Seismic Interpretation | What is seismic interpretation?

(adsbygoogle = window.adsbygoogle || []).push({}); Learning Objectives After completing this topic "Introduction to Seismic Interpretation", you will be able to: Define the ultimate goal of seismic prospecting. List the four basic steps in structural seismic interpretation. Identify where to pick the horizon on the seismic waveform and why. Describe why we initially follow the practice of timing at regular intervals of reflection time. Explain why migration changes the reflection times on dip lines, but does not change them on strike lines. Introduction to Seismic Interpretation The ultimate goal of seismic prospecting is to map certain features of the subsurface that can be identified as geologically significant in order to locate oil and gas targets for economical production. Seismic data is acquired by field crews during the acquisition phase using highly specialized equipment that records acoustic reflections of the subsurface. The main characteristics of seismic data are: It is band limited the low end is 5-10 Hz and the high end is 50-100 Hz. Subsurface spatial coverage is redundant yet incomplete. It produces large data volumes (up to tens of terabytes) with: 2000-4000 time samples per trace 6-12 second record length 100,000-1,000,000 spatial locations 12-1,000 fold redundancy Most seismic reflection techniques use only compressional waves because these are easier to acquire, resolution and data quality are generally better, and they are more sensitive to fluid properties. The use of shear and converted wave data is increasing in seismic acquisition because these waves may provide good images where compressional data cannot. These waves are sensitive to porosity and insensitive to pore fluid, and can be combined with compressional data, which provides more information about rock and fluid properties. As seen by seismic waves, earth properties are: Inhomogeneous Attenuative Anisotropic Porous Fluid filled (adsbygoogle = window.adsbygoogle || []).push({}); After acquisition, data processing produces images of the surface in time. The seismic reflections are then analysed (which is known as interpretation) in terms of geological elements. Interpretation is the conclusive process in seismic prospecting. Good quality seismic data can contribute extensively to the definition of some key subsurface characteristics and properties, for example: Depth Geologic setting - origin of rocks/fluids Geologic structure Geometry – thickness, areal extent, volume, and seals Rock type Heterogeneity – layering, faults/fractures, and compartments Porosity Fluid content/distribution Pressure distribution Mechanical strength Permeability Drive mechanisms Temperature The objective of seismic acquisition and processing is the accurate interpretation of seismic data. This includes the analysis and understanding of signal travel times, amplitudes, attributes, and ties to subsurface controls. Interpretation is complicated by several elements, including (to name a few): Noise or reflections from non-geological features Velocity variations in depth and lateral extent Variations of reflection angles Weak or discontinuous reflections Seismic interpretation is performed to understand subsurface structures, stratigraphy, lithology, fractures, pressure, pores, and fluids. Structural interpretation is the analysis of the geometry of the reflectors, whereas stratigraphic interpretation is mainly concerned with the character of the reflections that are related to lithology and depositional environment. The identified reflections are finally presented as depth profiles, depth contour maps, thickness maps, etcetera. Seismic data is the only lateral continuous information available for the subsurface. It provides an important basis for the up and down scaling process, to match the actual geology, or the simulator properties, or lithologic details. Seismic data resolution provides a link between reservoir simulations and well logs. Following are the typical resolutions for different data types: Simulations: hundreds of meters Seismic data: tens of meters Well logs: centimeters to meters Core samples: fractions of a centimeter Figure 1 is a graphical representation of the scaling process of geological characterization. (adsbygoogle = window.adsbygoogle || []).push({}); Figure 1 is a graphical representation of the scaling process of geological characterization. Process of Seismic Interpretation https://youtu.be/v9GddXq4UBs Process of Seismic Interpretation Seismic interpretation is the connection between geology and geophysics, where the truth about the subsurface environment becomes evident. As explained in the video, there are four basic steps in structural seismic interpretation: Picking: Identifying the horizon (the top surface of a rock layer) to be mapped. The structure can be defined, along the line of the section, by making measurements of the depth of the reservoir below the datum. Timing: Measurement of the reflection time from the datum to the picked reflection. Posting: Transferring