The problem of loading an external PPM

[ammcpherson]

Currently (as I understand it), the methodology for loading an external PPM into SPECFEM3D Globe is to create a text file that contains the Vs percent deviation from PREM with a transversely isotropic layer between the moho and the 220 km discontinuity, with oceans included.

I am a little confused on this, however. How does SPECFEM account for that anisotropy when I provide a flat Vs perturbation (not Vsh and Vsv)? I have yet to find an easy flag to choose to use the isotropic approximation for this perturbation as well, which would make sense for isotropic PPMs.

Is there any plan to allow external models in a similar fashion to the Cartesian code? Where you can provide both Vp and Vs directly from the external model?

Any insight would be greatly appreciated.

Thank you,
Amanda

[ammcpherson]

I was also wondering if there is a place where the values of PREM are printed out and stored, that I may use them in this percent deviation calculation and be sure that I am using the exact values of PREM that SPECFEM is expecting?

[ammcpherson]

I am also learning that if the external PPM does not incorporate a water layer, then (of course) the percent perturbation in the upper layer(s) of the model will be quite large and crash the mesher. It's simple enough to set those values to 0, but that would not reflect the model appropriately. Is it possible to choose PREM without a water layer?