SCATTERING FROM AN OBJECT IN A STRATIFIED MEDIUM .1. FREQUENCY DISPERSION AND ACTIVE LOCALIZATION

A signal scattered from an extended object is often very different fro m a point source signal emitted from the object. Hence active localiza tion based on matched-field/mode processing may not be feasible, depen ding on how the signal is modified by scattering. In a stratified medi um in which propagation of a signal can be expressed in terms of norma l modes, the effect of scattering is to induce mode coupling-an incide nt normal mode is converted to a series of outgoing normal modes by th e scatterer. This mode coupling is examined by ensonifying the object with a broadband pulse at some distance from the object. The time-freq uency dispersion of a scattered signal is compared with the dispersion of a passive signal traveling the same path (with the scatterer absen t). One finds that the coupled normal modes (created by scattering) ar rive at a different time than the uncoupled (original) normal modes an d can be identified using time-frequency analysis. For active source l ocalization broadband matched-mode processing is applied to the scatte red signal. It should be noted that in many problems, the often unknow n scattering function (of an unidentified object) prevents one from ca lculating the replica field for a scattered signal that is required fo r matched-field processing. For some problems, the relative phase dist ortion between the modes due to scattering is minimal even though made coupling is significant. For these cases, matched-mode processing suc cessfully localizes the target; matched-mode processing can tolerate t he lack of specific knowledge about the scattering function. Both inco herent and coherent broadband active processing are investigated. In g eneral, one can expect two peaks in the active range and depth ambigui ty surface corresponding to the locations of the target and the active source.