Reflection Seismology Overview

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Reflection Seismology Overview

  1. 1. REFLECTION SEISMOLOGY: A BASIC INTRODUCTION Tom Wilson January, 2003
  2. 2. TIME DISTANCE RECORDSBASIC DATA The in-class seismograph demo data were collected in a manner similar to anactual seismic survey. While we used only a simple 12 twelve channel seismograph andthe recording used single phones (i.e. no geophone groups), it was illustrative of the basicacquisition 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 bedistributed across the surface in several different ways but the crucial data for any phoneis its offset distance. Where was the phone relative to a given shot? Shot detonationinitiates recording on the seismograph. Ground motion sensed by each phone or group ofphones is passed to the seismograph and stored. Ground motion is sampled at constanttime intervals referred to as the sample rate. The raw data recorded on the seismographconsists of a number recorded at a certain time that describes in a relative sense themotion of the ground at that instant of time. As noted in the demo, the basic data may bemeasurement of surface velocity, acceleration or pressure. These parameters vary withdirection. In land surveys, measurements are usually made of the vertical component ofsurface velocity or acceleration. If multi-component phones are used then thesemeasurements are made in 3 separate orthogonal directions.Diagram of simple in-line 12 phone receiver stringTRAVEL PATHS What is recorded by the geophone and when it is recorded depends on what liesbeneath the surface. What happens and when it happens is easily illustrated using asimple single-layer model for starters (below). 2
  3. 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 theshot jiggle the geophones? There are different paths along which the mechanicaldisturbance can travel. What are they?Direct arrivalHow long will it take to get to a phone?Reflected arrivalHow long will it take to get to a phone?Critically refracted arrivalHow long will it take to get to a phone? 3
  4. 4. Diffracted arrivalHow 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 motionproduced by a given shot. Considering only the P-wave, there is still a confusing jumbleof events that will appear at a given receiver. The critical question is how does oneidentify them and later, how does one sort out useful information about the subsurfacefrom all these different arrivalsThe time distance plot provides a display of the basic data recorded in the field. It is aplot of travel time on the y-axis versus distance along the surface on the x-axis. Thepositive direction on the travel time axis is usually plotted downward (see below). 4
  5. 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 equalto the reciprocal of the interval velocity at the surface.The reflection event: A hyperbola 5
  6. 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 muchlike a geologic cross section. It’s not easy to interpret subsurface geology from such adiagram. 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 refersto the Rayleigh wave mentioned earlier. It is a real noise maker and acquisition and 6
  7. 7. processing efforts have to consider how to minimize its affect on data quality. It cutsthrough the reflection events that every interpreter wants to see clearly.REAL DATAThe situation becomes even more complex when you consider the presence of multiplelayers. The shot record below is from a shallow seismic survey using the equipment wedemonstrated in class. 7
  8. 8. The record below is from a Vibroseis survey conducted to image deeper stratigraphy andstructure within the Appalachian foreland area.SHOOTING GEOMETRYIn 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 sixgeophone split spread is illustrated below. In the diagram below, note that the spacingbetween reflection points is ½ the geophone spacing. 8
  9. 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 horizontalinterface. Note that the paths are symmetrical about the shot and that the time-distanceplot 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. 9
  10. 10. In the time-distance plot we still have the hyperbola, but its apex is offset in the up-dipdirection.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 dataso that an interpreter can look at it and begin to make geologic sense out of it. But – whatis common midpoint sorting? 10
  11. 11. COMMON MIDPOINT SORTINGWhen you go through the simple exercise of drawing in the reflection travel paths fromsource to receiver in the layout and you see that the reflection points are equally spacedalong 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. Drawin the reflection points and label them 1 through 6 across the bottom below the reflectinginterface. Now move the shot to the right over to the location of the first phone on thestring 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 samepoints of reflection associated with the first shot. Label those reflection points with theappropriate phone number. Keep moving the shot and geophone string along to the rightand 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 sourcereceiver combinations that provide information about a given reflection point.In this flat layer example, note that the three source-receiver combinations that provideinformation from a single reflection point share the same surface midpoint (see below). 11
