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

TW

O W

ay T

ime

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

tvAv

TARt G

etvCF

Av

)(

)(

t

Av

G

v

C : Scaling constant

: Average velocity

: Gain applied

: Absorption coefficientData Editing:

Some seismic data is harmful on the seismic record, so they have to be removedSuch as;1- Weak data2- Dead data3- 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-Fold24-Fold 48-Fold96-Fold

SP1

SP2 SP

3 G1G2G3

CDP

SP1

SP2

SP3 G1G2G3

CDP3-Fold, or %300, or

300 6-Fold, or %600, or

600

SP4

SP5 SP

6 G4G5G6

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 SS

54

CDP

S SS

321 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 GFrequency = 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

G1 G2 G3 G4 G5 G6 G7 G8 G1 G2 G3 G4 G5 G6 G7 G8

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 enhancedand incoherent events (Wind noise) are attenuated.

Before Coherency

After Coherency