Geoscience Reference
In-Depth Information
Fig. 2.7
Generation of multi-fold coverage during seismic surveys. As the survey proceeds,
different combinations of shot and receiver record the same reflection point. The difference
between the arrival times of the reflection from shot-receiver pairs with increasing separation
allows subsurface velocities to be estimated. The addition of different traces with the same
reflection point improves the signal to noise ratio of the final section.
areas of the survey where the boat changed shooting direction may lead to stripes in the
data. This is caused by the long time interval between these adjacent lines being shot,
so that there may be changes in tide, wave, currents or even water temperature (and
hence water velocity) between the two lines. Other patterns may appear in the data, for
instance owing to failure of part of the source gun array. Such striping may be difficult
to remove by later data processing.
As with 2-D operations, the same subsurface location is recorded many times by
traces having increasing separation between the source and receiver position
(fig. 2.7)
.
During processing, this multiplicity of data is used to increase the signal to noise ratio
of the final stack, to pick subsurface velocities and to discriminate between different
recorded events (such as primary and multiple reflections).
Modern boats are capable of towing as many as 12 cables each typically between
4 and 8 km in length, though it is unusual to see more than 10 actually deployed.
This large increase of data gathered with each pass of the boat means that, despite the
high cost of building and maintaining these vessels, their operational efficiency permits
large exploration surveys, covering thousands of square kilometres, to be routinely
acquired in a matter of weeks. At the time of writing, boats acquire up to 40 km
2
of
data daily at costs of around US $5000 per square kilometre. As a result of the high
initial cost of marine vessels and the need to have near-continuous operations to be
financially effective, almost all marine 3-D seismic boats are owned by specialised
service companies and not by energy companies.
In order to be able to tow the cables through the water off to the side of the boat, large
devices called paravanes are used to position the head of the outermost cables to the
