Lab 1 : Examine Seismic Data

Description of the Data Set:

The seismic data and its data must be carefully looked when meeting the seismic data for the first time. The seismic data and the header contains information and tabulation of parameters used to acquire the data. These data are store in the seismic data format SeismicData.mat. The data and the header is to be examined by using the MATLAB.

In this lab session, you are require to examine this data along with its header information, by using MATLAB. This will assist you in further analysis and processing of this real reflection seismic data, which will be performed in subsequent lab session. first you need to know the header information to understand  the processes  ..

OBJECTIVES:

• To examine and look into the seismic data and its header containing information and tabulation of parameters used to acquire the data
• To extract the useful information from the header using MATLAB function.
• To create several displays of shot gather

Header Information

• dt – Time sampling interval
• ns – number of time samples per trace
• offset – The offset
• tracl – Trace numbers
• fldr – The shot gather numbers

• sx & sy – source coordinate
• gx & gy – receiver coordinate
• gz – source elevation

Steps to do this practical are as below:

1. Loading the raw seismic data and its header

2. Extracting a certain shot gather or a group of shot gathers from the data matrix D and header structure H

3. Plotting the number of seismic traces versus the number shot gathers

 load SeismicData_A.mat

whos

load ('SeismicData_A.mat','H')

[sx,sy,gx,gy,shot_gathers,num_trace_per_sg,sz,gz]= extracting_geometry (H);

figure ,stem(shot_gathers,num_trace_per_sg)

xlabel('Shot gather numbers','FontSize',14)

ylabel('Number of traces/shot gather','FontSize',14)

axis([0,max(shot_gathers)+1,0,max(num_trace_per_sg)+2])

set(gca,'YMinorGrid','on')

4.  4. Plotting the number of sources x-axis locations versus the number of traces

figure ,plot(sx,'.')

xlabel('Number of traces','FontSize',14)

ylabel('Sources x-axis locations (ft)','FontSize',14)

axis tight 

5. Plotting the number of receivers x-axis locations versus the number of traces

figure,plot(gx,'.')

xlabel('Number of traces','FontSize',14)

ylabel('Receivers x-axis locations (ft)','FontSize',14)

axis tight

grid

6. Plotting the sources elevation versus the number of traces 

figure,plot(sz,'*-')
xlabel('Number of traces','FontSize',14)
ylabel('Sources elevation (ft)','FontSize',14)
grid

7 .Plotting the receivers elevation versus the number of traces

figure,plot(gz,'*-')

xlabel('Number of traces','FontSize',14)

ylabel('Receivers elevation (ft)','FontSize',14)

grid

8. Vewing the stacking chart

num_shots=length(shot_gathers);

stacking_chart(sx ,gx ,num_shots ,num_trace_per_sg);

9)  Plotting shot gathers and display in various formats

shot_num =8;

p=0;

[Dshot ,dt ,dx ,t,offset ] = extracting_shots(D,H,shot_num ,p);

scale =1;

mwigb (Dshot ,scale ,offset ,t)

xlabel ('Offset (ft)','FontSize' ,14)

ylabel ('Time(s)','FontSize' ,14)

figure ,simage_display(Dshot ,offset ,t,0)

xlabel ('Offset (ft)','FontSize' ,14)

ylabel ('Time(s)','FontSize' ,14)

figure ,simage_display (Dshot ,offset ,t,1)

xlabel ('Offset(ft)','FontSize' ,14)

ylabel ('Time(s)','FontSize' ,14)

Displaying a group of seismic shot gathers concatenated together can also be done by using the same function extracting_shots.m. The figure below shows the extracted shot gathers number 4-6 in their variable area display.

DISCUSSION:

The last Figure  shows the seismic trace amplitude is plotted as a function of time and a gray scaled image where the area under the shaded wiggle trace is functioning to make the coherent seismic events more evident. As, it is in density display where it shows the amplitude values by the intensity of shades of colors.

From all the figures above, it is proved that amplitude decays with time. Amplitude decreases due to wave attenuation because of energy loss. This energy loss can be due to multiple reason as wave propagates through subsurface.

However, there is an amplitude anomaly at trace 22 between 1.5 ms to2 ms time window. The trace may require muting before proceeding to next processing step. For instance, such amplitude anomaly will affect the result when conducting gain correction.

CONCLUSION:

• All objectives are achieved.
• The seismic data and its header information are examined before proceeding to the next processing step.
•  Able to extract information from the header.
• Able to create several display of shot gathers.