130
Walter Munk
transmitting for 3600 s at 210 dB (2500 W) of acoustic power gives about 10
7 J
of energy per transmission. This is comparable to the 3 x 10
7 J of acoustic energy
radiated by the 300-lb explosive charges in the Perth-to-Bermuda transmissions.
22 The
coded transmission has the advantage that it spreads this energy over 60 minutes. The
procedure for detection is to correlate the received signal with the stored transmission
code, leading to a processing gain equal to the degrees of freedom:
dof = bandwidth × coherence time = 10 Hz × 300 s = 3000 (35 dB)
Thus, detection can be achieved at a level well below the ambient sound level and
thus inaudible to marine life.
In the midst of preparations we were informed by NOAA’s National Marine
Fisheries Service that a permit would be required (there had been no such requirement
in the preceding 15 years). To make matters worse, the Australian authorities, who
had previously supported the test plan without raising the issue of a permit, now
decided one would be required. On very short notice, the sister ship Amy Chouest
with Ann Bowles as chief scientist was tasked to assist the Cory Chouest with a parallel
program of biological observations. A protocol provided for the termination of the
transmissions under certain prescribed circumstances. The biological add-on certainly
contributed to the complexity and expenses, yet it provided a welcome partnership
with a devoted group of observers who worked under very trying conditions.
Spindel had previously gone to sea on the Cory to tune the sources to the lowest
attainable frequencies centered on 57 Hz. The two vessels sailed from Freemantle
on 9 January with no permits in hand; postponement would have been tantamount to
cancellation. Andrew Forbes (an Australian oceanographer) and I were co-chief scientists, Matthew Dzieciuch (a Birdsall Ph.D.) took charge of the acoustic transmissions.
We received the American permit while underway one week before the start date,
and the Australian permit 21 hours prior to start time. The latter came with a message
from the Environmental Minister: “good luck and calm seas.” This was not to be.
Our Bermuda listening station and the station north of Seattle were both almost
halfway around the Earth, at 18,000 km or 3
1
2
hours acoustic travel time. The reader
will recall the 60 dB of a priori uncertainty, with little expectation that the signal would
be detected at the farthest sites. The evening prior to the scheduled start, technicians
aboard the Cory requested a five-minute checkout of the sources. Three and one-half
hours later we received an puzzled message from Metzger in Bermuda, demanding to
know what was going on. Fifteen minutes later a similar message arrived from Birdsall
at his station north of Seattle. The signal had arrived on the two opposite American
coasts halfway around the Earth, one traveling eastward past New Zealand, the other
westward past the Cape of Good Hope. The question of the feasibility of global-scale
transmission had been answered, and it was not yet Australia Day. (This was the most
exciting moment of my career, and the reader must excuse the detailed reporting.)
The scheduled transmissions commenced on time, and other stations began to
report receptions. On 31 January we encountered a gale with 10m seas. One suspended
Walter Munk
transmitting for 3600 s at 210 dB (2500 W) of acoustic power gives about 10
7 J
of energy per transmission. This is comparable to the 3 x 10
7 J of acoustic energy
radiated by the 300-lb explosive charges in the Perth-to-Bermuda transmissions.
22 The
coded transmission has the advantage that it spreads this energy over 60 minutes. The
procedure for detection is to correlate the received signal with the stored transmission
code, leading to a processing gain equal to the degrees of freedom:
dof = bandwidth × coherence time = 10 Hz × 300 s = 3000 (35 dB)
Thus, detection can be achieved at a level well below the ambient sound level and
thus inaudible to marine life.
In the midst of preparations we were informed by NOAA’s National Marine
Fisheries Service that a permit would be required (there had been no such requirement
in the preceding 15 years). To make matters worse, the Australian authorities, who
had previously supported the test plan without raising the issue of a permit, now
decided one would be required. On very short notice, the sister ship Amy Chouest
with Ann Bowles as chief scientist was tasked to assist the Cory Chouest with a parallel
program of biological observations. A protocol provided for the termination of the
transmissions under certain prescribed circumstances. The biological add-on certainly
contributed to the complexity and expenses, yet it provided a welcome partnership
with a devoted group of observers who worked under very trying conditions.
Spindel had previously gone to sea on the Cory to tune the sources to the lowest
attainable frequencies centered on 57 Hz. The two vessels sailed from Freemantle
on 9 January with no permits in hand; postponement would have been tantamount to
cancellation. Andrew Forbes (an Australian oceanographer) and I were co-chief scientists, Matthew Dzieciuch (a Birdsall Ph.D.) took charge of the acoustic transmissions.
We received the American permit while underway one week before the start date,
and the Australian permit 21 hours prior to start time. The latter came with a message
from the Environmental Minister: “good luck and calm seas.” This was not to be.
Our Bermuda listening station and the station north of Seattle were both almost
halfway around the Earth, at 18,000 km or 3
1
2
hours acoustic travel time. The reader
will recall the 60 dB of a priori uncertainty, with little expectation that the signal would
be detected at the farthest sites. The evening prior to the scheduled start, technicians
aboard the Cory requested a five-minute checkout of the sources. Three and one-half
hours later we received an puzzled message from Metzger in Bermuda, demanding to
know what was going on. Fifteen minutes later a similar message arrived from Birdsall
at his station north of Seattle. The signal had arrived on the two opposite American
coasts halfway around the Earth, one traveling eastward past New Zealand, the other
westward past the Cape of Good Hope. The question of the feasibility of global-scale
transmission had been answered, and it was not yet Australia Day. (This was the most
exciting moment of my career, and the reader must excuse the detailed reporting.)
The scheduled transmissions commenced on time, and other stations began to
report receptions. On 31 January we encountered a gale with 10m seas. One suspended