios

“…After the path identification the arrivals time difference are estimated to be further used in the localization system. The same data flows can be used to describe the two-hydrophones system presented in [1]. However without the beamformer step since this is composed by just two hydrophones.…”

“…In the following the arrays configuration, the bearnforming technique and the theoretical background are described (further details on the mathematics can be found in [1]). In the later, the signals past directly from the reception system to the direct and the surface reflected paths identification.…”

“…In Sec. III the error localization results which are obtained in different noisy channels in a homogeneous scenario are compared with those obtained by the two hydrophones methodology described in [1]. Finally in Sec.…”

“…The system was comprised of two basic parts: a short baseline system for direction calculation, for each array, and a long baseline system for 3D position calculation. In [1] a passive localization method of pulsed sound sources was presented, which required just the differential arrival times at a pair of hydrophones (at different depths) of the direct and the first surface-reflection paths to carry it out. It offered a low cost signal processing and it was appropriated for use in remote systems with power and computational limitations.…”

“…The proposed method is based on that presented by E. K. Skarsoulis [1] and it aims to develop a low computational complexity signal processing system, which can operate autonomously in remote buoys with power and computational limitations. This study consists on the analysis of the improvements provided by using beamforming theory on the method proposed in [1]. The eqnipment used mainly composed of two hydrophone arrays deployed near the surface.…”