WFTEM3D

WFTEM3D is a quick, flexible and extensible 3D finite-difference transient electromagnetic modeling open-source software. It is applicable to the tunnel model, the non-flat topography model, the multi-scale model, etc. WFTEM3D has two language versions: a Matlab version (in the WFTEM3D.Matlab folder) and a Fortran version (in the WFTEM3D.Fortran folder). A simplified Python version is also provided, but with slow calculation speed.

Method

First the scheme calculates the approximate initial field excited by whole-space magnetic dipole sources at an initial time after the current is switched off. Then the scheme steps Maxwell’s equations in time using a staggered grid and a modified DuFort-Frankel method. See the paper LiFei and JiulongCheng, 2023, 3D finite-difference transient electromagnetic modeling with a whole-space initial field, Geophysics, online: https://doi.org/10.1190/geo2021-0828.1 for detail.

Installation

The Matlab version: first the Matlab software should be installed, then the code can run in Matlab without installation.

The Fortran version: first the Visual Studio software and the Intel Visual Fortran software should be installed, then the code can run in Visual Studio without installation.

Usage

We use the example of a Conductive brick in a half-space to show how to use WFTEM3D. Detailed description of this model can be seen in the paper 3D Finite-difference Transient Electromagnetic Modeling with Whole-space Initial Field.

1. Edit the input file according to your model.

Example1_Conductive brick in a half-space.dat is the input file of the model a Conductive brick in a half-space. It is in the Data folder and it is a standard input file：

100
17,18,18
1280,640,320,160,80,40,20,20,20,20,20,40,80,160,320,640,1280
1280,640,320,160,80,40,20,10,10,15,20,20,40,80,160,320,640,1280
1920,960,480,240,120,60,30,15,15,15,15,15,15,30,90,270,810,2430
9,8,9
8,8
2400
64
0.7
3
1,17,1,18,1,18,0.1
1,17,1,18,1,9,0.0003
7,11,11,12,12,13,2

Description of the input file:

Line 1: Length of Tx loop (m), current is 1 A by default:
        100
Line 2: Number of cells in the x-, y-, and z-directions:
        17,18,18
Line 3: Grid size in the x direction (m):
        1280,640,320,160,80,40,20,20,20,20,20,40,80,160,320,640,1280
Line 4: Grid size in the y direction (m):
        1280,640,320,160,80,40,20,10,10,15,20,20,40,80,160,320,640,1280
Line 5: Grid size in the z direction (m):
        1920,960,480,240,120,60,30,15,15,15,15,15,15,30,90,270,810,2430
Line 6: Tx loop center (i0,j0,k0, at the bottom face center of this cell):
        9,8,9
Line 7: Receivers (from i0,Rx_st,k0 to i0,Rx_end,k0, at the bottom face center of these cells):
        8,8
Line 8: Maximum number of iterations:
        2400
Line 9: Number of subloops:
        64
Line 10: Coefficient used to calculate time step size (typically ranges from 0.4 to 0.8)：
        0.7
Line 11: Number of model subareas:
        3
Line 12: Subarea 1. Cells range (x=1–17,y=1–18,z=1–18) and conductivity (0.1 S/m) of the background: 
        1,17,1,18,1,18,0.1
Line 13: Subarea 2, Cells range (x=1–17,y=1–18,z=1–9) and conductivity (0.0003 S/m) of the air:
        1,17,1,18,1,9,0.0003
Line 14: Subarea 3, Cells range (x=7–11,y=11–12,z=12–13) and conductivity (2 S/m) of the conductive brick:
        7,11,11,12,12,13,2

2. Run the program.

The Matlab version: first open the Main.m file with Matlab, then click on the "Run" button in the toolbar or use the shortcut key F5 to run the program.

The Fortran version: first open the WFTEM3D.vfproj file with Visual Studio, then click on the "Run" button in the toolbar or use the shortcut key F5 to run the program.

3. View calculation results.

Results (dBz/dt at every time instants) will be output automatically in the Data folder when the calculation is finished. There are three output files:

• Result_time.txt

The time instants are saved in this file. The example is shown as follows:

1.087338257278733E-007
1.361303417188709E-007
...
1.000056346847610E-002
1.000888668992117E-002

The unit of time is s.

• Result_dBz.txt

The dBz/dt at receivers are saved in this file. The example is shown as follows:

-0.2916895094E-004
-0.2574270181E-002
... 
-0.5137402423E-009
-0.5129357698E-009

The unit of dBz/dt is V/Am^2.

• Run_time.txt

The run time is saved in this file. The example is shown as follows:

 Computation finished. Run-time is 0.40241540000000 s.

Contributing

Pull requests are welcome. We expect contributions via email with the corresponding author (email: figo1@163.com).