Geology Reference
In-Depth Information
Fig. 4.33
(a) A non-migrated seismic
section. (b) The same seismic section
after diffraction migration. (Courtesy
Prakla-Seismos GmbH.)
stricted to rays that have travelled in a single vertical
plane. In a three-dimensional survey, the disposition of
shots and receivers is such that groups of recorded ar-
rivals can be assembled that represent rays reflected from
an area of each reflecting interface. Three-dimensional
surveying therefore samples a volume of the subsurface
rather than an area contained in a vertical plane, as in
two-dimensional surveying.
In three-dimensional surveying the common mid-
point principle applies similarly, but each CMP gather
involves an areal rather than a linear distribution of shot
points and detector locations (Fig. 4.34). Thus, for ex-
ample, a 20-fold coverage is obtained in a crossed-array
three-dimensional survey if reflected ray paths from five
shots along different shot lines to four detectors along
different recording lines all have a common reflection
point.
On land, three-dimensional data are normally col-
lected using the
crossed-array method
in which shots and
detectors are distributed along orthogonal sets of lines
(in-lines and cross-lines) to establish a grid of recording
points. For a single pair of lines, the areal coverage of a
subsurface reflector is illustrated in Fig. 4.35.
At sea, three-dimensional data may be collected along
closely-spaced parallel tracks with the hydrophone
streamer feathered to tow obliquely to the ship's track
such that it sweeps across a swathe of the sea floor as
the vessel proceeds along its track. By ensuring that the
swathes associated with adjacent tracks overlap, data may
be assembled to provide areal coverage of subsurface re-
flectors. In the alternative
dual source array method
, sources
are deployed on side gantries to port and starboard of the
hydrophone streamer and fired alternately (Fig. 4.36).
Multiple streamers may similarly be deployed to obtain
both a wider swath and a denser fold of three-
dimensional data.
High-quality position fixing is a prerequisite of three-
dimensional marine surveys in order that the locations of
