1) Reflection seismology involves detonating a shot at a source point and recording the ground motions with geophones at receiver points. This produces a record of travel times versus distances known as a time-distance record. 2) Different types of seismic arrivals appear on this record, including direct arrivals, reflected arrivals, critically refracted arrivals, and diffracted arrivals. Each follows a different travel path and appears as a different event type. 3) To help interpret the subsurface geology, the records are sorted into common midpoint gathers based on the midpoint between each source and receiver. This stacking process organizes the seismic data to reveal features like dips and faults below the surface.

1. REFLECTION SEISMOLOGY:
A BASIC INTRODUCTION
Tom Wilson
January, 2003

2. TIME DISTANCE RECORDS
BASIC DATA
The in-class seismograph demo data were collected in a manner similar to an
actual seismic survey. While we used only a simple 12 twelve channel seismograph and
the recording used single phones (i.e. no geophone groups), it was illustrative of the basic
acquisition approach. A shot is detonated at some point (or points to form a source array)
and phones are distributed on the surface to record the different arrivals. Phones can be
distributed across the surface in several different ways but the crucial data for any phone
is its offset distance. Where was the phone relative to a given shot? Shot detonation
initiates recording on the seismograph. Ground motion sensed by each phone or group of
phones is passed to the seismograph and stored. Ground motion is sampled at constant
time intervals referred to as the sample rate. The raw data recorded on the seismograph
consists of a number recorded at a certain time that describes in a relative sense the
motion of the ground at that instant of time. As noted in the demo, the basic data may be
measurement of surface velocity, acceleration or pressure. These parameters vary with
direction. In land surveys, measurements are usually made of the vertical component of
surface velocity or acceleration. If multi-component phones are used then these
measurements are made in 3 separate orthogonal directions.
Diagram of simple in-line 12 phone receiver string
TRAVEL PATHS
What is recorded by the geophone and when it is recorded depends on what lies
beneath the surface. What happens and when it happens is easily illustrated using a
simple single-layer model for starters (below).
2

3. The 12-channel in-line receiver string sits at the surface to the right of the shot.
First imagine what must happen. When will the mechanical disturbance generated by the
shot jiggle the geophones? There are different paths along which the mechanical
disturbance can travel. What are they?
Direct arrival
How long will it take to get to a phone?
Reflected arrival
How long will it take to get to a phone?
Critically refracted arrival
How long will it take to get to a phone?
3

4. Diffracted arrival
How long will it take for the diffracted ray to get to a given geophone?
TIME DISTANCE RECORD
All geophones tied into the seismograph provide a record of ground motion
produced by a given shot. Considering only the P-wave, there is still a confusing jumble
of events that will appear at a given receiver. The critical question is how does one
identify them and later, how does one sort out useful information about the subsurface
from all these different arrivals
The time distance plot provides a display of the basic data recorded in the field. It is a
plot of travel time on the y-axis versus distance along the surface on the x-axis. The
positive direction on the travel time axis is usually plotted downward (see below).
4

