lecture - 07 march 27 2015 friday.unlocked

Department of Earth & Environmental Sciences,

Bahria University, Karachi Campus

www.bahria.edu.pk By: M. Hammad [email protected]

3D SEISMIC INTERPRETATION (GEO3D SEISMIC INTERPRETATION (GEO--518)518)

M. S (Geophysics)M. S (Geophysics)

By InstructorBy Instructor

M. Hammad ManzoorM. Hammad Manzoor

March 27, 2015 (Friday)March 27, 2015 (Friday) Lecture # 7Lecture # 7Lecture # 7Lecture # 7



3-D SEISMIC INTERPRETATION


(Structural Overview & Skelton Interpretation)



Structure Overview & Interpretation Skelton

A general sequence of events for interpreting a 3-D volume is to;

Tie well data and surface geology to seismic data Integrate other Geoscience software Use movies to scan the 3-D volume


Use movies to scan the 3-D volume Identify the faulting regime Divide the data using structural "blocking" Develop an initial geologic model Define a skeleton interpretation of the entire data volume Tie loops and infill lines Generate final contour and attribute maps Convert from time to depth





Structure Overview & Interpretation SkeltonTie Well Data & Surface Geology to Seismic Data




We start our interpretation with the absolutely critical step ofdisplaying our well logs and the synthetic seismograms we have generated foreach well. With time and effort, it is possible to get good correlations betweenwell data and seismic data, and it is worth the effort to do so at the beginningof the interpretation.


Wells should be tied to the seismic data nearest to the actual welllocation. Ensuring the correct position of all well heads and deviated well boreson a display is as much the responsibility of the interpreter as it is theresponsibility of the individual who loads the data.

Also, be sure to set your projection limits to reasonable distances toavoid confusion and unnecessary delays in extracting and posting wellinformation for sections as they are displayed.




Attention to detail at the beginning of the project will save youcountless headaches later. If your initial positions are wrong, nothing else willever be right. Take the necessary time and effort to check locations and makesure that they are correct on both the maps and seismic displays.

Synthetic seismograms can be posted at the well location either as


Synthetic seismograms can be posted at the well location either aswiggle or color variable density display traces, which are overlaid or insertedinto the seismic data at that point. Some systems can even display the synthetictrace down the track of a deviated well bore for more accurate matching ofdepth events to time.

Correlation capabilities for synthetic seismograms vary from system tosystem. The ability to correlate is, however, predicated on the basic paperfunction of being able to move the synthetic around on the section and to shiftit up and down to match the reflections derived from the well logs with thosethat appear on the seismic section.




Some systems allow the interpreter to fix or "peg" particular events onthe synthetic seismogram to events on the seismic, and to stretch and/orcompress either the entire synthetic or segments of the synthetic seismogrambetween defined points to make it correlate to the seismic data.

Poor correlations between synthetic and seismic are normally caused by:


Poor correlations between synthetic and seismic are normally caused by:

Incompatibility between the phase of the synthetic seismogram and the seismic data

Poor sonic log calibration poor quality sonic or density log data poorly processed seismic data




Another very effective tool for tying well data to surface seismic data andidentifying the lithologic origin of reflectors is the Vertical Seismic Profile (VSP):

The VSP acquires seismic data with geophones inserted into thewellbore, rather than along the surface. We present a detailed discussion of themethodology in the Vertical Seismic Profile module, contained in the series


methodology in the Vertical Seismic Profile module, contained in the seriesentitled 3-D and Other Geophysical Methods.

The VSP produces a measured seismic response of the subsurfacearound the well location, plotted against time and depth. The depth function isknown because we know the depth of the geophones within the borehole.Typically, measures are made with the geophone placed at 50-100 feet (15-30meter) intervals from the bottom of the well, up to a point where achievinggood data is prevented by multiple casing strings.




A VSP display is produced in the form of a corridor stack or transposedstack. The corridor stack looks like a synthetic seismogram and, like thesynthetic, has a known phase wavelet (generally zero) contained in it.

