Principles of seismic data interpretation m.m.badawy

M.M.Badawy
Principles of Seismic Data
Interpretation

Principles of Seismic Data Interpretation
M.M.Badawy
Page2
Principles of Seismic Data
Interpretation
Mahmoud Mostafa Badawy
Lecturer Assistant of Geophysics, Geology Department, Faculty of Science,
Alexandria University, Egypt

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Contents:
Fundamentals:
 Brief summary on seismic acquisition and processing
 Seismic Response
 Phase & Wavelet
 Polarity.
 Reflections.
 Reflection Coefficient
 Convolution Theorem
 Seismic resolution.
 Basic concept of seismic exploration
 Seismic events
 2D vs. 3D data
 Colour, display and 3-D visualization
Structural Interpretation Pitfalls:
 Pitfall Statics Busts
 Pitfall Fault Shadow
 Pitfall Multiplies
Seismic Interpretation workflows:
 Building Project Database
 Time Depth Relationships
 Synthetic Seismograms
 Check Shot
 VSP

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Structural Styles & Structural Interpretation:
 Normal faults:
 PLANARS
 LISTRIC
 Reverse or Thrust faults:
 Fault-bend fold
 Fault-propagation fold
 Inversion Structures
 Strike slip faults
Depth Conversion:
 Overview of seismic velocities.
 Time-to-depth conversion
Subsurface Mapping Techniques:
 Subsurface Structure Mapping
 Fault Polygon Definition
 The Mapping Process

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Fundamentals:
What Makes A Wiggle?
Seismic reflection profiling is an echo sounding technique. A controlled sound pulse is issued
into the Earth and the recording system listens a fixed time for energy reflected back from
interfaces within the Earth. The interface is often a geological boundary, for example the change
of sandstone to limestone.
Once the travel-time to the reflectors and the velocity of propagation is known, the geometry of
the reflecting interfaces can be reconstructed and interpreted in terms of geological structure in
depth. The principal purpose of seismic surveying is to help understand geological structure and
stratigraphy at depth and in the oil industry is ultimately used to reduce the risk of drilling dry
wells.
Wave is a disturbance which
travels in the medium or
without.

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What Is A Reflection?
 The following figure shows a simple earth model and resulting seismic section used to
illustrate the basic concepts of the method.
 The terms source, receiver and reflecting interface are introduced. Sound energy travels
through different media (rocks) at different velocities and is reflected at interfaces where
the media velocity and/or density changes.
 The amplitude and polarity of the reflection is proportional to the acoustic impedance
(product of velocity and density) change across an interface. The arrival of energy at the
receiver is termed a seismic event.
 A seismic trace records the events and is conventionally plotted below the receiver with
the time (or depth axis)
Snell's Law
The mathematical description of refraction or the physical change in the direction of a wave
front as it travels from one medium to another with a change in velocity and partial conversion
and reflection of a P-wave to an S-wave at the interface of the two media.
Snell's law, one of two laws describing refraction, was formulated in the context of light waves,
but is applicable to seismic waves. It is named for Willebrord Snel (1580 to 1626), a Dutch
mathematician.
Snell's law can be written as:

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Wave Propagation
For small deformations rocks are elastic, which is they return to their original shape once a small
stress applied to deform them is removed. Seismic waves are elastic waves and are the
"disturbances" which propagate through the rocks.
The most commonly used form of seismic wave is the P (primary)-wave which travels as a
series of compressions and rarefactions through the earth the particle motion being in the
direction of wave travel. The propagation of P-waves can be represented as a series of wave
fronts (lines of equal phase) which describe circles for a point source in a homogeneous media
(similar to when a stone is dropped vertically onto a calm water surface). As the wave front
expands the energy is spread over a wider area and the amplitude decays with distance from the
source.
This decay is called spherical or geometric divergence and is usually compensated for in seismic
processing. Rays are normal to the wave fronts and diagrammatically indicate the direction of
wave propagation. Usually the shortest ray-path is the direction of interest and is chosen for
clarity. Secondary or S waves travel at up to 70% of the velocity of P-waves and do not travel
through fluids.
The particle motion for an S-wave is perpendicular to its direction of propagation (shear stresses
are introduced) and the motion is usually resolved into a horizontal component (SH waves) and
a vertical component (SV waves).
Reflection: The energy or wave from a seismic source which has been reflected from an
acoustic impedance contrast (reflector) or a series of contrasts within the earth.
Refraction: The change in direction of a seismic ray upon passing into a medium with a
different velocity. The mathematics of this is defined by Snell’s law.

