Science Aims of HIFT
HIFT addressed three simple science questions:
Although these were the specific science goals of HIFT, it is important to bear in mind the overall
goal of developing long-range acoustic transmissions as tool for measuring climate change or
other large-scale ocean changes in ocean temperature.
Detectable? Can controlled, man-made acoustic signals travel the 10–20 Mm distances over the world's
oceans to be detected by remote hydrophones or hydrophone arrays?
Trackable? If those signals are detected, can some feature of the arrival pattern be found that repeats
reliably in a sequence of acoustic transmissions? The changes in travel times of that feature from
transmission to transmission could then be used as a measure of oceanic variability.
Identifiable? If such a consistent acoustic feature is resolved, can numerical calculations be used
to model or predict that feature? Can the acoustical sampling associated with the travel time of that
feature can be determined? In inverse theory, this is the "forward problem"; modeling the forward
problem successfully is a prerequisite to using the acoustic data to determine ocean properties.
Prior to HIFT, the various estimations of the attenuation of acoustic energy were widely varying, so
that the detection of the transmissions at great distances was not at all assured. Given the long
history of studies of long-range acoustics using small explosive charges (throughout 1950–1970
there were numerous experiments studying long-range acoustic propagation in the ocean using
explosive charges - 1 to 12 lb of TNT, called SUS charges for "Signal, Underwater Sound", c.f.,
the 1960 Perth to Bermuda experiment), it seems apparent that some sort of
acoustic signal would be detected even at the
longest ranges. What distinguishes the HIFT from this previous work was the use of controlled acoustic
sources used to send carefully crafted acoustic signals. The specially-sequenced signals are designed
to allow the resolution of travel times down to the millisecond level (travel time precision of order one
part in 107). Analysis of such data offered tremendous opportunities for new investigations
compared to previous experiments. One key question addressed by HIFT was the extent to which the
integrity of the coded signals would be maintained over very long ranges.
To answer these basic questions, many, many technical issues had to be resolved concerning
the nature of the acoustic signals to be sent, and the organization and synchronization of a
collaboration scattered over the globe. The main emphasis of the actual research for HIFT was
on acoustic sources and the signals that were to be sent, and an intensive effort on the technical
analysis of the recorded signals - signal processing. These efforts were an important step towards
developing acoustics as a remote sensing tool for oceanography.
Three types of HIFT signals as received at Bermuda (range 16 Mm, travel time 2.95 h) and at Ascension Island
(9.2 Mm, 1.71 h). Note the 60-Hz interference line for the M-sequence, and the 57-Hz carrier "afterglow" from
scattered arrivals. Spectra of the entire 60-min record are shown to the right. [After Munk et al., The
Heard Island Feasibility Test, JASA, 1994].
Summary of Results
(1) During the five days that acoustic sources operated (before they were all destroyed
by bad weather), a total of 35 acoustic transmissions were sent. The acoustic signals, and all
signal types transmitted, were easily detected on both coasts of the United States. In
fact, the signals of a brief, low-volume test of the acoustic sources the day before the
experiment was to begin were detected on both coasts of the United States.
At most receiver locations, some sort of repeatable pattern was noted in the acoustic
signals, although this is a complicated issue. Part of the problem was that the source
ship was forced to move at 1–2 knots because of bad weather. In addition, the array of 10
sources operated erratically because they were constantly started and stopped to
prevent the hydraulic oil within them from heating up. Thus, there was no repeatable pattern
of excitation, which is a prerequisite for a repeatable pattern of reception!
Other than the overall lump in the arrival pattern, no specific feature in
the receptions was identifiable or predictable at any of the receiver locations. For the
HIFT receptions, therefore, no inversion of the data for a determination of oceanographic
properties was possible. (Although the basic datum of the travel time of the overall arrival
pattern was potentially useful.) HIFT was focussed on the measurement of the travel times of
acoustic modes. We now know, however, that such travel times are impossible to measure because
of extensive "cross-talk" between the modes during the acoustic propagation. During the subsequent
decade-long ATOC experiment, stable, identifiable arrivals were apparent, but these arrivals
were the deep-turning, early-arriving acoustic rays, rather than the near-axis acoustic modes.
Acoustic Thermometry of Ocean Climate
The ATOC experiment was the follow-on project to HIFT. For this experiment, the measurements were not
global in extent, but confined to the North Pacific basin. See the page Epilogue
for more on ATOC.