3d seismic.doc

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General Introduction

The material covered in this module assumes your familiarity with the techniques covered in theSeismic Interpretation series. The material on 3D Techniques presents technology and methods ofincreasing importance in petroleum exploration; as such, this material is recommended for all seismictrainees involved in field wor, processing, and interpretation. It may also !e of !enefit to developmentgeophysicists and geologists.

"ew pro!lems invaria!ly require the development of newer techniques to solve them. During the

#$%&s, petroleum exploration !ecame a search for and investigation of more complex fields. Thissearch demanded a more precise definition of the su!surface, !oth for increased accuracy in detecting

hydrocar!on deposits and for delineating the extent of producing fields. The su!sequent developmentof three'dimensional (3D) seismic techniques met this need !y providing a new way to loo at thesu!surface.

3D data acquisition, processing, and interpretation.

Seismic Data *cquisition, an overview of the development of early 3D seismic technology and

the considerations involved in designing land and marine 3D surveys.

3D Seismic Data +rocessing loos at the !asics of 3D processing, including considerationsinvolved in !inning data.

3D Interpretation asics descri!es !asic sills needed for interpretation.

3D Interpretation In +ractice descri!es how these !asic sills are applied on the -o!

any of the techniques used in 3D seismic resem!le those used in /D. ut 3D technology provides newways of processing data that maes special use of the properties of a 3D data set (for example, 3D

D0, 3D re'datuming, and one pass 3D migration). In addition, new ways of presenting the processeddata afford us new insights into the su!surface !y adding a third dimension to our quantifia!le data.

In 3D seismic, the final processed traces are fit to an interpretation grid which usually has a regularspacing increment in the and the directions. The 3D data volume is therefore composed oftraces in the long axis, traces in the shorter axis and samples in the time or depthdimension

ecause we have a 3D volume of data, or a cu!e of data (1igure #2 * volume of 3D data), we cancreate vertical sections !y selecting ad-acent seismic traces from the data volume along a horiontalline in any orientation.
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Figure 1

The extracted seismic traces are then displayed similar to a traditional /D seismic line. y convention,

inline sections are those along the general direction of receiver lines. 4rosslines are another way to

sample and extract data in the 3D volume, resulting in a vertical section display that is perpendicularto the direction defined !y the inlines. * crossline may !e acquired in the field or it may !e createdfrom the 3D data volume !y taing traces from each of the inline sections and displaying themtogether to mae up a seismic section. 1or example, if trace num!er #&& is taen from each inline,together they form crossline #&&.

*nother unique 3D display is a line extracted from the data volume that does not follow the orthogonaldirections of inline and crossline, !ut rather cuts across the 3D data volume in an ar!itrary direction.

Traces are extracted or interpolated from the data volume to mae up the traces of this ar!itrary line,which may, for example, !e composed of line segments -oining several wells.

In viewing 3D data, we are never limited to a single vertical slice through the data volume. 4omposite

displays which com!ine several different slices may !e extracted. The data volume may also !e cuthoriontally, extracting data samples at every trace location at the same time value. The resultingdisplay is called a time slice. Interpreted data in the form of horion slices are also produced. oth timeslices and horion slices can !e processed using image enhancement techniques to clarify theinformation content (see the section on 5olume 5isualiation).

"ow that we6ve presented an overview of the finished product, let6s tae a loo at the real'lifeconsiderations that have prompted the development of 3D technology and the types of pro!lems it is

designed to solve.

Why We Need 3D

7e live in a three'dimensional world. +erhaps that is all the -ustification needed for conducting 3Dseismic surveys. The truth of the matter is that the complexity of pro!lems now !eing addressed !y 3D

techniques requires such sophistication. 8owever, this was not always the case. 7hen the seismicreflection technique was developed !y 9ohn 4larence :archer and others in the #$/&s, exploration


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o!-ectives were shallow and their stratigraphy was fairly flat, although faults, reefs, or shallow foldswere often present to create hydrocar!on traps. *t that time, a /D approach was a satisfactoryapproximation of the su!surface and a relia!le aid in finding new discoveries. In the deeper, morecomplex reservoirs we investigate today, the 3D method helps us to !etter define the reservoirgeometry !ecause of its three significant advantages over /D, namely2

1ocusing

+ositioning

esolution

3D Methods Described 3D seismic provides distinct advantages over /D. Sharper focusing ena!les us to more clearly

see what we want !y focusing energy that is spread over the 1resnel one. Improved

positioning ensures that geologic features appear in their correct places on seismic sections.*nd greater resolution means that even small features, !oth vertically (thinner !eds) and

horiontally (small lateral changes, such as faults, channels and local ones of porosity), can !edefined. In this section, we will discuss each of these features.

Focusing Improved focusing is an important enhancement availa!le in 3D migration techniques and its

pursuit has !een the primary driving force !ehind the development of 3D migration methods. Toillustrate what we mean !y focusing, let6s loo at a very simple model. If we were to !ury the

world6s largest cannon !all in a homogeneous medium so that the only acoustic !oundarypresent was the surface of the cannon !all, and then shoot a seismic traverse directly over this


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That is, if we design an acquisition system comprised of a ero'offset, omni'directional source

and receiver, and use an ideal source signal, then processing artifacts will !e minimal

The real world does not consist of isolated !odies sitting in a homogeneous earth. Instead, we

have numerous !odies !uried in a layered earth, with each !ody and layer having its own

acoustic impedance. 0n a real seismic section, the cannon !all diffractions descri!ed a!ovewould !e dispersed among neigh!oring diffractions, reflections, multiples and rever!erationsfrom all these other acoustic contrasts. igration resolves much of this complexity !y collapsingdiffractions, or focusing their energy, !ac to their points of origin. During migration, each

element of the cannon ball diffraction moves to the location of the cannon ball. Theseismic response of the cannon ball increases, and its diffraction tails are essentiallyeliminated.

igration, therefore, is a means of focusing acoustic energy that has !een scattered !y geologic

diffractors in the =arth. The diffraction pattern of the cannon !all may !e thought of as anacoustic um!rella. In order to properly reconstruct the diffraction source, this um!rella must !esampled in three dimensions with and as the two orthogonal horiontal axes of position

and as the single vertical axis of record time, as in (1igure /2 Diffraction umbrella).

Figure 2

y o!taining a grid of values over the cannon !all6s diffraction um!rella, a partial migration may

!e applied along one axis and completed along the orthogonal axis. This procedure will properlycollapse the diffraction pattern (indicated !y the arrows on the graphic) and locate the featurewithin the limits of resolution provided !y the grid used. (This assumes, of course, that we haveapplied correct velocities to each migration.)

Positioning 7hat happens if the seismic traverse does not pass over the cannon !all, !ut passes near!y> *

diffraction pattern is still recorded, !ut the curvature of the event is reduced and the apex of thehyper!ola appears at a later time. 7e can determine the lateral error of position of an


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situations. 1or instance, if we acquire seismic data in a !asin having shallow, flat'lying stratathat overlie lightly folded structures, the raypaths will remain nearly vertical in the shallow, flat'lying sediments and will curve only moderately as they pass through the deeper, foldedstructures (1igure A2Curved raypath effect).

Figure 4

This construction clearly indicates that we could reduce the lateral displacement computed in

(1igure 32 Horizontal migration distance and comparison of migrated events to unmigratedevents) by as much as one-half, or possibly even two-thirds. 0f course, nowledgea!le readersare surely familiar with many areas where this line of defense will not hold, and where theconstruction shown in 1igure 3accurately represents our liely findings. * third factorcontri!uting to the acceptance of /D data is the fact that the error in depth to the horion orpoint source is often small. In the case shown in 1igure 3, at a point vertically !elow the ero'

offset shot and receiver position, the horion is actually located only %B m !elow its apparent

unmigrated depth location. (In this figure, h, or %B m, is the distance !etween the apparentunmigrated depth and the actual depth.) This represents an error of only 3C, which is smallerthan or compara!le to the expected errors in the estimation of the seismic velocity. (@oodseismic velocities are considered to have errors in the 3'C range.) "evertheless, there is nodou!t that the utiliation of 3D techniques can significantly reduce errors in su!surface targetlocations.

