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Summary
Two large explosions were detonated in the North Sea about 80 km east of
Aberdeen on 29 September, 1987, as part of a collaborative project between
the British Geological Survey and the Admiralty Research Establishment.
Both charges were deployed and detonated on the sea bed by RMAS Goosander
at 57°06.98'N 00045.04'Y in water depth 76.125 m. The 4500 lb
(2041.27 kg) TNT charge was detonated at 11h 52m 33.225s GMT and the
900 lb (408.2 kg) TNT charge at 15h 22m 30.591s GMT.
The majority of offshore recording equipment was deployed and retrieved
by RMAS Salmaster. Shock waves from both explosions were recorded by a
variety of instruments, both offshore and onshore.
Vertical arrays of pressure gauges and shot timing equipment recorded
the shock waves in the water near to the shot point, ranges being about
550 m and 230 m respectively for the two shots. Five Pull-up Shallow
Yater Seismometers (PUSSes) were deployed at ranges from about 10 to
85 km. Each PUSS contained one three-component geophone and one
hydrophone and operated independently of any ship but was restricted to
recording within nine pre-set time windows, each of 10 minutes duration.
At 60 km range three instrument packages were deployed and connected by
cable to RMAS Salmaster: two vertical strings of hydrophones, sensitive
to higher and lower frequencies respectively and the BGS sea-bed package
containing a three component accelerometer and one hydrophone. Four
onshore near coastal seismograph stations were installed and recorded at
"soft" sites to investigate sediment to hard rock propagation effects of
the seismic waves. Seismic recordings were obtained at land stations in
the UK and Eire, the latter by the Dublin Institute of Advanced Studies
for investigations into the deep crust and lithosphere.
Predicted pressures from a 4500 lb explosion detonated at 5m above the
sea-bed were 818kPa at 0.5km range, 28kPa at 10km (closest planned PUSS
site) and 2kPa at 85km (furthest planned PUSS site). The pressures
observed at 0.55km over a range of depths were 210-660kPa; 8kPa was
observed on the sea-bed at 9.98km and .09kPa at 85.29km.
Seismic local magnitude estimated from onshore seismic stations are
2.9 ML and 2.3 HL for the 4500 lb and 900 lb TNT charge respectively.
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