Keynote address
Early Development of Acoustic Tomography and Future Prognosis
Robert C. Spindel
Applied Physics Laboratory
Universlty of Washington
Seattle, WA 98105
spindel@apl.washington.edu
Acoustic tomography developed largely as a result of underwater acoustic research sponsored by the US Navy during the 1960's and early 1970's. The Navy problem was the performance of ship and torpedo sonars at short ranges, and the passive detection and localization of submarines at long ranges. Numerous experiments revealed important unexplained signal amplitude and phase variations and sound speed variations. The problem was addressed with little real success by underwater acousticians who sought to discover relationships between ocean processes and observed acoustic variability. The term acoustic oceanography was coined during this era, and the field of ocean acoustics, rather than underwater sound, began to emerge. However, it was mainly a small group of physicists and oceanographers, many of whom were gathered together by the US defense department, that significantly advanced the field. They developed theories to explain observed acoustic effects in terms of known ocean processes. This, coupled with both an interest at that time in inverse theory and its application to oceanography, and a growing realization among oceanographers that the controlling scales of oceanic variability were impossible to observe with conventional ships and moorings, led directly to the development of acoustic tomography, and eventually to acoustic thermometry. Today, a time in which global oceanic observation is the goal of a plethora of national and international programs, it would seem that acoustic methods, with their inherent ability to provide long-range, basin-scale, in situ integral (path averaged) measurements--ideal for the task--would be embraced. There are positive signs that they will be.
The Beginnings-Acoustics
Fluctuations in acoustic signals, which are observed in essentially all ocean transmissions, were first recognized as important features during sonar development during WWII (1). They began to receive considerable attention following the war, and through the early 1960's, in an effort to improve the performance of two military systems, ship and torpedo sonars operating at short ranges and relatively high frequencies, and passive submarine surveillance systems operating at long ranges and much lower frequencies. Most of this early work focused on amplitude fluctuatlons because of the obvious connection to sonar performance. Phase fluctuations were considered less important, and research on them did not begin until much later. High frequencies were the initial subject of interest, and early research resulted in classic references by both US and Soviet authors such as Liebermann (2), Chernov (3), Tatarski (4), Potter and Murphy (5), Mintzer (6) and others. Research on fluctuations at lower frequencies started about ten years later, when long range ASW surveillance systems were first used. The earliest reported studies were experiments at 107 Hz by Clay, et.al.,over a very short 2 km path (7), and experiments at 270 Hz by Nichols and Young (8).