Resolution Seismic resolution depends on the digital time sampling and the spatial sampling of our data.

1or many years, geophysicists have understood the rules, advantages, and limitations ofdigitally sampling a stationary time series.

8owever, they have !een less inclined to apply the same concept spatially along the seismic line

and in the crossline direction. This is pro!a!ly !ecause the spatial sampling of /D data in thedirection of the line was more than adequate to resolve most discrete sampling issues. ut the
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distance !etween lines (that is, the line spacing) was so great that spatial sampling effects wererarely considered.

ecause of the poor spatial sampling used in /D surveys where line spacing might have !een on

the order of, say, one ilometer ' small, !ut important, targets and the details of larger targets

were often missed. The smaller inline (the primary direction of the lines !eing acquired) 4+spacing and the shorter perpendicular distance !etween inlines (crossline distance) provided !y3D surveys can significantly enhance the spatial resolution of the su!surface. 1or a 3D inlinesurface group interval of / m, the su!surface sampling rate, or 4+ interval, is #/. m (or, half

the surface group interval). 7ith spacing !etween inlines varying !etween / m and #&& m,resolution can !e improved !y an order of magnitude.

3D Seisic in Petroleu !"#loration *s we have seen, 3D seismic improves data density and resolves many of the pro!lems we face

when using /D sections, including out'of'plane reflections or


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wrong location. (1igure B2 Comparison of prospect map using "D and #D data, from $estvold,11, 1") illustrates the !enefits that 3D !rings.

Figure %

0n the left is an interpretation of a ey horion with the !enefit of /D data only. 0n the right is

the interpretation now supplemented !y 3D data. It can !e seen that the interpretation of !othfaults and structure has changed radically, and pro!a!ly in a way that will affect the way inwhich the field will su!sequently !e developed.

So, now that we6ve seen the needs that prompted the development of 3D seismic technology,

let6s move on to applying these techniques to petroleum exploration. 0ur first step will !e toexamine the factors involved in designing and completing a successful 3D data acquisitionprogram.

By

convention, 3-

D seismic

sections that

are parallel to

the generaldirection of

dataacquisition, or

along thelonger axis of

the survey

grid, are

called:

!" in line sections.

#. $n more complex reservoirs, the 3-D seismic method helps to define reservoir geometry better than #-D data

because of three significant advantages: improved focusing, positioning and resolution. The most important of

these advantages is:


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!" improved resolution.

3. %eismic interpretation problems associated &ith out-of-plane reflections can be resolved &ith:

B" 3-D seismic sections.

'. (mong the advantages that 3D seismic has over #D seismic are:

D" all of the above

&istory and !arly De'elo#ent7e employ the same types of sources and receivers used in /D surveys to acquire three'dimensional

seismic data. The energy sources include dynamite, vi!rators, and air guns; the receivers are eithergeophones or hydrophones. The main differences !etween /D and 3D surveys are in the layout of thesurvey and the quantity of data that is acquired. These differences in acquisition procedures affect theway we process and interpret our data.

7hen acquiring 3D data on land, multiple lines of geophones are active at the same time, and thegeophone array and shots do not fall on the same line. *s a result, reflection points are spread over an

area rather than !eing focused along a single, two'dimensional line.

*cquiring 3D marine data is similar to acquiring /D marine data, in that a !oat with an energy source

(such as an air gun) and a trailing streamer with many hydrophones are used. The significantdifference in 3D acquisition is the fact that we use multiple sources, multiple streamers and evenmultiple !oats.

!arly (once#ts

=arly 3D experiments !egan on land in the late #$B&s as an attempt to improve the efficiency withwhich oil fields were developed and to answer some of the conundrums that existed on /D data. *n

extension of the method into the marine environment soon followed. Eater, the method came to !eused in shallow'water surveying. Since those early days, the 3D technique has !een modified and

improved to !etter address exploration surveying and reservoir monitoring o!-ectives.

@eologists and geophysicists are trained to thin in three dimensions when interpreting geologicinformation, !ecause the earth is three'dimensional. *s a result, the search for new ways to apply 3Dcorrections to seismic data dates !ac to the earliest times of the industry. @eophysicists derived hand'drawn sections of ey horions from /D seismic profiles where the interpolation of data !etweeno!servations relied heavily on the sill of the interpreter. *ny feature that was smaller than the line

spacing could not !e interpreted with any degree of certainty. Then they migrated these horions !yhand, using wavefront charts and sometimes ingenious drawing devices. Fou might !e a!le to argue

that these maps, derived from a grid of /D migrated data, were a form of 3D representation.Technically, this statement is true, !ut the degree of error associated with these maps was often veryhigh. 0ne alternative to this method was to draw maps !ased on the interpretation of a /D grid of

unmigrated data and then use map migration techniques to produce a migrated version of theunmigrated map.

y the mid'#$B&s, computers had replaced the wavefront charts and ingenious drawing tools with

mathematical algorithms. This approach was much faster and more accurate, !ut did nothing to rectifythe inherent limitations of interpreting /D data taen from a 3D earth.

)nalog Systes

It is difficult to pinpoint the !eginning of true 3D acquisition and processing, as opposed to adaptationsof the /D method, since many of the early experiments !y competing seismic crews were shrouded in

secrecy. 0!viously, any true 3D survey must acquire seismic data over an area using close, regularlyspaced receivers. In #$%/, 7alton, et al pu!lished results of one of the earliest experiments in 3Dsurveying, conducted during the late sixties.


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7alton deployed receivers along one axis and sources along an orthogonal axis, forming a spread thatresem!led a cross (1igure #2Ac%uisition geometry).

Figure 1

Gsing this method with source and receiver spacing of equal distance, the common midpoints formed a

square. The same pattern was also achieved when the source and receiver axes were arranged in an E'shape.

7alton6s next step was truly ingenious. The field data were recorded on analog tape. * machine wasconstructed to read the tape and convert the signals into electrical voltage, which was used toilluminate light !ul!s arranged in a pattern that modeled the su!surface coverage. Then a movie wasmade, showing the response of the lights to a play!ac of the tape. The result was a time slice movie,

with each frame !eing a single time slice of the acoustic response. 7alton6s interpretation method wasalso ingenious. * time contour map of a given horion was interpreted from the movie. ecause no

normal moveout correction was applied, another map was made of the same horion assuming flatreflectors. y su!tracting one map from the other, a geologic time structure map was produced. Duringthe late sixties, digital processing techniques were esta!lished ' another ma-or step toward the modern3D survey.

S*aths )nd (roo+ed ,ines

* num!er of practical situations encountered during land surveys prompted the development of 3D

seismic acquisition techniques. 0ne of these situations was the need to acquire data from mountainous

terrain, where it is extremely difficult and expensive to acquire data along straight lines. Seismic crewsworing in these areas too advantage of local access roads, trails and river valleys as acquisitionroutes, in an effort to reduce costs. The use of these routes produced lines with sinusoidal orcurvilinear tracs over the surface. *s digital techniques developed, crossline information was derivedfrom these crooed lines. These curvilinear tracs, however, caused a lateral displacement of thesu!surface common midpoints and the staced data were often smeared.


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If there was little structural dip, smearing was minimal and the seismic results were generallyaccepta!le. If there was significant dip, however, remedial steps were required. 0ne solution was toconstruct a swath of lines parallel to the general direction of the original crooed line, separating theswath lines !y spacing of / m. This design ensured that the lines included all of the locations covered!y the crooed line. Then, all of the common midpoints could !e assigned to the nearest line, givingrise to a swath of su!stacs that minimied the reflection point smearing.

