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I,(/I/g rOl/ge sho('k lI'a\'es {ro/11 a large explosiol/ III/derwater. experimel1t al1d data
distance from a 4500 lb TNT (2041.2 Kg) charge are given in Table 2.3. A
water depth of 75 m is assumed with the charge on the sea-bed. Values
tabulated for the surface reflected wave equal those of the direct wave at
10 km range and are therefore not tabulated at greater ranges.
A maximum of 28 kPa pressure was expected at the closest proposed PUSS
at 10 km range and at the furthest proposed at 115 km range the peak
pressure should have dropped to about 1.8 kPa. Maximum pressure expected
at the pressure gauges close to the shot-point at the Goosander's
stand-off position at 0.5 km is about 818 kPa. In view of this expected
decay of peak pressure with distances it was decided to set the PUSSes at
sites 4-6 at an equal but highest gain, PUSSes at sites 2-3 at an
intermediate but equal gain, the PUSS at site 1 was to be set to the
lowest gain.
2.5.2 Seismic wav('s:
Jacob & Neilson (1977) investigate the seismic local magnitude (M L ) of
underwater explosions as a function of charge weight. Their data
(Fig. 2.5) adapted from Jacob & Neilson) show considerable scatter, but
the overall trend is very clear, and enveloping curves have been drawn
into Fig. 2.5 spanning ranges of observed ML for given charge size.
Examination of Fig. 2.5 shows empirically that the local magnitude
generated by a 4500 lb TNT charge lies in the observational range
2.6-3.74 ML. In view of this, a magnitude 3.74 ML was selected as being
the largest value compatible with the size of the charge.
Local magnitude is defined in terms of seismic ground motion by the
equation (Richter 1958):
ML = logA-logA o
(2.9)
A and A are ground displacement in mm (notionally on a standard torsion
o
Yood-Anderson seismometer). A is the observed seismic ground displacement
and A is a magnitude standardising distance correction term for local
o
magnitude calculation. There are tables of standard A values (see
o
Richter 1958). Forecasts of the extreme upper bound to the seismic
displacements, assuming 3.74 ML, are given in Table 2.4.
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This table also