The transposed stack is different in that, additionally, it containsinformation on the dip of the beds at the well location -information that is


information on the dip of the beds at the well location -information that isinherent in the VSP data. Having both event and dip correlation available isparticularly valuable when tying wells in complex structural regions or when adetailed stratigraphic correlation is needed.





Structure Overview & Interpretation SkeltonPreliminary Fault Correlation & Structural Blocking



Structure Overview & Interpretation SkeltonPreliminary Fault Correlation & Structural Blocking

The first active step in most interpretations is to get a sense of thefaulting regime.

The major faults in the data set are identified and roughed-in usinginlines, crosslines and time slices to tie the data together.


This does two things :First, it allows the interpreter to get a sense of the tectonics and structure ofthe prospect and,

Second, it helps to identify and delineate the major structural blocksthat make up the area. Fault digitizing at this point can be as detailed as theinterpreter chooses to make it, and faults may be stored in a single, generic fileor as named faults. Fault picking is invariably done in manual, point mode.





Structure Overview & Interpretation SkeltonInterpreting the Skelton




The 3-D interpretation proceeds by roughing in the limits of the surveyfor the horizons being picked. This process is sometimes called a Skeletoninterpretation because it forms the bare bones of the seismic interpretation,which will be fleshed out through the remainder of the interpretation process.There may be areas, or structural blocks, which cannot be easily tied in to theoverall skeleton due to lack of data, poor data, or large discontinuities.


overall skeleton due to lack of data, poor data, or large discontinuities.

However, the process does its best to establish the interpretationframework for the important horizons throughout the area of the 3-D survey.

The order of picking the skeleton framework given here is not cast instone, but rather it is offered as a guideline. Use whatever displays are necessaryin whatever order they are needed to establish the correlations of the skeletoninterpretation and to tie data across the survey.




Some geophysicists like to begin by picking in the inline direction. Ifthe survey was designed and oriented correctly, the inlines are likely torepresent the direction of primary dip.

Start the skeleton interpretation by bringing up an inline that eitherintersects or is within one line spacing of a synthetic seismogram.


intersects or is within one line spacing of a synthetic seismogram.

Use the synthetic and the well log data to determine which reflectorscorrespond with which geologic horizons.

Extend the horizons from the well, one at a time, by picking shortlateral segments on the inline for each horizon. Repeat this step for each wellthat falls on the line.




The next step is to extend the interpretation of the horizons across theinline section (Figure : Skeleton interpretation -picking the first Inline).





Because we obtained a firm tie between the well and the seismic data(in the first step), as well as a preliminary fault pattern analysis, it should not betoo difficult to pick the horizons across the full extent of the section.

If necessary, leave the areas around faults un-interpreted if the horizontermination cannot be accurately determined. Rough in the faults for now, and


termination cannot be accurately determined. Rough in the faults for now, andfine-tune your interpretation later.

Having picked the full inline section for all of the horizons of currentinterest, proceed to the crossline that intersects the well you used to tie theinline, or the crossline that passes nearest to that well.

Display the well log and synthetic seismogram, and make sure that thedisplay of the intersections of the picks (penetration points) made on the inlineis activated on the display.




Now proceed to make the correlation between the picks on the inlinesection and the corresponding horizon reflections on the crossline section(Figure : Skeleton interpretation -picking the first Crossline).





After We are certain of the horizon ties correspond between the inlineand the crossline, then complete picking each horizon across the full extent ofthe crossline section.

Check the database and redisplay the saved horizons to ensure that thesystem is storing your picks correctly.


system is storing your picks correctly.

After completing this step, you will have established correlations for allof your major horizons between the well log and lines that span the entireextent of your data volume in both directions.

Repeat this process for every well in the survey, tying the logs and thesynthetic seismograms to the seismic data along the nearest inline andcrossline.




You will begin to intersect previously picked lines, tied to syntheticseismograms in other wells, as you work through this process. Be careful to notewhether or not your picks on each new line tie correctly with the picks onprevious lines.

Make sure that your posted base map display is activated so that you


Make sure that your posted base map display is activated so that youcan watch your picks being posted in color-coded time values on the base mapas you work. This can also help you identify interpretation errors when theyoccur (Figure : Skeleton interpretation -inlines and crosslines picked for allwells).




