LAB 02 - AMPLITUDE GAIN

In this lab section, we will focus on trace editing and Gain Apllication. In order to enhance the seismic data, we need to apply amplitude gain correction. Amplitude loss occur due to transmission loss, geometrical divergence or absorption loss. Therefore, the gain application is applied to shot gather number 8 and the process are shown as below:-

MATLAB SCRIPT LAB 2.1

clear, close all;
load SeismicData_C

%% Assign Header Value and shot
shot_num=8;
p=0;
[Dshot,dt,dx,t,offset]=extracting_shots(D,H,shot_num,p);

%% Display before Gain
scale=1;
figure(1);
mwigb(Dshot,scale,offset,t)
xlabel('Offset(ft)','FontSize',14)
ylabel('Time(s)','FontSize',14)
title('Before Gain','FontSize',16)

Figure 1: Shot gather number 8, before applying amplitude correction gain method 

%% Applying Gain
pow=2; %power value
T=0; %time correction
Dg=iac(Dshot,t,pow,T);

%% Display after Gain
scale=1;
figure(2);
mwigb(Dg,scale,offset,t)
xlabel('Offset(ft)','FontSize',14)
ylabel('Time(s)','FontSize',14)
title('After Gain','FontSize',16)

Figure 2: Shot gather number 8, after applying amplitude correction gain method 

%% Display amplitude envelope gain
tnum=33;
seis_env_dB(Dshot,Dg,t,tnum)
seis_env_dB(Dshot,Dg,t)

Figure 3: the amplitude envelope gain for trace 33

Figure 4: the amplitude envelope gain for average trace

%% Apply new gain (AGCgain)

agc_gate=0.5;

T=1; %normalize with trace amplitude

Dg_AGC_T1=AGCgain(Dshot,dt,agc_gate,T);

%% Display Figure 5

scale=1;

figure(3);

mwigb(Dg_AGC_T1,scale,offset,t)

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

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

title('After AGC_T1','FontSize',16)

Figure 5: Shot gather num 8 after applying AGC using RMS method

%% Apply new gain (RMSgain)

agc_gate=0.5;

T=2; %normalize with trace rms value

Dg_AGC_T2=AGCgain(Dshot,dt,agc_gate,T);

%% Display Figure 6

scale=1;

figure(4);

mwigb(Dg_AGC_T2,scale,offset,t)

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

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

title('After AGC_T2','FontSize',16)

Figure 6: Shot gather num 8 after applying AGC using instantaneous based method

​​The quality control process prepare the data for further quality control and processing. Gain application is applied to account for amplitude losses due to spherical divergence and absorption. Whenever you want to display a seismic data, you may want to boost weak signals by adding more gain to the data.

The following MATLAB m-file uses the function iac.m to apply independent amplitude correction on shot gather number 8. You can clearly see the amplitude enhancements gained in Figure 2. You can further analyze the gain increase after the corrections by plotting the average trace amplitudes envelope in dB for both shot gathers.

The data dependent scheme relies on multiplying each time sample by a scalar derived from a window of data around the sample. Such technique is called automatic gain control (AGC). AGC technique includes:-

• RMS amplitude AGC: requires segmenting each trace into fixed time gates and then

1. calculate the RMS value in each gate
2. divide the desired RMS scalar by RMS value and multiply it by the amplitude of the sample
3. interpolate between these gate centers and multiply the resuly by the amplitude of corresponding time

• Instantaneous AGC