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Long range shock waves from a large explosion underwater: experiment and data
8. Discussion
8.1 MAJOR CHARACTERISTICS OF THE ACOUSTIC AND SEISMIC YAVEFORMS
An examination of the BGS vertical hydrophone string records, both the
analogue signal recorded on the jet pen and after digitisation at 2011.5
samples per second, has revealed that the signal at long range contains
high frequencies in excess of 2kHz in the first 1/10th sec from the first
arrival, obscuring any shape. It is apparent from the analogue playout
that the onset is not sharp, the maximum values occurring up to 0.2secs
after the first arrival. The two hydrophones nearest the sea bed,
although losing some sensitivity due to the effects of water pressure, did
detect the influence of the seismic waves, albeit weakly.
Unfortunately the only available sea bed
limited to a digital sampling rate of 256Hz.
instruments (PUSSes) were
They also have a limit on
the maximum increment between samples. The shape of the pulse at the
acoustic arrival is thus open to question. The acoustic arrivals, as
recorded by the PUSSes, also exhibit a high frequency component just after
the first arrival. Despite the limitations, clear arrival times can be
measured, and the peak amplitudes readily determined. It will be possible
to generate spectra from this data, bearing in mind the frequency
limitation of the instruments. This will be done for the next report.
8.2 OBSERVED SHOCK YAVE AMPLITUDE: EFFICIENCY OF THE SOURCE
The ratio of peak pressures measured for gauge 188 for the two explosions
is 0.66/1.5 - 0.44. Using the similitude equations, the calculated ratio
of peak pressures P max (4500Ib)/P max (900Ib) is 0.815 allowing for the
differences in distance. This is an indication that the present source
theory is inadequate when charges are detonated on the bottom. This may,
in part be a result of the differing conditions, in that the 900 lb charge
was lowered to the sea bed on a pallet and detonated while still on it,
whereas the 4500lb charge was detonated in a frame, allowing the shock
wave direct access to the soft sandy sediment of the sea bed. The much
reduced efficiency of the large charge could also partly be explained if
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