Spread Scattering and Propagation

Abstract

The sonar environment rarely produces the ideal conditions of point scattering in white (uncorrelated) Gaussian interference because the ocean is not a homogeneous medium. The salinity and temperature vary as a function of depth and location due to weather changes, solar heating, and fresh water influx from rivers and estuaries. These induce density variations that change the refractive index of the water and hence the propagation speed causing sound wave refraction. This is commonly called ray bending for sonars operating at medium to high frequency (1–100kHz) [ 1–3] and can cause a transmitted pulse and the resulting echo to propagate over several paths with different delays. Multiple boundary reflections from the surface and bottom are possible and the combined phenomenon is called multipath propagation. Figure 6.1 is an example of a summertime near surface downward refracting sound velocity profile (SVP) and raypath plot. The sun warms the surface waters generating a gradient of decreasing temperature with depth to about 100 ft depth. Thereafter temperature slowly increases with depth. Sound speed increases with water temperature causing the acoustic wavefront to refract downward near the surface but upward below creating a “duct” of converging raypaths at about 200 ft depth. Multiple reflections are also occurring at the surface and bottom.