p1-Fundamental of Geometry

7/29/2019 p1-Fundamental of Geometry

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FUNDAMENTALS OF 2D AND 3DFIELD SEISMIC GEOMETRIES

ASST. CHIEF SEISMOLOGIST

BGP/CNPC

BY

UBEKU JOSEPH


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OBJECTIVES

At the end of the presentation you willbe able to:

Describe the 2D techniques

Describe the 3D techniques Understand 2D geometry anomalies vs. 3D

geometry


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Presentation flow

2D Spread- methodology / terminology

2D Subsurface Coverage

2D Anomalies


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2D Spread- methodology / terminology

The set of active receiver used while

recording is called the SPREADThe following set of spread are applicable:

Shooting to the center with window (gapped split spread)

Shooting to the center without window (symmetrical splitspread)

Asymmetric Shooting (asymmetric split spread)

Off-end Shooting


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Occurs typically at the start/end of a line.

Is half the full spread operationally (channel count) andgeophysical (CMP count).

Channels always in-front/behind the shot point.

CMPs in one direction relative to the shot.


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Asymmetrical Spread

Occurs towards the start/end of a line.

Has greater channel count than off-end, but less than

or equal to symmetrical split spread.

Contains more channels on one side of the shot than on

the other.

CMP count is in both directions either side of the shot

but unequal.


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Coverage & Fold

The formulae for 2D fold is represented below:

where RP - Receiver point interval

SP - Shot point interval

CH n - Number of live channels

2D Fold

RP

SP

Ch n

2x

Fold = 1/2 x N x (group interval/source interval)where N is the total number of recording channels


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ReceiverPosition

Source Position 1

CMP Position

Receiver PositionSource Position 2

CMP Position

Receiver Position

Source Position 3

CMP Position

Receiver Position

Source Position 4

CMP Position

S1 R1 R 2 R 3 R 4 R 5 R 6 R7 R8 R9

1 1 1 1

2 2 2 1 1

R1 R 2 R 3 R 4 R 5 R 6 R7 R8 R9S2

3 3 2 2 1 1

R1 R 2 R 3 R 4 R 5 R 6 R7 R8 R9S3

Receiver Position

Source Position 5

CMPPosition

3

2

R1 R 2 R 3 R 4 R 5 R 6 R7 R8 R9S4

4 3 3 2 2 1 1R1 R 2 R 3 R 5 R 6 R7 R8 R9S5

4 4 3 3 2 2 1 1

4

5

1


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Advantages ofMultiple Coverage

Signal to noise ratio improvement

Multiple cancellation

Redundancy Residual static corrections

Velocity information (for NMO)


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Coverage Build-UP

Full Fold

Build-Up insome manner

Build-Down insome manner

Fold ofCoverage

This is what is requiredby the client


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Minimizing the End Effects

is done by:

Stacking (or Rolling) ON / OFF the spread

END-ON ASYMMETRIC SYMMETRICSplit Spread Split Spread

Spread is being rolled on

1 120

1

1

189

240120 121

68 69

R R

S

S

S

R

R


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Offsets

Near traces offset

Shows shallowest reflector of interest

Normally Shows data from the first group of

receivers from each source point


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Minimizing the End Effectsis done by:

Stacking (or Rolling) ON / OFF the spread

SYMMETRIC ASYMMETRIC OFF-ENDSplit Spread Split Spread

Spread is being rolled off

1121 1240

1000 12401120 1121

1051 12401168 1169

RR

S

S

S


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Offsets

Far traces offset

Approximately target depth

Beyond, not useful due to NMO muteShows data from the last group of

receivers for each source point

The first CMP for which data is recordeddepends on the offset to the far trace


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Effects of Offsets

mediumoffset

shortoffset

longoffset

Near trace offset (shot tonearest group distance) hasno effect on the coverachieved

The subsurface coverageis the same in each case

(This assumes that thecable configuration isunchanged)


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Effects of Offsets

mediumoffset

shortoffset

longoffset

Now with the shot in the sameplace each time you can see thatalthough the cover stays the same,the subsurface position fromwhich the data is obtained changes

With long offsets the signals havefurther to travel and becomeweaker and there is less detail inthe shallow reflections.

BUT unless you are very interestedin the shallow data, long offsetsare, in principle, preferred


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Although 2D seismic data is stillcommon (especially in frontier

areas), there is increasing useof 3D seismic acquisition and

processing, which solves some ofthe problems associated with 2Dseismic data.

What are these problems?

2D l


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2D Anomalies

Firstly, 2D seismic lines cover thin slices of the

sub-surface.


