page_title: Acoustic release
section_title: Project-specific selection criteria
hierarchical_section_title: Acoustic release / Project-specific selection criteria
caption_attribution_description: English: ROV with VLT-1 acoustic release interrogator (A).
context_page_description: 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, the battery housing, and a hook which is opened to release the anchor by high-torque electrical motor.
context_section_description: Applications for acoustic releases can vary substantially, and correspondingly the devices are designed and selected to best fit the requirements of a particular job.  Common design and selection characteristics are as follows:
  Acoustic transmission range and reliability: Acoustic command transmissions are used to issue the release command as sound travels easily through the water.  The transmission range must be sufficient to reach the device.  Individual releases are identified by unique identifier codes, and the number and security of available codes can be criteria when deploying many releases or in areas where accidental or unauthorized release may be a problem.  The command transmission system for shallow water releases must also be resistant to multi-path propagation (reverberations or echoes) which can corrupt a signal.
  Battery life:  Acoustic releases are generally powered by rechargeable or replaceable batteries. Battery life must be sufficient to cover the anticipated deployment period plus a reasonable margin of safety.  Depending on model, battery life may range from several weeks to a few years.
  Control station:  Acoustic releases can generally be controlled from the surface vessel, by lowering a sonar transducer into the water (figure 3).  However, some releases also offer the option to mount an interrogator on an underwater vehicle such as a ROV (figure 4). If a release should fail to surface, the underwater vehicle can be deployed and the ranging function can be used to home in on the stuck instrument, recovering it using the manipulator of the ROV or other methods.
  Depth rating: The acoustic release must withstand the water pressure at the operations site.  Depth ratings may range from 300m or less up to full ocean depth.
  Load rating:  Acoustic releases are designed to handle a certain maximum load.  The deployment of larger instruments generally requires a higher load rating.  A release may also have a minimum load rating, required for reliable operation of its mechanism.
  Resistance to failure: Failure modes for acoustic releases are both application and site specific.  Stainless steel components for example are subject to crevice corrosion in anoxic waters.  Releases used in shallow water sites are more subject to biofouling which can impede a mechanism than those used in fresh or deep water.  Shallow water sites are also more subject to mechanical forces on the mooring caused by surge.
  Ranging and status reporting capability: Some acoustic releases offer a remote ranging and status reporting capability.  Upon arrival on site, a specific release can be interrogated and its distance determined.  Operational parameters such as remaining battery capacity or the status of the release mechanism may be reported as well. This information can be used to position the surface vessel above the instrument for ease of recovery following release, or to evaluate the health and status of a device.
caption: Figure 4: An acoustic release interrogator (A) is mounted on a ROV, allowing the ROV to home in on the acoustic release to observe or recover in case of a release failure.
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