LAB 05 - NMO CORRECTION & STACKING

MATLAB SCRIPT LAB 5.3 

clear all,close all,clc
load SeismicData_gain_bpf_sdecon_gain_sorted_C.mat
load SeismicData_gain_bpf_sdecon_gain_sorted_velocities_C.mat

%% Apply NMO correction
max_stretch=10;
[Dsort,Hsort]=nmo_correction(Dsort,Hsort,v_stack,t_stack,cmp_start,cmp_end,cmp_step);

Stacking NMO corrected and stretched traces will severely damage the shallow seismic events. Therefore, we have to remove the stretching before we stack. We do this by muting the considerably stretched zones from the gather. The muted zone is usually set to threshold with typical value between 0.5 to 1.5. Selecting a small threshold value avoids stretching but might cause excessive loss of data while selecting large threshold value avoid loss of data but might introduce excessive stretching of the data. You can notice that the hyperbolic seismic events have become more flat for different layers (try to see the results from CMP number 220, 230 and 250). Now, you are ready to obtain the first approximation of the subsurface image using stacking.

%% Save NMO correction
save SeismicData_gain_bpf_sdecon_gain_sorted_nmo_corrected_C.mat Dsort,Hsort

The purpose of stacking is to enhance SNR ratio by eliminating coherent and incoherent noise and to reveal a first subsurface image approximation. The traces in the NMO-corrected CMP gather are stacked to produce one stacked trace that represent that particular CMP. The amplitude of the stacked trace can be the sum or the average of the amplitude of traces in the CMP gathers. 

%% Stacking
load SeismicData_gain_bpf_sdecon_gain_sorted_nmo_corrected_C.mat
[Dstacked,t,cmp_num]=sstack(Dsort,Hsort);

%% Save stacking
save SeismicData_gain_bpf_sdecon_gain_sorted_nmo_corrected_stacked_C.mat cmp_num t Dstacked

%% Display in wiggle
scale=1;
mwigb(Dstacked,scale,cmp_num,t)
xlabel(['CMP:',num2str(cmp_num),''],'FontSize',14)
ylabel('Time(s)','FontSize',14)

Figure 1: Wiggle display for stacked section of our seismic data

%% Display Gray scale
scale=1;
simage_display(Dstacked,cmp_num,t,0)
xlabel(['CMP:',num2str(cmp_num),''],'FontSize',14)
ylabel('Time(s)','FontSize',14)

Figure 2: Grey scaled display for the stacked section

%% Display Colored scale
scale=1;
simage_display(Dstacked,cmp_num,t,1)
xlabel(['CMP:',num2str(cmp_num),''],'FontSize',14)
ylabel('Time(s)','FontSize',14)​

Figure 3: Colored density display for the stacked section

You can see clearly the continuity of geological layers but yet this stacked section requires us to improve its horizontal (spatial) resolution. To improve the spatial resolution, this can be done by performing seismic migration.