Physics of 3-D scattering from rippled seabeds and buried targets in shallow water

Abstract

A new wave theory model (OASES-3D) providing consistent modeling of seabed insonification, three-dimensional target scattering, and rough seabed reverberation has been used to investigate the spatial and temporal characteristics of the multistatic scattering and reverberation from rippled, shallow-water seabeds with buried targets. It is shown that the highly polarized spectral characteristics of ripple fields is associated with a reverberation environment which is highly sensitive to both the frequency and insonification aspect relative to the ripples. The theoretical study has suggested that significant gains in subcritical detection performance for buried objects can be achieved by band-limiting the processing to frequencies below typically 3–5 kHz. The results of 3-D target scattering and reverberation measurements performed by SACLANTCEN in shallow-water environments with rippled, sandy bottoms have confirmed this hypothesis. However, the experiments have consistently shown higher low-frequency reverberation than predicted by the model. Here, OASES-3D has been used to demonstrate that a very small amount of low-wave-number roughness components superimposed on the sand ripples can account for the observed reverberation. This in turn suggests that the use of new low-frequency sonar concepts for buried target detection and classification must be accompanied by adequate environmental assessment incorporating the long-wavelength (λ>25 cm) seabed roughness components. [Work supported in part by SACLANTCEN and ONR, Ocean Acoustics.]