Page 14 - Fall 2005

well suited for measuring barotropic (depth-independent) currents, such as those associated with the ocean tides, because the steep acoustic ray paths naturally provide verti- cal averages.
Acoustic tomography was originally envisaged as a tool to map ocean mesoscale variability (the oceanic equivalent of atmospheric weather, with spatial scales of order 100 km and time scales of order 100 days) using many crossing acoustic paths. The analogy is to CAT scans. Acoustic thermometry can be considered to be a subset of acoustic tomography, in which a much sparser set of acoustic paths is used to obtain the average temperature of the intervening ocean.
Energetic ocean mesoscale eddies dominate measure- ments made at a point, making it difficult to measure vari- ability on the scale of the great wind-driven ocean gyres, which have spatial scales of thousands of kilometers. The advantage to using acoustic methods to measure gyre-scale variability is the inherent integrating nature of the acoustic transmissions. The acoustic travel time along a path from a transmitter to a receiver is determined by the average sound speed along the path. (Conversely, acoustic methods are not well suited to obtaining precise local information.) Ocean basin-scale transmissions provide horizontal and vertical averages that reduce noise due to ocean mesoscale variabili- ty, providing precise measurements of gyre-scale heat con- tent. Further, the heat content in a vertical section across an ocean basin can be rapidly and repeatedly measured at low cost using acoustic methods, once an acoustic measurement system has been installed.
A brief history
It all started with a discussion titled “Monitoring the Ocean Acoustically” by Munk and Worcester at a celebration of the thirtieth anniversary (1946–1976) of the Office of Naval Research3:
“The classical Physical Oceanographers cast their Nansen bottles and contoured dynam- ic heights, so that these would be available for computing geostrophic currents which are then published on permanent charts. The acoustician found it difficult to relate this delightfully simple view of a steady ocean interior to the complex and time- variable transmission of acoustic signals...”
At that meeting Munk described the results of reciprocal transmissions between the R/V Alexander Agassiz and the R/V Ellen B. Scripps at 25-km range. There was a clear distinction between the simplicity of the early deep arrivals and the sub- sequent complex multi-paths through the ocean’s upper layers.
Starting with the 25-km reciprocal transmission, the experiments moved in the direction of longer transmission ranges. For some years the emphasis was on mesoscale processes. The discovery in the 1960’s of a very active mesoscale was responsible in the first place for proposing Ocean Acoustic Tomography to meet some of the shortcom- ings of the traditional Expedition Mode. Much of the earliest
work took place in the northwest Atlantic with battery-pow- ered sources on autonomous moorings. Ranges were a few hundred kilometers, although even then receptions were recorded at longer ranges using bottom-mounted receivers that were part of the SOund SUrveillance System (SOSUS) belonging to the U.S. Navy. Between 1983 and 1989 Spiesberger and Metzger made intermittent long-range transmissions from a shallow source off Kaneohe, Oahu, to SOSUS receivers in the North Pacific, in the first systematic effort to acoustically monitor an ocean basin. In 1987 new HLF-5 hydroacoustic sources transmitting broadband signals at 250 Hz allowed the first megameter transmissions between autonomous moored instruments. In 1991 the global Heard Island Feasibility Test (HIFT) demonstrated that acoustic sources can be detected at antipodal ranges (20,000 km), but the arrival pattern is too garbled to permit precise determi- nation of travel time.4 From a climate point of view the very large ranges in HIFT are counter-productive, since they aver- age across very distinct climatic provinces. Following HIFT, we have emphasized shorter ranges, generally up to 5000 km.
Acoustic Thermometry of Ocean Climate (ATOC)
The completion of HIFT found us in an up-beat mood. We had transitioned from the earliest 25-km reciprocal transmissions to the documentation of a time-variable mesoscale to basin scale and now to a global scale. (The next step would be more difficult.) We were ready to tackle the problem of measuring the changing ocean heat content (cen- tral to the ongoing debate on climate change). Our first goal was to monitor the subtropical gyre of the northeast Pacific.
There were two problems: money and permits. At the behest of Senators Gore5 and Nunn, the Defense Department had initiated the “Strategic Environmental Research and Development Program” (SERDP). In late 1992, after prolonged negotiations, we secured a 30-month, $35-million grant for the Acoustic Thermometry of Ocean Climate (ATOC) project. The goals were (i) to determine the precision with which acoustic methods can measure large-scale changes in ocean temperature and (ii) to determine what effects, if any, the acoustic transmissions would have on marine mammals and other marine life. The hope was that ATOC would lead to a long-term, global program for measuring the changing ocean heat content. At Heard Island we had used a vertical array of powerful HLF-4LL transducers suspended from shipboard transmitting at 57 Hz; now we could backtrack to less power- ful and less expensive projectors. ATOC featured cable-con- nected, bottom-mounted HX-554 bender-bar sources on Pioneer Seamount off central California and off the north coast of Kauai (Fig. 2). The cable-connected sources allowed for the long time series needed.
The permit issue proved to be much more difficult. HIFT had been widely publicized under the unfortunate slo- gan “The Shot Heard Around the World.” It caught the imag- ination of the public, but also the attention of the environ- mental community. As a result, operation of the ATOC sources could not begin until approvals were granted and permits were issued. When the approvals were finally issued, the permits placed control of the sources under the
12 Acoustics Today, October 2005