Now, you have a loose grid of interpreted lines tying together eachwell with lines that extend across the entire survey area. If inconsistencies andmis-ties are found at the line ties, it will be necessary to go back and comparethe lines where these problems occur and, perhaps, re-correlate the syntheticseismograms to resolve these problems.


Once the skeleton structure of the orthogonal vertical section is inplace, proceed to interpreting time slices (Figure : Skeleton interpretation -timeslice picks added).




Looking at a typical vertical inline, select a time slice that intersects oneof the interpreted horizons and bring it to the screen.

The intersections of the picks made on the vertical sections(penetration points) will be displayed on the time slice to give you a guide forpicking the horizon across the time slice.


picking the horizon across the time slice.

Carefully pick the desired phase for the horizon across the time slice.Note that the time slice interpretation looks like a rough contour lineconnecting all of the other picks from the vertical sections.

Make allowances for faults and note the fault offsets to help clarify themovement patterns for your data set. Pick the obvious faults either as namedfault planes or store them in the generic fault file.




Repeat this process on at least one time slice for each horizon you arepicking. When this step is complete, you will have tied together all threedimensions of the data and tied all of the reflectors directly to the well data.

An excellent quality control tool, and the final arbiter of the correctnessof the ties of one well to another, is provided by an arbitrary well tie line (Figure:


of the ties of one well to another, is provided by an arbitrary well tie line (Figure:Skeleton interpretation -arbitrary well tie line added).




This line creates a seismic cross section between wells. It is notnecessary to wait until this point in the skeleton interpretation process to bringin these arbitrary lines. They should be used as needed to gain a full andcomplete picture of the data and how the wells tie together.

Bring up one or more well tie lines, of however many panels are


Bring up one or more well tie lines, of however many panels arenecessary. Interpret them as you did the inlines and crosslines, tying them to thedata in the well logs and on the synthetic seismograms.

Try to include single panel lines connecting each of the wells incombinations of two this is much more effective than trying to tie everythingon one, massive, multi-panel line.




Extend the lines all the way to the limits of the survey, if possible, togain the full advantage of the well-tie correlations throughout the data set(Figure: Skeleton interpretation -closed loop at survey boundaries).






Structure Overview & Interpretation SkeltonUsing Maps During the Interpretation




Four Types of Maps; Ribbon Maps, Quick Maps, Interpretative Maps, & Mis-tieMaps

Most systems can produce maps which are posted with color-coded

time values in real time as the interpreter works. These are called RibbonMaps


Maps. This type of map is an excellent quality control tool, because grosserrors show up on the map as incompatible color changes.

It is not very accurate, however, for small mis-ties like those usuallyfound in a 3-D interpretation. A number of ribbon maps are shown during thedata set interpretation segment of the video which accompanies this subtopic.




Another type of map, a Quick Map, allows the interpreter to look atthe interpreted data as color-filled, contoured surfaces or sometimes as plaincontours.

These algorithms use a quick-gridding process, often triangulation, toproduce the map surface. Contours are then calculated and displayed, so the


produce the map surface. Contours are then calculated and displayed, so theinterpreter can get a map view of what the surface looks like, and how theinterpretation is progressing.




Another map style, the Interpretative Map, is a contour map onwhich you may edit control points and draw contours by hand. The valuablefeature of this type of map is that the edited contours, either calculated orhand-drawn, may be loaded back into the database and displayed as a pickedsurface over the seismic data.


This gives you the opportunity to iterate between the rather sterileworld of picking sections and the art form and geological expression ofmapping. It also allows you to test whether the data support what you feel themap should look like. Conversely, the map information can help point outtrends and possibilities that you might otherwise overlook in the seismic data.




With correctly interpreted 3-D data, there should be no mis-tiesbetween picks on inlines, crosslines, time slices, arbitrary lines, etc. But, as we allknow, errors occasionally happen. One method of identifying such errors is to

generate a MIS-TIE MAP, which will help the interpreter isolate andhighlight differences in interpretation where lines intersect.


