"Phimean=51*1" means, that this variable has the sie [51, 1]. After
Vp=reshape(Vp,size(Phimean));
Cp has the same size also.
If i==2 in trhe 2nd iteration of the for loop, the expression:
dpr(:,i,:)= calcola_dati_pre_stack(Vp(:,i), Vs(:,i), Rho(:,i), wavelet, ang);
% ^^^^^^^
tries to access Vp(:, 2), but the 2nd dimension of Vp is 1, because it has the size [51, 1]. This is the meaning of the error message.
I cannot predict, what you want to do instead.
You can identify such problems by using the debugger:
dbstop if error
Then start the function again. When it stops at the error, check the sizes of the variables.
