[CIG-SEISMO] Noise simulation in SPECFEM2D

[Yang Luo]

Hi Jeroen & Yingzi,

I didn't implement the 2D version, which was actually done by Ryan
(cc-ed) based on the 3D code we have already had.
But I just checked the 2D code again and ran the example Yingzi mentioned,
all looks good to me.
The code I have may not be current, since I cannot use svn to get the
latest version.
But since the older version I have works, I assume the latest one should
work as well.

Yingzi, please see my following two cents, and make sure that you didn't
misunderstand what you have already obtained from your tests.

1. the positive branch --- it refers to the second half (i.e., 60 to 120 s
for the example) of the seismograms you obtained (not for t>0 in the
simulation).
In other words, our simulation actually should start from t=-60s, but to
avoid introducing this complexity, the time axis has been shifted.

Given this into consideration, I think what you saw (i.e., two envelops) in
your seismograms is correct.
You just missed the point that the time from the simulation (almost always
positive) is not the cross-correlation time (could be negative).
Tromp et. al. (2010) provides step-by-step figures, which I think you might
not fully understand yet.

2. the time_function_type --- you are right, the default should be changed
to the one that reads in the file S_squares.
But for benchmarking purposes, it is fine to put the one we have used in
our paper.
(Ryan, could you please fix that? just provide a default S_squres file
there using the time_function you hard coded).

3. nu_master --- it refers to the noise direction, which is trivial in 2D.

Yingzi, should you still be confused or claim that what you saw is totally
different from the paper,
please provide a picture of what you saw.

Thanks,
Yang









On Mon, Jan 28, 2013 at 11:45 AM, Jeroen Tromp <jtromp at princeton.edu> wrote:

>  Hi Yang:
>
> A question for you.
>
> Best regards,
>
> Jeroen
>
>
> Begin forwarded message:
>
>  *From:* Yingzi Ying <yingzi.ying at me.com>
> *Date:* January 28, 2013, 11:18:00 PST
> *To:* "cig-seismo at geodynamics.org" <cig-seismo at geodynamics.org>
> *Subject:* *[CIG-SEISMO] Noise simulation in SPECFEM2D*
> *Reply-To:* <cig-seismo at geodynamics.org>
>
>   Dear Tromp, Dimitri and all,
>
> I am interested in noise simulation with SPECFEM2D(V7.0.0) to get
> ensemble cross-correlation. I tried to reproduce the result in fig. 2(a)
> in Tromp's paper "Noise cross-correlation sensitivity kernels" by
> executing ./process.sh in EXAMPLES/noise_uniform/.
>
> My calculated interferogram after step 2 has two envelopes in the
> positive branch, and their positions are unconcerned with the time delay
> between two stations. This is totally different with the Tromp's result.
>
> I checked the source code "noise_tomography.f90" and found the value
> "time_function_type" is fixed to "4", which means the variable
> "time_function_type" in "DATA/SOURCE" will not take action and the
> "DATA/NOISE_TOMOGRAPHY/S_squared" will not be read. Furthermore, there
> is no "nu_master" in SPECFEM2D, which exists in 3D version.
>
> I don't know what goes wrong, and what I should do to get a correct
> ensemble interferogram. Anyone who knows please kindly give me a hint.
>
> Thanks in advance.
>
> Best,
> Yingzi
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>
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