Spatial and Temporal Variability in Bottom Roughness: Implications to High Frequency Subcritical Penetration and Backscatter

Abstract

Quantitative prediction of high frequency, low grazing angle penetration into, and scattering from, sand sediments requires knowledge of the roughness of the water/sand interface. Since the sediment roughness evolves due to hydrodynamic and biological processes, concurrent, co-located measurement of roughness and acoustic penetration/backscattering is essential for testing acoustic models or using such models to determine the likelihood of buried target detection. Here, we examine both roughness and acoustic measurements carried out during a six week sediment acoustics experiment in 1999 (SAX99). A ripple field was present throughout the experimental period but changed wavelength and orientation as a result of a storm event (i.e., the ripple field was temporally non-stationary). The predicted impact of this change in the ripple field on acoustic penetration at shallow grazing angles is presented. The small-scale roughness important for backscattering was measured at several locations near to, but not co-located with, acoustic backscattering measurements. These measurements indicate roughness changes with location. The effect of this spatial non-stationarity on tests of alternative backscattering models is discussed. Finally, simple sonar equation predictions of high frequency, low grazing angle, buried mine detection are made using various combinations of interface roughness conditions.