Subject 10: Lecturer: Objective

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Subject
10
:
Seismic Data Processing

Lecturer:
Dr. Bakhtiar Q. Aziz

Objective
:
This subject is very important for BSc

student so 4 hours are specified
to give the students an obvious look out about the digital sequence of essential and
optional processing
.
They take a flow chart of processing step by step, each
processing is clarify and illustrated by diagram.

Then the
students will practice on which optional data is necessary for certain
cases.



Scientific contents

1
-

Flow chart of essential data processing.

1
-

Flow chart of optional data processing.

3
-
Common depth point
.

References

1
.

Applied and environmental geophysics, 1999, Sharma,V.,P.

2
.

Introduction to geophysical prospecting, 1988, Durbin, M. B.

3
.

www.Geophysics.net

4
.

www.seismicexploration.net/

Digital processing of seismic data

It is a sequence of operations carried out according to a predefined program
to extract a useful information from a set of raw data.

It is summarized as follow:

1
-

Data copying.

2
-

data classification and storage.

3
-

Many types of computation and analysis.

4
-

Display of results
.

Display of raw data:

There are four types of displaying seismic data on paper:

1
-

Wiggles 2
-

Variable area 3
-

Wiggle+Variable area 4
-
Variable intensity

0

1

2

TWO Way Time

Distance (Trace)

Seismic section display as
Wiggle+Variable area


Seismic Field data

True Amplitude Recovery

Data Editing

CDP Gather

Static Correction

Dynamic Correction

Muting

CDP Stack

Section Display

Vibroseis Field data

Cross Correlation

Deconvolution

Filtering

Equalazation

Migration

Coherency

Depth Conversion

Data processing Sequence

It is divide into two parts:

1
-

Essential Processing.

2
-

Optional Processing.

Optional Processing
.

True Amplitude Recovery (TAR):

Involves the removal of the following effects by multiplying the trace with
the (F
(T)Tar
).

1
-

Spherical divergence.

2
-

Inelastic attenuation.

3
-

The net gain imposed by the recording station.

)
(
.
.
.
)
(
.
.
.
.
t
t
v
Av
TAR
t
G
e
t
v
C
F
Av



)
(
)
(
t
Av
G
v
C
: Scaling constant

: Average velocity

: Gain applied

: Absorption coefficient

Data Editing:

Some seismic data is harmful on the seismic record, so they have to be removed

Such as;

1
-

Weak data

2
-

Dead data

3
-

Reverse data

1
-
Essential Processing:

Muting on actual seismic section

Dead or
bad
traces

Bad
data
removed

Common depth point (CDP) or Common reflection point (CRP)

It is a group of wave path have one common depth point

In the past single shoot point and geophone is used, so every
point below the surface is covered once.

CDP1

SP1

G1

CDP2

SP2

G2

CDP3

SP3

G3

CDP4

SP4

G4

In this case fold of coverage = %100 or 100

Fold:

It is number of reflections from one common depth.

One fold of coverage may give wrong result. So the fold of coverage is
increased to:


%300, %600, %1200, %2400, %4800, %9600





9600
,
4800
,
2400
,
1200
,
600
,
300






3
-
Fold

6
-
Fold

12
-
Fold

24
-
Fold

48
-
Fold

96
-
Fold

SP1

SP2

SP3

G1

G2

G3

CDP

SP1

SP2

SP3

G1

G2

G3

CDP

3
-
Fold, or %300, or

300

6
-
Fold, or %600, or

600

SP4

SP5

SP6

G4

G5

G6

Concept of (CDP) stack

It is a process of adding all the seismic traces which return to one common
depth, it is always applied after static and dynamic corrections.

Before NMO Correction

After NMO Correction

After Stack

The importance of CDP
-
Stack

1
-
Enhancement of the signal

2
-

Attenuate the random noise

3
-

Control the multiples

CDP
-
Gather:

After missing all unwanted data, all traces that return to one common

depth point are gathered together to examine the process of data editing

S

S

S

5

4

CDP

S

S

S

3

2

1

6

1 2 3 4 5 6

Data Editing

CDP
-
Gather

1 3 6

Muting:

It is a kind of data editing by which the non
-
reflection events are removed, such as:

1
-

First part of traces which contained usually refracted data, it is also


called (first break suppression).

2
-

directed waves.

Direct and
Refracted events

Muting

2
-
Optional processing:


Deconvolution:

It is similar to TAR , but here we compensate for the loss of frequency components.

This loss is caused by the effect of the earth materials that act as a filter for high

Frequency.

Deconvolution also defined as a process by which we restore the observed reflection

Signal to the original form.

SP

G

Frequency = 10 KHZ

Frequency = 0.10 KHZ

Trace equalazation:

It is a process involves the adjacent of the gain of various channels so that their

Amplitudes are comparable.


Before Equalization

After Equalization

G
1

G
2

G
3

G
4
G
5

G
6

G
7

G
8

G
1

G
2

G
3

G
4
G
5

G
6

G
7

G
8

Migration:

It is a process that restore the reflettion events to their proper position, it is applied

In the following cases:

1
-

When the dip is large.

2
-

When there is a syncline.

Bow
-
Tie feature

Before migration

After migration

Coherency:

It is a process by which a coherent events (Reflection signal) are enhanced

and incoherent events (Wind noise) are attenuated.


Before Coherency

After Coherency