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 (read Ocean Acoustics for a discussion of how sound is produced and transmitted through the ocean). 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 multiple sonobuoys in a pattern, the location of the “target” can be determined. Sonobuoys have been used in ocean exploration as well to record marine mammal calls and listen for earthquake activity. The short life span of the device (a few hours) prohibits long-term monitoring of ocean sounds.
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. The arrays also do not cover the entire world's oceans as they 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.
The Sound in the Sea 2001 Expedition to Pioneer Seamount offshore of California installed the first long-term cabled acoustic observatory in the deep ocean. During the 1990s, an expensive underwater cable was laid to Pioneer Seamount to conduct an oceanographic experiment that has since concluded. In support of ocean exploration, NOAA has assumed responsibility for the cable. The Pioneer Seamount Acoustic Observatory is the first deep-water civilian (non-classified) hydrophone array for long-term monitoring of ambient ocean noises and their effects on the marine environment. The array consists of four hydrophone elements suspended vertically in the water above the sea floor. The acoustic data are collected at a small Air Force base in coastal California, and are immediately accessible to both scientists and the public via the World Wide Web.
In the mid-1990s, based on the success of earlier work with SOSUS, NOAA developed portable hydrophones that can be deployed anywhere in the world ocean. These devices consist of a single ceramic hydrophone attached to a water-proof pressure case that contains all of the batteries, computers, clocks, and other electronics required to maintain the hydrophone for several years. They have been used successfully in marine mammal studies and seismic studies, and have even been used to detect landslides on the south shore of Hawaii from a range of more than 5,000 km. These instruments have the advantage of portability; that is, they can be deployed anywhere in the world ocean. Another advantage is that these instruments are relatively inexpensive compared to a cabled system such as SOSUS. The major disadvantage is that data cannot be currently provided in real time; one must wait until a ship revisits the deployment site and recovers the instrument. It is anticipated that improvements in global cellular telephones in the coming years will make real-time transmission from inexpensive hydrophone moorings a reality. Portable hydrophone arrays are currently deployed in the equatorial Pacific, the Gulf of Alaska, and the North Atlantic south of the Azores (in partnership with NSF and Oregon State University). In 2002, Sound in the Sea in partnership with scientists in France, deployed a new hydrophone array in the Atlantic north of the Azores as part of the SIRENA project.
SOSUS provides excellent real-time acoustic coverage in areas of military interest. Opportunities exist for additional cabled acoustic observatories at several sites around the global ocean, from abandoned military arrays as well as commercial telecommunications cables. Over the next decade, NOAA and its partners plan to install numerous hydrophone arrays around the global ocean as part of the NOAA Ocean Exploration Program. These arrays will include additional cabled sites, similar to Pioneer Seamount, as well as autonomous hydrophone instruments and other new technologies for deployment in ocean regions inaccessible from cabled arrays. The scope of this effort will require additional partnerships, both with other U.S. agencies and universities, as well as international participation. The final result will be an observational system able to identify oceanic phenomena at a global scale.