INTERPRETATION OF SEISMIC REFLECTION DATA

INTRODUCTION

Differing procedures are adopted for the interpretation of two- and three-dimensional (2-D & 3-
D) seismic data. The results of 2-D surveys are presented to the seismic interpreter as non-
migrated and migrated seismic sections, from which the geological information is extracted by
suitable analysis of the pattern of reflection events. Interpretations are correlated from line to
line, and the reflection times of picked events are compared directly at profile intersections.
There are two main approaches to the interpretation of seismic sections: structural analysis,
which is the study of reflector geometry on the basis of reflection times, and stratigraphical
analysis (or seismic stratigraphy), which is the analysis of reflection sequences as the seismic
expression of lithologically-distinct depositional sequences. Both structural and stratigraphical
analyses are greatly assisted by seismic modeling, in which theoretical (synthetic) seismograms
are constructed for layered models in order to derive insight into the physical significance of
reflection events contained in seismic sections.

In the interpretation of 3-D survey data, the interpreter has direct access at a computer work
station to all the reflection data contained within the seismic data volume, and is able to select
various types of data for colour display, for example vertical sections or horizontal sections
(time slices) through the data volume. The two most important shortcomings of two-dimensional
interpretation are the problem of correlation between adjacent profile lines and the inaccuracy of
reflector positioning due to the limitations of two-dimensional migration. The improved
coverage and resolution of three-dimensional data often lead to substantial improvements in
interpretation as compared with pre-existing two-dimensional interpretation.

Structural Analysis

The main application of structural analysis of seismic sections is in the search for structural traps
containing hydrocarbons. Interpretation usually takes place against a background of continuing
exploration activity and an associated increase in the amount of information related to the
subsurface geology. Reflection events of interest are usually colour-coded initially and labelled
as, for example ‘red reflector’, ‘blue reflector’, until their geological significance is established.

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Subsurface reflectors may then be referred to by an appropriate stratigraphical indicator such as
‘base Tertiary’,‘top Lias’.

Most structural interpretation is carried out in units of two-way reflection time rather than depth,
and time structure maps are constructed to display the geometry of selected reflection events by
means of contours of equal reflection time (Fig. 1). Structural contour maps can be produced
from time-structure maps by conversion of reflection times into depths using appropriate velocity
(e.g. local stacking velocities derived from the reflection survey or sonic log data from
boreholes). Time-structure maps obviously bear a close similarity to structural contour maps but
are subject to distortion associated with lateral or vertical changes of velocity in the subsurface
interval overlying the reflector. Other aspects of structure may be revealed by contouring
variations in the reflection time interval between two reflectors, sometimes referred to as
isochron maps, and these can be converted into isopach maps by the conversion of reflection
time intervals into thicknesses using the appropriate interval velocity.

Fig. 1: Time-structure map of reflector

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Structural interpretation of three-dimensional data is able to take advantage of the areal coverage
of reflection 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 geological structures using three-dimensional data volumes
are illustrated in Plates 1 and 2. Interpretation of three dimensional data is often crucial to the
successful development of oilfields with a complex geological structure.

Plate 1: Three-dimensional data volume

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Plate 2: Seiscrop section

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 sedimentary sequences. The
principles behind this seismic sequence analysis are two-fold. Firstly, reflections are taken to
define chronostratigraphical units, since the types of rock interface that produce reflections are
stratal surfaces and unconformities; by contrast, the boundaries of diachronous lithological units
tend to be transitional and not to produce reflections. Secondly, genetically related sedimentary
sequences normally comprise a set of concordant strata that exhibit discordance with underlying
and overlying sequences; that is, they are typically bounded by angular unconformities variously
representing onlap, downlap, toplap or erosion (Fig. 2). A seismic sequence is the representation
on a seismic section 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 3.

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Fig. 2: Different types of geological boundary defining seismic sequences. (After Sheriff 1980)

Plate 3: A seismic section from the northern Amadeus basin, central Australia, illustrating a
depositional sequence bounded by major uncoinformities. (Reproduced from AAPG MemoirNo.
39, with the permission of the publishers.)

lithofacies that may be represented within it. This use of reflection geometry and character to
interpret sedimentary facies is known as seismic facies analysis. Individual seismic facies are
identified within the seismic sequence illustrated in Plate 3. Different types of reflection
configuration (Fig. 3) are diagnostic of different sedimentary environments. An example of
seismic stratigraphy based on threedimensional data is illustrated in Plate 4. The seiscrop of Plate
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4(a) shows a meandering stream channel preserved in a Neogene sedimentary sequence in the
Gulf of Thailand. The channel geometry and the distinctive lithofacies of the

Fig. 3: Various internal bedforms that give rise to different seismic facies within sedimentary
sequences identified on seismic sections. (After Sheriff 1980.)

