The reverberation for a broadband synthetic aperture sonar

By use of a broadband synthetic aperture sonar it has been possible to measure the reverberation with a resolution in the order of 1 cm along the sonar axis and 4 cm in the transverse direction. With such high resolution measurements it is possible to measure the acoustic response of low wavenumber structural features of the sediment which contribute to the scattering. While the wavenumbers of these features lie outside the band of the transmitted signal they can be observed via their modulation of the high wavenumber components in the backscattered signal. Using models developed to incorporate the modulation terms, together with the deterministic measurements of the bottom structure, it was shown that at the site where the measurements were carried out these terms did not significantly contribute to the variance of the backscattered signal for a sonar using a real array. For a broadband synthetic aperture sonar however these modulation components dominated the variance of the backscattered signal. This resulted in strong structure being present in the reverberation against which additional processing gain was realised by adaptively matching the filtering applied to the signal and the reverberation. This gain is additional to that predicted by the classical theory, which arises from the reduction in the insonified area of the bottom realised with a high resolution sonar.