Geoscience Reference
In-Depth Information
total survey is generally built up from continuous repetition of the basic acquisition
pattern or footprint. In
fig. 2.17
we see six receiver lines with shot lines forming an X
pattern. As the shots move position, different sections of the geophone lines are made
live by the recording instruments in order to maintain the same relationships between
sources and receivers as in
fig. 2.17
.
Figure 2.18
shows part of a full line swathe with a
full shot pattern in place. The whole arrangement is then moved for the next set of six
receiver lines
(fig. 2.19)
.
In this case there is an overlap of one line between one set of
six receiver lines and the next set. Special software is available to the designer of land
seismic surveys to allow determination of the fold, azimuthal coverage and distribution
of source-receiver distance at any location. Balances need to be struck between the
amount of equipment available and the need to continually pick up and move cables
and receivers. Usually in land acquisition it is not economic to get the simple uniform
geometry of marine operations and often there is a small overprint of the acquisition
design in the final processed data.
In areas that are difficult for vehicle access (e.g. mountains, forests), an explosive
source is used. Usually the area covered per day is not as great as achieved by vibrator
trucks. Holes need to be drilled into the earth to ensure the explosive charges are well
coupled to bedrock and not fired in the shallower weathered layers, which would cause
excessive amounts of noise in the data. This reduces the acquisition rates to somewhere
between 50 and 100 shotpoints per day. The lack of vehicles may mean the majority
of movement of equipment has to be by helicopter, an important issue for safety.
Another issue is the amount of time that may have to be spent cutting lines in forested
areas.
Whatever type of land source is used, the entire survey needs to be corrected for
the arrival time changes due to both topography and variations in the thickness of
the near-surface layer
(fig. 2.20)
. The near surface is generally heavily weathered and
usually has altered acoustic properties compared with the less affected deeper layers.
This generally results in a much slower velocity, but in areas of permafrost the near
surface may be substantially faster. Usually a separate crew is responsible for drilling
and measuring uphole times (the time required to travel from the bottom of the drilled
hole to the surface) throughout the area. These are then used to determine the depth
to the base of the weathered layer, and the velocities in the weathered and bedrock
layers from which the static corrections can be derived. Sheriff & Geldart (
1995
) give
an introduction to these corrections. It is impossible and uneconomic to drill holes
at every shot and geophone location, so first arrival times from the field records are
also used to estimate static corrections. In some circumstances, special refraction pro-
file crews are used to determine the near-surface velocity profile. When calculating
the statics to be applied, it is important to ensure consistency across the area. The
large redundancy in 3-D data means that there is often conflicting information about
the statics required at any one location and special software is used to generate the
corrections.
