% This program aims at making you familiar with the convolution concept and
% the equivalence of convolution in space-time and spectral domains

clear
close all

% Let us generate a theoretical "Green's function" for example, a time
% signal with one arrival of a certain amplitude at a certain time

dt=.01;
t=0:dt:10;  % time between 0 and 10 seconds with sampling of 10 Hz
s=(1:length(t))*0;
s(250)=1;

subplot(3,1,1)
plot(t,s)
%xlabel(' Time (s) ')
ylabel(' Amplitude ')
title(' Greens function ')


% Now let us generate an "arbitrary input signal", here we choose the first
% derivative of a Gaussian

sigma=.2;   
t0=1;
src=exp(-1/sigma^2*(t-t0).*(t-t0));
src=diff(src)/dt % simple finite differencing!


subplot(3,1,2)
plot(t(1:length(src)),src)
%xlabel(' Time (s) ')
ylabel(' Amplitude ')
title(' Input signal ')

% Now let us convolve the Green's function with the src

cs=conv(s,src);

% Note that the signal is now much longer but we only look at the original
% seismogram length

subplot(3,1,3)
plot(t,cs(1:length(t)))
xlabel(' Time (s) ')
ylabel(' Amplitude ')
title(' Response of the system ')