Geoscience Reference
In-Depth Information
Actual Cable Location
Planned Survey
Lines
Direction of Current
Fig. 2.11
Cable feathering due to action of current.
between each other and fixed receivers on the boat give a wealth of information on
their position relative to each other and the boat. This information is used by powerful
on-board computers to provide accurate measurements of absolute positions of every
shot and receiver throughout the survey. This information is later merged with the
seismic trace data during processing. On board, the navigation is also used to check
that the survey has actually been shot within the requirements defined by the client.
In many surveys it is necessary to re-shoot some lines because the original lines were
not positioned correctly due to wind/wave/current effects. This re-shooting, known
as infill, is an extra expense and every effort is made to keep it to a minimum by
accurate steering of the boat. On-board computer systems predict the heading required
to allow the boat to steer along the required trajectory taking into account the current
conditions, and ensure that the shots are automatically fired at the correct locations.
Despite all this, the effects of currents and winds usually mean that some infill is
required.
In areas where there is already infrastructure (platform and production facilities) in
place it is generally not possible to shoot the entire survey with the standard marine
configuration. A safe distance must be kept between the boat and any obstacles. In such
a case it is common to use techniques such as undershooting, where a source boat on
one side of the obstacle shoots into a receiver boat on the other side (see
fig. 2.12
). Care
must be taken during interpretation that any differences in the data in the undershot
area are not a result of the different acquisition parameters.
2.2
Marine shear wave acquisition
Most marine acquisition to date has been concerned with compressional (P) wave re-
flections from a P-wave source, since fluids (including sea-water) do not permit the
