Ocean acoustic tomography: Fortieth anniversary, 1976–2016

Abstract

The year 1976 saw the publication of “Monitoring the ocean acoustically” by Munk and Worcester [Science, Technology, and The Modern Navy, Thirtieth Anniversary 1946-1976, 497-508] and arguably the first ocean acoustic tomography experiment. The field got off to a stormy start when a reviewer of an early proposal wrote that “travel times along ray paths are meaningless in a saturated environment,” but a 1978 experiment showed that ray arrivals could be “resolved, identified, and tracked” at ~900 km range, just in time to save the field from an early demise. The need to measure and predict travel times with millisecond accuracy pushed the field of underwater acoustics to develop the requisite instrumentation and mathematical tools to accurately characterize the impulse response of the ocean. Important contributions to our understanding of the oceans include (1) the demonstration that internal tides are coherently generated by surface tides over the Hawaiian submarine ridge (and other similar benthic features) and can be traced for thousands of km to distant regions of dissipation, (2) measurements of oceanic deep convection, and (3) the discovery using trans-Arctic transmissions that the heat content in the Arctic was increasing. In the future, ocean acoustic tomography will likely be one component of multipurpose acoustic systems for acoustic remote sensing, navigation, communications, and passive listening, with special applicability in ice-covered regions.