ASA 124th Meeting New Orleans 1992 October

5pUW3. Low-frequency direct path surface/near-surface scattering measured using intermediate-length narrow-band and broadband waveforms.

Roger C. Gauss

Raymond J. Soukup

Naval Res. Lab., Washington, DC 20375

Joseph M. Fialkowski

Planning Systems, Inc.

Direct-path measurement in the surface/near-surface and air/sea interaction experiment in the Gulf of Alaska (February/March 1992) were designed to assess the Doppler, spatial, and temporal characters of very low grazing-angle backscatter as functions of frequency and environmental conditions. Fifty-four 30-min modules of data using cw, LFM, and PRN waveforms of intermediate duration (0.6 to 2.4 s) were collected during a variety of sea conditions, at wind speeds ranging from 4 to 19 m/s. Moreover, these data were collected simultaneously at two frequencies (usually 250 and 1000 Hz). The results show the contrast in spectral character, scattering strength, and ping-to-pong consistency between lower sea states where reverberation from fish is significant and higher sea states where bubble phenomena (clouds/plumes) are dominant. The data also illustrate the effect of subsurface bubbles on the elevation of observed surface strengths over predictions based on air--sea interface scattering theory.