Ocean Acoustic Tomography
131
source was torn loose and went to the bottom, and the others were severely damaged.
Fortunately there had been 28 successful runs before this untimely termination.
W. Kuperman, the editor of the Journal of the Acoustical Society of America,
invited us to collect our experience in a special issue,
23 and eighteen papers were published under the heading “The Heard Island papers: a contribution to global acoustics.”
There were some surprising results. In all the transmissions, the source ship Cory had
to be underway into winds and waves to avoid being broached. The measured Doppler
permits a very accurate determination of the geodetic launch angles (relative to the
ship’s course). These agreed closely with the a priori calculations of the geodetics,
except for the path through the Tasman Sea to the American west coast. Evidently the
direct route was blocked by unknown bathymetry, but a weak signal got through along
an alternate route passing east of New Zealand. For any fixed station the differential
Doppler between the early steep and late flat launch angles provided useful tools for
ray identification. The conclusion here is that the motion of the source ship (which
we regarded as a necessary evil) turned out to be a substantial asset.
Among the oceanographic results is evidence for stripping of high acoustic
modes by bathymetric features protruding into the sound channel, with subsequent
repopulation. There is strong mode coupling at sharp oceanic fronts (principally
the Antarctic Circumpolar front). The final paper by Bowles et al. deals with the
biological component of HIFT. Observations were restricted to times when the winds
were weaker than force 8. The study was severely limited by the short time available
for the baseline study prior to transmissions. The evidence suggests some behavioral
changes of the marine mammal population, but there was no indication of distress.
HIFT demonstrated that acoustic sources could be detected at antipodal ranges,
but the arrival patterns turned out to be too garbled to permit the precise determination
of identified travel times. From a climate point of view, the very large ranges are
counterproductive since they average across distinct climatic provinces. Following
HIFT we have emphasized shorter ranges, generally less than 5000 km.
ACOUSTIC THERMOMETRY OF OCEAN CLIMATE (ATOC),
1993–2000
The completion of HIFT found us in an upbeat mood. We were ready to take on the
problem of measuring the change in ocean heat content (central to the ongoing debate
on climate change). Our first goal was to monitor the subtropical gyre of the northeast
Pacific.
24
Plans were for a greatly reduced source level (195 dB versus 220 dB at HIFT),
at 75 Hz versus 57 Hz center frequency and at midlatitude axial depth of 1000 m
versus 175 m HIFT depth. The combined change in source intensity and depth leads
to a 35-dB reduction in the sound level within the biologically crucial surface layers.
Accordingly we thought that no further permit difficulties would be encountered. This
too was not to be.
131
source was torn loose and went to the bottom, and the others were severely damaged.
Fortunately there had been 28 successful runs before this untimely termination.
W. Kuperman, the editor of the Journal of the Acoustical Society of America,
invited us to collect our experience in a special issue,
23 and eighteen papers were published under the heading “The Heard Island papers: a contribution to global acoustics.”
There were some surprising results. In all the transmissions, the source ship Cory had
to be underway into winds and waves to avoid being broached. The measured Doppler
permits a very accurate determination of the geodetic launch angles (relative to the
ship’s course). These agreed closely with the a priori calculations of the geodetics,
except for the path through the Tasman Sea to the American west coast. Evidently the
direct route was blocked by unknown bathymetry, but a weak signal got through along
an alternate route passing east of New Zealand. For any fixed station the differential
Doppler between the early steep and late flat launch angles provided useful tools for
ray identification. The conclusion here is that the motion of the source ship (which
we regarded as a necessary evil) turned out to be a substantial asset.
Among the oceanographic results is evidence for stripping of high acoustic
modes by bathymetric features protruding into the sound channel, with subsequent
repopulation. There is strong mode coupling at sharp oceanic fronts (principally
the Antarctic Circumpolar front). The final paper by Bowles et al. deals with the
biological component of HIFT. Observations were restricted to times when the winds
were weaker than force 8. The study was severely limited by the short time available
for the baseline study prior to transmissions. The evidence suggests some behavioral
changes of the marine mammal population, but there was no indication of distress.
HIFT demonstrated that acoustic sources could be detected at antipodal ranges,
but the arrival patterns turned out to be too garbled to permit the precise determination
of identified travel times. From a climate point of view, the very large ranges are
counterproductive since they average across distinct climatic provinces. Following
HIFT we have emphasized shorter ranges, generally less than 5000 km.
ACOUSTIC THERMOMETRY OF OCEAN CLIMATE (ATOC),
1993–2000
The completion of HIFT found us in an upbeat mood. We were ready to take on the
problem of measuring the change in ocean heat content (central to the ongoing debate
on climate change). Our first goal was to monitor the subtropical gyre of the northeast
Pacific.
24
Plans were for a greatly reduced source level (195 dB versus 220 dB at HIFT),
at 75 Hz versus 57 Hz center frequency and at midlatitude axial depth of 1000 m
versus 175 m HIFT depth. The combined change in source intensity and depth leads
to a 35-dB reduction in the sound level within the biologically crucial surface layers.
Accordingly we thought that no further permit difficulties would be encountered. This
too was not to be.