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3-D VS 2-D SEISMIC

- THE END RESULT IS A DATA VOLUME

VS A DATA SLICE.

- MIGRATION OF 3-D SEISMIC DATA GIVES

A MORE ACCURATE SUBSURFACE IMAGE.

- 3-D SEISMIC DATA COSTS MUCH MORE.

3D 1

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2D VS 3D COVERAGE

3D 2

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3D 3

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LINE B 2-D MIGRATED

2

3

2

3

Tensor Geophysical Service Corporation

3D 63D 4

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RAW DATA

3D MIGRATION

2D MIGRATION

3D 53D 5

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3D COLOR DISPLAY OF CHANNEL

3D 43D 6

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LINE B 3-D MIGRATED

2

3

2

3

Tensor Geophysical Service Corporation

3D 7

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Important parameters in a 3-D CMP gather

1. fold

2. shot-receiver offset distributions

3. shot-receiver azimuth distributions

3D 8

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x

z

y

R

R

R

R

s

s

s

s

x

z

Rss ss RR R

Narrow-azimuth survey Wide-azimuth survey

Two general classes of seismic surveys

3D 9

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Common Midpoint (CMP) coverage

CMP bin size is 10m x 10m

8

7

6

5

4

3

2

1

9

10

Fold

3D 10

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a shot into a patch of receivers

receiver

shot

20 m

20 m

3D 11

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CMP fold for two shots

10

9

8

7

Fold

6

5

4

3

2

1

Y

C

O

O

R

D

3D 12

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Two patches shot

shot 2

shot 1

patch 1 patch 2

3D 13

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Example shot - receiver pairs into CMP bins

8

7

6

5

4

3

2

1

9

10

Fold

Y

C

O

O

R

D

3D 14

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" IN - LINE "

" CROSS - LINE "

TOTAL CMP FOLD =

IN-LINE FOLD X CROSS-LINE FOLD

3D 15

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Example Shot - Receiver pairs that contribute fold to a bin

8

7

6

5

4

3

2

1

9

10

Fold

Y

C

O

O

R

D

3D 16

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CMP fold for three shots

8

7

6

5

4

3

2

1

9

10

Y

C

O

O

R

D

Fold

3D 17

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. . . . ..

. . . . .

.. . . . .

.

. . . . .

.

. . . . ..

. . . . ..

. . . . ..

. . . . ..

*

*

*

*

2370 m

group

1 2 42

43

44

45

46

47

87

88

30 m

in-line direction

Patch roll

- in-line roll is 120 m (fourstations)

- cross-line roll is 120 m (4 lines)

Which yields

- in-line CMP fold = 11

- cross-line CMP fold = 4

- total CMP fold = 44

- 15m X 15m CMP bin size

15m

4 shots associatedwith each patch

30 m

Balam SE Geometry

3D 18

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3D 19

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SUBSURFACE SPACING

D. sin max

V

4 . F=

1

. DIPstk2 . F

Where :

V = Vrms (or stacking velocity to target event)

Fmax = highest useable frequency at target event

= steepest dip of interest

DIPstk =

3D 21

X

max max

= max dip of an event on the stacked section

max

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SUBSURFACE SPACING

D

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Example azimuth distributions for CMP gathers from a multi-azimuth survey

3D 22

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Example azimuth distributions for a narrow-azimuth survey

3D 23

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FAR OFFSET DEPTH OF DEEPEST TARGET

NOTE : IF WE KNOW THERE IS A MULTIPLE-PROBLEM, WE CAN

INCREASE THIS SOME, OR IF WE ARE MORE

INTERESTED IN THE SHALLOWER ZONES, WE CAN

DECREASE THIS SOME.

3D 24

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Example offset distributions from a narrow azimuth survey

3D 25

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EFFECT OF INCREASING SPATIAL SAMPLE INTERVAL

STACKED SECTION WITH TRACE SPACING = 106.4 M

3D 26

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EFFECT OF INCREASING SPATIAL SAMPLE INTERVAL

MIGRATED SECTION WITH TRACE SPACING = 106.4 M

3D 27

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EFFECT OF INCREASING SPATIAL SAMPLE INTERVAL

STACKED SECTION WITH TRACE SPACING = 53.2 M

3D 28

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EFFECT OF INCREASING SPATIAL SAMPLE INTERVAL

MIGRATED SECTION WITH TRACE SPACING = 53.2 M

3D 29

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EFFECT OF INCREASING SPATIAL SAMPLE INTERVAL

STACKED SECTION WITH TRACE SPACING = 26.6 M

3D 30

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EFFECT OF INCREASING SPATIAL SAMPLE INTERVAL

MIGRATED SECTION WITH TRACE SPACING = 26.6 M

3D 31

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MIGRATION APERTURE

Xmig

ZXmig= Z tan

If is themeasured (e.g. from dipmeter log) or calculated(e.g. from previous seismic) dip angle, and Z is the depth of

the dipping bed, then Xmigwill be an overestimate.

Xmig= (T V2 DIPmig)/4

An alternate, more accurate formula is:

Where:

DIPmig= x, the ratio of measured time over unit distance of the dipping event on a migrated section,

T = 2-way time to the deepest part of the dipping event.

V= Vrms (or migration velocity) to the dipping event.

3D 32

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MIGRATION APERTURE

3D 33

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MIGRATION APERTUREX mig

ZX mig = Z tan

An alternate, more accurate formula is:

T = 2 way time to the deepest part of the dipping event

3D 32

If is the measured (e.g. from dipmeter log) or calculated(e.g. from previous seismic) dip angle, and Z is the depth of

the dipping bed, then X mig will be an overestimate

X mig2

(TV DIP= mig ) /4

Where:

mig =DIP X , the ratio of measured time over unit distance

V = V rms (or migration velocity) to the dipping event

of the dipping event on an amigr atedsection

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Area to be imaged

Migration aperture

Fold taper zone

Full-foldboundary

SURFACE COVERAGE PROFILE