Detection and Classification of Marine mammals using an LFAS system

World 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 timefrequency
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).