[MSNoise] Autocorrelations in MSNoise

[Christos Evangelidis]

Hi Thomas,

As far as I understand you haven't changed anything since then in the code
concerning the pre-processing when computing autocorrelations. Right?

Greetings from Athens,

Christos




From: Thomas Lecocq <thomas.lecocq at ...
<http://gmane.org/get-address.php?address=thomas.lecocq%2d7ewb5K7nbqnl%2fIpH24cmtA%40public.gmane.org>
>
Subject: Re: Autocorrelations in MSNoise
<http://news.gmane.org/find-root.php?message_id=54DB931F.5080705%40seismology.be>
Newsgroups: gmane.science.geophysics.msnoise
<http://news.gmane.org/gmane.science.geophysics.msnoise>
Date: 2015-02-11 17:36:31 GMT (1 year, 14 weeks, 6 days, 11 hours and 16
minutes ago)

Hello Aurélien,

Le 11/02/2015 18:30, Aurélien Mordret a écrit :
> Hi Thomas,
>
> What is the pre-processing of the noise when computing autocorrelations? Is
> it the same as for the cross-correlations?
Yeah, currently it's the same, WHICH IS NOT GOOD ! (that's why it's
written somewhere NOT TO USE AUTOCORR), but that might not be trivial to
find...

Your suggestion makes absolutely sense ! Are you used to github etc ? If
yes, please fork and PR an updated version of whiten ! (including a
autocorr=False flag in the def, maybe?)!

Regards from Brussels

Thomas
>
> If yes, would it be better to not fully whiten the data before
> autocorrelation as the only information comes from the amplitude spectrum
> (and not the phase)?
>
> I suggest to modify a bit the whiten.py function to keep the structure of
> the whole code similar. Following Gorbatov et al. (2013), instead of
> putting the amplitude spectrum =1 (with tapper) I would use a normalisation
> with a water-level in the form
>
> A(freq) = A(freq)/max(A(freq),c*max(A(freq)))
>
> With A the amplitude spectrum and c the water-level factor (which is
> typically on the order of 1%).
>
> It should not change the results for the cross-correlations and improve the
> autocorrelations.
>
> Does it make sense or did I missed something?
>
> Cheers,
>
> Aurelien
>
> REF:
>
> Gorbatov, A., Saygin, E., & Kennett, B. L. N. (2013). Crustal properties
> from seismic station autocorrelograms. *Geophysical Journal International*,
> **192**(2), 861-870.
>