This method of assigning data to the nearest line is called binning. In practice, a grid of !ins isconstructed such that the !in width is equal to the line spacing and the !in length is equal to the trace

spacing (for example, / m !y #/. m). This grid overlays the desired straight line positions andsimplifies gathering data at the appropriate line location. The !inned data can !e gathered intosu!stacs which are then com!ined to construct the final staced line. This technique alone, however,is no !etter than a conventional stac that ignores the lateral displacement of the crooed line. Theadvantage of having su!stacs is that they can !e com!ined along different lines of crossdip when theyare processed. 7hen a correct (or nearly correct) crossdip is selected, the su!stacs com!ine to givean optimal seismic response.

Thus, the introduction of some innovative processing reduced the cost of data acquisition in a difficult

area, improved the quality of the data through an optimal com!ination of su!stacs and provided somecrossdip information. In addition, the easier access used !y crooed line acquisition helped avoid the

more difficult pro!lems of statics corrections and scattered noise which might have !een introduced !ythe severe terrain.

The crooed line approach is not truly 3D, !ut its introduction laid some groundwor toward a full'fledged 3D approach. The improved data quality of the additional su!stacs illustrated the !enefits thatwould eventually come with full 3D acquisition and processing. The development of the !inningtechnique and associated software ensured that these methods ' along with the experience required to

use them effectively ' were availa!le when the time came to plan and execute a full 3D survey.

Sur'ey Design (riteria - Field Paraeters

Designing a 3D survey requires us to examine several critical equations related to the geology of the

prospect and the !andwidth of the desired results. In this section, we will present equations and theirderivations for the 1resnel one, spatial aliasing, minimum migration aperture, and maximum

migration aperture. 7e can use these formulas to choose the desired su!surface sampling intervals.These intervals, in turn, lead us to a preferred line direction and line spacing. 7e will also discuss other

factors affecting the survey design, including how the surface environment dictates whether thepreferred line direction is reasona!le (or possi!le) and how costs affect whether we acquire a sparseline survey or a conventional 3D survey.

.he Fresnel /one

The first parameter we need to determine for a prospective horion is the 1resnel one radius,emem!er that the total o!served energy actually comes from an area on the surface of a reflector

'not a reflectionpoint. This area is called the 1resnel one, as shown in (1igure #2&eometricconstruction of the 'resnel zone).


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Figure 2

The process of migration applied during processing attempts to


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Figure 3

The same rules apply to the spatial sampling !ut it is important to differentiate the individual aspects

of the spatial measure. If we consider a particular reflector as, say, a series of folds having a constant

spatial period, then clearly, if we do not have at least two reflection points from each fold we will aliasthe structure. If such a condition occurred, then we would not !e a!le to recover the correct structureduring any su!sequent processing. Thus we would have to mae certain that our survey designprovided a high enough spatial reflection point density to properly sample the su!surface, and thisequates to a small enough in line and crossline 4+ spacing. *s it happens, the earth is rarely spatiallyperiodic in the way descri!ed, so the aliasing of the geology is generally handled within the otherspatial aliasing design criteria.

The types of spatial aliasing encountered most frequently in 3D surveys correct for dip and coherentnoise. It is important to recognie that these forms of aliasing are recovera!le, !ut generally at theexpense of resolution. 1or example, we can filter the data during processing so that acquisition spatialsampling is adequate for the highest frequency now present. This is !ecause these types of aliasing

occur during multichannel processing of the data, not during the acquisition. ut we do not want tolower the resolution after we have designed and paid for an acquisition program to achieve it. In somecases, spatial interpolation to create infill traces can prevent su!sequent aliasing without resorting tofiltering. It is important to understand !efore the survey design process, when and where such

techniques can !e used.

1or anticipating the effect of dip, the equation shown in (1igure A2 (patial aliasing e%uation) can be

utilized.


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Figure 4

5elocity, structural dip and 4+ spacing influence the spatial aliasing frequency at the depth of the

prospective horions. The frequency values derived in the following equation and ta!le are those

frequencies at which aliasing occurs. These values determine the required group interval and lineinterval spacing, which, in turn, dictate the amount of time and expense required to conduct a propersurvey. Spatial aliasing frequencies are computed separately for the inline 4+ interval (or,alternatively, the group interval) and the crossline (line spacing) interval.

Gsing the ta!le shown in (1igure 2 (patial aliasing table - with numbers inside the table cells

representing fre%uency, and showing e%uations for spatial aliasing, maimum aperture and minimumaperture), we can compute the aliasing frequency for the prospective horions for a range of dips and

4+ spacing. 7e should choose typical values for these parameters and chec some worst'casescenarios to ensure that no ma-or pro!lems will arise in the survey. +ersonal computers commerciallyavaila!le programs allow us to quicly generate a ta!le of aliasing frequencies for a range of dips andcommon midpoint intervals. 7e generally use a


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Figure $

* very important survey design issue is the presence of coherent noise, which causes aliasing with the

signal we wish to process. If we do not adequately spatially sample the noise as well as the signal,

then we may have to reduce the !andwidth of the data in order to separate the two components, thusreducing !oth temporal and spatial resolution. *lternatively, we may have to apply severe muting,which will degrade the signal to noise ratio and reduce the efficiency of the "0 and stac processdescri!ed later in the processing section. Typical coherent marine noise comes from direct waterarrivals and refractions. Similarly, on land, such noise comes from ground roll, air waves andrefractions.

Ideally, 3D survey design should involve an analysis of existing surface seismic data, modeling, and

availa!le well data (particularly 5S+), as well as the use of equations. 1or example, we have seen thatsome of the design criteria so far descri!ed involve assumptions a!out the achieva!le !andwidth. If wecan esta!lish this with minimum assumptions, from an examination of existing data, then we candesign


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Figure %

is used to calculate the dimensions of this padding one over which data should !e acquired

(1igure %2 *inimum and maimum migration apertures shown in dar+ and light green area).


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Figure 0

Solutions ( to this equation are included in 1igure B.

7hen determining the maximum migration aperture, we only need to consider dips at the margin ofthe prospect, as long as we are sure that steep dips towards the center of the prospect can !e properlymigrated. The aperture will !e small if there are small dips at the !orders of the prospect. If there is nodip, we use the minimum migration aperture equation (descri!ed in the following section). If we cannotmigrate the steeper dips over the center of the prospect properly, or if there are steep dips around thesurvey edges, the survey area may !e too small for the magnitude of the structure under investigation.

It is important to note here that we must tae great care around salt domes and other intrusions.

eflections from the steep flans of the dome (especially in the deeper portions) will !e recorded someilometers away, and we may need to cover an especially large survey area to fully migrate the data.

In addition to capturing sufficient data to migrate dipping events occurring close to the edge of the 3D

image, we also must consider the aperture required to sufficiently sample the diffracted energyoccurring from faults near the edge of the image, if the clear definition of such faults is a requirementof the survey. The shapes of diffraction hyper!olae are independent of the dip of the !eds. *s a rule ofthum!, the migration aperture needs to provide an angle of about 3) degrees from the apexof the deepest diffractor to be imaged. This equates to capturing around $C of the energy.igration apertures are usually not designed to allow the imaging of fault plane reflections at the edgeof the image; to do so would require very large apertures !ecause of the high angles involved.

Miniu Migration )#erture

The final equation we must consider when designing a 3D survey deals with the aperture required to

o!tain a satisfactory response from the migration algorithm in the presence of ero dip. This is theminimum migration aperture equation and its derivation is shown in (1igure H2*inimum migrationaperture construction).

Figure
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The minimum aperture value is compared to the maximum aperture value to determine how far to

extend the lines in order to get proper, full'fold migration of the lines within the central prospect area(1igure %2 *inimum and maimum migration apertures in stippled area).

Strictly speaing, the values for the minimum and maximum apertures should !e added together toderive the proper halo. In practice, this is rarely done. Instead, the larger of the two values is chosen.In the example shown in (1igure 2 (patial aliasing table), the halo would !e set around $&& m, whichis a typical order of magnitude for the minimum migration aperture. In general, halo dimensions aread-usted to the nearest integer multiple of the trace spacing and line spacing. To !e rigorous, there isalso a dip moveout (D&) effect that can affect the aperture, !ut it is not usually considered.

