Experimental evaluation of a dedicated underwater air gun for diver deterrence

To improve port security capability there is a need for systems for detection, warning and deterrence of unwanted divers in harbour environments. In a complex environment such as a harbour, these topics present challenging problems and as such are subject to research and development. Already a few dedicated systems are commercially available on the market, whether for underwater intruder detection, warning or deterrence. However, due to the complex nature of underwater environments and the difficulties of testing underwater, there is still a gap in knowledge on the performance of each of these systems in the actual operational environments of interest. A commercially available underwater air gun, which has been specifically developed for the purpose of diver deterrence, has been tested by order of the Netherlands Ministry of Defence (NL-MoD) in the naval harbour of Den Helder, The Netherlands. The main purpose of the test was to determine the effective range. For this, some specific metrics of the air gun pulse have been measured at various distances. Although still a lot is unknown about possible injuries or physiological effects on humans of high energetic impulsive underwater sound, and even less about the behavioural effects on determined underwater intruders, metrics like 'peak over pressure' and impulse of the received acoustic pulse are regarded as indicative. The tests have been performed in a straightforward manner. Due to the uncertainties on human impact of the air gun pulse, no divers were involved in the tests. Hydrophones were used to record the time series of the acoustic pulse at various distances. The acoustic pulses were fired by the air gun deployed at a fixed position. Due to the shallow water conditions in the harbour, the shape of the received pulse is very different from the actual transmitted pulse shape. Moreover, in the harbour environment, the received pulse shape is very much dependent on the distance between the source and the receiver. It has been found that the usual definition of the impulse metric that is used to assess the impact of underwater explosions on divers in free space or deep water environments, is of little practical value. Therefore another, more generally applicable definition of impulse was used, based on time integration of the absolute pressure over the time interval in which 95 % of the sound exposure has been reached.