  12. 12. They also have the same reflection point. Based on these relationships, this arrangementof sources and receivers has come to be known as common midpoint (CMP) or commondepth point (CDP). You may recall hearing of CDP data or CMP data in your structure orpetroleum geology class. That is just a reference to these geometrical interrelationships. Ifwe extract only those traces that share a common midpoint we have what is called acommon midpoint “gather.” The process of sorting out (or extracting) records that share acommon midpoint is called common midpoint sorting.What happens when we put some dip into the reflecting layer and go through this processof sketching in the reflection travel paths? After all, this is the more general and morerealistic case. The source receiver combinations still retain the common midpoint, butthey do not reflect from a common depth point on the dipping interface. Note that thereflection points walk up dip as the offset increases and none of the reflection points liebeneath the midpoint. For this reason these data are more appropriately referred to ascommon midpoint data than as common depth point data. Note that image points are usedto identify the reflection point.What does the time distance plot for the common midpoint reflection record look like? Inthe flat layer case we have another hyperbola that looks just like the one we had for theshot record. Distances are still source-to-receiver distances, but there is no commonreference point as there was in the case of the shot record. Each reflection has a differentreceiver and source location. 12
  13. 13. What about the appearance of the dipping layer reflection in the CMP gather? Recall thatthe dipping layer response of the shot record yields a hyperbola, but the apex of thehyperbola is displaced up dip. Source-receiver combinations sharing a common midpointdepicted above reveal that when the reflecting layer dips, the reflection points do notcoincide. As the source-receiver offset increases, the reflection point actually moveshigher up the dipping layer. Hence, information recorded on separate source-receivercombinations arises from different but adjacent points on the reflecting layer. A usefulcharacteristic 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 isvery 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 CORRECTIONThe first thing a geologist wants to do when they see a shot record or CMP gather is tostraighten out the reflection events. (The reflections are from the same or nearbyreflections points – so they should all arrive at the same time.) In the simplest case - thatfor the horizontal reflector - the hyperbolic increases in travel time from short to longoffsets 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 dueto changes in the locations of sources and receivers at the surface. The increase you see inarrival time (or travel time) with offset is called moveout. Take a minute and consider thediagram below showing the reflection travel paths in the common midpoint gather. Nextto it is the simple hyperbolic reflection event we would expect to see. And if we shotback (to the left) as well as forward (to the right) we could show both halves of thehyperbola. 13
  14. 14. The reflections all come from the same point. If the source and receiver were sitting righton 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 toshift all the arrival times so that they “appear” to have gone straight down and back up tothe surface.Making that shift is referred to as making the moveout correction and the difference fromthe “zero-offset” travel time and the actual travel time is called the moveout. To make themoveout correction we simply compute the moveout (∆tX1 or ∆tX2, below) and subtract itfrom the actual arrival time. The correction is often referred to as the NMO (normalmoveout) correction. 14
  15. 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 computationinvolves fitting a line to the hyperbola, determining velocity, calculating the ∆ts and thenshifting each trace by the corresponding ∆t. Computers are real good at doing moveoutcorrections. But the key to understanding how the correction is made lies inunderstanding the effect of velocity on “moveout.” Ask yourself what would happen ifyou increased the velocity? Decreased the velocity? Which time-distance plot goes with afaster velocity and which with the slower? Which is faster? Which is slower? CMP gather 15
