LAB 01 - SHOT GATHER NO 8

First of all, we were introduced to the MATLAB software that will be used throughout the learning process for this semester. We need to understand the basic knowledge in handling the seismic data. Header is denoted by 'H' while data set is denoted by 'D'. In this module, we were required to examine the 2D seismic data from land acquisition in East Texas, USA. The important parameters of the seismic data are shown as below:-

• Number of shots = 18.
• Source type = dynamite in 80-100-ft depth holes.
• Number of channels per shot = 33.
• Receiver type = Vertical-component geophones.
• Array type = 12-element inline.
• Number of traces in line = 594.
• Receiver interval = 220 ft.
• Shot interval is variable.
• Time sampling interval = 2 milliseconds (ms)
• Number of time samples per trace = 1501.
• Data format = SEG-Y.
• Byte swap type = Big endian.
• Data file name = data.sgy.
• Geometry has already been set up and recorded in the trace headers.
• Uphole times at shot locations have been recorded in the trace headers.
• An 8-64-Hz bandpass filter has been applied to the data in the field.

I was assigned to perform a processing on Data Set C. In order to display the header information, we need to load the seismic data using the function SeismicData_C.mat. After that, the function of extracting_geometry.m are used in order to display he geometrical acquisition parameters. The steps are shown as the script below:-

MATLAB SCRIPT LAB 1.1

load SeismicData_C.mat
whos

%% Extract geometrical acquisition parameters
load ('SeismicData_C.mat','H')
[sx,sy,gx,gy,shot_gathers,num_trace_per_sg,sz,gz] = extracting_geometry(H);

%% Display number of traces per shot
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 (gcs,'YMinorGrid','on')

Figure 1: number of traces per shot

%% Plot the number of sources x-axis location versus the number of traces
figure,plot(sx,'.')
xlabel('Number of traces','FontSize',14)
ylabel('Sources x-axis locations (ft)','FontsSize',14)
axis tight
grid

Figure 2: the source elevation profile for each trace

%% Display stacking chart

num_shots=length(shot_gathers);

stacking_chart(sx,gx,num_shots,num_trace_per_sg);

Figure 3: stacking chart plot of the seismic data

extracting_shots.m function are used to display the seismic data shot gather, in this case we were required to extract shot number 8. There are several ways of displaying seismic sections such as seismic shot gathers and CMP gathers. The most commonly used display are the following:-

1. the wiggle display: plots of seismic trace amplitude as a function of time
2. the variable area display: shades the area under the wiggle trace to make coherent seismic events evident.
3. the variable density display: represents amplitude values by the intensity of shades of gray (and sometimes in color)

The following are shown on how to display the wiggle, gray density and colored density area for the shot gather number 8:-

%% Display wiggle shot number 8

load SeismicData_C.mat

shot_num=8;

p=0;

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

scale=1;

Figure 4: variable area display shot gather no 8

%% Display grey scaled variable density display

mwigb(Dshot,scale,offset,t)

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

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

Figure 5: Gray-scaled variable density display shot no 8

%% Display color variable density display

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

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

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

Figure 6: Colored variable density display shot no 8

In a nutshell, this lab exercise is to familiarize the students with the MATLAB software and introduce basic functions used to display the various area display for the particular seismic shot gather. In next section, we will examine chosen number of shot gathers in further understanding the behavior of seismic trace.