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Reflection Coefficient:
(The ratio of amplitude of the reflected wave to the incident wave, or how much energy is
reflected). If the wave has normal incidence, then its reflection coefficient can be expressed as:
If the A.I of the lower formation is higher than the upper one, the reflection polarity will be
+ve and vice versa.
If the difference in A.I between the two formations is high, the reflection magnitude
(Amplitude) will be high.

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Tape Formats:
Several tape formats defined by the SEG are currently in use. These standards are often treated
quite liberally, especially where 3D data is concerned. Most contractors also process data using
their own internal formats which are generally more efficient than the SEG standards.
The two commonest formats are SEG-D (for field data) and SEG-Y for final or intermediate
products.
The previous figure shows the typical way in which a seismic trace is stored on tape for SEG-Y
format.
The use of headers is particularly important since these headers are used in seismic processing to
manipulate the seismic data. Older multiplexed formats (data acquired in channel order) such as
SEG-B would typically be demultiplexed (in shot order) and transcribed to SEG-Y before
processing.
In SEG-Y format a 3200 byte EBCDIC (Extended Binary Coded Decimal Interchange Code)
"text" header arranged as forty 80 character images is followed by a 400 byte binary header
which contains general information about the data such as number of samples per trace. This is
followed by the 240 byte trace header (which contains important information related to the trace
such as shot point number, trace number) and the trace data itself stored as IBM floating point
numbers in 32 byte format.
The trace, or a series of traces such as a shot gather, will be terminated by an EOF (End of File)
marker. The tape is terminated by an EOM (End of Media) marker. Several lines may be
concatenated on tape separated by two EOF markers (double end of file). Separate lines should
have their own EBCIDC headers, although this may be stripped out (particularly for 3D
archives) for efficiency. Each trace must have its own 240 byte trace header. Note there are
considerable variations in the details of the SEG-Y format.

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Processing Concept:
The purpose of seismic processing is to manipulate the acquired data into an image that can be
used to infer the sub-surface structure. Only minimal processing would be required if we had a
perfect acquisition system.
Processing consists of the application of a series of computer routines to the acquired data
guided by the hand of the processing geophysicist. There is no single "correct" processing
sequence for a given volume of data.
At several stages judgments or interpretations have to be made which are often subjective and
rely on the processors experience or bias. The interpreter should be involved at all stages to
check that processing decisions do not radically alter the interpretability of the results in a
detrimental manner.
Processing routines generally fall into one of the following categories:
 enhancing signal at the expense of noise
 providing velocity information
 collapsing diffractions and placing dipping events in their true subsurface locations
(migration)
 increasing resolution (wavelet processing)

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A Processing Flow:
Processing flow is a collection of processing routines applied to a data volume. The processor
will typically construct several jobs which string certain processing routines together in a
sequential manner.
Most processing routines accept input data, apply a process to it and produce output data which
is saved to disk or tape before passing through to the next processing stage. Several of the stages
will be strongly interdependent and each of the processing routines will require several
parameters some of which may be defaulted.
Some of the parameters will be defined, for example by the acquisition geometry and some must
be determined for the particular data being processed by the process of testing.
Factors which Affect Amplitudes