5. Time distance axes. Where do you think the different events will appear?
In the t-x plot, the direct arrival forms a straight line with zero intercept and slope equal
to the reciprocal of the interval velocity at the surface.
The reflection event: A hyperbola
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6. The critical refraction: a straight line tangent to the reflection.
The diffraction event: another hyperbola.
These different events get thrown together into a jumbled mess that doesn’t look much
like a geologic cross section. It’s not easy to interpret subsurface geology from such a
diagram. What’s a geologist to do? Things get even more complicated.
WHAT ABOUT GROUND ROLL?
In the above examples, we have only diagrammed the p-wave arrivals. Ground roll refers
to the Rayleigh wave mentioned earlier. It is a real noise maker and acquisition and
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7. processing efforts have to consider how to minimize its affect on data quality. It cuts
through the reflection events that every interpreter wants to see clearly.
REAL DATA
The situation becomes even more complex when you consider the presence of multiple
layers. The shot record below is from a shallow seismic survey using the equipment we
demonstrated in class.
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8. The record below is from a Vibroseis survey conducted to image deeper stratigraphy and
structure within the Appalachian foreland area.
SHOOTING GEOMETRY
In the above example we used what’s referred to as an off-end source-receiver layout.
One can also shoot using a split-spread layout or an asymmetrical split-spread layout.
These different shooting geometries are illustrated below.
The reflection path geometry and reflection point coverage obtained in a simple six
geophone split spread is illustrated below. In the diagram below, note that the spacing
between reflection points is ½ the geophone spacing.
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9. WHAT HAPPENS WHEN THE LAYER DIPS?
This is illustrated best using the split-spread layout. The first diagram (below)
illustrates how reflections travel down and back to the receivers over the horizontal
interface. Note that the paths are symmetrical about the shot and that the time-distance
plot portrays a hyperbola that is also symmetrical about the shot.
When the layer dips (below), notice how the travel paths are shorter up-dip than down-
dip. The pattern is asymmetrical in space and time.
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10. In the time-distance plot we still have the hyperbola, but its apex is offset in the up-dip
direction.
We said it would get more complicated, and that question – “What’s a geologist to do?”
becomes even more critical. Common midpoint sorting and stacking simplifies the data
so that an interpreter can look at it and begin to make geologic sense out of it. But – what
is common midpoint sorting?
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11. COMMON MIDPOINT SORTING
When you go through the simple exercise of drawing in the reflection travel paths from
source to receiver in the layout and you see that the reflection points are equally spaced
along the flat reflection surface at intervals equal to half the geophone spacing.
To make things even simpler, lets take a look at what happens for a 6-phone string. Draw
in the reflection points and label them 1 through 6 across the bottom below the reflecting
interface. Now move the shot to the right over to the location of the first phone on the
string and take that phone and move it out to the end. Maintain the geophone interval.
Draw in the reflection travel paths again and note that they reflect from some of the same
points of reflection associated with the first shot. Label those reflection points with the
appropriate phone number. Keep moving the shot and geophone string along to the right
and notice the pattern that builds up in the phone numbers that label each reflection point.
They begin repeating, and notice that in this setup there are never more than three source
receiver combinations that provide information about a given reflection point.
In this flat layer example, note that the three source-receiver combinations that provide
information from a single reflection point share the same surface midpoint (see below).
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12. They also have the same reflection point. Based on these relationships, this arrangement
of sources and receivers has come to be known as common midpoint (CMP) or common
depth point (CDP). You may recall hearing of CDP data or CMP data in your structure or
petroleum geology class. That is just a reference to these geometrical interrelationships. If
we extract only those traces that share a common midpoint we have what is called a
common midpoint “gather.” The process of sorting out (or extracting) records that share a
common midpoint is called common midpoint sorting.
What happens when we put some dip into the reflecting layer and go through this process
of sketching in the reflection travel paths? After all, this is the more general and more
realistic case. The source receiver combinations still retain the common midpoint, but
they do not reflect from a common depth point on the dipping interface. Note that the
reflection points walk up dip as the offset increases and none of the reflection points lie
beneath the midpoint. For this reason these data are more appropriately referred to as
common midpoint data than as common depth point data. Note that image points are used
to identify the reflection point.
What does the time distance plot for the common midpoint reflection record look like? In
the flat layer case we have another hyperbola that looks just like the one we had for the
shot record. Distances are still source-to-receiver distances, but there is no common
reference point as there was in the case of the shot record. Each reflection has a different
receiver and source location.
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13. What about the appearance of the dipping layer reflection in the CMP gather? Recall that
the dipping layer response of the shot record yields a hyperbola, but the apex of the
hyperbola is displaced up dip. Source-receiver combinations sharing a common midpoint
depicted above reveal that when the reflecting layer dips, the reflection points do not
coincide. As the source-receiver offset increases, the reflection point actually moves
higher up the dipping layer. Hence, information recorded on separate source-receiver
combinations arises from different but adjacent points on the reflecting layer. A useful
characteristic of the common midpoint record is that whether the layer is dipping or not,
we always get a hyperbola whose apex is centered at 0-offset (as shown above). This is
very important because of what is done next to the CMP data set.
Definition - CMP Gather: A collection of traces sharing a common midpoint.
NORMAL MOVEOUT CORRECTION
The first thing a geologist wants to do when they see a shot record or CMP gather is to
straighten out the reflection events. (The reflections are from the same or nearby
reflections points – so they should all arrive at the same time.) In the simplest case - that
for the horizontal reflector - the hyperbolic increases in travel time from short to long
offsets are due only to the increasing distance of the source from the receiver.
We want to see geologic changes – actual changes in the shape of the layer not ones due
to changes in the locations of sources and receivers at the surface. The increase you see in
arrival time (or travel time) with offset is called moveout. Take a minute and consider the
diagram below showing the reflection travel paths in the common midpoint gather. Next
to it is the simple hyperbolic reflection event we would expect to see. And if we shot
back (to the left) as well as forward (to the right) we could show both halves of the
hyperbola.
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14. The reflections all come from the same point. If the source and receiver were sitting right
on top of each other then the wave would travel straight down and back up to the surface.
This is the shortest possible travel path. The record would make more sense to us –
geologically speaking – if all the arrivals came in at the same time. To do this, we need to
shift all the arrival times so that they “appear” to have gone straight down and back up to
the surface.
Making that shift is referred to as making the moveout correction and the difference from
the “zero-offset” travel time and the actual travel time is called the moveout. To make the
moveout correction we simply compute the moveout (∆tX1 or ∆tX2, below) and subtract it
from the actual arrival time. The correction is often referred to as the NMO (normal
moveout) correction.
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15. The corrected reflection form a flat response in the CMP gather as suggested below.
The computation of the NMO correction is fairly straightforward. The computation
involves fitting a line to the hyperbola, determining velocity, calculating the ∆ts and then
shifting each trace by the corresponding ∆t. Computers are real good at doing moveout
corrections. But the key to understanding how the correction is made lies in
understanding the effect of velocity on “moveout.” Ask yourself what would happen if
you increased the velocity? Decreased the velocity? Which time-distance plot goes with a
faster velocity and which with the slower?
Which is faster?
Which is slower?
CMP gather
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16. If you increase the interval velocity, you flatten out or reduce the moveout on the
reflection hyperbola.
STACKING
At this point you’re probably wondering “Why go to all this trouble generating
redundant data, then sorting it and then flattening it? Why don’t we just flatten out the
reflection hyperbolae in the shot record? Why get all this additional data? That’s exactly
what geophysicists used to do and it worked quite well as shown in this data form over
the Rome trough in West Virginia. This is an old Exxon line that was reprocessed by
GTS. It was fine up to a point, and it didn’t always look this good. In many cases such
data are very noisy. Reflections are difficult to see and do not provide the interpreter with
very useful information. One of them main culprits is the noise. If geophysicists could
eliminate or even reduce the amount of noise in a data set then they might be able to get
clearer images of the subsurface – see things that couldn’t be seen before. That’s exactly
why geophysicists go to all the trouble of making seemingly redundant measurements.
The idea works like this. Imagine that you take a geophone and set it out on the
ground and turn on your seismograph, listen and record what you hear. You will hear the
16

