[CIG-SEISMO] velocity gradient specfem2D

[seisphd]

Dear developers and users
Downloading and starting using specfem2D I appreciated the fact that its free software and was amazed by its potentials.
I have a question though, which I can not solve through the manual.
Using an "internal velocity model" format;( like "simple_topography_and_also_a_simple_fluid_layer/interfaces_simple_topo_curved.dat").
Which parameters needs to be modified  into the Par_file to introduce a vertical velocity gradient?
So for example the nodes of the first layer instead being all 
3000.d0will start 2900 at the top and increase gradually with  depth to end with 3100 at the bottom.

Thank you in advance for your help and time


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