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New Data:
 Tape containing recorded seismic data (trace sequential or multiplexed)
 Observer logs/reports
 Field Geophysicist logs/reports and listings
 Navigation/survey data
 Field Q.C. displays
 Contractual requirements
Simple Processing Sequence Flow:
 Reformat
 Geometry Definition
 Field Static Corrections (Land - Shallow Water - Transition Zone)
 Amplitude Recovery
 Noise Attenuation (De-Noise)
 Deconvolution
 CMP Gather
 NMO Correction
 De-multiple (Marine)
 Migration
 CMP Stack

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The Seismic Method:
(Use acoustic waves (sound) to image the subsurface)
Measure
 Time for sound to get from surface to subsurface reflectors and back - Two-
way traveltime (twt)
 Amplitude of reflection
Wanted:
 Depth - Need to know subsurface velocities
 Rock properties (porosity, saturation, etc.)
Spherical Divergence:
 Due to the nature propagation of the energy on the shape of wave fronts, and with
increasing of the diameter of these waves, the energy decays through time so we
have to compensate this decay.
 The surface area of a sphere is proportional to the square of its radius so the energy
lost due to spherical divergence is proportional to 1/r2.

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Direct Waves:
They are source- generated due to the direct travel of these waves from the source to the
receiver and they are dominant in near offsets. They can be attenuated by normal move
out, muting and stacking.
Refraction:
They are generated by critically refracted waves from the near surface layers. They are
dominant in the far offsets. They can be attenuated by NMO, muting and stacking.
Ground Roll:
 It is a source noise coming from propagation of waves in particles of near surface
layers without net movement. It is dominant in the upper part from the data and
interfered with direct waves and refracted waves.
 Its characteristics: (low velocity, low frequency and high amplitude).
 It could be attenuated by F-K filter or Tau-p filter.
Zero phasing:
 It is a process that can be applied at the first steps or at the last but it is preferred to
be at first.
 Zero phases: (the maximum amplitude is at zero time).
 Zero phases is a mathematically solution but we can be close to it using vibroseis.
 Minimum phase: (maximum amplitude at minimum time, we can obtain it
with dynamite).
 Maximum phases: (maximum amplitude at maximum time).
 Mixed phases: (it is a mixed phase in between minimum phase and maximum
phase and we can get it with air gun).

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 Zero phasing is a process by which we can modify the position of peaks and
troughs to be at the reflector position instead of being above or below its real
position for facilitating the interpretation process.
To make zero phasing we should make:
1-Model source 2-cross correlation
 For air gun we get the source signature from the contractor.
 Then using software we determine the distance between the maximum amplitude
and zero time then we make shift toward zero time by a distance equal it from zero
time to max amplitude.
 And we can attenuate the bubble effect by designing the wavelet before shifting. And
by this step we designed a filter that we multiply it with the source signature to
ensure that the result is a zero phase signature. And then we apply this filter on
seismic data using cross correlation.
Source modeling can be for dynamite using the
charge, whole depth and the recorder model.
We also determine the polarity of the traces
either it is normal or reverse. For vibroseise
we don't do that step.

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Exercise 1:

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 These wavelets all have similar frequency content, but have different phase. The
ideal wavelet from the interpreter’s point of view is ‘zero phases’. In a zero phase
wavelet, each frequency component is lined up so that the wavelet is symmetrical.
This creates the shortest possible wavelet, and the main peak is aligned at the time
corresponding to the travel time to the reflector, facilitating correlation between
seismic data and geology.
 One aim of processing is to bring the data to zero phases. This is best done by
careful control of all the processes through stack and migration, followed by
calibration against one or, preferably, several wells. In the absence of well data, it is
possible to use a strong isolated reflector, such as a hard water bottom, chalk, or top
salt reflector, or calibrate against another seismic dataset of known phase.
 Explosive source data, such as marine air gun or dynamite, is close to minimum
phase when acquired. For a given frequency content, the minimum phase wavelet is
the wavelet that has its energy as close to zero time as possible with no energy
before zero. It is easy to transform a minimum phase wavelet to zero phases
mathematically and this is done during processing.
 We need to distinguish between the phase of the wavelet and the phase of the
individual frequency components. In the case of a zero-phase wavelet, all the
contributing frequencies have zero phases also.