This system works by looking for differences between lines -at a givenhorizon. Most mis-tie maps generate colored dots at line intersections wherethe difference in a picked value between two sections has a disagreement ofmore than some minimum value set by the interpreter. Using the mis-tie map,the interpreter still has to go back to the seismic data and resolve the conflicton a case-by-case basis, since no automated system for adjusting mis-ties canpossibly take into account the interpreter's subjective judgment regarding theaccuracy and quality of the picks.





Structure Overview & Interpretation SkeltonHorizon Interpretation



Structure Overview & Interpretation SkeltonHorizon Interpretation Generally, it is best to concentrate on one horizon at a time wheninterpreting a survey. We deliberately chose to pick several horizons whenbuilding up our framework during the skeleton interpretation phase, but thiswas primarily to avoid the unnecessary repetition of steps for correlatingsynthetic seismograms and well logs on each separate horizon on each line.During an actual interpretation, we normally focus on one segment of the


During an actual interpretation, we normally focus on one segment of thegeology at a time, so that we can have an undistracted view of what we aretrying to interpret and correlate.

Usually we begin infill picking on one boundary of the survey and stepthrough the sections in order, picking the horizon on each section in turn. Thisprocess is greatly facilitated on the workstation by the step function (sometimescalled a roll function) which allows the interpreter to display the next line of thesame general orientation (i.e., the next inline or the next crossline) by selectinga single menu function.



Structure Overview & Interpretation SkeltonHorizon Interpretation An increment may also be specified so that every other line or everyfifth line, etc., will be the next display called, depending upon the density ofpicking the interpreter is pursuing. Step functions work in all orthogonaldirections, and some systems even support stepping through a data setextracting sections at some interval parallel to an arbitrary line.


Composite displays that combine sections of more than oneorientation are also extremely useful for infill picking. These displays allow theinterpreter to carry correlations from one seismic display domain into anotherwhile confirming the validity of the ties at the same time. Vertical line-to-timeslice and concertina displays are the most common composite displays fortracking horizons over multiple sections.



Structure Overview & Interpretation SkeltonHorizon Interpretation Concertina displays (multi-window displays with different sections ordifferent attributes displayed in each panel) are frequently used to trackhorizons over parallel sections or define them more accurately by using morethan one attribute of each section. Some systems allow the interpreter toproject horizon and fault picks from other picked sections onto the currentsection to help reference and correlate horizons. These reference lines may also


section to help reference and correlate horizons. These reference lines may alsobe copied as live picks ready to be used or edited to fit the data on the newsection more accurately. This is a real time-saver in areas with little variationbetween sections and relatively flat reflectors.





Structure Overview & Interpretation SkeltonFault Interpretation



Structure Overview & Interpretation SkeltonFault Interpretation Faults are usually picked on sections when we pick horizons. Manyinterpreters like to pick faults first, in order to establish structural boundaries onthe section before trying to decipher how the horizons correlate. Often,however, it is necessary to concentrate on picking only faults for a few lines inorder to resolve small faults or complex fault movements. It is also helpful toassociate fault picks with fault planes.


associate fault picks with fault planes.

Most systems use a special fault symbol for horizons that terminateagainst faults. Some systems even calculate the fault heave and throw based onthe placement of these fault symbols! Some faults are obvious, but some arevery difficult to resolve and define. Usually a line along the discontinuity canrepresent a fault, but sometimes an indeterminate zone is left in theinterpretation because the precise definition of the fault is not possible. Also,fault geometries change, and frequently we see branching or healing alongfaults.





Structure Overview & Interpretation SkeltonCorrelating Data Across Faults



Structure Overview & Interpretation SkeltonCorrelating Data Across Faults Once a fault is identified, it is given a name and treated like a surface.These named fault planes are just as important as horizon picks (or more so).Fault plane mapping and tracing the intersections of fault planes with horizonsurfaces are basic techniques for isolating hydrocarbon traps which arecontrolled by faulting.


