Environmental factors governing shallow-water active sonar

Abstract

Recent experiments have measured the broadband sound transmission, coherency, reverberation and bottom backscattering strength in a variety of shallow-water environments with sand-silt bottoms. Ancillary range and time-dependent conductivity and temperature versus depth along with bathymetry were also obtained. The measurements were performed with especially designed impulsive sources, and vertical and horizontal receiving arrays. This paper presents key results on the spatial coherency of the received signals, the frequency dependence of the effective attenuation factors and the frequency dependent backscatter strength coefficient. The broadband coherency measurements, although variable from site to site, were found to be consistent with lengths of 30–35 wavelengths at 400 Hz at 40-km range. The frequency-dependent effective attenuation factor varied as frequency to a fractional power ranging between 0.15–0.25 dB/km at 100 Hz and 0.4–0.55 dB/km at 1 kHz. Bottom backscattering strengths at grazing angles on the order of 10 deg were found 10–15 dB lower at 200 Hz as compared to 1 kHz. The results are shown to be consistent with those of other investigators and are shown to place realistic limits on shallow-water sonar parameters. [Work supported by DARPA.]