Focal Regions in Spectrograms of Long Range Propagation
Bill Kuperman
Director of Marine Physical Laboratory, Scripps Institution of Oceanography
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 crossing through purely refractive to boundary reflecting phase speed regions. The result is that arrivals in this transition region line up in time across frequency; this combined 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, the ATOC data show this effect. These time markers, depending on auxiliary information, provide inversion capability.