128
Walter Munk
meeting of the European Geophysical Society where he received the Munk Award
given jointly by the U.S. Navy and The Oceanography Society.
HEARD ISLAND FEASIBILITY TEST (HIFT) OF 1991
The preceding fifteen years had seen a steady progression in ranges, from Worcester’s
25-km reciprocal transmission to the 100-km mesoscale-resolving experiments to
Spiesberger’s 4000-km climate-oriented transmissions. Forbes and I started to think
about an antipodal transmission of 20,000 km
21,22 (the next step would be more
difficult).
Birdsall had made some rough estimates that varied by as much as 60 dB;
they ranked global transmissions from trivially easy to impossible. The frequency of
choice was 57 Hz to avoid electric machinery noise at 50 and 60 Hz. Absorption is
not the problem at these low frequencies, varying from 3 dB per 20,000 km in Pacific
waters to 5 dB/Mm in Atlantic waters (because of the difference in dissolved salts).
Scattering might be. But global acoustics raises new problems. Geometric spreading
does not apply on a sphere and in fact great circles converge after 10,000 km. Further,
Longuet-Higgins had shown that even great-circle ray geometry fails catastrophically
within 50 km of the antipole where the departure from a spherical Earth is associated
with spheroidal caustics and multiple paths.
For guidance we studied what had been an almost casual addition to a geophysical survey conducted thirty years earlier. While Gordon Hamilton was tending his
Bermuda hydrophones, the R/V Vema and HMAS Diamantina used pressure detonators to fire 300-lb anatol charges near the sound axis off Perth, Australia. These were
clearly recorded in Bermuda. In fact, each of the explosions gave a double signature
presumably related to the spheroidal caustic (I had previously incorrectly attributed
the double signature to a reflection from the Bermuda coastline).
The Oceanographer of the U.S. Navy, Adm. Richard Pittenger, assigned the
acoustic source ship Cory Chouest to the HIFT. Cory had HLF-4 sources suspended
through a center well. The sources were limited to an operational depth of 300 m,
which dictated their deployment at high latitudes where the sound channel is shallow.
Heard Island in the Indian Ocean at 54
◦ S, 74
◦ E filled this requirement, and has the
unique location for providing great-circle windows into the North and South Atlantic,
North and South Pacific, the Indian Ocean, and Antarctica (Figure 8.6a). (As we later
learned, it also has the unique location of a maximum in global wave height statistics.)
Heard Island was discovered by the American Sea Captain Heard in 1853. It
is under Australian flag and uninhabited except for occasional visits to study the rich
sea life. We chose 0000 GMT 26 January (Australia Day) for a start time, and I
personally persuaded colleagues from nine countries (no organization) to be out at
sea with receivers suspended into the sound channel.
Birdsall had drawn up the transmission plan: starting on Australia day, 1 hour
out of every 3 hours for 10 days; intensity 210 to 220 dB re 1 µPa at 175 m depth:
Walter Munk
meeting of the European Geophysical Society where he received the Munk Award
given jointly by the U.S. Navy and The Oceanography Society.
HEARD ISLAND FEASIBILITY TEST (HIFT) OF 1991
The preceding fifteen years had seen a steady progression in ranges, from Worcester’s
25-km reciprocal transmission to the 100-km mesoscale-resolving experiments to
Spiesberger’s 4000-km climate-oriented transmissions. Forbes and I started to think
about an antipodal transmission of 20,000 km
21,22 (the next step would be more
difficult).
Birdsall had made some rough estimates that varied by as much as 60 dB;
they ranked global transmissions from trivially easy to impossible. The frequency of
choice was 57 Hz to avoid electric machinery noise at 50 and 60 Hz. Absorption is
not the problem at these low frequencies, varying from 3 dB per 20,000 km in Pacific
waters to 5 dB/Mm in Atlantic waters (because of the difference in dissolved salts).
Scattering might be. But global acoustics raises new problems. Geometric spreading
does not apply on a sphere and in fact great circles converge after 10,000 km. Further,
Longuet-Higgins had shown that even great-circle ray geometry fails catastrophically
within 50 km of the antipole where the departure from a spherical Earth is associated
with spheroidal caustics and multiple paths.
For guidance we studied what had been an almost casual addition to a geophysical survey conducted thirty years earlier. While Gordon Hamilton was tending his
Bermuda hydrophones, the R/V Vema and HMAS Diamantina used pressure detonators to fire 300-lb anatol charges near the sound axis off Perth, Australia. These were
clearly recorded in Bermuda. In fact, each of the explosions gave a double signature
presumably related to the spheroidal caustic (I had previously incorrectly attributed
the double signature to a reflection from the Bermuda coastline).
The Oceanographer of the U.S. Navy, Adm. Richard Pittenger, assigned the
acoustic source ship Cory Chouest to the HIFT. Cory had HLF-4 sources suspended
through a center well. The sources were limited to an operational depth of 300 m,
which dictated their deployment at high latitudes where the sound channel is shallow.
Heard Island in the Indian Ocean at 54
◦ S, 74
◦ E filled this requirement, and has the
unique location for providing great-circle windows into the North and South Atlantic,
North and South Pacific, the Indian Ocean, and Antarctica (Figure 8.6a). (As we later
learned, it also has the unique location of a maximum in global wave height statistics.)
Heard Island was discovered by the American Sea Captain Heard in 1853. It
is under Australian flag and uninhabited except for occasional visits to study the rich
sea life. We chose 0000 GMT 26 January (Australia Day) for a start time, and I
personally persuaded colleagues from nine countries (no organization) to be out at
sea with receivers suspended into the sound channel.
Birdsall had drawn up the transmission plan: starting on Australia day, 1 hour
out of every 3 hours for 10 days; intensity 210 to 220 dB re 1 µPa at 175 m depth: