On the possibility of detecting surfacing sperm whales at risk of collision using others' foraging clicks

Abstract

Cetaceans are prone to collisions with fast vessels, and in areas of high cetacean and vessel density such as in the Canary Islands, the sperm whale (Physeter macrocephalus) is of great concern. Sperm whales are highly vocal and can be localized with passive sonar, but, when at or near the surface, they tend to stop vocalizing, i.e. when they are most at risk. Regrettably, ship-borne active solutions have proven inefficient due to the short detection range and the ships' high-speeds. Our objective in this paper is to evaluate the efficiency of an original passive sonar solution that would use vocalizing whale clicks at depth as acoustic sources to detect silent whales. This solution could be a non-invasive complementary component of a more complex passive localization whale anti-collision system. To meet this aim, a simulation tool for 3D acoustic propagation was designed in which a wideband Nx2D ray solution of the wave equation simulates a passive solution consisting of an arbitrary number of active acoustic sources, an illuminated object, and a receiver, all positioned in a three-dimensional space with arbitrary bathymetry. Both curved and straight ray solutions were implemented, the latter providing greater computational speeds at the expense of temporal and angular precision. The software recreates the resulting sound mixture of direct, reverberated and target back-scattered signals arriving at the array sensors for any array configuration, any number of sources and one target. Simulations show the application of the concept for the Canary Islands, with a detection range upper bound of the order of one kilometre.