MAG3D is a program library for forward modelling and inversion of magnetic data over 3d structures. It was developed under the consortium research project JOINT/COOPERATIVE INVERSION OF GEOPHYSICAL AND GEOLOGICAL DATA, by the UBC–Geophysical Inversion Facility
MAG3D Ver 4.0 (June 2005) - changes to the code and the manual
As might be expected, more recent UBC-GIF codes have features that have been found important for solving practical problems but these features were not included in earlier program libraries. The upgrades described below address this issue. The revised codes are more uniform in capabilities and are more computational efficient.
Improvements since version 3.1:
- The user can input lower and upper bounds. This feature can be important in helping control the range of acceptable models.
- A new preconditioner for solving the Gauss-Newton system of equations results in significantly improved performance.
- All values, except for the stored sensitivity matrix, are now in double precision. This results in more accurate calculations.
- A file sensitivity.txt is output after running magsen3d.exe. It contains the average sensitivity for each cell. This file can be used for depth of investigation analysis or for use in designing special model objective function weighting.
- A file maginv3d_nopos.sus is output during the first part of the inversion. It contains the susceptibilities without the bounds constraints imposed.
- A file maginv3d_XX.sus is output after each beta iteration.
- In maginv3d.exe, the user can enter either alpha values or length scales.
- The reference model is included in the calculation of the model norm.
- The reference model is now scaled by the depth weighting before starting the no-positivity iterations.
- Sensitivity calculations carried out by magsen3d.exe are now more efficient.
Changes to run time files for MAG3D Version 4.0
- To implement the bound constraints the input file for maginv3d has an extra line where the user can choose one of three options:
- a filename: two-column file with lower and upper bounds for each cell
- 2 numbers: constant values for lower and upper bounds
- NULL: default bounds of 0 and 1.
- There are several additional output files created; see description above.
Changes to this manual for MAG3D Version 4.0:
- This introductory note.
- Additions to section 1.4 "Inversion Methodology" are included in the Appendix.
- Changes to the format for input file "maginv3d.inp" are described in the Appendix.
- The optional "bounds.sus" file that can be used to specify lower and upper bounds for susceptibility values in each model cell is described after page 22.
Notes on computation speed
- Time to run computations using MAG3D Ver3.2 and Ver4.0 are compared in the figure below for a moderately complicated problem. This was a real example involving 1564 data points and 417,582 cells in the model, and moderate topography.
- The Version 4.0 code is significantly faster in all cases.
- Speed also depends strongly on the computer. Times when using two computers are shown. Both were Pentium IV processors with 1 Gbyte of RAM. The significant difference however concerns the size of the CPU's cache memory. Most good computers sold since roughly 2005 have 1.0Mb of cache memory and this results in a significant improvement in time to complete computation-intensive jobs.
- The complexity of the problem also affects computation times. More complex problems include topography, complicated distributions of susceptibility, weighting functions, reference models, bounds, etc.
- These results are presented for illustration only. The time to compute any given problem is strongly dependent upon the number of data points, the size of the mesh, and how you set up all parameters for the inversion, including data, constraints, regularization, compression, etc.
The remaining sections of the manual are on separate pages: