The optical passive acoustic adaptive drifter system: A multisensor dynamically drifting mid-water biosurveillance platform

Conducting unobtrusive observations of the physical, acoustic, and biological properties of the mesopelagic zone remains challenging, despite the proliferation of mobile ROV, AUV, and glider platforms. Here, we discuss the Optical Passive Acoustic Adaptive Drifter System (OPAADS), a 2 m tall Lagrangian drifter platform that incorporates a CTD, stereo video cameras, vertical and tetrahedral hydrophone arrays, and acoustic vector sensors. The platform has also suspended a 91-m vertical thermistor array and acoustic doppler current profiler. The OPAADS uses a commercial buoyancy engine to dynamically adjust its target isobar via either a preprogrammed dive sequence or real-time commands relayed by an ultra-short baseline underwater tracking and communication system. Three platforms have been built and deployed over ten times since 2022, off both southern California and off the New England Seamounts, between depths of 100 and 700 m for durations between 24 and 72 h. We discuss the sensors, operations, and observations collected by OPAADS, including data useful for (1) characterizing bioacoustic chorusing, wind-driven surface noise directionality, and turbulence dynamics; (2) conducting species identification in the deep scattering layer; (3) performing marine mammal tracking; and (4) collecting pilot data studying potential approaches for long-range underwater navigation. [Work sponsored by ONR TFO.]