The aperture required is essentially equal to the diameter of the 1resnel one for the lowest frequencyof interest. The implied halo or padding one is half this value, the other half !eing availa!le from data

within the !oundaries of the original survey area. Typically, a value of is chosen for the lowestfrequency of interest. This represents a common low'cut filter for seismic data and is used in theexample in 1igure . 5alues lower than this significantly increases the sie of the halo and, therefore,increase the acquisition cost. arely do we come across examples where different values are chosen.8owever, lower frequencies are used where experience has dictated that these are necessary, whilehigher frequencies are used for shallower, high'resolution targets.

* common paradox is that the minimum aperture often exceeds the magnitude of the maximumaperture. The reason for this is simple. @iven low dip values, the maximum aperture equation may give

a result of, say, A&& m for the maximum aperture halo, !ut the minimum aperture halo for a frequencyof H 8 may !e as much as $&& m (1igure ). This means that we need a maximum aperture of A&& mto migrate a horion of a given dip, !ut that a $&&'meter minimum aperture is necessary to produce aseismic section with a pleasing appearance. If the aperture is severely limited in the migration process(which is liely in a :irchhoff time migration) the resulting seismic section may loo smeared.

Surace (onsiderations

So, now we6ve solved our four equations2

we now the sie of the 1resnel one,

we now when spatial aliasing will occur,

we now what the optimum values for the 4+ interval are, and

we now how far to extend the survey lines !eyond the immediate prospect area to avoid

migration edge effects.

7e should strive to meet or exceed the field parameter limits we determine from these calculations sothat we can achieve proper imaging and adequate resolution. Sometimes honoring these limits leads us

to extend the survey !eyond our original area and, if we also decide to use tighter 4+ intervals, thesurvey may cost more than we thought.

0nce the survey parameters have !een determined, acquisition depends on the surface features of thearea, and particularly, whether the survey is land or marine. 7e will descri!e the


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which way are we to shoot our survey relative to the properties of the su!surface> *s usual, it willdepend on a com!ination of logistics and survey o!-ectives. Eogistics equate to cost and surveyo!-ectives relate to the ultimate success or failure of the data to deliver. The two rarely exactlycoincide, so compromise is the order of the day. 7here we have to choose a single aimuth for ourdata acquisition then there are many factors to consider, some of which are often contradictory. Ideally,we might thin that we should design our survey so that the lines we acquire in the field (the inlines)

are in the direction of primary dip. 7e might hope that this would also !e in a direction that isorthogonal to the faults we wish to delineate.

1or exploration 3D where we may have !road structural imaging o!-ectives and wish to cover a largearea at minimum cost, it is liely that we would shoot in the predominant dip direction and with a widecrossline spacing. Shooting in the predominant dip direction (1igure $2 (eismic lines parallel to dipdirection) ensures that the spatial sampling set !y the streamer group interval (#/. to / m) isassigned to the steepest dip, where the most accurate control is desired.

Figure

If required, an occasional crossline can !e acquired for velocity control. Shooting in the dip directionwas considered less than ideal if *mplitude versus 0ffset (*50) data is a delivera!le. This is !ecausethe dip imposes an effect on the *50 that distorts the underlying trends we are looing for.

In such cases, if a predominant strie direction exists, then aligning the shooting direction with the

strie was the optimal way to acquire the data. 8owever amplitude preserving prestac time migrationis commonly utilied to mitigate against the effect of dip, and so this argument for shooting strie is

now less of an issue. *lso shooting in the dip direction, particularly if steep, creates non hyper!olic

moveout in the 4+ gathers which, unless accounted for in the "0 algorithm can reduce resolutionthrough a decrease in the signal to noise ratio due to poor alignment of primary energy into thestacing process.


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7e can imagine many cases that do not fit this neat image of regional dip. * more realistic geologicsetting is one dominated !y faulting rather than !y simple regional dip. The structural dip may !einsignificant and the interpretation goal may !e to clearly define the structural trap created !y faulting.:nowing that these faults will !e the source of many diffractions, as in /D seismic, we can pro!a!lyachieve the !est imaging of faults !y orienting lines perpendicular to the fault trend(1igure #&2 (eismic lines oriented normal to maor faults).

Figure 15

In an exploration setting with more than one direction of prominent structural dip or more than onefault trend, we may choose one critical trend to image, even if this choice compromises the !estimaging of one or more other trends. The ideal plan is to image the largest portion of the prospect with

the appropriate line direction and line spacing, and possi!ly change to finer line spacing over the areas

where the contrary dip or feature exists. 1or 3D surveys shot in producing fields or proven hydrocar!onplays, the potential compromises !ecome greater. *t the level of detail required from


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0ne way to image these structures is to tighten the line spacing to ensure that an areal coverage ofthe data is o!tained in two orthogonal directions. This approach is expensive, !ut accurate.

In marine surveys, the circular 3D shooting pattern (1rench, #$H) has !een an alternative approachto acquisition. The survey is shot as a series of concentric circles, with the line spacing chosen tosample the steep dips and achieve the required spatial resolution (as we will see later on in thissection). The constant radius of the circles ensures that the ca!le is pulled inward and sweeps a

continuous circular swath of midpoints. *s the diameter of the circle decreases toward the center, theca!le is drastically shortened. *s a result, this type of survey should !e centered over the top of the

structure. Then, the only reflections received near the center of the feature will !e the shallow returnsfrom its flatter top. 1inally, midpoints a!out the center of the structure should !e acquired !y severalcrisscrossing lines to provide additional structural control. 8owever, with multi'streamer technology,such structures now tend to !e imaged conventionally utiliing a close crossline spacing and parallel inlines.

Sometimes surface o!stacles such as drilling rigs or production facilities cause gaps in the 3D survey

coverage. Such areas of little to no coverage can !e filled in !y undershooting. 0ffshore, this techniqueinvolves using a separate source !oat shooting from one side of the o!struction to a !oat towing

streamers on the opposite side of the o!struction. Gndershooting provides a usa!le amount of data inthe 4+ !ins that lie !eneath the surface o!stacle.

Pre-)cuisition Modeling

3D seismic surveys are expensive so every effort should !e made to ensure that the planned surveywill give good quality results at the lowest cost. 4omputer modeling is an important tool for this effort.

It may !e performed using a relatively simple program that allows estimates of the fold in a !in or amore sophisticated finite difference model that shows the degree of illumination on a horion !eneath acomplex salt structure. Design programs help to lay out sources and receivers taing account of thelocal environment and topography using maps and satellite images. 0nce laid out the survey can !eshot and acquisition attri!utes such as fold, offset and aimuth distri!ution analyed. The programscan also !e used to analye migration halo, spatial sampling, line spacing, offsets, and a host of othergeophysical calculations that are critical for properly imaging the geophysical o!-ectives. The followingexamples are only a small num!er of the possi!ilities offered !y the design programs and general

modeling software.

.arget )nalysis

In many cases, 3'D surveys are acquired after a preliminary interpretation !ased on /D seismic. Inthese cases, a geological model can !e constructed that may guide the design of the 3D survey. 1or

example, ray tracing is performed to calculate the migration aperture required. This value is then usedto extend the outline of the desired survey image area. *n example of such an analysis is shown

!elow. It is a plan view of a target horion with colors displaying the radius of the migration aperturerequired to properly migrate a horion with that structure and velocity over!urden. This aperture is

calculated from the local depth of the horion and the velocity structure of theover!urden. 1igure #2 *igration aperture re%uired for target horizon Courtesy .*$/ #D0 Design

*odeling (oftware, &2DC.).


