ASA 124th Meeting New Orleans 1992 October

5aUW2. Measurements of low grazing angle bottom backscatter at frequencies from 220 to 1000 Hz in the central Mediterranean Sea.

Michael T. Sundvik

Naval Undersea Warfare Ctr., Code 3112, New London, CT 06320

Sean M. Reilly

Tracor Analysis and Simulation Dept., New London, CT 06320

In June 1991 bottom scattering strength measurements were obtained with directional sources and receivers using short CW pulses at frequencies between 220 and 1000 Hz from three locations in the central Mediterranean. Two sites were at the southern edge of the Messina Cone in the Ionian basin (3400- to 3800-m water depth), and one site in shallow water (500-m water depth) at the eastern edge of the Medina bank. Reverberation data were processed using a sonar equation approach (using a flat bottom approximation) to obtain scattering strengths versus grazing angle. Average scattering strengths in the basin calculated from 1-s CW pulses and equivalent beam geometries vary 6 dB between 225 and 880 Hz, and show little dependence on grazing angle from 8 to 25 deg. Scattering strengths show a dependence on pulse length at all frequencies sampled, which can be interpreted as due to the dominance of sub-bottom effects. Scattering strengths from the shallow site show a similar dependence on pulse length, but a relationship to grazing angle similar to Lambert's rule. Comparisons of the results to both the empirical Lambert's rule and a high-frequency acoustic composite roughness and volume scattering model [Jackson et al., J. Acoust. Soc. Am. 79, 1410--1422 (1986)] met with limited success, implying a need for a more complete description of bottom and sub-bottom properties in treatment of bottom scattering at these frequencies. [Work supported by SPAWAR.]