How to Calculate Vertical Wavelength

Hi,

In the attached file "Signal" I have three (3) columns of signals. Please I want to calculate the wavelength?. The dt is 0.02 km. the data ranges from 20 to 40 km.

thanks

Answer 1

[Star Strider]

This should get you started:

D = load('Signal.txt');
dt = 0.02;
L = size(D,1);
t = linspace(0, 1, L)*dt;
Fs = 1/dt;
Fn = Fs/2;
FTD = fft(D - mean(D))/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);

[Amax,idx] = max(abs(FTD(Iv,:))*2);
FreqAtMaxAmplitude = Fv(idx)

figure
subplot(3,1,1)
plot(Fv, abs(FTD(Iv,1))*2)
xlim([0 5])
grid
subplot(3,1,2)
plot(Fv, abs(FTD(Iv,2))*2)
grid
xlim([0 5])
subplot(3,1,3)
plot(Fv, abs(FTD(Iv,3))*2)
grid
xlim([0 5])

The wavelength is the propagation velocity divided by the frequency in radians/time unit. The ‘FreqAtMaxAmplitude’ gives the frequencies of the peaks in Hz, so multiply these by to get the radian frequencies. You will then have to provide the propagation velocity in order to calculate the wavelength:

lambda = propagation_velocity ./ (2*pi*FreqAtMaxAmplitude)

[TTA]

Thank you very much. Please how can know the propagation velocity. I used the default wave velocity of 299 792 458 m/s, but my values are too large. Please I get a hint?

[Star Strider]

I would use that propagation velocity as well, for electromagnetic waves. You might be able to get better frequency resolution with:

FTD = fft(D - mean(D), 4.^nextpow2(L))/L;
Fv = linspace(0, 1, fix(size(FTD,1)/2)+1)*Fn;
Iv = 1:numel(Fv);

then:

[Amax,idx] = max(abs(FTD(Iv,:))*2);
FreqAtMaxAmplitude = Fv(idx)

producing:

FreqAtMaxAmplitude =
     3.161430358886719e-02     9.374618530273438e-02     3.080368041992188e-02

and:

propagation_velocity = 299792458;
lambda = propagation_velocity ./ (2*pi*FreqAtMaxAmplitude)

producing:

lambda =
     1.509236205638623e+09     5.089641934579894e+08     1.548952947892271e+09

They definitely appear to be very large. This may be a problem with the units, so check them carefully starting with ‘dt’ (that determines the frequency vector, ‘Fv’) and continuing through all of them.

[TTA]

Thank you very much. I really appreciate

[Star Strider]

As always, my pleasure!