Spatial Coherence of Sonar Reverberation

Abstract

This paper describes experiments performed to determine some of the spatial characteristics of reverberation signals as encountered in pulsed echo-ranging sonar systems. A sonar system operating at 112 kHz was set up in a tank and used to obtain reverberation signals from smooth sand surfaces (particle sizes≪λ). Backscattered signals for small grazing angles (4°–12°) and narrow beamwidths (8°, 15°, and 22°) were recorded. Spatial correlations of the reverberation fluctuations were plotted for various spatial translations of the transducers relative to the scattering surface. The correlations of the fluctuations became small for lateral displacements (displacements normal to transducer axis) on the order of a wavelength and longitudinal displacements (displacements parallel to transducer axis) on the order of a pulselength. Further analyses of the recorded signals indicated that the average reverberation pressure diminished as a function of range as r−2.7 for one grade of sand and as r−21 for another grade of sand. Amplitude analyses of the recorded signals indicated that with 95% confidence the amplitudes were Rayleigh-distributed. [Research was conducted at Defense Research Laboratory, The University of Texas.]