PANDA; A self-recovering shallow water acoustic logger

Abstract

An increasing focus on monitoring the coastal environment and the need to do so without leaving any surface expression to hinder vessel traffic or attract unintended recovery or collateral damage from other marine activities has led to a several developments in building bottom-mounted observation platforms that release to the surface on command or after a fixed delay. Typically these consist of a pressure housing with internal electronics package, a release mechanism, deadweight anchor, buoyancy unit and perhaps a spooled line. If no line is used, the deadweight is released and left on the bottom. This may be unacceptable in terms of environmental impact and undesirable for other reasons, particularly in sensitive areas. A second major disadvantage is that a free-floating released package drifts with current and can easily be lost. If the package is small (desirable if it is be cheap and easily deployed and recovered) it is then difficult to see on the surface, particularly in rough seas. Expensive VHF transmitters, GPS receivers, strobes, etc., are then required to ensure pick-up. This drives up the cost. The spooled line approach not only recovers the entire system, but also maintains the package tethered to the bottom at a fixed location until pick-up. With GPS standard positioning now accurate to some 5 m rms, this is sufficient to locate even small packages. We present an inexpensive package design that is small, light, deployable by two people and that consists only of a single combined buoyancy, pressure casing and spooled line unit plus an anchor. This is a development of an earlier Pop-up Ambient Noise Data Acquisition (PANDA) system built in 1996. The new unit is based on COTS components, costs under $10k and has been tested at depths from 17-100m acquiring ambient noise and to monitor oceanographic acoustic sources. The design permits various electronics payloads in a modular cage, inserted into a cylindrical pressure housing which has its own release mechanism, around which is wrapped 250m of release line. The PANDA is presently released on a preprogrammed timer. The conceptual design, components and example data from the South China Sea are presented.