Scattering from an object in a stratified medium. I. Frequency dispersion and active localization

Abstract

A signal scattered from an extended object is often very different from a point source signal emitted from the object. Hence active localization based on matched-field/mode processing may not be feasible, depending on how the signal is modified by scattering. In a stratified medium in which propagation of a signal can be expressed in terms of normal modes, the effect of scattering is to induce mode coupling—an incident normal mode is converted to a series of outgoing normal modes by the scatterer. This mode coupling is examined by ensonifying the object with a broadband pulse at some distance from the object. The time-frequency dispersion of a scattered signal is compared with the dispersion of a passive signal traveling the same path (with the scatterer absent). One finds that the coupled normal modes (created by scattering) arrive at a different time than the uncoupled (original) normal modes and can be identified using time-frequency analysis. For active source localization broadband matched-mode processing is applied to the scattered signal. It should be noted that in many problems, the often unknown scattering function (of an unidentified object) prevents one from calculating the replica field for a scattered signal that is required for matched-field processing. For some problems, the relative phase distortion between the modes due to scattering is minimal even though mode coupling is significant. For these cases, matched-mode processing successfully localizes the target; matched-mode processing can tolerate the lack of specific knowledge about the scattering function. Both incoherent and coherent broadband active processing are investigated. In general, one can expect two peaks in the active range and depth ambiguity surface corresponding to the locations of the target and the active source.