  16. 16. If you increase the interval velocity, you flatten out or reduce the moveout on thereflection hyperbola. STACKING At this point you’re probably wondering “Why go to all this trouble generatingredundant data, then sorting it and then flattening it? Why don’t we just flatten out thereflection hyperbolae in the shot record? Why get all this additional data? That’s exactlywhat geophysicists used to do and it worked quite well as shown in this data form overthe Rome trough in West Virginia. This is an old Exxon line that was reprocessed byGTS. It was fine up to a point, and it didn’t always look this good. In many cases suchdata are very noisy. Reflections are difficult to see and do not provide the interpreter withvery useful information. One of them main culprits is the noise. If geophysicists couldeliminate or even reduce the amount of noise in a data set then they might be able to getclearer images of the subsurface – see things that couldn’t be seen before. That’s exactlywhy 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 theground and turn on your seismograph, listen and record what you hear. You will hear the 16
  17. 17. earth creek and groan as cars drive by, as the wind blows, rain falls, water flows by in thestream, as a cow steps on your geophone, etc. All these things happen more or less atrandom. If you repeat this experiment you will get another record that will be completelydifferent from the first. If you were listening for a reflection to make its way back to thesurface this noise just gets in the way. It’s like listening to a faint signal on the radio. Thenoise 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 anddown as the wave made its way back to the surface (see A below). But in reality, there isa lot of noise there. You might not have been able to pound as hard as you would haveliked. Perhaps you wanted to use 50 pounds of dynamite but the local landowners wouldonly let you use a couple ounces. Instead you keep hitting the ground and making recordsthat 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 thesame time. What about the noise? The noise vibrations, if they are random, will jigglethe phone in one direction during the first recording and then in another differentdirection during the next. It is very unlikely that random vibrations of the ground willshake the phone in the same direction during subsequent recordings. When severalrecordings are summed together, the noise gets smaller and smaller in amplitude. Noisevibrations at one time partially cancel those recorded at another time. The signal, on theother hand, continues to build in amplitude in direct proportion to the number of recordsthat 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. 17
  18. 18. The general level of improvement is illustrated by comparing the quality of reflections inthe 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. 18
  19. 19. FOLDIn the stacking chart diagram shown previously (see also below) for the simple 6-phonegeophone array, three reflection observations are obtained from each midpoint. This isthe maximum number of records or observations that can be obtained of that reflectionpoint with this acquisition geometry. That number of records, the maximum number ofrecords of a given reflection point, obtained from the common midpoint gather of tracesis 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 thenremains 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. Thesevariations occur because of bends in the road (see figure below). They can also occur instraight “cross country” lines when rivers or other barriers result in gaps in the shooting,recording or both. 19
  20. 20. The problems of sorting into common midpoint bins can become complicated by the linegeometry 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 pointSIGNAL TO NOISE RATIOAs noted in the discussion of stacking, the redundancy of observations helps improve thequality and amplitude of the signal while minimizing the deleterious effects of noise. Thedegree 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 thefold 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 stepstake the walker somewhere other than their starting point?” The problem is often posedanecdotally in the form of the drunken sailor experiment. The common expectation is thatthe stumbler gets nowhere, but in fact the stumbler makes progress proportional to thesquare-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 acompromise between data quality and economics. In the example shown below, note the improvement in reflection continuityobtained from stacking the noisy traces. 20
  21. 21. Stack tracesPre-stack single fold recordsTHE STACK TRACEThe stacked seismic trace simulates data acquisition conditions that in reality did notexist. The stacked seismic trace represents a record that would have been acquired if theshot and receiver were located in the same position. Such a record is often referred to as acoincident source and receiver record or CSR record for short. As noted earlier (seefigure below), the process of NMO correction shifts the arrival time so that reflectionevents at different source-receiver offsets appear to have traveled straight down and backto geophones located at the midpoint of the CMP gather. 21
