Detection and classification of marine mammals using an LFAS system
Keywords:Acoustic waves, Detectors, Fisheries, Problem solving, Signal theory, Sonar, Low frequency active sonar (LFAS), Marine mammals, Plot structures, Submarine echoes
AbstractWorld wide a concern is emerging about the influence of man-made sound in the sea on marine life, and particularly about high power active sonars systems. Most concern lies with marine mammals, which fully depend on sound in their natural behaviour (foraging, navigation and communication). One of the sonars under debate is the Low Frequency Active Sonar (LFAS). This type of system is designed for long range detection of submarines. It consists of a powerful source and a towed array receiver. Incidents with marine mammals could be avoided if the receiver that is dedicated to detection of submarine echoes, is equipped with Detection, Classification and Localisation capabilities for marine mammals as well. In this paper the development of a prototype transient detector and classifier for the TNO-FEL LFAS array (named CAPTAS) is described. A broadband beamformer is developed that creates 8 beams (sectors) that are equally wide over the whole frequency band. A multi-beam LOFAR display is presented. On the normalised data a Page's test detector is applied that is "optimum" for signals with unknown duration. Detected transients are sent to a classifier that tries to discriminate between biological and man-made or natural transients. Time-frequency analysis is performed and in the resulting time-frequency plot structures are determined by means of cluster analysis after which the sound is classified. Detection results of the prototype are very good, the Classification module is under development and the Localisation module is part of future research. Part of this research is sponsored by the Royal NetherLands Navy (RNLN).
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