The use of active sonar to study cetaceans

Abstract

Cetacean species face serious challenges worldwide due to the increasing noise pollution brought to their environment by human activities such as seismic exploration. Regulation of these activities is vaguely defined and uncoordinated. Visual observations and passive listening devices, aimed at preventing conflicts between human wealth and cetaceans’ health have some fundamental limitations and may consequently fail their mitigation purposes. Active sonar technology could be the optimal solution to implement mitigation of such human activities. In my thesis, the proper sonar unit was used to test the feasibility to detect cetaceans in situ. Omnidirectional sonars could be the optimal solution to monitor the presence of cetaceans in the proximity of potential danger areas. To use this class of sonar in a quantitative manner, the first step was to develop a calibration method. This thesis links in situ measurements of target strength (TS) with variation trends linked to the behavior, morphology and physiology of cetacean. The butterfly effect of a cetacean’s body was described for a fin whale insonified from different angles. A relationship between whale respiration and TS energy peaks was tested through a simple prediction model which seems very promising for further implementation. The effect of lung compression on cetacean TS due to increasing depth was tested through a basic mathematical model. The model fit the in situ TS measurements. TS measurements at depth of a humpback whale, when post-processed, correspond to TS measurements recorded at the surface. Sonar technology is clearly capable of detecting whale foot prints around an operating vessel. Sonar frequency response shows that frequencies between 18 and 38 kHz should be employed. This work has established a baseline and raised new questions so that active sonar can be developed and employed in the best interest for the whales involved in potentially harmful conflicts with man.