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
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.
Walter Munk therefore conjectured that long-range acoustics, employed on a global scale, could provide a
direct way to measure oceanic climate change.
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.