Acoustic Thermometry, or Have you heard Heard?

The Perth test and the analysis of the properties of acoustics to account for the observations raised the interesting notion of using the characteristics of sound at very long ranges in the ocean to measure small changes in the average ocean temperature. Such measurements address the question of the warming of the oceans in response to climate change.

Speed of sound is a function of temperature, with a change of 1°C corresponding to a change in sound speed of about 4 m/s. (Sound speed is also a function of the salinity of seawater, but that effect is very small in practice.) By measuring the travel times of acoustic pulses very accurately and knowing the range between the source and the receiver, the changes in the speed of those pulses can be inferred (speed × time = distance). The change in speed can, in turn, be used to infer the change in the ocean temperature. The technique of ocean acoustic tomography has exploited this technique to measure ocean variability since the early '80's (see the book by Munk, Worcester and Wunsch, Ocean Acoustic Tomography, 1995).

The advantage of using sound in this way is that it inherently measures the average temperature over the acoustic path. Long-range acoustic measurements give a natural measure of average ocean temperature. Attempting to make such measurements by "conventional" measurements, such as observations by many single thermometers, would require many, many instruments. In addition, the ocean is highly variable - the ocean has a "weather" of its own that causes temperatures to fluctuate by 1–2°C at any one point from one month to the next. There are also "internal waves" - rapid vertical movements of the ocean's layers - that cause temperatures to fluctuate. In contrast to all this variability, changes in the ocean's average temperature as a result of climate change are expected to be tiny - about 0.005°C per year at 1000 m depth is a nominal expectation. Measuring such small changes conventionally is difficult, while the acoustical approach appears to be ideally suited for the measurement.

Professor Walter Munk in a photo taken in the Southern Ocean during the 1991 HIFT experiment.

Acoustic sources were readily available and had been used for oceanographic research for many years. (Such acoustic sources are basically loudspeakers that send low-level, but lengthy, carefully-crafted signals, rather than the single very loud pulses from explosives.) However, there were a number of questions that arose about using such sources for the purposes of the acoustic measurements over ocean-basin-scale ranges. The HIFT experiment was designed as a feasibility study to answer some of those questions.

Sound made near the sound channel axis at 1000 m depth travels along a discrete set of acoustic paths between the source and the receiver. Measurement of the travel times of the arriving acoustic pulses can be used to infer the change in temperature averaged over the acoustic paths.