[CIG-SEISMO] [SPAM: 7.000] Re: get_attenuation_scale_factor error

Sat Aug 27 14:01:48 PDT 2016

Hi Robin,

Q = 25 or above is classical and works fine, no problem.
However there is likely an error in the routine you use to build your 
velocity model, since it gives you mesh points at which Q = 4, and in 
that case yes SPECFEM3D can become unstable (easy to fix, see 
http://komatitsch.free.fr/preprints/GJI_Lombard_2016.pdf , and already 
fixed in the 2D code; we never took the time to fix the 3D code as well 
because such very low Q values never occur in practice).

It is likely your velocity model routine that gives fictitious very low 
Vs or Vp values at at least one location.

Best regards,
Dimitri.

On 27/08/2016 18:10, Robin Lee wrote:
> Hi Dimitri,
>
> Thank you very much for your answer, and apologies for the (very) late reply, I have been busy with a few other things but am back to addressing this issue with attenuation now.
>
> I'm afraid I'm a bit unsure about what this Q value specfem refers to represents. For our velocity model, the lowest Vs is 0.5km/s and we are using the correlations from Graves (2010), Qs = 50Vs and Qp = 2Qs, which for this case is Qs = 25 and Qp = 50. Is this attenuation too high for specfem3D? Or am I misinterpreting something?
>
> Regards
> Robin
>
> ________________________________________
> From: Dimitri Komatitsch [komatitsch at lma.cnrs-mrs.fr]
> Sent: Friday, July 29, 2016 7:48 PM
> To: cig-seismo at geodynamics.org; Robin Lee
> Subject: [SPAM: 7.000] Re: [CIG-SEISMO] get_attenuation_scale_factor error
>
> Hi Robin,
>
> Q = 4 means attenuation is extremely large. You maybe then need to
> switch to the "SolvOpt()" routine that we put in the 2D code. See
> http://komatitsch.free.fr/preprints/GJI_Lombard_2016.pdf and
> https://github.com/geodynamics/specfem3d/issues/742
>
> You should also check your reference frequency
> (ATTENUATION_f0_REFERENCE) because the correction factor you get (0.37)
> is extremely big (the reference value being 1.).
>
> Are you sure the viscoelastic wave equation remains a valid model for
> such a medium?
>
> Best regards,
> Dimitri.
>
> On 29/07/2016 06:31, Robin Lee wrote:
>> Hi all,
>>
>> I've been attempting to implement attenuation in a tomography model
>> which I have been running. It seems to run fine without attenuation, but
>> once I add in Qp and Qs, and set the attenuation to .true. in the
>> Par_file, I get an error in the gen_databases step which states the
>> following:
>>
>> error : in get_attenuation_scale_factor()
>>    scale factor:   0.37110551337678144       should be between 0.7 and 1.3
>>    Q value =    4.3168292045593262       central frequency =
>> 5.3879655900887585E-002
>>    please check your reference frequency ATTENUATION_f0_REFERENCE in
>> constants.h
>>  unreliable correction factor in attenuation model
>>  Error detected, aborting MPI... proc            0
>> --------------------------------------------------------------------------
>> MPI_ABORT was invoked on rank 0 in communicator MPI_COMM_WORLD
>> with errorcode 30.
>>
>> NOTE: invoking MPI_ABORT causes Open MPI to kill all MPI processes.
>> You may or may not see output from other processes, depending on
>> exactly when Open MPI kills them.
>> --------------------------------------------------------------------------
>>
>> I am currently using a simple correlation of Qs=50Vs and Qp=2Qs where Vs
>> is in km/s. I have looked in the constants.h file but cannot actually
>> find the ATTENUATION_f0_REFERENCE parameter. Has anyone experienced this
>> error before, know what is going wrong or know how to fix it?
>>
>> Regards
>> Robin
>>
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>
> --
> Dimitri Komatitsch
> CNRS Research Director (DR CNRS), Laboratory of Mechanics and Acoustics,
> UPR 7051, Marseille, France    http://komatitsch.free.fr
>
> This email may be confidential and subject to legal privilege, it may
> not reflect the views of the University of Canterbury, and it is not
> guaranteed to be virus free. If you are not an intended recipient,
> please notify the sender immediately and erase all copies of the message
> and any attachments.
>
> Please refer to http://www.canterbury.ac.nz/emaildisclaimer for more
> information.
>

-- 
Dimitri Komatitsch
CNRS Research Director (DR CNRS), Laboratory of Mechanics and Acoustics,
UPR 7051, Marseille, France    http://komatitsch.free.fr