  22. 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 downto the reflector point and back along a path which is normally incident on the reflector, asshown above. However, reflectors are often deformed into complex structures, and thedepositional patterns, themselves, can give rise to complex variations in reflectorgeometry. The figure below portrays normal incidence reflections returned to a single ,coincident, source and receiver point. In this example, there are only three normalincidence paths: one, down and back from point B and two others, down and back frompoints A and C. 22
  23. 23. For this reason, the coincident source-receiver record is also often referred to as a normalincidence record. As you can quickly appreciate, the events that appear on a normalincidence seismic record may not represent actual vertical relationships in depth beneaththe midpoint (see below) since the reflection events do not originate from points directlybeneath the midpoint or directly beneath the imaginary coincident source-receiver. A C BThis can lead – particularly in areas of complex structure – to considerable distortion inthe representation of subsurface structure and structural interrelationships.The relationships implied by the names: coincident source and receiver record or normalincidence record, are useful too understanding the nature of the data presented in this typeof record. However, the reference you are most likely to encounter when talking toseismic interpreters is that of the CDP stack section or CDP seismic section. MODELINGThe paths along which reflection events travel are referred to as ray paths. The ray pathsin the normal incidence seismic section are normal incidence ray paths. Processors dotheir best to eliminate the geometrical distortions appearing in the stack section using aprocess referred to as migration, which we will discuss later. Regardless of theconfidence one has in the subsurface view provided by the seismic section it is often thecase that more than one interpretation of the subsurface is possible. For this reason theinterpreter like to generate model seismic surveys across their interpretations to see how 23
  24. 24. well the seismic expression of their geological interpretation matches actual seismic dataacross the area.The process of simulating the seismic response of the interpreter’s model requiresknowledge of subsurface interval velocities and densities. Velocities and densities areobtained from sonic and density logs of a well that is preferably located near the areawhere seismic data is being acquired. Knowledge of velocity is necessary because theseismic section is basically a representation of the time it takes for seismic energy totravel down to a reflecting interface and back to the surface. Velocity and density arecombined to provide a measure of the strength of the reflection. The measure ofreflection strength is the reflection coefficient and its value Z 2 − Z1 R= Z1 + Z 2where Z is acoustic impedance and is equal to the product ρV, where V representsinterval velocity and ρ, interval bulk density. The subscripts refer to the layer number. Rtells the interpreter how large the amplitude of a given reflection will be and also, howthe reflection strength of a reflector from one interface will compare with that fromanother. The pool player learns early on to violate this law using “English” (placing aspin on the ball) (below left). Only for pool playersThe basic mathematical relationships governing how rays travel from source to receiverare the reflection and Snell’s laws. For reflection, the angle of incidence equals the angleof reflection (see figure below). 24
  25. 25. Snell’s law (see below) is one known very well by every spear fisherman. Because thevelocity of light in water is less than that in air the fish appears beyond its actual location.RAY TRACINGThe first step in converting the interpreter’s subsurface representation into a seismic viewis 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 sourceand receiver were located at the same point on the surface the calculation of two-waytravel times is simplified. As mentioned above, the coincident-source-receiver travel pathis one along which reflection takes place at normal incidence to the reflecting interfaceDipping Reflector Horizon: The coincident-source-receiver format of the data yields anaccurate representation of subsurface structural interrelationships only for the trivial caseof horizontal layers as noted earlier. When the reflecting surface dips, ray paths travel tothe receiver from points up-dip (see figure below). The seismic image of the reflector (therecord surface) suggests that the reflector is longer than actual and has less dip. 25