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Answer 1:
What is the dominant frequency of the seismic data in the interval between 1500
and 1600 ms? If the velocity is 5000 m/s, what is the tuning thickness? If it is
possible to detect a bed down to 1/16 of the wavelength, what would that be?
Dominant frequency:
About 4 ½ cycles in 100 ms
= 45 cycles/second
= 45 Hz
Tuning thickness:
Frequency = 45 Hz, Velocity = 5000 m/s
Wavelength = 5000/45
= 111 m
Tuning thickness = ¼ x 111 = 28 m

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The practice of seismic rock physics:
The practice of seismic rock physics depends to a large extent on the application. In some
cases, simply fluid substituting the logs in a dry well and generating synthetic gathers for
various fluid fill scenarios may be all that is needed to identify seismic responses
diagnostic of hydrocarbon presence.
On the other hand, generating stochastic inversions for reservoir prediction and
uncertainty assessment will require a complete rock physics database in which the elastic
properties of various lithofacies and their distributions are defined in an effective pressure
context. Either way, the amount of knowledge required to master the art of seismic rock
physics is a daunting prospect for the seismic interpreter.

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Kinds of Velocity:
• Average velocity: at which represent depth to bed (from surface to layer). Average velocity is
commonly calculated by assuming a vertical path, parallel layers and straight ray paths,
conditions that are quite idealized compared to those actually found in the Earth.
• Pseudo Average Velocity: when we have time from seismic & depth from well
• True Average Velocity: when we measure velocity by VSP, Sonic, or Coring
• Interval Velocity: The velocity, typically P-wave velocity, of a specific layer or layers o rock,
• Pseudo Interval Velocity: when we have time from seismic & depth from well
• True Average Velocity: when we measure velocity by VSP, Cheak shot
• Stacking Velocity: The distance-time relationship determined from analysis of normal move
out (NMO) measurements from common depth point gathers of seismic data. The stacking
velocity is used to correct the arrival times of events in the traces for their varying offsets prior
to summing, or stacking, the traces to improve the signal-to noise ratio of the data.
• RMS Velocity: is root mean square velocity & equivalent to stacking velocity but increased by
10%
• Instantaneous Velocity: Most accurate velocity (comes from sonic tools) & can be measured
at every feet
• Migration Velocity: used to migrate certain point to another (usually > or < of stacking
velocity by 5-15%)

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Convolution:
Is a mathematical way of combining two signals to achieve a third, modified signal.
The signal we record seems to respond well to being treated as a series of signals superimposed
upon each other that is seismic signals seem to respond convolutionally. The process of
DECONVOLUTION is the reversal of the convolution process.
Convolution in the time domain is represented in the frequency domain by a multiplying
the amplitude spectra and adding the phase spectra.

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The Power of Stack:
 Relies on signal being in phase and noise being out of phase i.e. primary signal is ‘flat’
on the cmp gather after NMO corrections
 A spatial or K- filtering process
 Data reduction - usually to [almost] ‘zero-offset’ trace
 Attenuates coherent noise in the input record (to varying degrees)
 Attenuates random noise relative to signal by up to N; where N = number of traces
stacked (i.e. fold of stack)
 K filter - filtering of spatial frequencies by summing/mixing
 K-filter - Apply an ‘all-ones’ filter and output the central sample.
 To apply a spatial K-filter to a record we must first collect the series of samples having
the same time values from each data trace - ie. form a common-time trace.
 This is the input data which must be convolved with our chosen filter to produce the
filtered output. The process is applied to each common-time trace in turn (0 msec, 4
msec, 8 msec, etc.).
 The summing filter is a high-cut spatial filter. It passes energy close to K=0, ie.
effectively dips close to 0ms per trace. Therefore, if signal has been aligned to zero dip
(as in NMO corrected data), signal will be passed.
 Organized noise contained in steeper dips will be suppressed - except at low temporal
frequencies or if the noise aliases and wraps-around through K=0.
 If we increase the number of filter points - ie. increase the fold - then the filter becomes
more effective at passing only energy close to K=0, or dips closer to zero.