17. earth creek and groan as cars drive by, as the wind blows, rain falls, water flows by in the
stream, as a cow steps on your geophone, etc. All these things happen more or less at
random. If you repeat this experiment you will get another record that will be completely
different from the first. If you were listening for a reflection to make its way back to the
surface this noise just gets in the way. It’s like listening to a faint signal on the radio. The
noise or “static” could be so loud that you never hear your reflection.
Now, as another experiment, assume that rather than just listening to the noise,
that you bang on the ground and listen for a reflection from some layer you know is there.
If there were no noise it would come in at some time. The ground would wiggle up and
down as the wave made its way back to the surface (see A below). But in reality, there is
a lot of noise there. You might not have been able to pound as hard as you would have
liked. Perhaps you wanted to use 50 pounds of dynamite but the local landowners would
only let you use a couple ounces. Instead you keep hitting the ground and making records
that you save. On any one record you can’t see the reflection event very well – if at all.
But- if you sum them together – then what happens? The reflection always arrives at the
same time. What about the noise? The noise vibrations, if they are random, will jiggle
the phone in one direction during the first recording and then in another different
direction during the next. It is very unlikely that random vibrations of the ground will
shake the phone in the same direction during subsequent recordings. When several
recordings are summed together, the noise gets smaller and smaller in amplitude. Noise
vibrations at one time partially cancel those recorded at another time. The signal, on the
other hand, continues to build in amplitude in direct proportion to the number of records
that are summed together.
Sometimes the noise can be coherent as in the case shown below. In this example,
seismic recordings were made over an underground longwall mining operation.
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18. The general level of improvement is illustrated by comparing the quality of reflections in
the shot record (Figure A, below) to the quality of the final stack section (Figure B,
below).
Figure A: Vibroseis Shot Record
Figure B: Stacked seismic traces.
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19. FOLD
In the stacking chart diagram shown previously (see also below) for the simple 6-phone
geophone array, three reflection observations are obtained from each midpoint. This is
the maximum number of records or observations that can be obtained of that reflection
point with this acquisition geometry. That number of records, the maximum number of
records of a given reflection point, obtained from the common midpoint gather of traces
is referred to as the “fold”. In this simple example, 3 is the maximum “fold” of the data.
On the ends of the profile the fold increases from 1 to the maximum of 3. The fold then
remains constant until the right end of the profile is reached.
Unlike the fold in this simple example, the fold along a seismic line can often vary. These
variations occur because of bends in the road (see figure below). They can also occur in
straight “cross country” lines when rivers or other barriers result in gaps in the shooting,
recording or both.
19

