Seismic reflection surveying - An Introduction to Geophysical Exploration - page 67

the removal of ground roll from shot gathers. This
leads to marked improvement in the subsequent stacking
process, facilitating better estimation of stacking veloci-
ties and better suppression of multiples.Velocity filtering
can also be applied to portions of seismic record sections,
rather than individual shot gathers, in order to suppress
coherent noise events evident because of their anom-
alous dip, such as diffraction patterns. An example of
such velocity filtering is shown in Fig. 4.26.
It may be noted that individual detector arrays operate
selectively on seismic arrivals according to their apparent
velocity across the array (Section 4.4.2), and therefore
function as simple velocity filters at the data acquisition
stage.

migrated sections into reflector depths, using one-way
reflection times multiplied by the appropriate velocity,
yields a reflector geometry known as the record surface .
This coincides with the actual reflector surface only when
the latter is horizontal. In the case of dipping reflectors
the record surface departs from the reflector surface; that
is, it gives a distorted picture of the reflector geometry.
Migration removes the distorting effects of dipping
reflectors from seismic sections and their associated
record surfaces. Migration also removes the diffracted
arrivals resulting from point sources since every dif-
fracted arrival is migrated back to the position of the
point source. A variety of geological structures and
sources of diffraction are illustrated in Fig. 4.27(a) and
the resultant non-migrated seismic section is shown in
Fig. 4.27(b). Structural distortion in the non-migrated
section (and record surfaces derived from it) includes a
broadening of anticlines and a narrowing of synclines.
The edges of fault blocks act as point sources and typical-
ly give rise to strong diffracted phases, represented by hy-
perbolic patterns of events in the seismic section.
Synclines within which the reflector curvature exceeds
the curvature of the incident wavefront are represented
on non-migrated seismic sections by a 'bow-tie' event
resulting from the existence of three discrete reflection
points for any surface location (see Fig. 4.28).
Various aspects of migration are discussed below using
the simplifying assumption that the source and detector
have a common surface position (i.e. the detector has a
zero offset, which is approximately the situation
involved in CMP stacks). In such a case, the incident and
reflected rays follow the same path and the rays are
normally incident on the reflector surface. Consider a
source-detector on the surface of a medium of constant
seismic velocity (Fig. 4.29). Any reflection event is
conventionally mapped to lie directly beneath the
source-detector but in fact it may lie anywhere on the
locus of equal two-way reflection times, which is a
semi-circle centred on the source-detector position.
Now consider a series of source-detector positions
overlying a planar dipping reflector beneath a medium of
uniform velocity (Fig. 4.30). The reflection events are
mapped to lie below each source-detector location
but the actual reflection points are offset in the updip
direction. The construction of arcs of circles (wavefront
segments) through all the mapped reflection points
enables the actual reflector geometry to be mapped.This
represents a simple example of migration.The migrated
section indicates a steeper reflector dip than the record
surface derived from the non-migrated section. In gen-

4.9 Migration of reflection data
On seismic sections such as that illustrated in Fig. 4.22
each reflection event is mapped directly beneath the
mid-point of the appropriate CMP gather. However,
the reflection point is located beneath the mid-point
only if the reflector is horizontal. In the presence of a
component of dip along the survey line the actual reflec-
tion point is displaced in the up-dip direction; in the
presence of a component of dip across the survey line
(cross-dip) the reflection point is displaced out of the
plane of the section. Migration is the process of recon-
structing a seismic section so that reflection events are
repositioned under their correct surface location and at
a corrected vertical reflection time. Migration also
improves the resolution of seismic sections by focusing
energy spread over a Fresnel zone and by collapsing
diffraction patterns produced by point reflectors and
faulted beds. In time migration , the migrated seismic
sections still have time as the vertical dimension. In
depth migration , the migrated reflection times are con-
verted into reflector depths using appropriate velocity
information.
Two-dimensional survey data provide no information
on cross-dip and, hence, in the migration of two-
dimensional data the migrated reflection points are con-
strained to lie within the plane of the section. In the
presence of cross-dip, this two-dimensional migration is
clearly an imperfect process. Its inability to deal with
effects of cross-dip mean that, even when the seismic line
is along the geological strike, migration will be imperfect
since the true reflection points are themselves out of the
vertical section.
The conversion of reflection times recorded on non-