Subcritical Detection of Targets Buried Under a Rippled Interface: Calibrated Levels and Effects of Large Roughness

摘要

The paper describes recent results from an ongoing modeling and measurement investigation of shallow grazing angle acoustic detection of targets buried in sand. Measurements were performed in a 13.7-m deep 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 sand with a machined rake. Broadband (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 techniques. Acoustic backscatter data were acquired at subcritical grazing angles for various ripple wavelengths and heights. 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. The acquired transmitted data and the backscattered data collected using the free- field sphere allowed more stringent comparisons of predicted to measured target backscatter levels.