20. The problems of sorting into common midpoint bins can become complicated by the line
geometry as shown in the more realistic example below..
Along crooked survey lines,
the common midpoint gather
includes all records whose
midpoints fall within a
certain radius of some point
SIGNAL TO NOISE RATIO
As noted in the discussion of stacking, the redundancy of observations helps improve the
quality and amplitude of the signal while minimizing the deleterious effects of noise. The
degree of enhancement is described quantitatively in terms of the signal to noise ratio.
This ratio is directly proportional to the square root of the fold of the seismic data. If the
fold is increased from 1 to 4 then the signal to noise ratio is increased by a factor of 2.
This problem was originally solved by Einstein and is often described in terms of a
“random” walk. The random walk poses the question – “ will a series of random steps
take the walker somewhere other than their starting point?” The problem is often posed
anecdotally in the form of the drunken sailor experiment. The common expectation is that
the stumbler gets nowhere, but in fact the stumbler makes progress proportional to the
square-root of the number of steps taken. Noise can be attenuated but - if truly random -
cannot be eliminated entirely. The decision of what fold to use is often based on a
compromise between data quality and economics.
In the example shown below, note the improvement in reflection continuity
obtained from stacking the noisy traces.
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21. Stack traces
Pre-stack single fold records
THE STACK TRACE
The stacked seismic trace simulates data acquisition conditions that in reality did not
exist. The stacked seismic trace represents a record that would have been acquired if the
shot and receiver were located in the same position. Such a record is often referred to as a
coincident source and receiver record or CSR record for short. As noted earlier (see
figure below), the process of NMO correction shifts the arrival time so that reflection
events at different source-receiver offsets appear to have traveled straight down and back
to geophones located at the midpoint of the CMP gather.
21

22. When reflectors are flat the resulting seismic section (lower graph in figure below)
accurately portrays “structural” information.
Reflection events appearing on a CSR record appear as though they have traveled down
to the reflector point and back along a path which is normally incident on the reflector, as
shown above. However, reflectors are often deformed into complex structures, and the
depositional patterns, themselves, can give rise to complex variations in reflector
geometry. The figure below portrays normal incidence reflections returned to a single ,
coincident, source and receiver point. In this example, there are only three normal
incidence paths: one, down and back from point B and two others, down and back from
points A and C.
22