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Figure 1

In this example it can !e seen that at the south and east !oundaries the migration aperture is !etween

#&&&m to #A&&m. To ensure an adequate migration into the desired image area the !oundary of the

survey must !e expanded !y these amounts. The north and western !oundaries need HA&m or less.Gsing tools such as this can ensure good survey results while minimiing costs.

ay trace models can also !e used for illumination analysis. Irregularities in the over!urden can causesevere ray path distortion that lead to minimal source energy reaching the target horion. Typically, 3Dsurveys are designed with a uniform layout of sources and receivers on the surface. ay tracing may

suggest alternative surface geometry or shooting directions and can test the !enefit of multi'aimuthsurveys. * typical 3D model is shown !elow. 1igure /2*odel from eisting interpretation to be analyzed

by ray tracing Courtesy .*$/ #D0 Design *odeling (oftware, &2DC.).


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Figure 2

The upper horions can then !e made invisi!le to mae the target horion easier to see and a

simulated 3D survey can !e shot over the model using ray tracing. This allows the calculation of the

fold of reflection points on the surface of the target horion that would result from that specificacquisition geometry as seen !elow. 1igure 32 /llumination of the target horizon Courtesy .*$/ #D0Design *odeling (oftware, &2DC.).


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Figure 3

It is seen that the western flan of the structure has poor illumination; !lue is indicating a fold of A or

less and white shows there to !e no reflection points at all. The survey designer now must consider

how the situation can !e improved. It could !e that the survey area is not large enough to capture thereflections as might happen if the migration apron is set too small. The graphic shows that reflectionscan only !e o!tained from the desired image area from a down dip surface location. 1igure A2 Dippingreflectors re%uire ac%uisition from offset locations that may be outside the survey area).


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Figure 4

In order to understand the pro!lem, the designer can select individual shots and ray trace from these

to understand !etter how to modify the survey plan as shown !elow. 1igure 2 3ay tracing from a

single shot can help eplain illumination problems Courtesy .*$/ #D0 Design *odeling (oftware,&2DC.).


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Figure $

Dealing *ith 6bstructions

The example !elow shows the simple case where the designer needs to adapt a regular orthogonal gridsurvey to compensate for an area where sources cannot !e used. This might !e due to the presence ofresidential housing or industrial complex or perhaps a lae. *dditional shot'points will !e required tomaintain the fold as far as possi!le. Two alternatives are shown for offsetting theseshots. 1igure B2 Compensating for shot eclusion zone Courtesy C&&4eritas).


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Figure %

The figures show the fold in each !in colored according to the !ar on the left. The pale one within the

!lue !oundary line is the area where the source is not allowed. lac dots indicate the shot points; !lue

indicates the receiver points. It might seem o!vious to place the additional compensating shots asclose as possi!le to the !oundary of the exclusion one as on the left result. 8owever, this led tostripes of high and low fold which would adversely affect the migration of the data. The figure on theright shows an alternative approach using short additional shot lines parallel to the original shot lines.This leads to a much more accepta!le result.

In the next o!struction example, there is a lae in the middle of the survey. In the middle right isshown the acquisition template in pale !lue. Some shot points near the o!struction have !een moved

up to half a receiver group interval to minimie the loss of fold !ut this had minimalimpact. 1igure %2 .rthogonal survey grid and la+e eclusion zone with template shown to the right,Courtesy .*$/ #D0 Design *odeling (oftware, &2DC.).


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Figure 0

7hen this survey is shot in the computer model it results in the fold !elow. 1igure H2 'old resulting

from eclusion 5one with #"66m offsets Courtesy .*$/ #D0 Design *odeling (oftware, &2DC.).


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Figure

The maximum fold with this layout is $& and this is only reduced to $ at its lowest value. This mightseem an accepta!le level !ut it is only appropriate for deeper events that can use the full offsets. If a

shallower target is also of interest then stretch mute and noise trains may limit the useful offset. In theexample !elow, a #&&m offset limit has !een set. "ow the maximum fold has reduced to A& !ut theminimum value is as low as H. This is far from accepta!le. 1igure $2 'old resulting from eclusion 5onewith 1!66m offsets Courtesy .*$/ #D0 Design *odeling (oftware, &2DC.).

Figure

The next figure shows detail of the previous figure with a change in color scale to highlight the foldvariations. * num!er of extra shot lines are shown which are proposed to improve the fold in the areaof the exclusion one. 1igure #&2 Detail of previous figure showing proposed additional shot-pointsCourtesy .*$/ #D0 Design *odeling (oftware, &2DC.).


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Figure 15

7hen these additional shotpoints are included there is a ma-or change in fold. The lowest fold is now

%& and the highest #&& as shown !elow. 1igure ##2As previous figure but with additional shot-points

fired Courtesy .*$/ #D0 Design *odeling (oftware, &2DC.).


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Figure 13

)dditional 7in )ttributes

odeling software also allows the survey designer to see more than -ust fold within a !in. * fewexamples are shown !elow2

"earest offset display which shows the offset to the nearest receiver contri!uting to the stac

within a !in. This is particularly useful for shallow horions where the mute distance may !e

shorter than this nearest offset leading to no visi!le reflections from thehorion.1igure #A2 $ear offset display for a bric+ 7ayout8 /f the mute in processing for ahorizon is shorter than the longest minimum offset shown here then that horizon will not bevisible in the stac+ for that bin8


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Figure 14

Eongest offset display. This ena!les a determination of whether the longest offset is suita!le for

the target and whether the varia!ility is excessive so that it might lead to a footprint in the

data.

ose diagram showing fold as a function of offset and aimuth. This is useful when scatterednoise or illumination is an issue or studies of anisotropy are important. 1igure #2 3osediagram for the same bric+ pattern8 9he poor azimuth distribution results from the relatively

narrow ac%uisition template8.


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Figure 15

0ffset histogram showing the num!er of occurrences of a particular offset range within the

survey. This is useful as a regular offset distri!ution might lead to !etter linear noiseattenuation whereas an offset distri!ution that shows an excess of long offsets might lead to

!etter multiple attenuation. 1igure #B2.ffset histogram resulting from the bric+ layout8 9hereis a rapid nearly linear increase in the number of traces at any given offset up to 1166m andthen a gentle fall off to the maimum offset which is at #166m8 9his might be an appropriatedistribution if there is strong linear noise on the near offsets and not much multipleenergy. 1igure #%2 .ffset distribution from a wide azimuth orthogonal survey8 /n thisac%uisition template, there are about ":! traces within the offset range 1:66m to 1:!6m but

only ;! from "!6m to #66m8 9his may be an appropriate distribution if there is strongmultiple energy.


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Figure 16

Figure 17


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Stac array response showing how the arrays at the source and receiver in com!ination with

stacing attenuate linear noise. 7hen we thin of coherent noise at a common midpoint withina !in, we normally associate one ray path from the center of gravity of the source to thecenter of gravity of the receiver. In practice, there are liely to !e !oth source and receiverarrays. 1or example, in the setch !elow there is an array of % receivers and an array of Bsources. =ach of the source elements will have % rays, one to each receiver element. *s there

are B source elements there will !e a total of ray paths. This is for

only one source receiver pair. 1or a B& fold stac, the num!er of ray paths will !e

ray paths. =ach ray path will have a different length so the signalwill arrive at a different time. The result is that there will !e an attenuation of coherent noisethat will !e difficult to quantify without a modeling program. 1igure #H2 Horizontal rays fromone source element to a receiver array.

Figure 18

4omputer modeling will allow an estimate of this attenuation and changes to the layout may !e madeto ensure that the attenuation covers the wavelength of the most serious coherent noise. *n example

response is shown !elow. 1igure #$2 (tac+ array response for an orthogonal #D survey.


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Figure 1

In this figure, the degree of attenuation in d is shown as a function of wave num!er. 1or a typical low

frequency ground roll, the wave num!er might !e arranged to fall in the notch shown !y the red arrow.

1or an airwave, it might fall in the area of the green arrow.