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Relating Density to Compression-Wave Velocity:
A popular relation between density (ρ) and P-wave velocity (PV) seems to be that of Gardner
et al. (1974). The relation takes the following forms, depending on the units of PV (in all cases
the units of density are gm/cc):
Ft/sec: (1) ρ=0.23 VP0.25
Km/sec: (2) ρ=1.74 VP0.25
M/sec: (3) ρ=0.31 VP0.25
Their relation is simply an approximate average of the relations for a number of
sedimentary rock types, weighted toward shales. The relation comes from the figure in
Gardner et al. (1974):

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Exercise 2:

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Exercise 3:

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Exercise 4:
Convert this data from depth domain into time domain?

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INTERPRETATION
It is the last step in the seismic method. It means the transformation of seismic data
presented on seismic sections into geological information.
Seismic interpretation is an art that needs to be based on a clear knowledge of highly
developed technology and a proper understanding of what actually can happen within the
earth.
In the past, interpretation was mainly directed to detection of geologic subsurface
structures. In the present time, interpretation has been extended to include the detection
and mapping sand bodies and stratigraphic traps.
 Seismic Ties To Well Data:
When the interpreter comes to establish a tie between the seismic sections and a borehole
section; S/he faces the problem of making a direct correlation between pattern of
reflectors which are scaled vertically in terms of two way reflection time and the realities
of subsurface geology.
 Well Velocity Survey:
The Well Velocity Survey is the most direct method of identifying the relationship between
subsurface geology and the seismic reflection data. The technique involves detecting sound
from a near surface source with a pressure geophone at selected levels within the
borehole. These levels are usually chosen with reference to major changes in formations in
the geological section.

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 Vertical Seismic Profiling:
Through the using of digital acquisition equipment, it is possible to derive additional data
than those required to produce a calibrated sonic log. If sufficient time interval is sampled,
the data from each test level provide a record which is equivalent to a reflection seismic
trace with a deeply buried detector, because the hydrophone is buried, both upward and
downward travelling waveforms will be recorded from reflecting horizons above and
below the detector’s location, as well as multiples generated in the time progression.
The product, after processing, is displayed in a form similar to that of a variablearea
seismic section as a Vertical Seismic Profile (VSP)

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 Synthetic Seismograms:
The synthetic seismogram is considered to be of great value to the interpreter and it is best
presented by splicing it to an interpreted seismic section through the well location. The
acoustic impedance is calculated by multiplying seismic velocity by the density, and
reflection coefficients are calculated from impedance changes.
For comparison with the seismic trace, the reflection coefficient series must be convolved
with a suitable wavelet. Choice of the wavelet is critical for the appearance of the final
synthetic seismogram
Reflectivity
Density
Transittime
Primariesonly

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General Principles, [Seismic Facies Parameters]:
 Continuity:
It is the criteria observed on seismic section of the waveform, which is the seismic arrival
of a reflection, and can be recognized on successive traces, perhaps with small changes in
arrival time from trace to trace.
These repeated pulses create an alignment, and this alignment has continuity which can be
followed. The length of continuity represent, an “island of confidence”, from which one can
work in both directions.
The visual impression is dominated by the alignment not by individual pulses Seismic
continuity of a reflection is not an expression of the continuity of a geologic unit. It is an
expression of the continuity of two geological units one following immediately on top of
the other, at their contact is the interface at which the reflection is produced
 Correlation:
It is pattern recognition. The pattern may be a single pulse distinguished by its length ,
amplitude or shape , also characteristics of individual reflections, the spacing between
them It is used primarily to relate one area of confidence to another.
Correlation is:
 Shape of individual pulses
 Sequence of reflections and their spaces
The sequence of reflections is a very reliable basis for correlation. The spacing of
reflections is less reliable. Thickening and thinning change in seismic velocities
unconformities and other features tend to change the spacing of reflections.

