3D meshes

In 3D the forward problem is solved by ModEM, but the mesh still has to come from somewhere. In this section we build a 3D mesh from a data file, move a model from one mesh to another, and pull the topography and water out of a model.

Build a 3D mesh

MakeMesh3D reads a ModEM data file and writes a start model and a matching ModEM covariance file:

julia --project=. examples/make_mesh3D.jl data.dat start.rho dem.tif

or, from Julia:

MakeMesh3D("data.dat"; out_model = "start.rho", topo_file = "dem.tif", topo_crs = "EPSG:26918", mode = :nogui)

The model (mesh_start_model.rho unless you name it) and the covariance (C3.dat) are written next to each other. mode = :gui opens a window where you can change the settings and see the mesh update.

topo_file is optional. It is a digital elevation model (DEM) in the coordinate system topo_crs. With it, the ground surface is cut into the mesh as air cells and the station depths in the data are set from the DEM.

Setting	Default	Meaning
cell_width_frac	0.5	core cell width, as a fraction of the station spacing
n_pad, pad_factor	12, 1.5	number of padding cells on each side, and their growth
first_layer_div	4	first layer = skin depth at the highest frequency / this
vertical_factor	1.2	growth of the layers with depth
depth_mult	3	model depth = this × skin depth at the lowest frequency
air_layers	6	number of air layers above the highest ground
cov_value	0.3	smoothing written to the covariance file

Move a model to another mesh

Often you want to start one inversion from the result of another, on a finer or coarser mesh. MeshToMesh resamples a model onto a target mesh:

MeshToMesh("vfsa_best.rho", "data.dat", "fine_mesh.rho")

It writes three files into a new timestamped folder next to the target mesh: the resampled model, a copy of the data with the station depths moved onto the new ground surface, and a matching covariance file.

The target mesh keeps its own topography and water. Only its earth cells are filled from the source model. method = :nearest (the default) keeps sharp contrasts, and method = :linear smooths them.

Topography and water from a model

A model with topography stores the air as very high resistivity. You can extract the ground surface, or the sea floor, as a table:

m = load_ws3d_model("model.rho")
topo  = extract_topography(m)
bathy = extract_bathymetry(m; water_log10 = 0.3)   # cells below 2 Ω·m at the surface are water
write_bathymetry("bathymetry.dat", bathy)

A bathymetry file like this tells 3D VFSA which cells are sea and should stay fixed.