hello
see my little demo below
I extracted the portion of data of interest - you can do computation or smoothing and put back the smoothed portion in lieu of the original data
my plot is horizontal but that does not impact the demo principle
code
% dummy data
depth = (1870:2200);
VP = 1+rand(size(depth));
VP(1917-1870:1976-1870) = VP(1917-1870:1976-1870) + 2; % create a local step
%% main code
ind = find(depth>=1917 & depth<=1976);
VP_extract = VP(ind);
% math
VP_extract_mean = mean(VP_extract);
% smooth
VP_extract_smooth = smoothdata(VP_extract,'gaussian',20);
% replace original VP_extract with VP_extract_smooth
newVP = VP;
newVP(ind) = VP_extract_smooth;
plot(depth,VP,depth,newVP);