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 Tracing A Seismic Horizon (Phantom):
The primary purpose of most seismic surveys is to determine structure; and this can be
achieved by tracing identifiable seismic horizons on cross-sections.
 Miss-tie at lines intersections changes interval time due to change of the water table
in land or large tidal movement at sea.
Change in stacking velocities.
Errors in survey
Recording and or processing changes (parameters)
Noise.
 Splitting Of Reflections:
The sequence is thickening.
The sequence is changing.
Over-step relationship at unconformity.
Overlap.
 Naming Of Reflections:
The identification of a seismic reflection requires two geological names, the rock above
and below the contact which generates the reflections.
Drilled wells
Outcrops
Tie to another survey

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Interpretation Process:
Data:
 Available surveys.
 Versions of the seismic sections.
 Base maps.
 Velocity (NMO, Migration, depth conversion).
The Interpretation:
 Data review Q.C, overall impression of the geology, side label.
 Seismic data quality.
 Seismic panel.
 Data quality map, to select lines, areas of easy interpretation, work schedule.
 Geological review and well to seismic.
 Identification of seismic sequence.
 Identification of seismic boundaries.
 Well tie, synthetics.
 Horizon selection.
 Objective horizon plus one above and one below it.
 Interpretation of the seismic sections.
 Section folding at all intersections.
 Picking.
 Line tying and correlation.
 Digitizing.
 Contouring.

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X
Loop Tying
Contouring Rules:
 Recognize trends, establish regional dip, search for dip reversals and seek a geological
rationale for trends in anomalies (folds, faults, reefs,).
 Contour from dense data and simple geology toward sparse data and more complex
geology.
 Locally reduce the contour interval in complicated areas if the structure form is
unclear.
 Be suspicions of closed high within a low.
 Be suspicious of a closed low on a top of a high.
 Look twice at a low trend.
 Be wary of like contours which run parallel over considerable distance.
 Be wary of contours that bear relationship with the seismic grid.
 Check the interpretation against the seismic sections, especially in regions of complex
structure.
 Contouring for locating a well or delineating a field should be done with the maximum
objectivity, (be optimistic).

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Geological Structural Styles

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Fault:
A break or planar surface in brittle rock across which there is observable displacement.
Depending on the relative direction of displacement between the rocks, or fault blocks, on
either side of the fault, its movement is described as normal, reverse or strike-slip.
According to terminology derived from the mining industry, the fault block above the fault
surface is called the hanging wall, while the fault block below the fault is the footwall.
Given the geological complexity of some faulted rocks and rocks that have undergone more
than one episode of deformation, it can be difficult to distinguish between the various
types of faults. Also, areas deformed more than once or that have undergone continual
deformation might have fault surfaces that are rotated from their original orientations,
so interpretation is not straightforward. In a normal fault, the hanging wall moves down
relative to the footwall along the dip of the fault surface, which is steep, from 45o to 90o.
A growth fault is a type of normal fault that forms during sedimentation and typically has
thicker strata on the downthrown hanging wall than the footwall. A reverse fault forms
when the hanging wall moves up relative to the footwall parallel to the dip of the fault
surface. A thrust fault, sometimes called an over thrust, is a reverse fault in which the fault
plane has a shallow dip, typically much less than 45o.
Normal fault:
A type of fault in which the hanging wall moves down relative to the footwall and the fault
surface dips steeply, commonly from 50o to 90o. Groups of normal faults can
produce horst and graben topography, or a series of relatively high- and low-standing fault
blocks, as seen in areas where the crust is rifting or being pulled apart by plate tectonic
activity.
A growth fault is a type of normal fault that forms during sedimentation and typically has
thicker strata on the downthrown hanging wall than the footwall.

