Abstract
A series of underwater acoustic localization experiments were conducted in the Eastern Mediterranean Sea to test the performance of a Bayesian method for localization of pulsed acoustic sources exploiting time differences between direct and surface-reflected arrivals at two hydrophones of known depth. The experiments involved a controlled source (pinger) at various depths/ranges as well as vocalizing sperm whales encountered off southern Crete. The localization method provides primarily range and depth information. In addition, if the location of the hydrophones in the horizontal is known, horizontal localization can be performed as well, subject to left–right ambiguity; this was applied for whale localization. The localization results confirmed the anticipated behavior of range, depth, and bearing estimation errors, which, according to theory, depend mainly on the source azimuth. In particular, range and depth estimation errors are larger for source locations close to broadside to the array and smaller towards endfire, and they increase with range. Conversely, bearing estimation errors are larger close to endfire and smaller towards broadside. Localizations in this paper were performed to ranges of about 3.5 km. The limiting factors for localization to longer ranges were the loss of ability to resolve direct and surface-reflected arrivals as well as the self-noise of the hydrophones.
Highlights
Accurate localization in the underwater environment is crucial for a variety of applications such as monitoring of equipment deployed underwater [1], marine mammal behavioral studies [2], security monitoring of divers, search and rescue operations following accidents at sea [3], etc.Sound, in contrast to light and electromagnetic waves in general, propagates efficiently in the ocean interior, because acoustic waves are subject to low absorption in water
If the hydrophone depths and the sound speed profile are known, from independent measurements, the range and depth of the source can be estimated from the three time difference of arrival (TDOA), between direct and surface-reflected arrivals at each hydrophone and between direct arrivals at the two hydrophones [20]
For ranges longer than 3 km the boundary of the area is constrained not because of the time difference panel on the right of Figure 4 focuses on the upper 200 m and shows the area where the TDOA between direct and surface-reflected arrivals is larger than 7 ms, whereas the colors denote the ray-theoretic transmission loss (TL) in dB according to the scale on the right
Summary
Accurate localization in the underwater environment is crucial for a variety of applications such as monitoring of equipment deployed underwater (e.g., autonomous underwater vehicles—AUVs, towed devices) [1], marine mammal behavioral studies [2], security monitoring of divers, search and rescue operations (e.g., search for black boxes—voice and data recorders) following accidents at sea [3], etc. In the framework of a recent study [20], a Bayesian formulation has been introduced accounting for uncertainties of the arrival times and hydrophone locations, as well as for depth-dependent uncertainties in the sound speed profile, and using these to provide estimates for localization uncertainty. In this approach, which is based on ray theory, the depth and range (horizontal distance) estimation problem is decoupled from the bearing (azimuth) estimation problem, and the localization is carried out in two steps.
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