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with sufficient buoyancy to hold the charge taut at Sm from the sea bed.
(Fig 2.4). The purpose of detonation 5 m above the sea-bed is to instigate
against energy loss in simple rock fracturing, probable with detonation on
the sea-bed. Detonation 5 m above the sea-bed, a distance less than the
bubble radius, ought to achieve slap-down of the pressure pulse
impulsively on the sea-bed without extensive rock fracturing. This is
seismically more efficient.
The shot point for practical purposes at sea was defined as the shackle
ring shown in Fig 2.4. The mooring arrangement consisted of two Class 5
buoys wired together with each buoy anchored to the sea-bed so that the
whole system was held under tension. The mid-point of the wire connecting
the two buoys consisted of a shackle ring which lay below sea level. The
shackle ring was connected to a free floating orange marker buoy so that
it could easily be retrieved when required, for example, to locate the
precise shot point when the charge was to be lowered.
The detonation cable was to be reeled out from a drum with small
sponge-like floats
through the shackle
distances for the
attached every few metres (see photographs), and fed
ring down to the charge. Anticipated stand-off
Goosander at shot-time were 1500' and 750' for the
larger and smaller charge size respectively.
2.5 PREDICTED SIGNAL AMPLITUDES AND EXPLOSION SOURCE PARAMETERS
It was expected that the equipment deployed on the sea-bed at different
distances from the explosions would need to span a wide range of signal
amplitudes. It was necessary to forecast the approximate amplitude of
direct pressure waves and seismic ground motions at each proposed site so
that instrumental gains could be set accordingly.
2,:;.' Press/I/"(' wav(':
Standard expressions given by Cole (1948) for the direct shock or pressure
wave P(t) MPa from an underwater explosion, detonated at time t = 0, are
of the form:
P(t)
12
(2.1)