N = length(Hs1_eta0);
vel1 = (Hs1_eta0(2:N) - Hs1_eta0(1:N - 1)) / dt;
vel2 = (Hs2_eta0(2:N) - Hs2_eta0(1:N - 1)) / dt;
vel3 = (Hs3_eta0(2:N) - Hs3_eta0(1:N - 1)) / dt;
vel4 = (Hs4_eta0(2:N) - Hs4_eta0(1:N - 1)) / dt;
vel5 = (Hs5_eta0(2:N) - Hs5_eta0(1:N - 1)) / dt;
vel6 = (Hs6_eta0(2:N) - Hs6_eta0(1:N - 1)) / dt;

N = length(vel1);
acc1 = (vel1(2:N) - vel1(1:N - 1)) / dt;
acc2 = (vel2(2:N) - vel2(1:N - 1)) / dt;
acc3 = (vel3(2:N) - vel3(1:N - 1)) / dt;
acc4 = (vel4(2:N) - vel4(1:N - 1)) / dt;
acc5 = (vel5(2:N) - vel5(1:N - 1)) / dt;
acc6 = (vel6(2:N) - vel6(1:N - 1)) / dt;


Fs = 1/.05;
Nfft = 4096;
window = hanning(Nfft);
Noverlap = 0.5 * Nfft;
pwelch(Hs1_eta0, window, Noverlap, Nfft,Fs);