Finite Dierence Modeling

ay trace modeling has its limitations. In reality, there is no such thing as an acoustic ray. Sound does

not travel along a narrow !eam lie a laser pointer. If it did, we would only !e a!le to tal to oneperson at a time? In seismic, we define a ray to !e a line that is perpendicular to the acoustic wave

front at all times. This maes it easier to visualie and calculate the !ehavior of waves. This wors wellwhen the wavelengths are short relative to the variations in the su!surface velocity. 1or this reason,ray tracing is referred to as 6a high frequency approximation to the wave equation6 (high frequencymeans short wavelength). ay trace modeling does not adequately descri!e the wave'front !ending

that occurs at sharp changes in the su!surface which means that it may exaggerate the impact ofillumination pro!lems. 7hen these conditions apply, it is necessary to use finite difference modelingwhich will provide a more realistic result and include diffractions and multiple events. 8owever, thetechnique is very compute intensive. 4omputing time increases with the fourth power of frequency?

"evertheless, the cost and effort may !e worthwhile relative to the cost of acquiring a multi'aimuth3D survey.

The following example is taen from 6* modeling approach to wide'aimuth design for su!salt imaging6!y 4*E 9. =@0"=, +, 8ouston, GS* pu!lished in the Eeading =dge in Decem!er /&&B. 5ariousacquisition techniques were modeled and included conventional towed streamer, ocean !ottom nodesand wide aimuth methods. 0ne of the results is shown !elow for the 7ide *imuth Towed Streamermethod using two different acquisition patches. 1igure /&2


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Figure 25


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Figure 21

4omparisons of results of this ind allow a -udgment to !e made of the relative value of different

acquisition methods. They also provide a useful data set to guide and experiment with differentprocessing techniques on data with a nown velocity model and structure.

Wide )8iuth 7eneits

*fter /&&, considera!le attention was given to the !enefits of wide aimuth acquisition, especially inmarine. 0n land, where it was easier to implement, it was already popular, largely as a result of thesymmetrical sampling concepts introduced !y @-is 5ermeer. *t sea, it was harder to implement anddata quality was generally much !etter already.

There are four main !enefits of wide aimuth surveys2

etter illumination,

etter multiple attenuation,

eduction in scattered noise,


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*imuthal velocity information.

There is also the mixed !lessing of anisotropic velocity. If velocity changes as a function of aimuth itcreates difficulties in processing as it will !e necessary to analye and apply velocity corrections as afunction of aimuth in wide aimuth surveys. The two 4+ gathers !elow show the seriousness of thispro!lem. The first gather has !een corrected with an isotropic "0 program, that is, a program thatuses a single velocity for all aimuths. 1igure #2C*> gather corrected with $*. correction using asingle velocity function.

Figure 1

The next 4+ gather uses a "0 program that accounts for velocity changes that are a function ofaimuth. 1igure /2 C*> gather corrected with $*. correction using a velocity function that varies with

azimuth.


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Figure 3

1igure A2 (alt Dome model with additional shallower horizons Courtesy >&().

Figure 4


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ay tracing was then used to simulate the actual acquisition that was employed in the existing 3D

survey using the positioning information from that survey to duplicate all source and receiver locations.The result showed that there was an area of the su!surface near the salt dome where no rays could

reach. This 6illumination hole6 is crucially in the most interesting target location. 1igure 2 (alt Dome

model showing the density of ray paths stri+ing the target horizon8 &() 7ines were ac%uired southwest - northeast8 $otice also the ac%uisitionfootprint can be seen on the horizon8

Figure $

The model was then reacquired with the shooting direction perpendicular to the first and the result isshown !elow. 1igure B2 (alt Dome model showing the density of ray paths stri+ing the target horizonwith shooting direction perpendicular to previously Courtesy >&().


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Figure %

There is still an illumination hole !ut it is not in exactly the same place as !efore and is smaller. 7hen

the results of the two shooting directions are com!ined, the hole is much reduced. 1igure %2 (alt Dome

model showing the density of ray paths stri+ing the target horizon with two perpendicular shootingdirections Courtesy >&().


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Figure 0

This model wor ena!led the oil company to mae a decision on whether the !enefits of shooting in a

second direction were worthwhile. These studies are routinely made when multi'aimuth surveys are

contemplated.

* similar study using finite difference modeling compared wide aimuth towed streamer acquisitionagainst conventional narrow aimuth acquisition. The results shown !elow areremara!le. 1igure H2 *odel comparison of wide azimuth data against conventional narrow azimuth.


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Figure

Multi#le )ttenuation

7hen a narrow aimuth survey is acquired, most offset ranges have approximately the same num!erof traces at every offset. This can !e seen !y referring to the left hand side of the following figure.Thus a multiple6s residual move'out curve will !e sampled relative evenly in offset. 8owever, when theresidual move'out curve of a multiple is considered, it is o!vious that, at short offsets, the timedifferent !etween traces is small !ut that, at long offsets it is large. Thus a regular sampling of offsetsleads to an irregular sampling of time increments. In fact, in severe cases the multiple move'out curvecan !e aliased. 1igure $.


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Figure

0n the other hand, for a wide aimuth survey, the num!er of traces that will fall in a given offset range

increases rapidly as the offset increases as seen on the right side of the figure. In this case, the

multiple residual move'out curve is !eing sampled more finely in distance as the offset increases. Thismeans that the time sampling of the move'out is more regular and leads to !etter multipleattenuation.

The following figure from the previous section on pre'acquisition modeling shows -ust how significantthe increase in traces as a function of offset can !e with a wide aimuth survey. The chart shows how

many traces fall within each &m range of offsets. In this acquisition template, there are a!out /%traces within the offset range #%&&m to #%&m !ut only A from /&m to 3&&m. 1igure #&2 .ffset

distribution from one patch of a wide azimuth orthogonal survey.


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Figure 15

The result of this more regular time sampling of the residual move'out curve using wide aimuth

surveys is that the multiple may !e !etter attenuated.

Scatered Noise )tttenuation

The near surface is often complex. 1or example, there may !e arsting, sand dunes, permafrost, andinfilled river valleys. In the @ulf of exico, salt may nearly penetrate the sea floor or cause ma-or

changes in the water !ottom profile; there may also !e gas seepage. In the "orth Sea, there are oftenterminal moraine deposits. *ll these can scatter seismic energy, some of which will !ounce !ac to the

receivers as noise. Gsually the most significant energy in a seismic record will !e refractions or groundroll on land or refractions and trapped modes at sea. It is this energy that creates the greatestpro!lems. The noise often appears on stac sections as shown !elow. 8owever, sometimes when thescattering sources are randomly distri!uted, the noise may appear to !e random and mistaenly !e

assumed to !e am!ient noise. 1igure ##2 (tac+ section showing scattered noise energy.


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Figure 11

The reason that these events appear on the stac section is that they can have "0 that is similar to

that of reflections despite the fact that they travel with different velocity to the reflections. *n

examination of the figure !elow shows that the geometry of the reflections and scattered events issimilar in a 4+ gather. 1igure #/2 2%uivalence of a vertical section and a plan view with near surfacescattering events seen at a C*> gather.


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Figure 12

0n the left, the setch represents a vertical slice through the earth with three reflectors as seen !y a

4+ gather. * is steeply dipping, is dipping !ut less steeply and 4 is flat. 0n the right is shown a plan

view of a 4+ gather with energy !eing scattered !ac from three surface discontinuities, * is near tothe line, 4 is perpendicular to the line at the 4+ and is at a!out A& from the seismic line measuredfrom the midpoint. Since the ray path geometry is the same, the same equations will apply for "0correction; the only difference !eing the velocity that is used. @enerally the surface velocity will !elower than that used to stac the reflections. 8owever, it is nown that the stacing velocity increasesfor a dipping event. Thus, there will !e occasions when the velocity that will stac horiontallytravelling energy from the point is the same as that needed to stac a reflector from a flat event. It

is this that causes the scattered energy to !e enhanced !y the "0 and stac process. There will !eones a!out the seismic line where energy from scattered events will !e enhanced. These are

represented !y the !lue triangles shown in the following figure. 1igure #32 5ones around a C*> whereenergy from scattered event will be enhanced by $*. and stac+.