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2D A li


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2D Anomalies


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3 D Seismic


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At the end, you will be able to:

Describe the 3D technique

Calculate 3D Fold

Describe various field layouts

Offset Source and Receiver Points

Aim


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Presentation flow

Review of 2 D multiple Coverage

Why 3D?

Objectives of 3D Multiple Coverage

Rules for 3D Design3D Technique

Fold/Coverage

Field LayoutsOffsets and compensations

f l l


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Review of 2D Multiple Coverage

End-on technique

Split spread

Stacking on/off

Coverage, CMPs

Offsets


29/692D Covera e

ReceiverPosition

Source Position 1

CMP Position

Receiver PositionSource Position 2

CMP Position

Receiver Position

Source Position 3

CMP Position

Receiver Position

Source Position 4

CMP Position

S1 R1 R 2 R 3 R 4 R 5 R 6 R7 R8 R9

1 1 1 1

2 2 2 1 1

R1 R 2 R 3 R 4 R 5 R 6 R7 R8 R9S2

3 3 2 2 1 1

R1 R 2 R 3 R 4 R 5 R 6 R7 R8 R9S3

Receiver Position

Source Position 5

CMPPosition

3

2

R1 R 2 R 3 R 4 R 5 R 6 R7 R8 R9S4

4 3 3 2 2 1 1

R1 R 2 R 3 R 5 R 6 R7 R8 R9S5

4 4 3 3 2 2 1 1

4

5

1

Wh 3D C ?


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Why 3D Coverage?


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3D Objective

3D Data Volume (x,y,z)

No acquisition footprint

Homogeneous data volume

constant S/N ratio in volume - can only expectconstancy over CMP's not in time/depth

Next Generation

4D Data Volume (x,y,z,t)

Change of reflection coefficient with time

Increasing value of seismic in the reservoir

Th O ti 3D


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The Optimum 3DSymmetric Sampling criteria are:

Equal shot and receiver intervals Equal shot and receiver patterns (Arrays) Equal numbers of receivers per shot as shots

per receiver Split spread acquisition

3D symmetric sampling provides single-

fold 3D subsets that are eminentlysuited for pre-stack processing (e.g. FKfiltering)

R l f 3D D i


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Rules for 3D Design

Spatial continuity

Equal shot and receiver intervalsEqual shot and receiver patterns (Arrays)Equal numbers of receivers per shot as shots per

receiver

Split spread acquisition Deepest horizon to be mapped defines max offset Shallowest horizon defines line spacing Resolution defined by station interval

Noise suppression according to multiplicity In case of obstacles use smooth solutionsDo we need spatial continuity to this extent?

3D L


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3D Layout terms Box

CMP Bin Cross-line direction In-line direction

Fold Patch Template Swath

Zipper Xmin Xmax


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3D CMP / BIN


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3D CMP / BIN

CMPs

Defined by CMP

spacing in the 2

orthogonaldirections X,Y

D CMPX

= 1/2 SPI

D CMPY = 1/2 RPI

CMPs

S/L

R/L

Sourcepoint

Receiver

station

Y

X

SPI=Source point interval RPI=Receiver Point Interval

3D CMP / BIN


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3D CMP / BIN

But in 3D, we think more in

Bins of sizeD CMPX ; D CMPY

with ideal CMPs in the centre

CMPs

BINs

3D CMP / BIN


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3D CMP / BIN

In the real world not all CMPs hit exactly on

the theoretical position.A variety of reasons exist, but few are

highlighted below:

o Survey tolerance

o Source position error

o Drill errorso Offsets due to obstacles

Shot Template


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Shot Template

Xmax = Maximum Offset ~ Depth todeepest Target

Coverage patch

Building Block

Roll this wayThenthisway

Swath


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Swath

Width of the area over which source points are

being shot without any cross-line rolls.

S/L - SOURCE LINER/L - RECEIVER LINE

S/L S/L S/L S/L S/L S/LS/L

R/L

R/L

R/L

R/L

S/L

Template & Roll Pattern


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Template & Roll Pattern

Roll up 1 S/Lspacing

R/L

S/L

S/L

S/L

S/L

S/L

Swath 1

Swath 1

Swath 2

R/L

NEW

R/L

Then roll 1

R/L

and

comebackdown

Here a grid of sourcelines and receiverlines are shot withour Geometry= Template + Roll

pattern

S/L

Live Spread

Zipper design


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Zipper design

The Zipper design is a 3D layoutstrategy for large surveys whichuses overlapping swaths.


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Review of 2D/3D Coverage


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Review of 2D/3D Coverage

2D Fold comes from the length ofthe coverage area divided by themove up distance for a repeat of

that coverage unit.