the measured reflection times to the map. The depth values can be posted at the corresponding shotpoints on a shotpoint location map. Contouring: Showing the structure or the relief of the chosen horizon on the map. The maximum hydrocarbon content is then defined by the spill-point contour, for example, the oil-water contact (OWC) shown in Figure 2. The closure and the maximum area of pay can be read from the map. Figure 2: Contour map with oil/water contact (OWC) and gas/oil contact (GOC) (adsbygoogle = window.adsbygoogle || []).push({}); There are several important principles related to the geological message in the seismic trace. The seismic method measures only physical properties and their changes; geological properties can be obtained only by inference. Reflections evident on a seismic section have a time and a character: The reflection time is an expression of the depth and dip of a geological surface, and of the velocity to it. That surface, in almost all cases, locally approximates the surface of the solid earth at some time in the past; thus it may be a bedding plane (representing a one-time surface of deposition) or a local portion of an unconformity (representing a one-time surface of erosion or non-deposition). Studies of reflection time allow us to see the folding and faulting of a geological surface, and therefore, to recognize structural traps. Further, they allow us to see angular unconformity between two surfaces so we can recognize truncation and erosion. They also allow us to see patterns of surfaces so we can identify certain depositional mechanisms; from these, in turn, we can make inferences about facies. The reflection character is a combination of amplitude, waveform and polarity. It is an expression of the physical properties above and below the reflecting surface, and of interference between reflections from closely-spaced reflecting surfaces. Studies of reflection character allow us to map variations in the nature of the rocks, to supplement our inferences about their type and condition. With respect to basic seismic interpretation, our primary concern is with studies of reflection time, and with the correct representation of geological surfaces. This is of paramount importance, of course, to the location of structural traps. Thereafter, we may pay more and more attention to reflection character, and to its implications for stratigraphic traps. (adsbygoogle = window.adsbygoogle || []).push({}); Where to Pick (adsbygoogle = window.adsbygoogle || []).push({}); Determining where to pick the horizons in seismic requires knowledge and skill. Initially, we should stress the importance of looking at all the seismic data of the survey, and of getting an overall feeling for the behavior and character of important and key reflectors over the whole area. One of the functions of wavelet processing is to shape the reflection pulse to a form that is symmetrical about the maximum of its envelope. Figure 1 will help us explain this process. A discrete reflection, like that in image A before wavelet processing, would look like image B after this part of the wavelet-processing action. This manipulation is termed phase-zeroing or bringing to zero phase. After wavelet processing, the reflection pulse (and its envelope) are nominally symmetrical about the envelope maximum. The envelope maximum may coincide with a white trough (image B), or, if the initial wavelet has a form like image C, the maximum may coincide with a black peak (image D). Figure 1: Before and after wavelet processing The visual effect of the zero-phase process is not major; reflections move earlier, and their forms become a little sharper. However, the process (to the degree that it works) does make it much easier for the interpreter to decide where to pick. On marine sections shot in deep water, the success of the zero-phase process can be assessed from the sea-floor reflection; because this reflection is usually fairly discrete, its form should approximate zero-phase symmetry. Where we have the benefit of a discrete reflector, there is no doubt where we should pick a reflection: we must pick the peak or the trough which lies at the envelope maximum. No other pick is acceptable on a discrete reflection. If the envelope maximum is at a trough, we must pick the trough; if it is at a peak, we must pick the peak. All our work in other topics (concerned with timing accuracy, velocities, and multiple reflections) will confirm the validity of this rule. (adsbygoogle = window.adsbygoogle || []).push({}); Of course, reflections are not always discrete; often we can see a band or complex of reflections which merge together into one long wave train. Then, we scan all the lines of the survey, and determine whether (in general) the reflection character shows just one envelope maximum, or if there is a "saddle" in the envelope. If there is a saddle, we are justified in making two picks, corresponding to the two envelope maxima provided. Of course, the extra interpretation effort brings some exploration benefit. A particular situation where we