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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 42
Long range shock waves from a large explosion underwater: experiment and data some of the energy were expended in digging a crater. This would also lead to the gauges nearer to the sea bed being sheltered from much of the shock wave, more so in the case of the large charge. This is borne out by the observations given in Table 7.2. The peak pressures measured at the various instruments deployed at sea are compared with the predicted values from equation 2.4 for P using max the constants for TNT given in equations 2.6 and the measured depth and range of each instrument (Britt 1984). The results are listed in Table 8.1 and displayed in Figs 8.1 and 8.2. It is noted that the measured values do not exceed the the calculated values for the 4500 Ib charge but for the 900lb charge, for which the depth was closer to the optimum, calculated maximum pressures were exceeded at the site of the Goosander. 43
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Appendix A: Proposed schedule for L
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16:47:00 16:47:20 16:47:30 16:47:34
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08:12:45 08:12:50 08:13:00 08: 13:2
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PUSS 3 (INSTRUMENT 11) REDEPLOYED 2
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PUSS 6 (INSTRUMENT 14) REDEPLOYED 2
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Appendix C: Locations of sites at s
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(2) PUSS LOCATIONS SITE 1 (PUSS 2)
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Fix: 56°42.810N aN = .001N +1 m 01
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Appendix D: MODELS FOR COMPUTER SIM
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(c) Permanent land stations with a
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4 13 5 14 Hydr. z x y Hydr. z x y R
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5 14 Hydr. Z X y Hydr. z X y First
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F.3.9 PUSS 10 at 9.982 km, 9001b sh
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Fig. F.2.16 BGS Sea Vertical Hydrop
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I-' -oJ w Fig. F.3.6 BGS Sea Bottom
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I-' ()) 0 Fig. F.3.13 BGS Sea Botto
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Start of record = 15:22:05.0 Filena
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BGS VHS Start of record = 11:53:13.
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Plate 16 Plate 17 Plate 18 Plate 19
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Plate 4 Plate 5 Plate 6
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Plata 10 Plate 1 1 Plate 12
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Plate 16 Plate 17 Plate 18
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