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Figure 13

If a source of scattering exists within one of these ones it will !e enhanced !ut only on the particular

4+ aimuth. *nother source receiver aimuth is liely to exclude that scattering point and so will not

enhance the energy. If a survey is shot with many different aimuths, it is liely that only a few of theaimuths will enhance the noise and the ma-ority will re-ect it. 7hen all the different aimuths aresummed, the result will !e a relative attenuation of the noise.

*n example of this is shown !elow. The improvement is clear. 1igure #A2 Comparison section of datafrom a single conventional towed streamer and the same location repeated using three different

azimuths courtesy >&().


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Figure 14

I#leentation o Wide )8iuth

0n land, it is relatively simple to achieve multiple aimuths in a 3D survey !y the use of widetemplates provided sufficient equipment is availa!le. The following setch shows the most commonwide aimuth techniques at sea. *n alternative using circular or spiral towed streamer is also in use."aturally, these methods all cost more than conventional 3D surveys. 1or example the three aimuthsurvey shown a!ove requires the survey to !e shot three times. 8owever, the cost is not liely to !ethree times as much since2

There is only one mo!iliation.

Eess weather downtime as it may !e possi!le to change shooting direction when swell noise

causes pro!lems.

Turn times may !e reduced.

Eess infill shooting often !y half. 1igure #2Alternative methods to achieve wide azimuths in

marine ac%uisition.


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Figure 1$

,and )cuisition

The earliest 3D surveys and experiments were conducted on land, where it was easier to positionreceivers and sources, and to experiment with different areal layouts and com!inations. The 3D land

techniques !ecame esta!lished when higher'channel recording systems (greater than AH channels)were developed. 4learly on land, if the environment permits it, we could achieve our ideal 3D coveragewhere every 4+ !in contains data acquired from all offsets, each along a full range of aimuths.

The pro!lem on land is that source and receiver effort is expensive. 1or example, the num!er of


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Figure 1

The geophones and the source locations were laid completely around the loop, and the prospect was

covered !y laying out successive contiguous loops. This method was fast and economical, !ut it

suffered from several severe draw!acs. *lthough the plan of contiguous loops permitted total arealcoverage of the su!'surface, often at dense spacing, the trace assem!lage revealed large variations infold coverage, source'receiver aimuth and offset. *s a result, multiples were attenuated eithersuper!ly or not at all, signal'to'noise ratios varied, velocity information was varia!le in quality andthere was usually insufficient data redundancy for resolving statics errors. The method has largelyfallen out of use.

.he Rectangle .echniue

* variation of the loop technique was a regular plan of squares or rectangles, which were used to fitroad systems or land su!divisions (1igure /2 3ectangle techni%ue)


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Figure 2

This rectangle techni%uealso had applications in the -ungle, where it reduced costs !y requiring much

less line cutting. The shots and receivers were laid completely around each individual square or

rectangle in the same manner as for the loop. ecause of the regularity, this method wored !etterthan the loop. "evertheless, the same criticisms of too much varia!ility in offset, fold, and aimuthapplied to the rectangle technique, and this method also fell into disuse. It is possi!le that, under theright conditions, particularly simple near'surface weathering effects, the loop and rectangle approachesmay !e revived someday.

S*ath Shooting

*fter 7alton6s pu!lished experiment in #$%/, the first practical land 3D surveys used a system ofparallel lines, which !ecame nown as the swath techni%ue. *nother variation on 7alton6s approach,which uses an areal spread rather than orthogonal lines and crosslines, is called thepatch techni%ue.

oth techniques are still in continuous use and development.

The idea of swath shooting is to !uild fold coverage in !oth the inline and crossline directions. Severallines of receivers are used, with each line having a large num!er of receivers in order to achieve theoffsets required with a reasona!ly small group interval. This approach is useful in resolving weathering

layer statics pro!lems !ecause the inline and crossline redundancy can !e used when statics correctiontechniques are applied. 7ith #,&&& channels or more in the recording system, we can cover the

geometry of our prospect comprehensively, with enough channels for several lines of receivers.

*s an aid in visualiing a swath shoot, a plan for a #$/'channel swath comprised of four lines of AHchannels each is shown in (1igure 32 1"-channel swath, with fold plot). This small channel num!er ischosen for descriptive purposes only; in current practice there would typically !e #B lines of /A&

channels each.


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Figure 3

If a shot is taen on every fourth group as the spread is rolled in the inline direction, and the next

swath overlaps !y two lines of receivers, then the maximum fold coverage (away from the edge effects

of the ramp) is B'fold inline and A'fold crossline, giving a total fold of /A. The A'fold crossline coveragecan !e improved if extra lines of shots are placed !etween the lines of receivers. This gives H'foldcoverage in the crossline direction, for a total of AH'fold coverage when all the data are com!ined inthe stac.

y scaling this plan upward to AH& channels, it is possi!le to lay out H lines of B& channels each, or B

lines of H& channels each. This improves the crossline coverage and increases the inline coveragewithout having to shoot on every group. In some cases (such as, if this means cutting extra lines in the

-ungle), it is not convenient to place lines of shots !etween the lines of ca!les. In these situations, theH x B& plan can !e used. y shooting on alternate groups on the receiver lines, we can o!tain A'foldcrossline and #'fold inline coverage, for a total coverage of B&'fold.

Typical practices vary, depending on the nature of the terrain and the sie of the survey. In the iddle=ast for example, with large survey areas it is common to utilie receiver lines spaced typically at 3&&to && m, with a group interval of & m for an exploration survey. 1or higher resolution, the receiverline spacing can !e /&&'3&& m, and the receiver group interval can !e reduced to / m. The shootinggeometries vary, !ut are usually orthogonal to the receiver lines with typically a &'meter shot interval

and /&&'to && m spacing !etween shot lines.

6rthogonal Grid

The most common layout in use on land for ocean !ottom acquisition is the orthogonal grid. eceiverlines are laid out perpendicular source lines. Typical practices vary, depending on the nature of the

terrain and the sie of the survey. In the iddle =ast, for example, with large survey areas it iscommon to utilie receiver lines spaced typically at 3&& to && meters, with a group interval of &

meters for an exploration survey depending on the target depth and velocities. 1or higher resolution,the receiver line spacing can !e #&&'/&& meters, and the receiver group interval can !e reduced to /

meters.


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* schematic of this layout is shown !elow. This setch only shows a sparse layout for illustrationpurposes and, in practice, the num!er of lines may num!er in the hundreds and a template maycomprise /& or more lines. 1igure A.

Figure 4

ecause it is not usually practical to acquire the entire survey at one time due to equipmentlimitations, individual templates are shot and then repeatedly moved to a new position and shot again.The template is a su!set of the survey. 0ften the template will !e approximately square; in this casethe survey will !e considered wide aimuth as there will !e rays in all direction and the fold will !esimilar in all directions.

5ariations include the !ricwor and igag techniques. In the !ricwor technique, the source lines

are staggered !etween ad-acent receiver lines !y half the source line spacing (1igure 2'old plot for abric+ pattern survey).


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Figure $

*gain for higher resolution, the shot interval can !e reduced to / m with the source line spacing

reduced to /&&'3&& m. 0ther source geometries include single and dou!le igag, in which the source

lines move diagonally !acwards and forwards !etween receiver lines. =ach variation !rings a differentpattern of offset, aimuth and fold distri!utions with the !ins in each swath. If the source and receivergroup spacing is the same in these orthogonal geometries, then the direction of shooting relative to thesu!surface structure is not usually an issue. If the geometry is asymmetric with, say, a / m receiverspacing and a & m shot spacing, then the receiver lines may !e aligned with the predominant dip toprovide the !est sampling in this direction. Such asymmetric geometries are particularly useful in areasof steep dip with a predominant direction.

0f course the terrain may have a very significant influence in where receivers, and particularly sources,can !e positioned. This is especially true in ur!an or su!ur!an environments. It is common practice too!tain maps or satellite images of the area over which the 3D is to !e performed and then


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If the desired survey is relatively small, the patch shooting technique, rather than the swath, can !eutilied. 7alton6s original experiment was, essentially, a patch survey that gave single'fold coverageover a square area.(1igure B2 'rame a -patch layout) illustrates a portion of a patch survey that has $Bgroups laid out in an H x #/ spread.

