1D and 2D meshes

Before we can model or invert data we need a mesh: the cells in which the resistivity is defined. A good mesh is fine near the stations and at shallow depth, where the data see the most detail, and coarse far away. In this section we build meshes directly from a data file.

1D: nothing to do

A 1D inversion builds its own layering for every site, so usually you do not need this step. MakeMesh1D makes layers that start thin at the surface (a fifth of the skin depth at the highest frequency) and grow with depth. You can call it yourself to change the settings:

meshes = MakeMesh1D(load_data2d("data.dat"); first_layer_div = 5, vertical_factor = 1.1, depth_mult = 4)

Build a 2D mesh

MakeMesh2D reads a data file and writes everything a 2D inversion needs:

julia --project=. examples/make_mesh2D.jl data.dat topo.dat out_dir          # opens a window
julia --project=. examples/make_mesh2D.jl data.dat topo.dat out_dir batch    # no window

or, from Julia:

MakeMesh2D("data.dat"; out_dir = "mesh", topo_path = "topo.dat", mode = :batch)

In out_dir you will find:

• model.start and model.prior: the start and reference models

• fwd.ctrl, inv.ctrl, cov.ctrl: the inputs of Gauss–Newton and NLCG

• vfsa.ctrl, mask.ctrl: the inputs of VFSA

• the data rotated to the strike, data-r.dat

• Mesh.png and MeshCore.png: pictures of the whole mesh and of its core

The tool also prints a summary with advice, for example when two stations share a cell or when the padding is shorter than the skin depth at the lowest frequency.

With mode = :gui every setting is a slider with a live preview. Save inputs writes the files.

The main settings

Setting	Default	Meaning
cell_width_frac	0.5	core cell width, as a fraction of the station spacing
n_pad, pad_factor	12, 1.3	number of padding cells on each side, and their growth
first_layer_div	5	first layer = skin depth at the highest frequency / this
vertical_factor	1.1	growth of the layers with depth
depth_mult	4	model depth = this × skin depth at the lowest frequency
strike	nothing	nothing estimates it from the data; or degrees clockwise from north

All settings are listed in the control file reference.

Topography and water

Give MakeMesh2D a topo.dat and it cuts the ground surface into the mesh. topo.dat holds WGS84 lat lon elevation points along the line, in metres above sea level. Where there is a lake or sea, give the elevation of its bottom and mark the water:

MakeMesh2D("data.dat"; out_dir = "mesh", topo_path = "topo.dat",
           water = [(y_range = (-5500.0, -2500.0), level = 100.0)], water_resistivity = 200.0)

Here a lake lies between −5.5 and −2.5 km along the profile, with its surface at 100 m. The tool places every station on the ground of its column, and the depths in the data file (Z) are set to match.

Masks

The mask says which cells the inversion may change. Gauss–Newton and NLCG read it from cov.ctrl, VFSA from mask.ctrl. Both hold the same values:

Value	Cells
0	air, or cells you want to keep fixed
9	water: fixed, and not smoothed across
any other	free

You can build a mask yourself, for example to fix everything below 30 km:

mask = Mask2D(ReadModel2D("model.start"); fixed_below_m = 30e3)
WriteMask2D("mask.ctrl", mask)