Two-dimensional mapping of impulsive ambient sound on a coral reef using vector sensors

Abstract

Coral reef health is an important focus of tropical marine park management. Evidence exists that passive acoustic measurements of reef sound are associated with reef health and ecodiversity, but its utility would be enhanced by an ability to localize and map source distributions. Since much reef noise is impulsive, arising from both fish and crustaceans, localization could be achieved by collecting relative time-of-arrival information across synchronized spatially separated hydrophones. However, the sheer number of bioacoustic pulses in many reef environments can overwhelm attempts to associate sounds between sensors, Here we discuss how directional acoustic vector sensors sidestep these issues and permit 2-D mapping of thousands of pulses an hour, using data collected from two locations off the Big Island of Hawaii in 2020. At one location, three sensors spaced 20 m apart found acoustic activity was only present along the current-facing side of a 100 m long block reef. A second location set in the middle of more complex topography found several acoustic “hotspots” that remained stable over a month’s worth of measurements. A new generation of vector instrumentation is permitting this approach to be extended to crustacean noise in the kilohertz range. [Work sponsored by ONR and DARPA.]