[CIG-SEISMO] Question: Acoustic simulation equivalent to a curl-free Elastic one (both homogeneous media)? SPECFEM2D v. 6.1

[Duran Neme Javier Alejandro]

Dear all

I am comparing the results obtained in an acoustic simulation with a curl-free elastic one (both homogeneous media).  I would want to know if both numerical experiments are equivalent on SPECFEM2D (I’m using version 6.1)?

In the acoustic case (vs=0m/s), I sent a plane wave (generated from several pressure point sources aligned horizontally) and I measure the seismograms at a distant point.
In the elastic case I sent also a plane P-wave, generated from several explosive sources aligned horizontally (An explosive source should generate only P-waves in a homogeneous elastic medium).  For the sources I used moment tensors in this case (Mxx=Mzz=1, Mxz=0) and measure the seismograms at the same point as before. I observed that Curl(u)~0  (which is expected if there are almost no S-waves) and Div(u) is non zero.

In the acoustic case I can only use Pressure sources and my receiver is an Hydrophone. In the elastic one my sources are moment tensors and my receiver is a geophone , thus I can measure displacement u,  Curl(u) and Div(u).  In both scenarios I used Ricker time function with the same amplification factor and frequency.
My question is:

•       Are both experiments equivalent (acoustic <--> elastic with curl(u)=0?) even though sources and receivers are different?  If they are equivalent, how can I compare both synthetic data (Pressure and Div(u) ) to make sure they are the same?    I tried Pressure=-Bulk_Modulus*Div(u), but they are several orders of magnitude off.

•       In the acoustic case, which are the units of the source?  [Pascals_2D]?=[N/m]?

I appreciate your reply. I’m using this information in order to compute the scattering cross section of a localised velocity anomaly in both scenarios. If both are equivalent, I expected to obtain the same cross section, but so far it is not the case.

With kind regards,
Alejandro


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