23. For this reason, the coincident source-receiver record is also often referred to as a normal
incidence record. As you can quickly appreciate, the events that appear on a normal
incidence seismic record may not represent actual vertical relationships in depth beneath
the midpoint (see below) since the reflection events do not originate from points directly
beneath the midpoint or directly beneath the imaginary coincident source-receiver.
A
C
B
This can lead – particularly in areas of complex structure – to considerable distortion in
the representation of subsurface structure and structural interrelationships.
The relationships implied by the names: coincident source and receiver record or normal
incidence record, are useful too understanding the nature of the data presented in this type
of record. However, the reference you are most likely to encounter when talking to
seismic interpreters is that of the CDP stack section or CDP seismic section.
MODELING
The paths along which reflection events travel are referred to as ray paths. The ray paths
in the normal incidence seismic section are normal incidence ray paths. Processors do
their best to eliminate the geometrical distortions appearing in the stack section using a
process referred to as migration, which we will discuss later. Regardless of the
confidence one has in the subsurface view provided by the seismic section it is often the
case that more than one interpretation of the subsurface is possible. For this reason the
interpreter like to generate model seismic surveys across their interpretations to see how
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24. well the seismic expression of their geological interpretation matches actual seismic data
across the area.
The process of simulating the seismic response of the interpreter’s model requires
knowledge of subsurface interval velocities and densities. Velocities and densities are
obtained from sonic and density logs of a well that is preferably located near the area
where seismic data is being acquired. Knowledge of velocity is necessary because the
seismic section is basically a representation of the time it takes for seismic energy to
travel down to a reflecting interface and back to the surface. Velocity and density are
combined to provide a measure of the strength of the reflection. The measure of
reflection strength is the reflection coefficient and its value
Z 2 − Z1
R=
Z1 + Z 2
where Z is acoustic impedance and is equal to the product ρV, where V represents
interval velocity and ρ, interval bulk density. The subscripts refer to the layer number. R
tells the interpreter how large the amplitude of a given reflection will be and also, how
the reflection strength of a reflector from one interface will compare with that from
another. The pool player learns early on to violate this law using “English” (placing a
spin on the ball) (below left).
Only for pool players
The basic mathematical relationships governing how rays travel from source to receiver
are the reflection and Snell’s laws. For reflection, the angle of incidence equals the angle
of reflection (see figure below).
24

25. Snell’s law (see below) is one known very well by every spear fisherman. Because the
velocity of light in water is less than that in air the fish appears beyond its actual location.
RAY TRACING
The first step in converting the interpreter’s subsurface representation into a seismic view
is to compute travel times to and from the reflector(s) represented in the interpretation.
Because NMO correction and stack simulate seismic data as it would appear if the source
and receiver were located at the same point on the surface the calculation of two-way
travel times is simplified. As mentioned above, the coincident-source-receiver travel path
is one along which reflection takes place at normal incidence to the reflecting interface
Dipping Reflector Horizon: The coincident-source-receiver format of the data yields an
accurate representation of subsurface structural interrelationships only for the trivial case
of horizontal layers as noted earlier. When the reflecting surface dips, ray paths travel to
the receiver from points up-dip (see figure below). The seismic image of the reflector (the
record surface) suggests that the reflector is longer than actual and has less dip.
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26. Syncline: The distortions become more serious with increased structural complexity. The
seismic expression of a syncline (below), for example, leads to the appearance of an
anomalous anticline beneath the axis of the syncline. The limbs of the syncline, AB and
CD appear down dip. Since they dip in opposite directions they can actually appear to
cross over each other (below). Normal incidence reflections across the axis of the
syncline (reflection points 1 through 5 in figure below) are reflected back to the surface
in reverse order, right to left. Ray paths cross each other at a focal point. Travel time
down and back from the hinge of the syncline are shorter than those to either side. The
net effect is that the seismic image portrays the hinge area of the syncline as an apparent
anticline (see below). In addition the lateral extent of the syncline has been reduced. The
pitfall in this for the interpreter is obvious and more than one unsuspecting company has
drilled the apparent anticline (reverse branch) only to find themselves in the depths of a
syncline. The reverse branch arises when the focus is located beneath the surface.
Anticline: Normal incidence reflections across an anticline (below) shows that ray paths
are spread out from the limbs of the anticline in the down dip direction. The net effect is
that the seismic appearance of an anticline (in time, below) has much broader aerial
extent. Again the seismic appearance is a misleading representation of the subsurface. If
uncorrected, the seismic view suggests more extensive closure and reservoir capacity.
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27. Fault: The seismic expression of faulted horizons can be quite varied. The simple case
shown here (see below) portrays normal offset of a layer accompanied by minor uplift
leading to diverging dips on opposite sides of the fault. Reflections from the faulted
horizon produce an apparent shift of horizon segments down dip. The apparent fault gap
appears wider than it actually is.
Diffractions arising from faulted edges of the horizon (see figure below), fan out across
the surface leading to the appearance of hyperbolic events in the seismic section. These
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28. diffractions may suggest the presence of rollover into the fault. In general the interpreter
finds the diffractions helpful, since their apex accurately locates the position of the fault.
A line drawn to connect the diffraction apex defines the location of the fault plane.
GEOMETRICAL PITFALLS
The above models illustrate a few “pitfalls” that are classified as geometrical in nature.
Their effect is to distort the appearance of subsurface structure.
COMPUTER GENERATED MODELS
Seismic modeling is routinely undertaken at the computer workstation. Computer models
of the above examples are shown below. A single flat layer has been added beneath the
deformed horizon in each of these models to illustrate additional distortions that arise
from velocity variation. In each model the velocity above the deformed horizon (Va) is
15,000 feet per second and that below (Vb), 20,000.
Syncline: The ray-tracing here (below) is much more thorough than in the preceding
example. The computer can compute and draw these ray paths much more quickly than
we can. Note the familiar features in the diagram including the buried focus and the travel
of reflection events from the reflector surface to receivers down dip. Reflections from the
lower horizon are incident at right angles and return to the receiver along their downward
28

