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Long range shock waves from a large explosion underwater: experiment and data 3.1.3 Vertical Hydrophone String (VHS) A general impression of the VHS is shown in Fig. 3.2, (see also Plate 22). At the position marked BGS (Figs 2.2 and 2.3), ten Geospace type MP-24L hydrophones were deployed in a vertical configuration at 5m intervals, with the first hydrophone placed at 5m from the sea-bed. The string was anchored to the sea bed by a sinker weight and held approximately vertical by submerged plastic floats. Signal cables were routed along the sea-bed and up to the recording systems on Salmaster. The cables were terminated in an underwater junction box which, in the event that the ship had to leave station, could be buoyed-off and later retrieved. The recording configuration and response specifications for the VHS are given in Figs 3.3, 3.4 and Table 3.3 The method of deployment (Fig. 3.5), which was devised to eliminate the possibility of entanglement, was successfully implemented at sea. 3.1.4 BGS Sea Bottom Package (SBP) The SBP was also deployed at the position marked BGS (Figs 2.2 and 2.3). The package was originally designed to monitor earthquakes in the North Sea near to the oil fields (Turbitt et al 1983). The pressure sealed package (Plate 17) contains a 3-component set of piezoelectric seismometers (Teledyne Geotech type S500) and one piezoelectric hydrophone (Graseby Dynamics type 770/401/PZT-4), along with the associated electronics for signal conditioning. The multicore signal cable was routed along the sea-bed and up to the recorders on Salmaster. This cable was also terminated in an underwater junction box (Plate 20) which could be buoyed-off if necessary. The system configuration and response specifications are given in Figs 3.6, 3.7 and Table 3.4. The final arrangement at the BGS station is shown in Fig. 3.8. 3.1.5 Pull-Up Sea-bottom SeiSmometers (PUSSes) The complete digital recording system is contained in a 4ft long pressure sealed tube. Each PUSS contains a 3-component set of geophones and an externally mounted piezoelectric hydrophone (Plate 12). The general 20
Long range shock waves from a large explosion underwater: experiment and data impression is shown in Fig. 3.2 and the system details are described in Owen & Barton (1986). For this experiment the 90 minutes of available recording time was devided into nine pre-selected time windows of ten minutes duration (see Appendix A). To allow for any slight loss of synchronisation between PUSS windows and shot times, and also shock-wave travel times across the array, the explosions were to be detonated no sooner than 2 minutes and no later than 4 minutes into any PUSS recording window (Appendix A). This would ensure that, the fastest waves at the closest PUSS (10Km) and the slowest waves at the most distant PUSS (115Km), would be recorded. PUSS sensitivities (Table 3.5) at each site were arranged according to the expected pressures and ground velocities given in Tables 2.3 and 2.4. The specifications for the hydrophone and geophones are given in Tables 3.6 and 3.7. 3.1.6 Onshore Seismic Stations The explosions were recorded on-shore using standard seismological stations of the BGS. The main network used was LOYNET (Crampin et al 1970) in the Scottish Midland Valley with extensive additional observations coming from other BGS networks (Browitt 1979). All LOYNET stations, which use the Villmore MkIII-A seismometer, are shown in Fig. 2.1. Response specifications are given in Turbitt & Stewart 1982. For this experiment additional special stations were installed at four coastal sites, the details of which are given in Table 3.7. These sites were chosen near to the sea and deliberately placed in soft sediments in an attempt to obtain the closest coupling to the seismic waves propagating in the sea-bed sediments. Strata were chosen to give the best geological match to the extension of the submarine strata just offshore. At each site, the data from a 3-component set of seismometers placed on soft ground and a vertical reference seismometer on a nearby hard rock site, were recorded on a Geostore which provided one week of continuous recording per tape. 21
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Table 8.1 Comparison of measured ex
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90 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
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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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.... N Table D2 Paraaeters of preli
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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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I-' W en Fig. F.1.2 PUSS 11. RANGE=
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.... w
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.... t Fig. F .1.10 PUSS 11, RANGE=
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.... (JI .... Fig. F.2.2 PUSS 11. R
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I-' U1 CD Fig. F.2.1O PUSS 10, RANG
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.... m I\) Fig. F.2.13 PUSS 13, RAN
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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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.... (J) 0 Fig. F.3.13 BGS Sea Bott
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Start of record = 11:52:05.0 Filena
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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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Plate 22 Plate 23
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Plate 28 Plate 26
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