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Long range shock waves from a large explosion underwater: experiment and data
3.2 REMOTELY OPERATED VEHICLE (ROV)
The BGS sealed Package and the PUSSes do not contain a facility to
determine their orientation after deployment, and although they can be
held lengthwise, as much as possible, along the array line while lowering
them to the sea-bed, the final alignment on the sea-bed is not known. In
an attempt to verify the orientation of the instruments, a bright line was
painted along the length of each package (Plates 12,17). Subsequently,
using a remote controlled submersible (SUTEC: Sea Owl Hkll) (Plate 23),
it was anticipated that, by following the PUSS tethering cable or the SBP
recovery rope down to the sea-bed and along to the package, the
orientation could be determined from the visual lines on the instruments
and the compass bearing indicated on the Sea Owl control unit.
By fixing the grab-hand around the cable near the surface the ROV could
successfully reach the decoupling weight but could not successfully follow
the cable along the sea-bed to the package. This was quite conceivable as
the cables between the decoupling weights and the packages were
deliberately loose and winding on the sea-bed. It was also apparent that
the ROV was under-powered for this particular application, because of the
strength of the sea currents.
3.3 NAVIGATIONAL SYSTEMS
The slower seismic surface waves were expected to have a velocity similar
to or slightly less than that of the main shock wave in the water, that is
about 1.5 km s-l or less. The arrival time of such waves would be
resolvable to a few hundredths of a second, implying an uncertainty in
path length not exceeding a few tens of meters at most. As phase arrival
time resolution could not be usefully improved beyond this, it was
concluded that the position-fixing scheme should not introduce a larger
error than that implicit in reading the seismic phase arrivals. This
meant that position-fixing had to achieve accuracy of the order of a few
tens of meters.
Positions were fixed by simultaneous ranging by Sercel Syledis B to any
three of four precisely coordinated shore reference stations. Shore
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