[CIG-SEISMO] specfem2d basin model mesh generation

[Phil Cummins]

Hello,

I'm new to specfem2d and am trying to help a student use it to do 2D 
modelling of seismic waves in a sedimentary basin. We started by trying 
to make a mesh of a simple, rectangular basin (see attached figure, 
whole domain is 30 km x 5 km, basin is 20km x 1 km). We used Gmsh so 
that we can have much smaller elements in the basin than outside. The 
only way we were able to get this to work with Gmsh was to define a 
rectangular domain, cut out a hole where the basin is, then insert a 2nd 
rectangular domain for the basin, and finally recombine the surfaces 
interior and exterior to the basin. (see attached .geo file if your 
interested - I know it sounds more calculated than it should be).

At the end of the day, when we use the script LibGmsh2Specfem.py to 
convert the gmsh output to specfem2d format, we appear to lose all the 
elements in the basin, and none of the elements are recognized as being 
along the free surface.

I would be very grateful if anyone could show us what we are doing 
wrong. What might work better though, is to have a simple example to 
follow. Has anyone worked on a similar problem and if so would they be 
willing to send me their gmsh (or perhaps cubit) file that I could use 
as an example?

Many thanks,

- Phil


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cl__1 = 150;
cl__2 = 50;
Point(1) = {0, 0, 0, 150};
Point(2) = {30000, -5000, 0, 150};
Point(3) = {30000, 0, 0, 150};
Point(4) = {0, -5000, 0, 150};
Point(5) = {7500, 0, 0, 150};
Point(6) = {22500, 0, 0, 150};
Point(7) = {22500, -1000, 0, 150};
Point(8) = {7500, -1000, 0, 150};
Point(9) = {7500, 0, 0, 50};
Point(10) = {22500, 0, 0, 50};
Point(11) = {22500, -1000, 0, 50};
Point(12) = {7500, -1000, 0, 50};
// Left wall
Line(1) = {1, 4};
// Bottom of the box
Line(2) = {4, 2};
//Right wall
Line(3) = {2, 3};
//Top of the box
Line(4) = {3, 6};
// Right Boundary
Line(5) = {6, 7};
// Lower Boundary
Line(6) = {7, 8};
// Left Boundary
Line(7) = {8, 5};
//Top of the box
Line(8) = {5, 1};
// Top of the box
Line(9) = {9, 10};
//Lower Boundary
Line(10) = {11, 12};
// Left Boundary
Line(11) = {9, 12};
// Right Boundary
Line(12) = {10, 11};
//Top of Domain
Line(13) = {1,3}
Line Loop(14) = {9, 12, 10, -11};
// overlying box
Plane Surface(14) = {14};
// underlying box
Line Loop(16) = {8, 1, 2, 3, 4, 5, 6, 7};
Plane Surface(16) = {16};
Recombine Surface{14,16};
Mesh.SubdivisionAlgorithm=1;
Physical Line("Top") = {13};
Physical Line("Left") = {1};
Physical Line("Bottom") = {2};
Physical Line("Right") = {3};
Physical Surface("M1") = {14};
Physical Surface("M2") = {16};Line Loop(17) = {9, 12, 10, -11};
Plane Surface(18) = {17};