The term “Passive Acoustic Monitoring System” (PAMS) describes a developmental sensing and data-acquisition system for recording underwater sounds. The sounds (more precisely, digitized and preprocessed versions from acoustic transducers) are subsequently analyzed by a combination of data processing and interpretation to identify and/or, in some cases, to locate the sources of those sounds. PAMS was originally designed to locate the sources such as fish of species that one knows or seeks to identify. The PAMS unit could also be used to locate other sources, for example, marine life, human divers, and/or vessels.

The underlying principles of passive acoustic sensing and analyzing acoustic-signal data in conjunction with temperature and salinity data are not new and not unique to PAMS. Part of the uniqueness of the PAMS design is that it is the first deep-sea instrumentation design to provide a capability for studying soniferous marine animals (especially fish) over the wide depth range described below. The uniqueness of PAMS also lies partly in a synergistic combination of advanced sensing, packaging, and data-processing design features with features adapted from proven marine instrumentation systems. This combination affords a versatility that enables adaptation to a variety of undersea missions using a variety of sensors.

The interpretation of acoustic data can include visual inspection of power-spectrum plots for identification of spectral signatures of known biological species or artificial sources. Alternatively or in addition, data analysis could include determination of relative times of arrival of signals at different acoustic sensors arrayed at known locations. From these times of arrival, locations of acoustic sources (and errors in those locations) can be estimated. Estimates of relative locations of sources and sensors can be refined through analysis of the attenuation of sound in the intervening water in combination with water-temperature and salinity data acquired by instrumentation systems other than PAMS.

A PAMS Is Hoisted into the water, then guided into position by a diver.

A PAMS is packaged as a battery-powered unit, mated with external sensors, that can operate in the ocean at any depth from 2 m to 1 km. A PAMS includes a pressure housing, a deep-sea battery, a hydrophone (which is one of the mating external sensors), and an external monitor and keyboard box. In addition to acoustic transducers, external sensors can include temperature probes and, potentially, underwater cameras. The pressure housing contains a computer that includes a hard drive, DC-to-DC power converters, a postamplifier board, a sound card, and a universal serial bus (USB) 4-port hub.

Typically, a PAMS is lowered into the water by use of a deck hoist, then guided to its assigned position by divers for a shallow deployment (see figure) or by crew members in miniature submarines for deployment at a greater depth. Alternatively, if great precision is not required, a PAMS can be simply allowed to sink to the selected location. The PAMS is then left in place to record data for a predetermined time or until it exhausts battery energy or data-storage capacity. Currently, the PAMS can be deployed at sea for four days, but the deployment time and sampling are battery and hard-drive dependent. For example, at a sample rate of 22,050 Hz, the data acquired was 3.5GB per 24 hours using a 40GB hard drive. However, at 44,100 Hz sample rate, the data acquired was 7.1GB per 24 hours.

The PAMS is subsequently retrieved and its recorded data are downloaded to a computer, which can be used to either process the data or record the data on a disk for transfer to another computer for processing. The PAMS can then be prepared for another deployment.

This work was done by Michael Lane and Steven Van Meter of Kennedy Space Center; Richard Grant Gilmore, Jr., of Estuarine, Coastal and Ocean Science, Inc.; and Keith Sommer of the United States Air Force. For further information, contact the Kennedy Innovative Partnerships Office at (321) 861-7158. KSC-12634.


NASA Tech Briefs Magazine

This article first appeared in the March, 2007 issue of NASA Tech Briefs Magazine.

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