[FULL] Technologies for Ocean Acoustic Sound Monitoring - 'Undersea Surveillance'

• Report rights infringement
• published: 14 Jun 2014
• views: 1380

Chris Fox, Director Acoustic Monitoring Project NOAA Pacific Marine Environmental Laboratory Just as microphones collect sound in the air, underwater hydrophones detect acoustic signals in the ocean. Most hydrophones are based on a special property (piezoelecticity) of certain ceramics that produce a small electrical current when subjected to pressure changes. When submerged in water, a ceramic hydrophone produces small-voltage signals over a wide range of frequencies as it is exposed to underwater sounds propagating from any direction. By amplifying and recording the electrical signals produced by a hydrophone, sound in the sea can be measured with great precision. Although a single hydrophone records sound arriving from any direction, several hydrophones can be simultaneously deployed in an array, and the resulting signals can then be manipulated to "listen" in any direction with even greater sensitivity than a single hydrophone element. Whether within an array or as a single element, the hydrophone is the basic sensor of underwater acoustics. Currently, several technologies are available for acoustic exploration of the ocean. For decades, the U.S. Navy has used a device called a sonobuoy to record the sound of enemy submarines. This simple device can be deployed either from an aircraft or a surface ship. The sonobuoy includes a single underwater hydrophone and a radio transmitter to send the recorded signals back to the aircraft or ship. By deploying a pattern of sonobuoys, the location of the "target" can be determined. Sonobuoys have been used in ocean exploration as well, e.g., to record marine mammal calls and listen for earthquake activity, but the short life span of the device (a few hrs) prohibits long-term monitoring. A much more expensive, but permanent, technology for acoustic exploration is the installation of a hydrophone array connected to an underwater communications cable. Since the 1960s, the U.S. Navy has operated such a SOund SUrveillance System (SOSUS) for military applications in many areas of the world ocean. With the fall of the Berlin Wall in 1989 and the end of the Cold War, the U.S. Navy offered the civilian scientific community "dual use" of SOSUS to evaluate its value in ocean environmental monitoring. Since 1991, NOAA has successfully used these arrays to detect submarine volcanic eruptions in the northeast Pacific and blue whale movements in the same area. The range of the system is such that volcanic tremors from south of Japan have been successfully detected and located using SOSUS arrays deployed off the coasts of Oregon and Washington. Access to SOSUS is restricted, both in the sense that the data are classified and can only be used in a secure facility, and also by the fact that the arrays are deployed only in areas of military need. The cabled nature of SOSUS allows real-time acquisition of the acoustic data, but at a high cost; the total investment in SOSUS is estimated at more than $16 billion.