29. path. As the rays travel back to the surface they pass from the deeper high velocity layer
into the lower velocity surface layer and are refracted toward a line drawn normal to the
reflector surface.
In the time display (below) the reverse branch and crossing synclinal limbs are expected
based on our previous discussion. However, the appearance of the underlying reflector
suggests that it may also have experienced a similar level of deformation. Ray paths
toward the edges of the model travel through a greater thickness of the higher velocity
medium than do rays traveling down
through the hinge area. While the lengths
of the travel paths do not vary greatly, the
time taken to travel these different paths is
less in proportion to the distance traveled in
the high velocity layer. Rays make their
way down and back more quickly high on
the limbs of the syncline than do rays
which travel through a much greater
thickness of the low velocity medium
occupying the hinge area of the syncline.
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30. Anticline: Computer ray tracing was performed acros the more complex anticlinal
structure shown below. Based on the preceding discussion, we expect the reflections
from the crest of the anticline to fan out and produce an anticline with much broader
appearance in the time section. However, note that we have a tight syncline sitting
between two anticlines.
Subsurface structural interpretation across the Summit Field north of
Morgantown along the Chestnut Ridge anticline.
Raytracing through the syncline shown below shows that we have a buried focus
event, and what we should expect to see in time is another anticline – not a syncline.
30

31. Normal incident rays rising from the lower interface are refracted toward the
normal in accordance with Snell’s law. Travel times to and from the underlying flat
horizon (Figure) decrease below the anticlinal hinge and increase down the limbs taking
on an anticlinal form.
Can you spot the reverse branch and apparent anticline arising from the
base of the syncline?
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32. VELOCITY PITFALLS
Along with the class of geometrical pitfalls there are also pitfalls or distortions associated
with subsurface velocity distribution. Did you notice anything unusual about the time
section across the simple syncline in our first ray-tracing example (reproduced below)?
Reflections from the shallow syncline and deeper – flat –
reflector.
The velocity in the layer beneath the synclinally shaped shallow reflector is much faster
than in the overlying layer. Thus, two-way travel times to the deeper reflector on either
side of the syncline arrive much earlier than do reflections from the same depth that
travel through the axis of the syncline. Velocity distribution in the syncline model
produces a “sag” in the reflection from the deeper flat horizon beneath the syncline, since
the syncline contains a much thicker section of low velocity strata.
The example below illustrates a combination of velocity and geometrical pitfalls inherent
in the seismic time section. In this example, a seismic line crosses a reef.
32

