Subcritical detection of targets buried under a rippled interface: calibrated levels and effects of large roughness

This paper describes recent results from an ongoing modeling and measurement effort investigating shallow grazing angle acoustic detection of targets buried in sand. The measurements were performed in a 13.7-m deep, 110-m long, 80-m wide test-pool with a 1.5-m layer of sand on the bottom. A silicone-oil-filled target sphere was buried under a rippled surface with contours formed by scraping the sand with a machined rake. Broad band (10 to 50 kHz) transducers were placed onto the shaft of a tilting motor, which in turn was attached to an elevated rail that enabled this assembly to be translated horizontally, permitting acquired data to be processed using synthetic aperture sonar (SAS) techniques. Acoustic backscatter data were acquired at subcritical grazing angles for various ripple wavelengths and heights. In addition, the backscattered signals from a calibrated free-field sphere and the transmitted signals received with a free-field hydrophone were recorded. For each bottom configuration, the seabed roughness over the buried target was measured to determine the ripple parameters and to estimate the small-scale roughness spectrum. This roughness information is used in scattering models to calculate the backscattered signal levels from the target and bottom. In previous work, measured signal-to-reverberation ratios were found to compare well with model predictions, demonstrating the accuracy of first-order perturbation theory (for the ripple heights used in those experiments) for frequencies up to 30 kHz. By taking advantage of the backscattered data collected using the free-field sphere and of the acquired transmitted data, more stringent comparisons of predicted buried target backscatter levels to measured levels are made here. Results of a second series of measurements using larger ripple heights to investigate the impact of higher-order scattering effects on buried target detection are presented.