Passive acoustic detection of a rebreather using a random array

Divers, including closed circuit (rebreather) divers constitute a potential threat to waterside infrastructures. Active diver detection sonars are available commercially but present some shortcomings, particularly in highly reverberant environments. This has led to research on passive sonar for diver detection. Passive detection of open-circuit UBA (underwater breathing apparatus) has been demonstrated, but detection of rebreathers is more challenging due to their quiet operation. A study was conducted using an array of broadband hydrophones (100 kHz bandwidth). 10 hydrophones were arranged in a random array of 1.5 m baseline in order to achieve a favorable tradeoff between angular resolution (requiring a large array), side-lobe rejection (requiring low hydrophone spacing), and cost (requiring a limited number of sensors). The array was deployed from a pier in a basin that is nearby a busy shipping lane in the Port of Rotterdam. A diver equipped with an Inspiration Classic rebreather from Ambient Pressure Diving (a popular recreational rebreather) approached the array, towing a surface GPS that was used for validating detections. The collected data was processed using conventional beamforming and 4th order cumulant based beamforming. The detection of the rebreather at range up to 120 m in presence of shipping traffic was reported and validated against GPS ground truth. This result illustrates the potential of higher order statistics to improve passive acoustic detection. The relatively large detection range suggests that quieter military rebreathers may be detectable at closer range. Furthermore, the limited number of hydrophones used (10) compared to commercial active diver detection sonar, that include typically hundreds of sensors, indicates a strong potential for improvement.