wish to do this is at the top and base of a reservoir layer. When a layer thins, we may obtain the reflection appearance of Figure 2. The reflections from the top and base can be seen separated at the right-hand side; there is a saddle in the envelope. Using envelope maxima, we pick the top as a white trough, and the base as a black peak. As will be discussed in other related courses, this does not quite provide the best or correct answer, but any other approach suggests an interpretation which is not optimal. Figure 2: Wavelet responses to a thinning layer What if the layer is not thick enough, anywhere, to show us the top and base separately—we do not have the three or four traces on the right of Figure 2 as a basis for a starting point. We go to pick the envelope maximum, and find that it does not correspond to a peak or a trough. This tells us that we have a thin layer, and that our reflection is actually an interference between two reflections. If the layer is our target, we pick both the trough and the peak; again we shall find that this is not perfect, but it is the best we can do. If we have a long reflection train that does not show any saddle in its envelope, the train is almost certainly composed of three or more reflections. We pick what seems reasonable, taking into account whatever is known about the geology and the importance of that level in the exploration play. Most interpreters would pick the first major peak or trough to show good continuity, and label that pick "near-top-yellow complex." Typically picks are given color names until we correlate these with an event on a well or tie with another section. Yellow is often the first color used, then other colors follow. Now what about older sections without wavelet processing, or modern sections in which the success of the process is in doubt? The traditional approach has been to pick the first clear trough of each reflection; the trough in order to make the color visible, and the first trough to minimize the lag. Since any reflection pick in this situation is subject to a lag, it may be better to accept a later pick (and a somewhat larger lag) if the continuity is thereby improved. And, to the degree that the continuity depends on the signal-to-noise ratio, the best continuity is always found near the envelope maximum. The general picking recommendation is to pick any convenient point on the reflection-peak, trough, or zero-crossing which is near the envelope maximum. All of this must be balanced with common sense. In many poor-record areas, interpreters have no choice. They thankfully pick anything at all which has some appearance of continuity. Finally, we should note again the importance of looking at all the data of the survey, and of getting an overall feeling for the behavior of important reflections over the whole area. (adsbygoogle = window.adsbygoogle || []).push({}); Where to Time (adsbygoogle = window.adsbygoogle || []).push({}); In discussing the basics of interpretation, we initially follow the practice of timing at regular intervals of reflection time; in fact, we select the timing at the contour interval. This is done for simplicity of explanation, because it eliminates the need to discuss the interpolation (sampling) of time values. In fact, the normal practice of interpreters is to time the picked reflection at a regular progression of shotpoints, rather than at a regular progression of times, and then to interpolate the contour times between the timed shotpoints. The problem of faithfully recording a continuous variation (such as reflection time along a line) by a series of discrete measurements made at regular intervals is the process called sampling. This process is subject to the sampling theorem. It makes no difference whether we sample at regular intervals of shotpoints or at regular intervals of time, provided that the measurements are made at appropriate intervals. Thus a long and simple monocline can be specified by just two widely-spaced measurements, but a rough eroded surface requires at least two measurements per cycle of the corrugation. Further, according to sampling theory, an additional step (reconstitution) is necessary before values can be interpolated between the measured values. The reasonability of the sampling theorem is illustrated in Figure 1. In image A, we see an anticline whose crest has collapsed (blue line) because of irregular extensional faulting. The black dots are the time values corresponding to a regular progression of shotpoints across the anticline. The increment of the progression of shotpoints is too coarse because, when we post and contour these values, the structure appears as shown by the dashed green line, and all memory of the collapsed crest has disappeared. In image B we see a major high, a second high, and a small saddle between them. The time values are again represented by the black dots, the shotpoint values corresponding to regular intervals of reflection time. Again, the interval is too coarse because when we post and contour these values, it is not possible to determine