Geology Reference
In-Depth Information
Shot - detector
midpoint
(a)
S
4
S
3
S
2
S
1
CMP
D
1
D
2
D
3
D
4
CDP
(b)
S
4
S
3
S
2
S
1
CMP
D
1
D
2
D
3
D
4
Fig. 4.9
Common mid-point (CMP)
reflection profiling. (a) A set of rays from
different shots to detectors reflected off a
common depth point (CDP) on a
horizontal reflector. (b) The common
depth point is not achieved in the case of a
dipping reflector.
random and coherent noise. Combining all the traces in
a CMP together will average out the noise, and increase
the signal-to-noise ratio (SNR). This process is termed
stacking
.
Strictly, the common mid-point principle breaks
down in the presence of dip because the common depth
point then no longer directly underlies the shot-
detector mid-point and the reflection point differs for
rays travelling to different offsets (see Fig. 4.9(b)). Never-
theless, the method is sufficiently robust that CMP
stacks almost invariably result in marked improvements
in SNR compared to single traces.
In two-dimensional CMP surveying, known as
CMP
profiling
, the reflection points are all assumed to lie
within the vertical section containing the survey line;
in three-dimensional surveying, the reflection points
are distributed across an area of any subsurface reflector,
and the CMP is defined as a limited area on the surface.
offset distances from a shot point. As discussed in Chap-
ter 3, this requirement is complicated in practice by the
fact that the reflected pulses are never the first arrivals of
seismic energy, and they are generally of very low ampli-
tude. Owing to this and other problems to be discussed
later, each individual reflection survey is designed
specifically to optimize the data for the required pur-
pose. It is essential that the geologists and interpreting
geophysicists commissioning a survey understand this,
and communicate their requirements to the geophysical
contractor performing the survey.
In
two-dimensional surveys
(
reflection profiling
), data are
collected along survey lines that nominally contain all
shot points and receivers. For the purpose of data pro-
cessing, reflected ray paths are assumed to lie in the verti-
cal plane containing the survey line. Thus, in the
presence of cross-dip the resultant seismic sections do
not provide a true representation of the subsurface struc-
ture, since actual reflection points then lie outside the
vertical plane.Two-dimensional survey methods are ad-
equate for the mapping of structures (such as cylindrical
folds, or faults) which maintain uniform geometry
along strike.They may also be used to investigate three-
dimensional structures by mapping lateral changes
4.4 Multichannel reflection survey design
The basic requirement of a multichannel reflection sur-
vey is to obtain recordings of reflected pulses at several
