Geoscience Reference
In-Depth Information
Surface
V
w
Uphole
Uphole
Base of Weathered Layer
Bedrock
V
b
Seismic Datum Plane
Fig. 2.20
Weathering layer corrections for land data. For land acquisition the surface topography
and thickness of the weathered zone must be corrected for, since otherwise the underlying structure
would have an overprint of elevation and weathering layer changes. Uphole information together
with refraction records is used to build a model of the near surface. Once the velocity of the
bedrock and weathering layer and the depth to the base of the weathering layer have been
determined, the correction is simply the sum of the shot and receiver statics. The static at any
location is simply (
E
s
−
E
w
)
/
V
w
+
(
E
w
−
E
d
)
/
V
b
, where
E
s
is the surface elevation,
E
w
is the
elevation of the base of the weathered layer,
E
d
is the elevation of the seismic datum, and
V
w
and
V
b
are the velocities in the weathered and bedrock layers.
2.4
Other types of seismic survey
Marine surveys require a minimum depth of water and land surveys must terminate in
areas close to the shore. The zone in between is termed the transition zone and surveys
involve a variety of specialised equipment such as specially designed swamp buggies
with shallow draughts. Although the operation in these areas is more time consuming
and has some extra constraints such as the timing of tides, they can be considered as a
mixture of marine and land operations.
4-D seismic surveys are repeat seismic surveys, generally over a producing field,
that are specially shot at incremental time periods to determine differences in seismic
response due to production. The fourth dimension referred to in the title is time, the
other three dimensions being the standard ones of two horizontal co-ordinates and
one vertical spatial co-ordinate. Both pressure and fluid within the reservoir may change
during production and both of these affect the acoustic properties of the rock. Even the
absence of a change in part of the reservoir may indicate compartmentalisation due to
faulting, etc. These 4-D surveys may be land or marine, with one to four component
receivers, although the majority to date have been conventional towed-streamer marine