Plate 4(a): Seiscrop section at 196 ms from a three-dimensional survey in the Gulf of Thailand
area, showing a meandering stream channel.

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channel fill lead to its clear identification as a distinctive seismic facies. Use of such seiscrops
over a wider area enables the regional mapping of a Neogene deltaic environment (Plate 4(b)).

Plate 4(b) Diagrammatic map of a former prograding delta system within the Gulf of Thailand
survey area, based on interpretation of seiscrop sections 1, 2 and 3 shown on map. (Both
illustrations reproduced from AAPG MemoirNo. 42, with the permission of the publishers.)

Hydrocarbon accumulations are sometimes revealed directly on true-amplitude seismic sections

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Fig. 4: Part of a true-amplitude seismic section containing a seismic bright spot associated with a
local hydrocarbon accumulation. (From Sheriff 1980, after Schramm et al. 1977.)

Seismic Modeling

Reflection amplitudes may be normalized prior to their presentation on seismic sections so that
original distinctions between weak and strong reflections are suppressed. This practice tends to
increase the continuity of reflection events across a section and therefore aids their identification
and structural mapping. However, much valuable geological information is contained in the true
amplitude of a reflection event, which can be recovered from suitably calibrated field recordings.
Any lateral variation of reflection amplitude is due to lateral change in the lithology of a rock
layer or in its pore fluid content. Thus, whilst the production of normalized-amplitude sections
may assist structural mapping of reflectors, it suppresses information that is vital to a full
stratigraphic interpretation of the data. With increasing interest centring on stratigraphic
interpretation, true-amplitude seismic sections are becoming increasingly important.

Synthetic seismograms and synthetic seismic sections can be compared with observed data, and
models can be manipulated in order to simulate the observed data. By this means, valuable
insights can be obtained into the subsurface geology responsible for a particular seismic section.
The standard type of synthetic seismogram represents the seismic response to vertical
propagation of an assumed source wavelet through a model of the subsurface composed of a

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series of horizontal layers of differing acoustic impedance. The synthetic seismogram comprises
the sum of the individual reflections in their correct travel-time relationships (Fig. 5).

Fig. 5: The synthetic seismogram

Seismic Attribute Analysis

Conventional seismic reflection sections are displayed in variable-area format where positive
half-cycles of the waveforms of seismic traces are filled in black. This has the desirable effect of
merging the shaded areas from trace to trace to form continuous black lines across the section.
These black lines guide the eye of the interpreter to correlate features across the section, and
hence make a structural interpretation. The undesirable effect of this display is that the precise
amplitude and shape of the waveform, which has been the subject of so much effort during data
acquisition and processing, is lost. The amplitude of a normally reflected wave is directly related
to the reflection coefficient at the interface, and hence the physical properties (density and
velocity) of the Formations. Thus, variations in amplitude along a reflector should indicate
changes in the properties of the Formations.

These properties can be viewed by presenting an image of the seismic section where the
amplitude of the seismic wave is displayed as a colour scale. Changes of amplitude along a
continuous reflector will then be emphasized by the colour change, rather than hidden in a broad

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black line. Such amplitude changes may be related to changes in the pore fluid in the rocks, and
in favourable circumstances can be direct hydrocarbon indicators (DHIs). Amplitude is merely
the simplest example of a property (attribute) of the seismic wave which can be examined for its
geological significance. Others include the seismic wave phase and the frequency content. From
the waveform amplitudes the acoustic impedance of each formation can be estimated, and if S-
wave data are available Poisson’s ratio can be found. On a yet more detailed level, the amplitude
variation of reflected wavelets with source–receiver offset (AVO) within each CMP gather can
be analysed. This AVO effect can be particularly diagnostic in distinguishing between amplitude
effects due to rock matrix variation and those due to pore fluids. An excellent review of this
complex subject is given in Castagna and Bachus (1993).

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