  26. 26. Syncline: The distortions become more serious with increased structural complexity. Theseismic expression of a syncline (below), for example, leads to the appearance of ananomalous anticline beneath the axis of the syncline. The limbs of the syncline, AB andCD appear down dip. Since they dip in opposite directions they can actually appear tocross over each other (below). Normal incidence reflections across the axis of thesyncline (reflection points 1 through 5 in figure below) are reflected back to the surfacein reverse order, right to left. Ray paths cross each other at a focal point. Travel timedown and back from the hinge of the syncline are shorter than those to either side. Thenet effect is that the seismic image portrays the hinge area of the syncline as an apparentanticline (see below). In addition the lateral extent of the syncline has been reduced. Thepitfall in this for the interpreter is obvious and more than one unsuspecting company hasdrilled the apparent anticline (reverse branch) only to find themselves in the depths of asyncline. The reverse branch arises when the focus is located beneath the surface.Anticline: Normal incidence reflections across an anticline (below) shows that ray pathsare spread out from the limbs of the anticline in the down dip direction. The net effect isthat the seismic appearance of an anticline (in time, below) has much broader aerialextent. Again the seismic appearance is a misleading representation of the subsurface. Ifuncorrected, the seismic view suggests more extensive closure and reservoir capacity. 26
  27. 27. Fault: The seismic expression of faulted horizons can be quite varied. The simple caseshown here (see below) portrays normal offset of a layer accompanied by minor upliftleading to diverging dips on opposite sides of the fault. Reflections from the faultedhorizon produce an apparent shift of horizon segments down dip. The apparent fault gapappears wider than it actually is.Diffractions arising from faulted edges of the horizon (see figure below), fan out acrossthe surface leading to the appearance of hyperbolic events in the seismic section. These 27
  28. 28. diffractions may suggest the presence of rollover into the fault. In general the interpreterfinds 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 PITFALLSThe 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 MODELSSeismic modeling is routinely undertaken at the computer workstation. Computer modelsof the above examples are shown below. A single flat layer has been added beneath thedeformed horizon in each of these models to illustrate additional distortions that arisefrom velocity variation. In each model the velocity above the deformed horizon (Va) is15,000 feet per second and that below (Vb), 20,000.Syncline: The ray-tracing here (below) is much more thorough than in the precedingexample. The computer can compute and draw these ray paths much more quickly thanwe can. Note the familiar features in the diagram including the buried focus and the travelof reflection events from the reflector surface to receivers down dip. Reflections from thelower horizon are incident at right angles and return to the receiver along their downward 28
  29. 29. path. As the rays travel back to the surface they pass from the deeper high velocity layerinto the lower velocity surface layer and are refracted toward a line drawn normal to thereflector surface.In the time display (below) the reverse branch and crossing synclinal limbs are expectedbased on our previous discussion. However, the appearance of the underlying reflectorsuggests that it may also have experienced a similar level of deformation. Ray pathstoward the edges of the model travel through a greater thickness of the higher velocitymedium than do rays traveling downthrough the hinge area. While the lengthsof the travel paths do not vary greatly, thetime taken to travel these different paths isless in proportion to the distance traveled inthe high velocity layer. Rays make theirway down and back more quickly high onthe limbs of the syncline than do rayswhich travel through a much greaterthickness of the low velocity mediumoccupying the hinge area of the syncline. 29
  30. 30. Anticline: Computer ray tracing was performed acros the more complex anticlinalstructure shown below. Based on the preceding discussion, we expect the reflectionsfrom the crest of the anticline to fan out and produce an anticline with much broaderappearance in the time section. However, note that we have a tight syncline sittingbetween 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 focusevent, and what we should expect to see in time is another anticline – not a syncline. 30
  31. 31. Normal incident rays rising from the lower interface are refracted toward thenormal in accordance with Snell’s law. Travel times to and from the underlying flathorizon (Figure) decrease below the anticlinal hinge and increase down the limbs takingon an anticlinal form. Can you spot the reverse branch and apparent anticline arising from the base of the syncline? 31
  32. 32. VELOCITY PITFALLSAlong with the class of geometrical pitfalls there are also pitfalls or distortions associatedwith subsurface velocity distribution. Did you notice anything unusual about the timesection 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 fasterthan in the overlying layer. Thus, two-way travel times to the deeper reflector on eitherside of the syncline arrive much earlier than do reflections from the same depth thattravel through the axis of the syncline. Velocity distribution in the syncline modelproduces a “sag” in the reflection from the deeper flat horizon beneath the syncline, sincethe syncline contains a much thicker section of low velocity strata.The example below illustrates a combination of velocity and geometrical pitfalls inherentin the seismic time section. In this example, a seismic line crosses a reef. 32
