On the environmental impact of continuous active sonar (abstract only)

Continuous Active Sonar (CAS) is an emerging technology in anti-submarine warfare operations. The feasibility of the technology has been demonstrated and it has been shown that CAS has a potential to reduce false alarm rates in reverberation-limited conditions. Now that the feasibility has been shown and further use of the technology is considered, it becomes important to consider the environmental impact. There is a special concern for marine mammals, since typical anti-submarine warfare (ASW) sonar has the potential for introducing physical effects and behavioral responses in particular. Compared to conventional pulsed sonar (CPS), there are two main differences: (i) CAS can operate at lower source levels compared to CPS, (ii) CPS operates at low duty cycles, whereas CAS uses a full duty cycle. For the assessment of the influence on marine mammals, the received sound exposure level (a measure of received sound energy level) is usually considered in addition to the peak pressure level. The sound exposure and peak pressure levels are thought to determine the potential of the sonar to induce hearing threshold shifts, where received sound pressure levels, that may be lower for CAS compared to CPS, are thought to be relevant for behavioral responses. However, masking of marine mammal hearing by CAS might be a new issue. Our conclusion therefore is that a detailed assessment of the environmental impact of CAS, and especially the influence on marine mammals, is required.