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Reverse fault:
A type of fault formed when the hanging wall fault block moves up along a fault surface
relative to the footwall. Such movement can occur in areas where the Earth's crust is
compressed. A thrust fault, sometimes called an over thrust if the displacement is
particularly great, is a reverse fault in which the fault plane has a shallow dip, typically
much less than 45o.
Growth fault:
A type of normal fault that develops and continues to move during sedimentation and
typically has thicker strata on the downthrown, hanging wall side of the fault than in the
footwall. Growth faults are common in the Gulf of Mexico and in other areas where
the crust is subsiding rapidly or being pulled apart.
Growth faults are a particular type of normal fault that develops during ongoing
sedimentation, so the strata on the hanging wall side of the fault tend to be thicker than
those on the footwall side.
Antithetic fault:
A minor, secondary fault, usually one of a set, whose sense of displacement’s opposite to its
associated major and synthetic faults. Antithetic-synthetic fault sets are typical in areas of
normal faulting.
Synthetic fault:
A type of minor fault whose sense of displacement is similar to its associated major fault.
Antithetic-synthetic fault sets are typical in areas of normal faulting.

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Inversion Tectonics
The reversal of features particularly features such as faults by reactivation. For
example a normal fault might move in a direction opposite to its initial movement.
Basic Inversion terminology:

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Criteria of inversion:
 Less dip of growth fault.
 Normal – Null.
 Normal – Reverse.
 Reverse – Null.
 Kink fold.
 Short steep limp, long gentle limp.
 Horst – Graben.
 Half Grabens.
[The Main Benefits of Inversion Is To Know (Basin Shift)]

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Interpreting Seismic Amplitude:
 In areas with favorable rock properties it is possible to detect hydrocarbon directly
by using standard 3-D seismic data.
 Amplitude interpretation is then very effective in reducing risk when selecting
exploration and production drilling location.
 Not all areas have such favorable rock physics, but it is always useful to understand
what seismic amplitudes may be telling us about hydrocarbon presence or reservoir
quality.
 As well as amplitudes on migrated stacked data, it is often useful to look at pre-stack
data and the way that amplitude varies with source-receiver offset (AVO).
 The first step is to use well log data to predict how seismic response will change
with different reservoir fluid fill (gas or oil or brine), with changing reservoir
porosity, and with changing reservoir thickness.
AVO [Amplitude versus Offset]:
 AVO stands for amplitude variation with offset, or amplitude versus offset.
 The AVO techniques use the amplitude variations of pre-stack seismic reflections to
predict reservoir fluid effect.
 The AVO response is depending on the properties of P-wave velocity, S-wave
velocity and density in the porous reservoir rock.
 The calibration of amplitude to reflectivity is possible from a well tie, but the
calibration is valid only over a limited interval vertically.
 In any case, it is a good idea to inspect the entire section from surface to the target
event and below, if amplitude anomalies at target level are seen to be correlated
with overlying or underlying changes [high or low amplitudes due to lithology or
gas effect, or over burden faulting, as an example].
 Following the amplitude anomaly through the seismic processing sequence from the
raw gathers may be helpful; this may reveal an artifact being introduced in a
particular processing step.

M.M.Badawy
Page45
DHI [Direct Hydrocarbon Indicators]:
 AGC {Automatic Gain Control} cause a low opportunity for studying amplitude [e.g
Bright Spot].
 Important considerations in seismic data processing for DHI are:
 Polarity, Phase, Amplitude and Spatial extent.
 Frequency, Velocity, Amplitude/Offset and Shear wave information help in the
positive identification of DHI.
 Flat spot is a fluid contact reflection.
 Bright spot reflects gas accumulations.
 Termination of flat and bright spot at the same point increases the confidence of
hydrocarbon presence.

Principles of seismic data interpretation m.m.badawy

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