ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

1aAO6. The internal waves effect on the vertical noise directionality in shallow water.

T. C. Yang

Dirk Tielbuerger

Naval Res. Lab., Washington, DC 20375

Acoustic environment is known to influence the vertical directionality of the ambient noise. For example, in shallow water with a downward refractive (summer) sound-speed profile, the noise vertical directionality is expected to show a deep (>10 dB) null in the horizontal direction for wind-generated noise below 1 kHz. This ``horizontal null'' is a consequence of the fact that low-order modes are weakly excited by the noise sources which are located near the ocean surface. This phenomenon has been experimentally observed and is not expected to occur in an upward refractive (winter) sound-speed profile environment. It is noted, however, that when internal waves are present, the deep null may disappear or sufficiently weaken due to the ``strong'' mode coupling effect caused by the scattering of the noise field from the internal waves. In the internal wave fields, the conversion of higher-order modes (generated by the surface noise sources) to low-order modes by the internal waves can fill in the deep (>10 dB) null which would otherwise be expected (theoretically). Numerical calculations of the vertical noise directionality at <500 Hz for typical shallow water environments with rock (low loss) and sediment (lossive) bottoms are presented and the potential effects of internal waves on the ambient noise directionality are investigated. Practical situations are discussed for which the smearing of the ``horizontal null'' in the noise vertical directionality can be experimentally verified. [sup a)]On leave from Forschungsanstalt der Bundeswehr fur Wasserschall-und Geophysik, Klausdorfer Wef 2-3=24, 24148 Kiel, Germany.