ASA 124th Meeting New Orleans 1992 October

3aAO11. Possible uses of submarine canyons in acoustic thermometry.

Warren W. Denner

EOS Research Associates, P.O. Box 1378, Carmel Valley, CA 93924

Khosrow Lashkari

Monterey Bay Aquarium Res. Inst.

James H. Miller

Ching-Sang Chiu

Naval Postgraduate School

High-power sources and high-gain receivers are used in long-range ocean acoustic thermometry. Because of power supply and data telemetry the sources and receivers are often placed near coastal facilities. There may be some significant benefits associated with taking advantage of natural terrain features to protect the sources and receivers from fisheries, and to optimize certain aspects of their performance. Sources and receivers are generally moored in deep water, in the axis of the sound channel, on the order of 100 to 1000 m. The shortest distance from shore to deep water is frequently associated with submarine canyons. For example it is less than 15 km from Point Pinos to water depths in excess of 2000 m in Monterey Submarine Canyon, less than half the distance along continental borders without submarine canyons. Fishermen do not fish inside the canyon walls for fear of rapid depth changes. Therefore, the canyons provide a natural barrier to the threat of fishing activity to moored sources and receivers. It is obvious that the canyon restricts the ray paths between a source or receiver in the canyon. However, if the source and receiver geometry is carefully selected this may not be a significant limitation. Oceanic noise arrivals in a submarine canyon can be restricted to those in the direction of deep water. If for example the canyon opens at an angle of 30 deg from the receiver and if the noise is omnidirectional at the axis of the array, the terrain barrier restricts noise to that 30-deg window. This would provide an optimum noise reduction of 7.8 dB. On the source side of the problem, placing the source in the canyon will limit the high-intensity blast to the energy going out the canyon mouth into deep water. In this way, marine mammals in the area of the source would be shielded from the direct blast unless they happen to be in a restricted direction from the source, or very near the source. If the source could be made so that it was directional, the volume of the direct blast would be even more restricted. Certainly, there are some possible downsides to placing sources and receivers in submarine canyons. These will be explored in the presentation.