Focal regions in spectrograms of long range propagation

Abstract

A source near the deep sound channel axis excites mode groups (or paths) that involve both deep sound channel and boundary interacting propagation. Modal group speeds have a functional transition when passing through purely refractive to boundary reflecting phase speed regions. The result is that arrivals in this transition region line up in time across frequency. The combination of this alignment together with a similar, though broader coincidence of the last deep sound channel arrivals provide two time markers on a single phone spectrogram (intensity as a function of arrival time and frequency). Indeed, data from the Acoustic Thermometry of the Ocean (ATOC) program [ATOC Consortium, Science 281, 1327 (1998)] show this effect. These time markers, depending on auxiliary information provide source localization or inversion capability. [Work supported by ONR and DTRA.]