FFT Spectrum from 2D array

[rpursley8 · Accepted solution]

There is a function in the palette for unevenly sampled data.  It can be found in Signal Processing>>Spectral>>Unevenly Sampled Signal Spectrum

[Magma]

---

These measurements are real measurements .. take it from a real well ... The samples are manually extracted from a real well and the depth correspond to real manually extraction. I am not doing DAQ with a system. Lets said for example that I have been manually measuring the porosity on the correspoding depth that I am showing to you. I cannot extrapolate the data in this case.

---

As noted, numerically identical values of alternating sign are not physically realistic. Understand and correct your measurements before you attempt to calculate a spectrum. Also as noted, the full development version of LabVIEW contains a VI that uses the Lomb normalized periodogram and avoids the need for evenly sampled data found in most (all?) FFT implementations.

[altenbach]

---

Magma wrote:

As noted, numerically identical values of alternating sign are not physically realistic.

---

 They could be real. They simply mean that we have a very strong spectral component near the Nyquist frequency. (Which is poorly defined for unevenly sampled data)

[altenbach]

---

rpursley8 wrote:

There is a function in the palette for unevenly sampled data.  It can be found in Signal Processing>>Spectral>>Unevenly Sampled Signal Spectrum

---

 Excellent. I remember seeing that in the past but did not find it when writing my first reply. Of course the output is not an FFT, but a power spectrum. Maybe that's sufficient for the OP?

[Magma]

altenbach, I suppose that's mathematically possible, but I suspect the data file consists of differential voltage measurements from a single or paired sensor and for some reason includes both normal and reversed polarities. I usually prefer the simplest explanation that fits.

[Alvaro_Ortiz]

Excellent .. That is what I need !! ... and I could find it !! .. Thanks a lot .. know my problem is to try to identify the values of Frequency (if I am having depth instead of time) ...

Do you know a way that I could be doing an inverse of this spectrum ?? I suppose that is going to give me the original depth.

Thank you ..

By the way .. the amplitudes are not Voltage measurements .. the are values from Log (porosity, permeability, etc) .. for that reason I have different polarities 

[Magma]

Alvaro, I'm a geophysicist- porosity and permeability cannot have negative values. And to be precise what you have are digitized readings from logging tool sensors (neutron, gamma, sonic or other) that you hope are related to the physical properties in question, not the properties themselves. Find out how the data was collected and what the values represent, otherwise risk garbage in, garbage out.

[Alvaro_Ortiz]

Magma,

I appreciatte your help and concern on this problem. As I told you Im not a geophysics or petroleum eng. I am just helping them on a project class.

They explain me that the data that I have are Wavelet coefficients or one of the Wavelet coefficients of the original log data.

That is all I know .. How was processed ?? .. I dont know .. I am just trying to make them a program that automatically analyze the frequency spectrum and  plot a filter part of that spectrum on depth vs amplitude (or whatever they have).

I really sorry for the confussion and I appreciatte your help ..

[altenbach]

LabVIEW contains some wavelet tools, but you might want to look at the advanced signal processing toolkit.

It is not clear at all what you are trying to do with all this. 

[dilaekayati]

Hello all, I've tried to get fft from 2d array data ( 1st column is time, in csv format) .  

Is there something wrong with my diagram? Why the fft doesn't come out ? The fft graph show nothing but blank. 

Please help..