33. The ray path diagram shown below suggests that the recordings of reflection travel times
in the normal incidence format simulated by the CMP stack trace will be complicated and
not directly related to the structural features portrayed in the depth section above.
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34. How might the time section shown below, lead to incorrect interpretation of subsurface
structure?
Seismic section over the reef.
SEISMIC WAVELETS, DECONVOLUTION AND STRATIGRAPHIC
INTERPRETATION
The seismic wavelet refers to the mechanical disturbance, generated by the seismic
source that travels through the subsurface. The impact of a hammer produces a jolt of
energy that passes quickly. A charge of dynamite when detonated rapidly deforms the
surrounding area and sends out a shock wave which may be felt as a rapidly passing
shake of the ground. It is the temporal characteristics of this pulse of deformation
produced by the seismic source that we refer to as the seismic wavelet, seismic pulse, or
just wavelet. An example of a seismic wavelet is shown below. Note that time plots left
to right.
WAVELET A
Basic Seismic Wavelet
Wavelets come in many different shapes and sizes. Another wavelet is shown below.
34

35. Note that this wavelet is more compact or has shorter duration than the one above.
WAVELET B
Seismic data processing is a fascinating field of study. There are many techniques applied
to seismic data that enhance the quality of the seismic image and help improve the
resolution of subtle geological features – both structural and stratigraphic.
One very important seismic data processing procedure is known as deconvolution.
Deconvolution can be thought of as a pulse compression technique; in other words, it is a
process applied to seismic data to reduce the duration of the seismic wavelet. It is a
process which can transform wavelet A into wavelet B shown above. The benefits of
deconvolution become evident when we think about resolving the top and bottom of a
layer. If wavelet A is reflected back to the surface from the top and bottom of a reflective
interval, note that the long duration of the reflection event from the top of the layer will
probably overlap or interfere with the reflection from the base of the layer, making it
difficult to distinguish between the two.
Let’s take a look at some of the difficulties that can arise. Examine the section below –
and before turning the page make an interpretation of this small seismic section.
Section A
The section above is actually a synthetic or model (made up) seismic data set. The
structural and stratigraphic features in the model are representative of graben structures
encountered in the North Sea. Note the obvious stratigraphic pinch-out. This would make
a nice stratigraphic trap. Now take a look at the seismic section below.
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36. Section B
What happened to that pinch-out? The geologic model of the area is shown below. The
reflective properties of each layer are defined by the velocity contrasts shown in the cross
section. This geologic model was transformed into the seismic displays shown above.
The only difference between the two model seismic displays is in the wavelet that was
reflected from the interfaces between layers. In the first seismic section, Wavelet A was
used; in the second, Wavelet B.
Note that the Upper Jurassic Hot Shale and Callovian Shale are capped by a basal
Cretaceous marl/limestone unit. A complex deformation history is revealed by the
variations in thickness of the different units across the normal faults bounding the horst
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37. and across the top of the horst-block itself. The Hot Shale and Callovian Shale do not
pinch-out against the basal Cretaceous. So why does a pinch out appear in the Section A?
Go back and take a close look at Wavelet A. This wavelet has a long duration; the first
two cycles in the wavelet have relatively high amplitude. When this wavelet reflects from
the basal Cretaceous interval across the top of the horst-block, the initial reflection is
accompanied by all the cycles in the wavelet. (Wavelet A). In section A the follow cycles
of the reflections from the basal Cretaceous follow beneath and drop with the reflector
left to right across the horst-block; and as they do, they intersect the reflection event from
the base of the Hot Shale and Callovian Shale. The result gives the illusion that these
Jurassic shales pinch out againts the basal Cretaceous.
When the seismic data is deconvolved (i.e. when wavelet A is transformed into wavelet
B, the long tail is eliminated from the wavelet. Reflections in the deconvolved section
(Section B) consist of a single sharp reflection event with no following cycles to
complicate the appearance of the seismic section.
Deconvolution produces significant improvement in the resolution of geologic features in
the seismic section. However, even with this simplified, more compact wavelet, we are
still faced with resolution limitations when the two-way travel times separating reflectors
are less than the duration of the seismic wavelet. Overlap becomes a problem again, and
we loose the ability to identify relatively thin layers.
To sharpen your interpretation skills try your hand with the section below. On a separate
sheet of paper sketch your interpretation of the geology producing this seismic response.
Can you find the sand channel? Can you find a velocity anomaly? Do stratigraphic
intervals continue across the axis of the anticline?
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38. 38