whether the high is a single or double. Figure 1: Regular sampling of anticlines (adsbygoogle = window.adsbygoogle || []).push({}); To be completely sure, therefore, it is necessary to time the reflection at least twice for each cycle of undulation of the reflection surface. It is also necessary to apply the process of reconstitution (restoring the structure) before interpolating between these times for contouring purposes. The best practice when working at an interpretation workstation is to follow the sampling theorem, because then the interpretative software program can handle the resulting large volumes of data. In practice, however, interpreters disregard the sampling theorem when interpretation is done by hand because the volume of interpretation work would be too much. So, we bypass the sampling theorem by making some reasonable judgements, and by applying some reasonable standard processes: During picking, we ignore or smooth all small-scale trace-to-trace variations which we interpret to be due to noise, poor static corrections, or artefacts of the seismic method. In doing this, we are applying the criterion of geological plausibility. We look at the sections overall, in terms of the exploration play, and judge how much of the relief in the reflection surface is material; anything less than this we judge to be an artefact of the acquisition process and therefore of no importance to finding oil or gas. We select a contour interval which provides at least two contours (and preferably three or four) over this minimum significant relief. If we wish to time the picked reflection at a regular progression of times, we do this at the chosen contour interval. In this case, no interpolation of times is necessary in the contouring, and so we are not at risk for failing to reconstitute the data samples. If we wish to time the picked reflection at a regular progression of shotpoints, we look at the sections again, and decide what convenient shotpoint interval corresponds approximately to the contour interval on the steepest flanks of the smallest material relief. In this case we are at risk for failing to reconstitute. This can be seen in Figure 1A; if it happens that the times at A and B are 2.007 and 1.987 seconds respectively, then a simple interpolation of the 2.000 second contour is significantly in error. (As noted above, this problem does not arise if we time at the chosen contour interval.) When we elect to time on a regular progression of shotpoints, therefore, we err on the side of caution, and adopt shorter intervals if there is any doubt. Whether we sample regularly in time or in shotpoints, the major protection against our disregard of the sampling theorem is that we mark on the map the position and time of every significant high and every significant low, and the position and throw of every significant fault. This step saves us from mistakes like those in Figure 1. This should be a standard practice. Finally, we gain additional confidence and insurance by marking on the map zones of extended monoclinal dip and of zero dip. Sensible modification of these rules may be indicated in practice, depending on the scale of the sections, the eye and hand of the interpreter, the software program available, the admissibility of skipping some measurements over featureless zones, and whether the timing is to be done by the interpreter or by a technician. As indicated by the examples in the text, the important thing is to learn by doing; a variety of examples quickly shows the dangers of violating the sampling theorem, and quickly provides practical reasons for the choice between regular intervals of time and regular intervals of shotpoints. (adsbygoogle = window.adsbygoogle || []).push({}); Migrated and Unmigrated Sections and Line Ties (adsbygoogle = window.adsbygoogle || []).push({}); Animation 1 reminds us that on a dip line the reflection occurs updip from the vertical through the point of observation on the surface, but that an unmigrated section displays the reflection on the vertical. The process of migration moves the reflection back, by a distance of h, to where the reflection should be. Location of a reflection on a dip line and a normal unmigrated section Animation 1 https://youtu.be/uGC4zkxYo_w Location of a reflection on a dip line and a normal unmigrated sectionAnimation 1 Figure 1 is an idealized counterpart of Animation 1A. Figure 1: Migration moving the reflection back to its proper location to the right (adsbygoogle = window.adsbygoogle || []).push({}); Figure 2 extends the situation into three dimensions. We see that on a strike line (Line 2) the reflection still comes from updip of the vertical plane of the section, but is again presented as being in that vertical plane. However, the reflection appears to have no dip, and therefore the process of migration leaves it substantially unchanged. Thus, migration changes the reflection times on dip lines, but does not change them on strike lines. It follows that sections which tie properly on unmigrated sections do not tie, in general, on migrated