Signal processing of ideal echoes resonantly scattered by underwater structures

Abstract

A study is presented of the scattering of sound waves that results when penetrable underwater obstacles of various types are insonified by various incident transient signals. The targets are elastic shells, although the case of a rigid sphere was also studied for comparison. The deformation of the shell is described both exactly, by means of the three-dimensional equations of elasticity, and approximately, by means of Donnell’s shell theory. The goal is to identify significant signal processing parameters that are useful at the source, the target, and the receiver, to properly and quantitatively estimate the nature of the backscattered echoes from submerged structures. These parameters play an important role in the ultimate goal of active target identification. The methodology presented has similarities to that most commonly used for impenetrable targets (i.e., perfect conductors)—in the radar literature. The analysis is first introduced here for targets away from boundaries, and in noiseless media. It is believed that the type of echo received from a sonar target should be known, before such return is distorted and corrupted by the environment. As presented here, this idealized situation is well suited to later account for the presence of noise in the medium, as well as for the proximity to environmental boundaries. The methodology already takes advantage of the machinery of statistical communication theory (viz., matched filters, ambiguity and correlation functions, etc.) to extract desired signals from signals mixed with undesirable noises. The work is naturally divided into descriptions and characteristics of: (a) the incident pulses, (b) the elastic targets themselves, and (c) the backscattered returns. The various quantities are illustrated at all stages with many computed displays.