William M. Carey
DARPA, 3701 North Faifax Dr., Arlington, VA 22203
Peter G. Cable
BBN Systems Technology, New London, CT 06320
Henry Cox
Orincon, Arlington, VA 22202
The propagation and reverberation of sound in shallow water under downward refracting conditions is governed by the geoacoustic and scattering properties of the bottom. We develop the active bistatic sonar equation for this case and show that, when the transmission to the target and the bottom are approximately the same for reverberation-limited conditions, a simpler parametric form of this equation results. That is, the signal excess depends on the effective bottom backscattering strength, the coherent signal gain, and a reverberation area for a given target strength and threshold. A parametric analysis is presented that focuses on the frequency-dependent characteristics of the bottom backscattering strength and the effect of coherence on receiving array aperture. Recent reverberation and signal coherence measurements are reviewed and the needs for additional studies delineated. The coherence issue for the waveguide is stressed and the measurement problem of coherence is discussed and compared to the measurement of signal versus the receiver length in wavelengths.