Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 369

Short-period multiples have paths that are only slightly
longer than their primaries, caused by reverberations in
thin layers or by a ghost re ection of the source at the
ground surface. Their travel times are only slightly greater
than that of the primaries to the extent that only one
(longer-wavelength) arrival is recognisable (see Section
6.3.2 ) , which reduces the resolution of the survey data
(see Section 6.7.1 ).

Three-dimensional surveys
When a 3D survey is conducted on land, ideally the
spread comprises parallel detector-lines and sources
located along a perpendicular line, as shown in Fig.
6.17a . Coverage comprises a grid of re ection points. As
shown in Fig. 6.17b and c , recording using various com-
binations of detector and source positions allows repeated
recording of re ections from the same points in the
subsurface.

6.5.1.2 Survey design
A key objective of reflection data acquisition is to create a
uniform coverage of subsurface reflection points across the
area of interest. It is normal to assume that the interfaces
reflecting the seismic waves are horizontal, in which case
the surface projection of the point of re

6.5.2 Data processing

The three operations fundamental to the processing of
seismic reflection data are stacking, deconvolution and
migration. All other processing operations are designed
to improve the effectiveness of these three operations,
either by attenuating noise or by making the data better
conform to the assumptions inherent in the operations.
Deconvolution is a means of manipulating the charac-
teristics of the signal wavelet, in particular reversing some
of the detrimental consequences of its passage through the
subsurface. Stacking combines traces recorded at

ection is halfway
(the midpoint) between source and detector (as can be seen
in Fig. 6.15 ) . Recall that to enable stacking, data acquisition
is designed so that re ections from the same point in the
subsurface are repeatedly recorded.

Two-dimensional surveys
For 2D surveys, recordings are made using a linear spread
along one or more traverses, ideally forming a regular dip-
and strike-line (grid) network of perpendicular lines. An
approximate grid arrangement may be all that is possible
in areas of limited access, often dictated by the network of
roads and tracks in the survey area. For a straight line
traverse and an end-on spread source, the reflection points
form a straight line and are spaced at half the distance
between the individual detectors. Figure 6.16 illustrates 2D
seismic profiling using an end source and six detectors
(A

finite

source detector offsets into a much smaller number of

equivalent zero-offset recordings and in so doing improves
the signal-to-noise ratios. This means that post-stack pro-
cessing operations are applied to the greatly reduced
number of traces, offering considerable savings on com-
puter resources compared with pre-stack operations. How-
ever, stacking makes important assumptions about the
nature of the subsurface and if these are significantly vio-
lated the data may be degraded. In these cases pre-stack
processing operations are preferable. Migration is a process
that moves features in the seismic section so that they are
in their correct relative positions, and the section better
resembles the subsurface geology.

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F). In reality, the number of detectors would be much
larger. For simplicity, it is assumed that the subsurface
comprises two layers, both having constant velocity but
different acoustic impedance (

-

), and separated by a planar
horizontal interface. Only re ected arrivals, with their
characteristic curvilinear moveout (see Section 6.5.1.1 ) ,
are shown in the shot gathers, which are from eight spread
locations. Note how re ections from the three coloured
reference points on the interface are repeatedly recorded as
the spread is moved, i.e. re ections at the red location are
recorded by receiver F from source 1, by receiver E from
source 2 and so on. The number of occasions when re ec-
tions are recorded from the same point on the interface is
known as the fold of the stack. There are six reflections
from the red location so the fold of the stack is six in this
case, also described as 600%.

ζ

6.5.2.1 Common processing operations
Before describing the three fundamental processing oper-
ations we brie y summarise some of the other commonly
used processing methods. As with any type of data
enhancement, noise suppression involves identifying some
property of the noise that differs from the signal. In seismic
processing, properties that are exploited include random-
ness/coherence, frequency, moveout and periodicity. For
example, frequency filtering is routinely applied to remove
low-frequency surface waves, as is shown in Fig. 2.26 . The
different velocities of the different types of waves, causes