  33. 33. The ray path diagram shown below suggests that the recordings of reflection travel timesin the normal incidence format simulated by the CMP stack trace will be complicated andnot directly related to the structural features portrayed in the depth section above. 33
  34. 34. How might the time section shown below, lead to incorrect interpretation of subsurfacestructure? Seismic section over the reef.SEISMIC WAVELETS, DECONVOLUTION AND STRATIGRAPHICINTERPRETATIONThe seismic wavelet refers to the mechanical disturbance, generated by the seismicsource that travels through the subsurface. The impact of a hammer produces a jolt ofenergy that passes quickly. A charge of dynamite when detonated rapidly deforms thesurrounding area and sends out a shock wave which may be felt as a rapidly passingshake of the ground. It is the temporal characteristics of this pulse of deformationproduced by the seismic source that we refer to as the seismic wavelet, seismic pulse, orjust wavelet. An example of a seismic wavelet is shown below. Note that time plots leftto right. WAVELET A Basic Seismic WaveletWavelets come in many different shapes and sizes. Another wavelet is shown below. 34
  35. 35. Note that this wavelet is more compact or has shorter duration than the one above. WAVELET BSeismic data processing is a fascinating field of study. There are many techniques appliedto seismic data that enhance the quality of the seismic image and help improve theresolution 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 aprocess applied to seismic data to reduce the duration of the seismic wavelet. It is aprocess which can transform wavelet A into wavelet B shown above. The benefits ofdeconvolution become evident when we think about resolving the top and bottom of alayer. If wavelet A is reflected back to the surface from the top and bottom of a reflectiveinterval, note that the long duration of the reflection event from the top of the layer willprobably overlap or interfere with the reflection from the base of the layer, making itdifficult 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 AThe section above is actually a synthetic or model (made up) seismic data set. Thestructural and stratigraphic features in the model are representative of graben structuresencountered in the North Sea. Note the obvious stratigraphic pinch-out. This would makea nice stratigraphic trap. Now take a look at the seismic section below. 35
  36. 36. Section BWhat happened to that pinch-out? The geologic model of the area is shown below. Thereflective properties of each layer are defined by the velocity contrasts shown in the crosssection. 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 wasreflected from the interfaces between layers. In the first seismic section, Wavelet A wasused; in the second, Wavelet B.Note that the Upper Jurassic Hot Shale and Callovian Shale are capped by a basalCretaceous marl/limestone unit. A complex deformation history is revealed by thevariations in thickness of the different units across the normal faults bounding the horst 36
  37. 37. and across the top of the horst-block itself. The Hot Shale and Callovian Shale do notpinch-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 firsttwo cycles in the wavelet have relatively high amplitude. When this wavelet reflects fromthe basal Cretaceous interval across the top of the horst-block, the initial reflection isaccompanied by all the cycles in the wavelet. (Wavelet A). In section A the follow cyclesof the reflections from the basal Cretaceous follow beneath and drop with the reflectorleft to right across the horst-block; and as they do, they intersect the reflection event fromthe base of the Hot Shale and Callovian Shale. The result gives the illusion that theseJurassic shales pinch out againts the basal Cretaceous.When the seismic data is deconvolved (i.e. when wavelet A is transformed into waveletB, 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 tocomplicate the appearance of the seismic section.Deconvolution produces significant improvement in the resolution of geologic features inthe seismic section. However, even with this simplified, more compact wavelet, we arestill faced with resolution limitations when the two-way travel times separating reflectorsare less than the duration of the seismic wavelet. Overlap becomes a problem again, andwe loose the ability to identify relatively thin layers.To sharpen your interpretation skills try your hand with the section below. On a separatesheet 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 stratigraphicintervals continue across the axis of the anticline? 37
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