Science Aims of HIFT

HIFT addressed three simple science questions:

1. 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?
2. 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.
3. 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.

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.

(2) 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!

(3) 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.