[MSNoise] 1Bit and Whitening: Order of operations

[Thomas Lecocq]

Hi Francesco,


Le 08/02/2016 16:53, Francesco Civilini a écrit :
> Hello all,
>
>
> I would like to suggest an option in MSNoise to change the order of operations from Bandpass -> 1Bit Norm -> Whitening to Bandpass -> Whitening -> 1 Bit Norm in the s03_compute_cc.py routine.
>
> I have observed that for datasets where there is significant correlation in the higher frequencies by a cultural source (for example machinery near two stations), but the frequencies of interest are lower, the resulting correlation will be completely dominated by the high frequency signal if 1Bit Normalization occurs prior to whitening.
Could you define "higher frequencies" ? In that specific case indeed, 
that could be an issue, but on the other hand, if your data has a strong 
earthquake, or even worse, a big glitch, it'll smash the FFT.
>
>
> This could lead inexperienced users with noisy datasets to think that there are no low frequency signals propagating across their array. By doing 1Bit normalization prior to Whitening, it will no longer be possible to filter the cross correlations to further narrow the target frequencies, which is not the case if Whitening occurs before 1 Bit.
Interesting. Could you provide some demo data to demonstrate the effect 
? (the data, the parameters used, and some pictures? )
>
>
> A workaround for the current version of MSNoise is to change the original bandpass to a frequency range that excludes the problem frequencies, but this requires prior knowledge of what the cross correlations will look like for a wide frequency band.
Welllll... We usually assume poeple know what they have in hand... And 
have at least done a spectrum or spectrogram of their data. But I agree, 
it can be very tricky to identify before actually seeing the CCFs.

Cheers

Thomas

>
> Does anyone have any personal examples for datasets that support or contrast these observations?
>
>
> Thanks,
>
> -Francesco
>
>
> Francesco Civilini, M.S.
> Geophysics PhD Candidate
> Victoria University of Wellington
> Cotton Building Room 505
> Wellington 6012, New Zealand
> (027) 868-5939
> Francesco.Civilini at vuw.ac.nz<mailto:Francesco.Civilini at vuw.ac.nz>
> www.francescocivilini.com<mailto:Francesco.Civilini at vuw.ac.nz>
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