Figure %

This spread can !e rolled in the same way as one rolls a conventional /D spread, to give B'fold inlineand A'fold crossline for /A'fold total coverage. This is a more !alanced arrangement than the swathgeometry and avoids the pitfalls of over'sampling the inline direction and under'sampling the crosslinedirection. This patch spread may !e scaled upwards to a 3/ x 3/ square of #,&/A channels with H'foldcoverage in each of the two orthogonal directions, for a total fold of BA.

*nother technique was later developed using a patch made up of rectangles, with alternate rectanglescontaining (or not containing) geophones. * shot is placed at the corner of each rectangle. Single fold

coverage over the patch is achieved when two shots are used, as shown in the large patch depictedin 'rame bof the a!ove graphic. *dditional shots and a roll'along method are used to increase foldcoverage and offset range. This is an efficient means of providing detailed and equal coverage in twoorthogonal directions.

.he 7utton Patch

* variation on the patch technique descri!ed a!ove is one nown as the lanning 7and #D (eismic (urveys, pu!lished !ythe S=@.

*ccording to 4ordson, et al., the buttonsconsist of a tight pattern of receivers, arranged in squares orrectangles ranging from BxB to BxH or HxH. *nd apatchof receivers is formed !y com!ining a num!erof receiver !uttons into a checer!oard arrangement.


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ultiple source points are fired into the receiver patch, after which the patch is rolled to its nextlocation. This su!sequent receiver patch is laid out so that it overlaps the preceding patch to a certainextent. * similar pattern of sources points is then fired into this new receiver patch.

eceiver !uttons are spread over the area to !e imaged at full'fold. utton patches typically use small!ins with lower fold. Source points are placed !eyond the full'fold area to o!tain a fold taper aroundthe edges, thus achieving longer offsets without spreading additional equipment !eyond the planned

area of the survey.

eceiver !uttons and source points can !e irregularly distri!uted in deference to surface o!structions.

8owever, !y repeating source points for different !uttons this technique is a!le to improve staticscoupling. *nd staggering source points !etween previous locations yields !etter mid'point distri!ution.

This technique tends to improve migration and D0 as seismic amplitudes contained on these longoffsets contri!ute energies to many trace gathers. 8igh resolution can !e achieved !y using closerreceiver spacing.

*. +roblems associated &ith data acquisition in mountainous terrain prompted development of a technique to

minimie the reflection point smearing in a crooed-line survey. ne solution &as to construct a s&ath ofparallel lines and then assign all of the common midpoints to the nearest line. This method of assigning data to

the nearest line is called:

(*) swathing.

() !inning.

(4) migrating.

(D) curve fitting.

#. Because the migration process produces edge effects, a halo or padding one must be included around theborder of the survey so that edge effects do not affect the seismic data over the central survey area. (lthough

rarely done, to derive the proper halo, the migration aperture distance is calculated from:

(*) the value for the minimum migration aperture alone.

() the value for the maximum migration aperture alone.

(4) the sum of the values for the minimum and maximum apertures.

(D) the ratio of the minimum over maximum apertures.

3. (lthough the character of the land surface sometimes forces a seismic survey to be shot in a particulardirection, &hen designing a 3-D survey, the ideal shooting direction for lines acquired in the field is:

(*) parallel to the direction of primary structural dip.

() perpendicular to the direction of primary structural dip.

'. /our techniques &ere developed for acquiring 3-D seismic data on land - loop, rectangle, s&ath and patch

acquisition patterns. T&o of these techniques are not often used because of problems associated &ith foldcoverage, source-receiver aimuth and offset. The t&o &idely used techniques that overcome these problems

are:

(*) loop and patch acquisition patterns.

() rectangle and swath acquisition patterns.

(4) swath and patch acquisition patterns.

(D) rectangle and patch acquisition patterns.

0. 1hen obstacles, such as drilling rigs or platforms, are present in the area of a marine survey, a particulartechnique enables a seismic acquisition cre& to acquire data from beneath the obstacle and to obtain

continuous coverage over the survey area. This technique is called:


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(*) sideshooting.

() overshooting.

(4) undershooting.

2. (s a rule of thumb, deep-&ater seismic survey vessels &ith a draft of 0 meters can safely acquire data in &ater

depths no shallo&er than:

(*) & meters.

() #& meters.

(4) meters.

. $n &hich shallo&-&ater acquisition method is the streamer cable laid on the sea floor &hile a source boat

travels do&n a line of recording cables4

(*) The fixed ca!le method.

() The !ay ca!le method.

(4) The !ottom ca!le method.

5. $n modern land seismic positioning surveys, elevations are not referenced to mean sea level, but to another

feature that is a mathematical approximation of the general shape of the earth excluding local topography.

This feature is referred to as:

(*) the @eoid.

() the =llipsoid.

(4) the *stereroid.

(D) the =llipse

6. There are three important components of marine seismic location positioning, or navigation. T&o of these are

no&ing the location of the seismic vessel and no&ing the location of the sources and streamers. The third

component is:

(*) !inning of the data as it is acquired so that vessel course corrections can !e made.

() the location of the shoreline.

(4) the location of the prospect.

*). True or /alse:1hile #D and 3D seismic surveys can both employ the same types of sources and receivers, the main difference

bet&eeen these t&o methods lies in the physical layout of the survey, and in the amount of data acquired.

(*) True

() 1alse

**. ( variation on the +atch Technique employs a tight pattern of receivers, arranged in squares or rectangles toform a checerboard arrangement. Then multiple sources are fired into the receiver patch. This method is

no&n as:

(*) the checer!oard technique

() the moving square technique

(4) the patch fold technique

(D) the !utton patch technique


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*#. True or /alse:

Because it is more economical for marine seismic vessels to shoot the inline direction along the longer axis of

the prospect the shooting plan may 7T be aligned in the direction of steepest dip.

(*) True

() 1alse

*3. True or /alse:1hen compared to marine seismic acquisiton, the land seismic acquisition is able to cover more area at a

faster rate.

(*) True

() 1alse

*'. 8adio telemetry bouys &ould most probably be used by

(*) land acquisition crews

() transistion one acquistion crews

(4) marine acquisition crews

(D) all of the a!ove

Marine )##lications

arine 3D data acquisition is a high'volume, high'productivity technology. The introduction of marine3D acquisition was delayed until the mid'#$%&s, when a means of tracing the streamer feathering

angle was developed using small magnetic compasses inserted into the streamer. y monitoring theaimuth of the streamer at each compass location, the shape of the ca!le and the location of each

group relative to the seismic vessel could !e determined. This method remains the !asis of today6smore sophisticated streamer tracing process. *lthough there are still errors inherent in the streamer

location system, the effects of these errors can !e minimied. Eocation errors resulting from errors inthe estimate of the local magnetic deviation and in compass cali!ration tend to !e consistent. These

errors can !e minimied if large contiguous portions of the survey are collected with the same streamerfeathering direction.

,ine Shooting )nd )cuisition Producti'ity

The conventional, single'vessel marine acquisition method calls for parallel'line shooting. This type ofsurvey is designed as a !loc of closely spaced, parallel lines (1igure #2 *arine shooting plan), typically(!ut not necessarily) oriented in the direction of steepest dip.


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Figure 1

1ew marine 3D prospects are square in areal form and, if they are rectangular, it is more economical to

orient the inline direction parallel to the longer axis of the prospect. 1or larger prospects, the

rectangular shape is wasteful since we do not need coverage everywhere, so the area is


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appropriate processing sequence, noise levels as high as H& to #&& micro!ars can !e tolerated if thenoise is short in duration and occurs randomly.

9ndershooting )nd Gra8ing .echniues

0!stacles are common pro!lems on 3D surveys, since many surveys are conducted over existing oil

and gas fields. 7hen rigs, platforms, or other o!stacles are present in the survey, an auxiliary shooting!oat may !e called in to assist in undersho

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