In an earlier blog “A brief history of the RAFOS float” I mentioned the development of the SOFAR float and its use in the 1973 Mid-Ocean Dynamics Experiment (MODE), and later in polyMODE. It was a collaboration between me and Doug Webb, an amazing engineer I was most fortunate to work with. Doug’s challenge was enormous: to develop the SOFAR float which as it drifts freely at depth would tell us about the ocean currents we cannot see. In principle the concept was simple: as it drifts it would transmit precisely timed acoustic signals which could be used to determine its position. My task was to listen for those signals and determine their arrival times at hydrophones (underwater microphones) at Bermuda, Bahamas, Grand Turk, and Puerto Rico. Doug joked that his task was to put the sound in the water, mine was to get it out. But make no mistake, his challenge was far greater than mine. Behind those brief words hides an amazing period of engineering to develop the SOFAR float: its conceptualization, construction, and testing, addressing shortcomings, and testing again. a continuing process both in the shop and later at sea. After just one opportunity to test a completely re-designed float in 1972, Doug built 20 floats to be deployed in MODE. It was an immensely successful study, but what the scientific publications don’t tell you is what a close call it was.
MODE was a game changer in physical oceanography. It was one of several major initiatives of the International Decade of Ocean Exploration (IDOE) funded by NSF, ONR and NOAA. Virtually all instruments in MODE were novel developments, some barely out of the shop. Besides the SOFAR float, there was the vector-averaging current meter, the Neil Brown cable-lowered CTD (conductivity, temperature, and depth profiler), two different very high-resolution bottom pressure sensors to measure slight variations in the weight of the overlying water column, and the inverted echo sounder that measures water column heat content acoustically. There was also a British participation called mini-MODE, which used cleverly designed floats and CTDs to zoom on the details of eddy motion.
Doug began deploying floats on the first leg of a 5-leg cruise of the NOAA vessel Researcher. As soon as a float was deployed, we would immediately hear it at our listening stations at each of which I had observers on watch. The beauty of this arrangement was that we could follow all floats in real-time. Diane Dow, who oversaw the coding and processing of the arrival time data at Yale University, worked up the positions and faxed them to the entire MODE community on a regular basis. That is the good news. But it wasn’t long before a float’s signal disappeared simultaneously from all stations – clearly a float problem and not an acoustic propagation problem. We radioed to Doug that a float had gone silent, and soon after several others went silent too. Now comes what you don’t read about in the scientific reports.
Each float was equipped with a 12 kHz acoustic beacon and a command system to drop ballast so it would return to the surface. It couldn’t have been an easy decision, but we are immensely grateful to the MODE steering committee to give Doug permission to redirect the Researcher from its scheduled hydrographic work to the last known position of the float. Since we had tracked up till then we had a pretty good idea of how it might continue to drift over the next couple of days. The acoustic tracking was accurate enough that he could hear the 12 kHz beacon when he arrived at the expected float position. He used it to position the Researcher near the float at which point he sent the drop ballast command. It took perhaps a ½ hour for the float to surface. After retrieving the float, he quickly found that one of the resonator pipes had developed a leak that shorted the high voltage source which quieted the other pipe as well. He disconnected the bad pipe, put on a new ballast weight, and relaunched the float! Although the acoustic power was cut in half (3 dB), it was still quite adequate for tracking. This scenario repeated itself with several other floats.
Knowing that the Researcher had an excellent machine shop onboard, Doug went back to WHOI to develop a jig with which he could re-machine and seal the endplates of the resonator tubes to prevent them from leaking. Over the last two legs of the Researcher he retrieved the floats, one after another, disassembled the transducers, re-machined and resealed them, put on new ballast and sent them back on their missions. While MODE itself was a four-month study, we continued to track the floats for 1, 2 and a few even for 3 years as they spread out across the western North Atlantic between the US mainland and Bermuda. We learned so much from those trajectories. We owe MODE management and Doug a big thanks for that!