Deep-water ocean acoustic propagation: Observations

There is a rich history of long-range, low-frequency, deep-water ocean acoustic propagation measurements extending back to the discovery of the deep sound channel by Ewing and Worzel in 1944. Experiments up to the 1970s focused on measuring parameters in the sonar equation, including transmission loss and ambient noise, motivated in part by the development of the U.S. Navy Sound Surveillance System (SOSUS). These measurements generally used wideband explosive sources or narrowband transducers. Beginning in the 1970s, low-frequency, broadband transducers and vertical receiving arrays that could operate autonomously and be moored for extended periods were developed in connection with the new field of ocean acoustic tomography. These developments allowed individual multipaths and modes to be resolved and long time series collected, so that the fluctuations in the received signals could be quantified. At about the same time, advances in characterizing ocean internal wave and mesoscale variability provided information on the causes of the acoustic fluctuations that could be used in theoretical calculations. The study of deep-water propagation and the development of ocean acoustic tomography have been intertwined ever since. The applications of deep-water propagation now extend beyond military uses to ocean acoustic remote sensing (active and passive), communication, and navigation.