Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Approved for public release; distribution is unlimited.
PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
SPONSOR / MONITOR'S ACRONYM(S) 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES)
SPONSOR / MONITOR'S REPORT NUMBER(S)Multistatic active system performance can be driven by reverberation from the ocean boundaries and biologics. Providing accurate sonar performance predictions of reverberation, in turn, relies on providing accurate estimates of bistatic scattering strengths. This report presents new three-dimensional models that provide physics-based estimates of the dependence of scattering strength on the incident and scattered grazing angles, the bistatic angle, the acoustic frequency (10 to 10000 Hz), and physical descriptors of the environment (such as bottom properties for the bottom model, wind speed for the surface model, and fish properties for the volume model). The bottom model describes scattering from rough, elastic interfaces, while the surface model describes scattering from both the rough air-sea interface and subsurface bubbles. The volume models describe scattering from dispersed bladdered fish, including boundary-interference effects. For all, parameter studies along with data-model comparisons demonstrate the importance of using physics-based scattering models to describe the complex acoustic interaction processes at the ocean boundaries. These broadband models can enhance sonar performance prediction capabilities through their inclusion as submodels in both active performance/reverberation models
Direct-path measurements of low-frequency (200–1000 Hz) and low-grazing-angle (≤10 deg) acoustic surface scattering were made in the Gulf of Alaska in April of 1990 and in March of 1992. Short-duration (0.6–2.4 s) cw and PRN waveforms were used to quantify the spectral character of surface/near-surface reverberation as a function of frequency and environmental conditions. Measures include spreading and peak-Doppler-shift statistics versus azimuth. Results over the range of wind speeds (10–35 kns) have revealed a dominant zero-Doppler component and a weaker-than-expected dependence of spread on both sea conditions (wind speed and relative direction of the seas) and frequency. The results are consistent with sub-surface bubbles as the primary mechanism for surface reverberation when white caps are present, and give insight as to the nature of the bubble clouds that could give rise to the observed acoustic scattering.
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