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points, the improved resolution associated with three-
dimensional migration and the improved methods of
data access, analysis and display provided by dedicated
seismic work stations. Examples of the display of geolog-
ical structures using three-dimensional data volumes are
illustrated in Plates 4.1 and 4.2. Interpretation of three-
dimensional data is often crucial to the successful devel-
opment of oilfields with a complex geological structure.
An example is the North Cormorant oilfield in the UK
Sector of the North Sea, where three-dimensional seis-
mics enabled the mapping of far more fault structures
than had been possible using pre-existing two-
dimensional data, and revealed a set of NW-SE trending
faults that had previously been unsuspected.
4.14.2 Stratigraphical analysis
(seismic stratigraphy)
Seismic stratigraphy involves the subdivision of seismic
sections into sequences of reflections that are interpreted
as the seismic expression of genetically related sedimen-
tary sequences.The principles behind this seismic sequence
analysis are two-fold. Firstly, reflections are taken to de-
fine chronostratigraphical units, since the types of rock
interface that produce reflections are stratal surfaces and
unconformities; by contrast, the boundaries of diachro-
nous lithological units tend to be transitional and not
to produce reflections. Secondly, genetically related
sedimentary sequences normally comprise a set of con-
cordant strata that exhibit discordance with underlying
and overlying sequences; that is, they are typically
bounded by angular unconformities variously repre-
senting onlap, downlap, toplap or erosion (Fig. 4.48). A
seismic sequence is the representation on a seismic sec-
tion of a depositional sequence; as such, it is a group of
concordant or near-concordant reflection events that
terminate against the discordant reflections of adjacent
seismic sequences. An example of a seismic sequence
identified on a seismic section is illustrated in Plate 4.3.
Having subdivided a seismic section into its con-
stituent sequences, each sequence may be analysed in
terms of the internal disposition of reflection events
and their character, to obtain insight into the deposition-
al environments responsible for the sequence and into
the range of lithofacies that may be represented within
it.This use of reflection geometry and character to inter-
pret sedimentary facies is known as seismic facies analysis .
Individual seismic facies are identified within the seismic
sequence illustrated in Plate 4.3. Different types of
reflection configuration (Fig. 4.49) are diagnostic of dif-
ferent sedimentary environments. On a regional scale,
Fig. 4.46 Corridor stack of the zero-offset VSP section (Fig.
4.45(c)) reproduced eight times and spliced into a conventional
seismic section based on surface profiling data from the vicinity of
the borehole site. Comparison of the VSP stack with the surface
recorded data enables the primary events in the seismic section to
be reliably distinguished from multiple events. (From Cassell
1984.)
closed loop of survey lines reveals any errors in the iden-
tification or correlation of a reflection event across the
area of a seismic survey.
Reprocessing of data, or migration, may be employed
to help resolve uncertainties of interpretation, but addi-
tional seismic lines are often needed to resolve problems
associated with an initial phase of interpretation. It is
common for several rounds of seismic exploration to be
necessary before a prospective structure is sufficiently
well defined to locate the optimal position of an explo-
ration borehole.
Structural interpretation of three-dimensional data is
able to take advantage of the areal coverage of reflection
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