Geology Reference
In-Depth Information
reflector, the survey coverage is said to be
single-fold
.
Each seismic trace then represents a unique sampling of
some point on the reflector. In common mid-point
(CMP) profiling, which has become the standard
method of two-dimensional multichannel seismic sur-
veying, it is arranged that a set of traces recorded at dif-
ferent offsets contains reflections from a common depth
point (CDP) on the reflector (Fig. 4.14).
The
fold
of the stacking refers to the number of traces
in the CMP gather and may conventionally be 24, 30, 60
or, exceptionally, over 1000.The fold is alternatively ex-
pressed as a percentage: single-fold = 100% coverage,
six-fold = 600% coverage and so on.The fold of a CMP
profile is determined by the quantity
N
/2
n
, where
N
is
the number of geophone arrays along a spread and
n
is
the number of geophone array spacings by which the
spread is moved forward between shots (the
move-up
rate
). Thus with a 96-channel spread (
N
= 96) and a
move-up rate of 8 array spacings per shot interval
(
n
= 8), the coverage would be 96/16 = 6-fold. A field
procedure for the routine collection of six-fold CMP
coverage using a single-ended 12-channel spread
configuration progressively moved forward along a
profile line is shown in Fig. 4.14.
The theoretical improvement in SNR brought about
by stacking
n
traces containing a mixture of coherent in-
phase signals and random (incoherent) noise is .
Stacking also attenuates long-path multiples. They have
travelled in nearer-surface, lower velocity layers and have
a significantly different moveout from the primary re-
flections. When the traces are stacked with the correct
velocity function, the multiples are not in phase and do
not sum. The stacked trace is the equivalent of a trace
recorded with a vertical ray path, and is often referred to
as a
zero-offset
trace.
D
2
S
1
Path 1
D
4
S
2
Path 2
D
6
S
3
Path 3
D
8
S
4
Path 4
D
10
S
5
Path 5
D
12
S
6
Path 6
n
Common
depth (reflection)
point
Fig. 4.14
A field procedure for obtaining six-fold CDP coverage
with a single-ended 12-channel detector spread moved
progressively along the survey line.
4.5 Time corrections applied to seismic traces
Two main types of correction need to be applied to re-
flection times on individual seismic traces in order that
the resultant seismic sections give a true representation
of geological structure. These are the
static
and
dynamic
corrections, so-called because the former is a fixed time
correction applied to an entire trace whereas the latter
varies as a function of reflection time.
4.4.4 Display of seismic reflection data
Profiling data from two-dimensional surveys are con-
ventionally displayed as seismic sections in which the in-
dividual stacked
zero-offset
traces are plotted side by side,
in close proximity, with their time axes arranged verti-
cally. Reflection events may then be traced across the
section by correlating pulses from trace to trace and in
this way the distribution of subsurface reflectors beneath
the survey line may be mapped. However, whilst it is
tempting to envisage seismic sections as straightforward
images of geological cross-sections it must not be forgot-
ten that the vertical dimension of the sections is time, not
depth.
4.6 Static correction
All previous consideration in this chapter on reflected
seismic traces has assumed that the source and detector
