Surface and volume scattering measurements using broadband explosive charges in the Critical Sea Test 7 experiment

Abstract

Extensive measurements of low-frequency (70–1000 Hz) sea surface backscattering strengths have been made as part of the Critical Sea Test (CST) 7 experiment. These measurements were made for a variety of wind speeds from 3.0 to 17.5 m/s and for mean grazing angles from about 5 to 30 deg. The results from the CST-7 tests support the conclusions based on surface scattering strengths measured in previous CST SUS tests regarding the mechanisms controlling surface scattering and the frequencies and environmental conditions at which the mechanisms are important. As in previous tests, three scattering regimes were noted: a regime where results are generally consistent with scattering from a rough surface, a second regime where the frequency and grazing-angle dependences are much greater than expected from air–sea interface scatter but are consistent with scattering from subsurface bubble clouds, and a third regime that forms a transition zone between the other two regimes. The new measurements extend the range of environmental conditions over which these regimes are seen to substantially higher wind speeds than were obtained in the CST tests prior to CST-7. Volume scattering was observed to play a significant role in the measured results, with scattering from salmon the principal source of volume reverberation. Correlations of the measured scattering strengths were calculated for various environmental parameters and showed that the predictive ability of several wind speed-related parameters was improved by using a 2- to 4-h backaverage rather than the values taken during the runs.