[MSNoise] compute mwcs error
Aurélien Mordret
mordret at mit.edu
Mon Dec 20 21:27:20 UTC 2021
Dear all,
Sorry for jumping this late in the conversation... I somehow missed the
first exchanges (I guess they when to my spam box...).
I just wanted to add a little clarification about the numerical values that
Dylan was referring to (the order of magnitude variations between bmin and
bmax).
Actually, I mostly chose bmax roughly equal to 2 times bmin or close to
this range (from 1.2 to 3 times). However, bmin and bmax are really
inter-station distances dependant and it's where the order of magnitude
step comes in.
For distances around 500m - 5km, the values of bmin and bmax will be
optimal around bmin=0.1 and bmax=0.3 (still with the 1.2 to 3-time factor).
Then, if you jump one order of magnitude in distance (5 to 50 km) you will
have to decrease by one order of magnitude bmin and bmax (0.01 and 0.03
respectively).
These rules are only rules of thumbs, and I never took the time to
formalize why they work or build an empirical relationship between
bmin/bmax and distance for a large set of cross-correlations, so use them
with care always remembering to explore a bit the values of bmin and bmax
before going bling-picking.
Cheers,
Aurelien
Le lun. 20 déc. 2021 à 20:09, Martha Savage <martha.savage at vuw.ac.nz> a
écrit :
> Thanks very much Dylan!
>
> It's great that you took the time to explain these meanings, despite the
> holidays and possibly AGU.
>
> Have a happy holiday!
>
> Martha
>
> Martha Savage
> Professor of Geophysics, SGEES
> Victoria University of Wellington
> Te Whare Wananga o te Upoko o te Ika a Maui
> Cotton 529
> Box 600, Wellington 6140
> New Zealand
>
> Email: Martha.Savage at vuw.ac.nz
> DDI: +64 (0)4 463-5961
> Mobile: +64 (0)21-262-7516
> Makere ana nga here
>
>
> -----Original Message-----
> From: msnoise-bounces at mailman-as.oma.be <msnoise-bounces at mailman-as.oma.be>
> On Behalf Of Dylan Mikesell
> Sent: Monday, 20 December 2021 8:42 PM
> To: Python Package for Monitoring Seismic Velocity Changes using Ambient
> Seismic Noise <msnoise at mailman-as.oma.be>
> Cc: Dagim Yoseph <dagim.yoseph at aau.edu.et>; El Mestel <el.mestel at vuw.ac.nz
> >
> Subject: Re: [MSNoise] compute mwcs error
>
> Hi Martha et al.,
>
> By 1 day, I meant 1 week I guess. Sorry about that. Here are my notes from
> Aurelien's original message about the FTAN parameters.
>
> The bmin and bmax parameters are the bandwidth min and max of the gaussian
> filters used in the FTAN algorithm.
> The FTAN applies a set of Gaussian filters to the seismograms with
> different central frequencies.
> The width of the filter (in Hz) for the minimum central frequency is bmin
> and vice versa for bmax.
> The width for the intermediate filters increases linearly between bmin and
> bmax.
> So you can determine the beta for each central frequency (f) as, *beta =
> linspace(bmin, bmax, nfreqs)*
>
> These parameters are equivalent to the "alpha" parameters in this paper
> defining the FTAN:
> Herrmann, R. B. (1973). Some aspects of band-pass filtering of surface
> waves. *Bulletin of the Seismological Society of America*, *63*(2), 663-671.
>
> If you look into the cpp source code in libfta.cpp (starting at line 31)
> you see the Gaussian filter definition.
>
>
>
>
>
> *double filtre_Gaussien(double f, double fh, double beta){//printf("freq :
> %f filtrage : %f beta : %f\n",f, fh, beta);return
> 1.0/beta*exp(-0.5*(f-fh)*(f-fh)/(beta*beta));}*
>
> Keep in mind that in this msnoise-tomo case, alpha/beta changes with each
> frequency. If I recall correctly, Aurelien's recommendation was to have the
> bmin/bmax vary by orders of magnitude (e.g. bmin=0.03, bmax=3.0). I can't
> find his email stating this or the reason, but this is what I recollect. If
> you play with the values you will see how the bandwidth changes the
> structure in the FTAN image. In the end, this changes how the autopicking
> picks.
>
> Please let me know if you have any other questions.
>
> Dylan
>
>
> On Wed, Dec 8, 2021 at 8:15 AM Dylan Mikesell <
> DylanMikesell at boisestate.edu>
> wrote:
>
> > Hi Martha,
> >
> > Yes, I do have some documentation about this. Give me a day to get it
> > sorted out. My last two MS students wrote some things about this. It
> > comes from the original papers that Aurelien Mordret wrote.
> >
> > Dylan
> >
> > On Wed, Dec 8, 2021 at 3:57 AM Martha Savage <martha.savage at vuw.ac.nz>
> > wrote:
> >
> >> Dear Thomas,
> >>
> >> Can you tell us what stage your paper "Lecocq, T., Mordret, A. (in
> >> prep), MSNoise-TOMO." Is in? We are trying to figure out how to use
> >> MSnoise-tomo and are having trouble understanding what some of the
> >> parameters are and therefore what would be good values to use. We
> >> hope that some of the answers might be in that paper.
> >>
> >> Otherwise, do you have any other documentation or hints? Right now
> >> the parameter we are most concerned with is the Gaussian filter
> >> bandwidth bmin and bmax.
> >>
> >>
> >> Thanks,
> >>
> >> Martha
> >>
> >> Martha Savage
> >> Professor of Geophysics, SGEES
> >> Victoria University of Wellington
> >> Te Whare Wananga o te Upoko o te Ika a Maui Cotton 529 Box 600,
> >> Wellington 6140 New Zealand
> >>
> >> Email: Martha.Savage at vuw.ac.nz
> >> DDI: +64 (0)4 463-5961
> >> Mobile: +64 (0)21-262-7516
> >> Makere ana nga here
> >>
> >>
> >> -----Original Message-----
> >> From: msnoise-bounces at mailman-as.oma.be <
> >> msnoise-bounces at mailman-as.oma.be> On Behalf Of Thomas Lecocq
> >> Sent: Wednesday, 24 November 2021 8:53 PM
> >> To: msnoise at mailman-as.oma.be
> >> Subject: Re: [MSNoise] compute mwcs error
> >>
> >> Hi,
> >>
> >> This usually means that no points for computing the linear regression.
> >> Did you set up a very very narrow filter, narrower than the number of
> >> points available in the spectrum ?
> >>
> >> Thomas
> >>
> >> Le 23/11/2021 à 14:49, Michelle Lee a écrit :
> >> > Hello everyone!
> >> >
> >> > Hope everyone is doing well! I ran into an error while trying to
> >> > run compute_mwcs. I am not quite sure what the error fully means
> >> > and was wondering if anyone could help me with the error or point
> >> > me in the right direction to troubleshoot and resolve the error.
> Thanks!
> >> > [image: Screen Shot 2021-11-23 at 8.43.25 AM.png] Best Wishes,
> >> > Michelle Lee
> >> > -------------------------------------------------------------------
> >> > ---
> >> > ------
> >> > Ph.D. Candidate
> >> > Columbia University: Lamont-Doherty Earth Observatory Department of
> >> > Earth and Environmental Science Marine Geology and Geophysics
> >> > c: (206)397-2488
> >> > alt. email: mkl2158 at columbia.edu
> >> >
> >> >
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--
==========================
Postdoctoral associate
Department of Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology
54-526
Cambridge, MA 02139-4307
phone: 617-253-5051
e-mail: mordret at mit.edu
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