The correlation of seismic data across faults is a much more intenseproblem that requires original thought and flexibility on the part of theinterpreter. Occasionally, correlating across a fault is simple because of smallamounts of dip-slip movement and some dominant characteristic of a horizonthat permits it to be identified on both sides of the fault.





Structure Overview & Interpretation SkeltonDepth Conversion



Structure Overview & Interpretation SkeltonDepth Conversion - Introduction




Sources of Depth Conversion

Well Velocity

Stacking Velocities

Pseudo Velocities

Single Velocity Function

Layer Cake Method

Interval VelMethod

Avg VelMethod

Structure Overview & Interpretation SkeltonDepth Conversion


Velocity Survey /

Check Shot

Vertical Seismic Profile

Velocities Velocities Function Method Method Method



Structure Overview & Interpretation SkeltonSource for Depth Conversion




Velocity Survey or Check Shot Survey





Vertical Seismic Profile





Stacking Velocities

The distance-time relationship determined from analysis ofnormal moveout (NMO) measurements from common depth



normal moveout (NMO) measurements from common depthpoint gathers of seismic data.

The stacking velocity is used to correct the arrival times ofevents in the traces for their varying offsets prior tosumming, or stacking, the traces to improve the signal-to-noise ratio of the data.



Time V RMS V int DEPTH V ave

0 6500 6500 0 6500

200 7250 7250 725 7250

1500 10000 10358 7458 9944

2000 11000 13565 10849 10849

2250 11750 16570 12920 11485

2700 12250 14494 16181 11986

3460 13250 16314 22381 12937

3950 13750 16664 26513 13424

4860 14750 18474 34918 14370

Horizon Depth SubseaSeismic Time Seismic Time (Sec)

A 2196 2153 475.7166 0.4757166

B 4059 4016 835.4225 0.8354225

C 4925 4882 1002.629 1.002629

D 6368 6325 1281.242 1.281242

E 8047 8004 1586.848 1.586848

TD 11281 11238


4860 14750 18474 34918 14370

5000 15500 18357 45381 15127

TD 11281 11238

T1 1500

T? ?

T2 2000

del T 500

D1 7458

D cal 8004

D2 10849

del D 3391

Unit Method = del T/del D

Unit Value => D unit= 0.147449

Dcal - D1 = 546

(D unit) X (Dcal - D1)

D? = 80.50723

T? = 1580.507



vint=((vrmsn^2*tn-Vrms n-1^2*tn-1)/(tn-tn-1)^1/2

Interval Velocity Vint & Average Velocity Vavg


Vave,n = [(Vint,n * Tn - Tn-1) + (Vave,n-1 * Tn-1)]/Tn



Single Velocity Function






Layer Cake ConversionLayer Cake ConversionLayer Cake ConversionLayer Cake Conversion




Int Vel ConversionInt Vel ConversionInt Vel ConversionInt Vel Conversion




Avg Vel ConversionAvg Vel ConversionAvg Vel ConversionAvg Vel Conversion





Structure Overview & Interpretation SkeltonMapping



Structure Overview & Interpretation SkeltonMapping Mapping should never be considered as an end product only. Creatingand using maps while interpreting lines and time slices helps to clarify what ahorizon surface looks like, how it is broken by faults, and which segments arelikely to be related.

Structural maps are displayed and updated while we interpret the


Structural maps are displayed and updated while we interpret theseismic data. On a dual monitor system, the map should be displayed on onescreen while you are interpreting seismic data on the other screen. On a singlemonitor system, one window will contain your seismic display and the otherwindow will display your map. The map serves as an interpretation tool, as aquality control aid used while picking horizons, and as an aid in understandingthe structure and characteristics of the prospect.



Structure Overview & Interpretation SkeltonContour Parameters And Map Editing

In defining the way maps are generated and displayed, you should adhere tothe following general rules of thumb.

First, the gridding increments should never be much larger than theinterval between interpreted lines.


interval between interpreted lines.

Second, a contouring interval should be selected which allows thesurface relief to encompass the range of colors available to display it.

It is also helpful, particularly in areas of complex faulting, to project thesurface in space as a movable and ratable wire mesh or solid surface.



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lecture - 07 march 27 2015 friday.unlocked

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