Acoustic Release

An acoustic release is an oceanographic device for the deployment and subsequent recovery of instrumentation from the sea floor, in which the recovery is triggered remotely by an acoustic command signal.

A typical release consists of the hydrophone (see dark gray cap in the figure), the battery housing (long gray cylinder), and a (red) hook which is opened to release the anchor by high-torque electrical motor.

Early use of acoustic releases for oceanography are reported in the 1960s, when it was recognized that deep ocean currents could more accurately be measured with sea floor mounted rather than ship board instruments. An obvious means of recovery was the use of a surface marker buoy linked to the sea floor instrument, but in areas of high ship traffic or the presence of ice bergs, this proved problematic. The acoustic release became a method to solve that problem, allowing the current meters to remain unattended on the seafloor for weeks or more, until the research vessel returned and triggered the release of the instrument by remote command, allowing it to float to the surface. In the book Descriptive Physical Oceanography, authors Pickard and Emery vividly describe the recovery phase:

Upon returning to the general location of the deployed mooring the scientist will reactivate the acoustic system on the release and use it to better locate the mooring and assure its condition as being ready for release. When ready, the release or wire-cutting mechanism is activated and the mooring is free to rise to the surface. There are many tense moments while waiting for the mooring to come to the surface; it may be difficult to spot as it floats low in the water so it usually carries a radio transmitter and a light to assist in locating it.

Today, acoustic releases are widely used in oceanography and offshore work alike. Applications are varied and range from individual instrument recovery, to salvage operations. More recent technological advances have resulted in the introduction of smaller devices that are now deployed in large numbers. For example, the Pfleger Institute of Environmental Research has deployed an array of 96 acoustic receivers for the monitoring of fish migrations in California's Channel Islands, with acoustic releases used to recover receivers beyond diver depth in regular intervals for data download and service.

...
Wikipedia