3D Fold is based on the same idea,but in 2 directions.

Coverage and Fold


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Coverage and Fold

Coverage shows the extent of fold while the actualfold itself is the number of times a CMP - in thiscase a Bin - receives a signal.

In 3D, fold is considered in source (cross-line -x)

and receiver (in-line - y) directions.

3D Fold

RX

2R

Yx R = Rolls

3D Fold calculation


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3D Fold calculation

Orthogonal grid

In-line fold = in-line patch dimension2 X Source Line Interval

Cross-line fold = cross-line patch dimension2 x Receiver Line Interval

Total nominal fold= (In-line fold) x (Cross-line fold)

3D Fold calculation


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3D Fold calculation

Orthogonal grid

In-line fold = Number of live channels per line * Bin sizeShot line Interval

Cross-line fold = Number of live lines) * Bin sizeShot point Interval


Exercise


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Exercise

Consider the following Orthogonal grid

Receiver point Interval = 60mSource point interval = 60m

Receiver line spacing = 360mSource line spacing = 360m

Patch: 10 lines (receiver) with 72 active stations on each

Calculate the total fold

Solution


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Solution

= 10 X 30 = 560

= 72 X 30 = 6360

In line fold = Number of live channels per line * Bin size

Shot line Interval

Cross-line fold = Number of live lines * Bin sizeShot point Interval


= 6 X 5 = 30

3D Fold Calculation


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3D Fold Calculation

X cov= n active RP per RL * n SL(traverses) / active spread2 * n of R/L intervals after rolling of the active R/L spread

Y cov= n SP/SL * n RL used / swath2 * n of shots between 2RL n lines rolled per swath

Total coverage is =Xcov*Ycov

RP=Receiver points SP=Source pointsRL=Receiver lines SL=Source lines

Offset and Azimuth distribution


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Offset and Azimuth distribution

SPI

Box or RPI

"Unit Cell"

CMP Bin

SLI

cross-line RLI

Template (Patch)

in-line

Offset distribution stick diagram


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Offset distribution stick diagram

Azimuth distribution


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Azimuth distribution


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Orthogonal layout


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Orthogonal layout

Even near surface informationPeg numbering system simple

Operationally easy

S/L1

S/L2

S/L3

R/L1 R/L2 R/L3

Brick layout


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Brick layout

Better near surface informationwith respect to straight linepattern

Less environmentally friendlySimple peg numbering systemLogistically, slightly more difficultMore operational travel time

R/L1

R/L2

R/L3

S/L2

S/L1

S/L3


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Slant pattern


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Slant pattern

Travel / detour time reducedEnvironmentally friendly

Numbering of pegs complexGood near surface information

Omissions / Obstacles


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Om ss ons / Obstacles

Roads

Installations

Pipelines

Water wells

Houses

Irrigation Canals

etc..

The HSE Manual.

The Client. Local Government Regulations.

These are set bySafe Shooting Distances from:

The Safe Shooting Distance parameters

could depend on the Source type,size and or depth

Omissions / Obstacles


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m n /

The basic order ofpriority will change with:contract specifications

and options available.

Different source patterns

Different source size

Different source depth

Different source type

Omit / skip

Skid

OffsetOffset and Compensate,

Undershoot.

Options for Source Points

only!

Skid


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Sk d

Moving source points along away from itstheoretical position but not as an Offset.

OR

The movement of a source point without itsCMP leaving the associated Bin

Skid


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Skid

This is dependant on accumulation oferrors:

Positioning of sources and receiversin the Survey

Receiver / Phone layout

Drilled points and Vibrator locations

Skid


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X

Y

RS

The surface moves are X/2 ; Y/2

Combined surface errors of 5m (source & receiver )leads to an equal 5m subsurface shift.

If X, Y = 25m ( Bin dimensions)

Then pegging tolerances = 5mon source and receivertotal error on subsurface = 5m,.. 7.5m left.

If 2.5m is allowed for safety, this leaves 5m.If sources and receiver positions are good to5m on the surface, then we have nothing left.

THIS IS WHY ITS BETTER TO OFFSET INGROUP INTERVALS

Compensation


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Compensation

In this case, the point must be moved to a differentbin

Compensation


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Equal and opposite spread move

live

spread

l

ive

spread

Compensation

Compensation


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Equal and the same spread move

live

spread

live

spread

ompensat on

Undershooting / In-fill


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Undershooting / In fill

For large omissions

Undershooting / In-fill


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cannot recover near surface

Undershooting / In fill


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Thank You

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