sections. Figure 2: 3D view of the migration geometry Let’s discuss how to handle these issues. We like the improved clarity of the faults on migrated sections. We accept that where a reflecting surface is very rough (Figure 3, for example), migration is necessary before we can even begin to pick it. But we are reluctant to lose that confidence-building check that we get by tying unmigrated sections at the line intersections. Comparison of Unmigrated and Migrated Seismic Sections compare the two images Figure 3: Unmigrated and migrated sections of the same line The optimum process depends on the circumstances: (adsbygoogle = window.adsbygoogle || []).push({}); Where the structures are smooth, gentle and unfaulted, and there is no problem of following the continuity of the target reflection(s), we prefer to use unmigrated sections. This gives us our line ties. It places the structural axes correctly, but makes the anticlines broader than they should be, and the synclines narrower, as seen in Figure 4. We time, post and contour the unmigrated reflection(s), thus obtaining an unmigrated time structure map. Finally we migrate the contour map, and so obtain the best of both worlds. Figure 4: Unmigrated versus migrated sections showing anticline (left) and syncline (right) structural changes In fact, there are few prospective areas where the structures are smooth, gentle and unfaulted. Consequently, the vast majority of seismic sections on dip lines, or across faults, are clarified by the migration process.Many interpreters call for both migrated and unmigrated sections on all lines, whatever their orientation. This invariably brings up the concern as to what to do when the unmigrated sections can be picked, but the migrated sections are clearer locally. We prefer to pick the unmigrated sections, but we use the migrated sections to guide our picking. Then we proceed, as before, to the migration of the unmigrated contour map. In this last stage, it helps if we can specify, from the migrated sections, the true position and throw of the faults. As the complexity of the geology increases, we come to some point where the improved clarity of the migrated section outweighs the comfort of exact line ties. If we have a grid of lines which are close to being on dip and on strike, and if the faults are generally on strike with the structure, we pick and time the migrated dip lines first. Next, we pick and time any strike lines which happen to cross the dip lines in zones of small dip; there we expect good ties. Then, in the vicinity of the other line intersections, we search the dip-line sections for any identifiable reflection feature (perhaps a small fault) that is recognizable on both migrated and unmigrated sections (Figure 3), and we assess how far the migration process has moved it. The distance h tells us where the line intersection really is in the subsurface.We then time the reflection on the strike line. We do not use the absolute reflection times, but only the differences of reflection time relative to the times at the line intersections. These differences are then posted on the map in the time subsurface position of the strike line (which is, of course, the distance h updip from its surface position). Finally, we contour the absolute reflection times from the dip lines, using the time differences on the strike lines to guide our contouring between the dip lines. In the real world, many lines are neither on strike nor on dip, reflection quality is variable, and faults cross all the lines at unfavorable angles, so interpreters do the best they can. Aiding interpreters is the first rough map which shows the highs, the lows, the steep dips, the shallow dips, the flat areas, the fault locations, the zones of bad data, and any other obvious anomalies. From these indications, we have a crude picture of the structure, and the likely effects of migration and poorly migrated events. It is important that interpreters carefully think about their method of incorporating migrated and unmigrated sections and understand the allowances they make at the line intersections. (adsbygoogle = window.adsbygoogle || []).push({}); Assessment Seismic Interpretation (adsbygoogle = window.adsbygoogle || []).push({}); 1. Identifying the horizon to be mapped is the definition of which of the following processes of seismic interpretation? A .PostingB .ContouringC .TimingD .Picking 2. On marine sections shot in deep water, the success of the ______________ process can be assessed from the sea-floor reflection. 3. What is the process called of faithfully recording a continuous variation (such as reflection time along a line) by a series of discrete measurements made at regular intervals? A .ConvolutionB .SamplingC .MigrationD .Deconvolution The seismic method measures only ___________ properties and their changes; __________ properties can be obtained only by inference. Recommended for You (adsbygoogle = window.adsbygoogle || []).push({}); Interpretation with Seismic Attributes Seismic Interpretation Methods Seismic Stratigraphy Structural Interpretation