[CIG-SEISMO] Modelling seafloor roughness with SPECFEM3D

[Jean Lecoulant]

To whom it may concern,

  I seek to model the emission of T-waves by a rough seafloor with
SPECFEM3D.

To do this, I need to mesh a random topography on the crust/ocean
interface, with a topography wave-length an order of magnitude smaller than
the typical wave-length of T-waves. The wave-length of the T-waves emitted
by my seismic source is about 1 km, therefore I need a topography with a
100 m wave-length. The discretization shall be done with a space-step dx ~
10 m to obtain a smooth topography.

The calculation domain for the simulations is a parallelepiped, 10x10 km
wide and 6 km thick. It is horizontally divided in a 3 km thick fluid
medium (the ocean) underlain by a 3 km thick solid medium (the Earth
crust). All sides, except the top (sea) surface, are absorbing layers. I
use Gmsh to build meshes and I run the appropriate python and fortran
routines to convert these meshes. I have been using different
discretization, varying the number of finite elements both along the
horizontal axes and along the vertical axis. In the horizontal plan, I have
tested 1001x1001 (dx = 10 m), 501x501 (dx = 25 m), 251x251 (dx = 40 m) and
201x201 (dx = 50 m) meshes. For each of these discretizations, I have
tested different numbers of finite elements along the vertical axis in the
crust and in the ocean: 31 (dz = 100m), 51 (dz = 60m) and 91 (dz = 33.33m).

Running the SPECFEM3D routine xgenerate_databases with those meshes, I
encounter two different errors. When there are too few elements along the
vertical: 'there is an error in separation of beta_x, alpha_y, alpha_z'.
When there are too many elements along the vertical: 'Access to an
undefined portion of a memory object'. Hence I cannot find a satisfying
mesh.

Does anyone know a way to generate a mesh able to model a rough seafloor?

Best regards,

Jean Lecoulant
Ph.D. student in geophysics
Laboratoire Géosciences Océan
Institut Universitaire Européen de la Mer
Université de Bretagne Occidentale, France
Tel: +33 (0)2 98 49 88 94
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