# 1. MAG3D v5 Package Overview¶

## 1.1. Description¶

is a program library for carrying out forward modelling and inversion of surface, airborne, and/or borehole magnetic data in the presence of a three dimensional Earth. The program library carries out the following functions:

1. Forward modelling of the magnetic field anomaly response of a 3D volume of susceptibility contrast.
2. The model is specified in the mesh of rectangular cells, each with a constant value of susceptibility. Topography is included in the mesh. The magnetic response can be calculated anywhere within the model volume, including above the topography to simulate ground or airborne surveys. There is also a capability to simulate and invert data collected beneath the surface (e.g. borehole surveys) and combinations of ground and borehole surveys.
3. Assumptions:
• This code assumes susceptibilities are small enough that the effects of self-demagnetization can be neglected.
• Remanent magnetization is not directly accounted for, although anomaly projections can be included with the observations.
4. Inversion of surface, airborne, and/or borehole magnetic data to generate 3D models of susceptibility contrast:
• The inversion is solved as an optimization problem with the simultaneous goals of (i) minimizing a model objective function and (ii) generating synthetic data that match observations to within a degree of misfit consistent with the statistics of those data.
• To counteract the inherent lack of information about the distance between source and measurement, the formulation incorporates depth or distance weighting.
• By minimizing the model objective function, distributions of subsurface susceptibility contrast are found that are both close to a reference model and smooth in three dimensions. The degree to which either of these two goals dominates is controlled by the user by incorporating prior geophysical or geological information into the inversion. Explicit prior information may also take the form of upper and lower bounds on the susceptibility contrast in any cell.
• The regularization parameter (controlling relative importance of objective function and misfit terms) is determined in one of three ways, depending upon how much is known about errors in the measured data.
• Implementation of parallel computing architecture (OpenMP) allows the user to take full advantage of multi-core processors on a CPU. A cluster-based code using Message Passing Interface (MPI) is also available. Notes on computation speed are found at the end of this section.
5. The large size of 3D inversion problems is mitigated by the use of wavelet compression. Parameters controlling the implementation of this compression are available for advanced users.

The initial research underlying this program library was funded principally by the mineral industry consortium “Joint and Cooperative Inversion of Geophysical and Geological Data” (1991 - 1997) which was sponsored by NSERC and the following 11 companies: BHP Minerals, CRA Exploration, Cominco Exploration, Falconbridge, Hudson Bay Exploration and Development, INCO Exploration & Technical Services, Kennecott Exploration Company, Newmont Gold Company, Noranda Exploration, Placer Dome, and WMC.

The current improvements have been funded by the consortium “Potential fields and software for advanced inversion” (2012-2016) sponsored by Newmont, Teck, Glencore, BHP Billiton, Vale, Computational Geoscience Inc, Cameco, Barrick, Rio Tinto, and Anglo American.

## 1.2. Program library content¶

### 1.2.1. Executable programs¶

This package consists of five major programs:

• PFWEIGHT: calculates the depth/distance weighting function
• MAGFOR3D: performs forward modelling
• MAGSEN3D: calculates the sensitivity matrix
• MAGPRE3D: multiplies the sensitivity file by the model to get the predicted data
• MAGINV3D: performs 3D magnetic inversion

### 1.2.2. Graphical user interfaces¶

GUI-based utilities for these codes include respective viewers for the data and models. They are only available on Windows platforms and can be freely downloaded through the UBC-GIF website:

• GM_DATA_VIEWER: a utility for viewing raw surface or airborne data (not borehole data), error distributions, and for comparing observed to predicted data directly or as difference maps.
• MeshTools3D: a utility for displaying resulting 3D models as volume renderings. Susceptibility volumes can be sliced in any direction, or isosurface renderings can be generated.
• GUI: a GUI to run MAG3D v5.0 on either Linux or Windows. NOTE: The download does not contain the inversion/modelling codes.

## 1.3. Licensing¶

A constrained educational version of the program is available with the IAG package (please visit UBC-GIF website for details). The educational version is fully functional so that users can learn how to carry out effective and efficient 3D inversions of magnetic data. However, RESEARCH OR COMMERCIAL USE IS NOT POSSIBLE because the educational version only allows a limited number of data and model cells.

Licensing for an unconstrained academic version is available - see the Licensing policy document.

NOTE: All academic licenses will be time-limited to one year. You can re-apply after that time. This ensures that everyone is using the most recent versions of codes.

Licensing for commercial use is managed by third party distributors. Details are in the Licensing policy document.

## 1.4. Installing¶

There is no automatic installer currently available for this package. Please follow the following steps in order to use the software:

1. Extract all files provided from the given zip-based archive and place them all together in a new folder such as

• Do not store anything in the “bin” directory other than executable applications and Graphical User Interface applications (GUIs).
• A Message Pass Interface (MPI) version is available for Linux upon and the installation instructions will accompany the code.

## 1.5. Highlights of changes from version 3.2¶

The principal upgrades, described below, allow the new code to take advantage of current multi-core computers and also provide greater flexibility to incorporate the geological information.

Improvements since version :

1. A new projected gradient algorithm is used to implement hard constraints.
2. Fully parallelized computational capability (for both sensitivity matrix calculations and inversion calculations).
3. A facility to have active and inactive (i.e. fixed) cells.
4. Bounds are specified through two separate files, rather than one two-column file.
5. Additional flexibility for incorporating the reference model in the model objective function facilitates the generation of smooth models when borehole constraints are incorporated.
6. The maginv3d.log file has been simplified and detailed information on the inversion can be found in the maginv3d.out file.
7. An alternative version of the software compatible with Message Pass Interface (MPI) is available for Linux.
8. Backward compatibility: The new version has changed the input file format and the bounds file. Data, mesh, model, and topographic file formats have not changed.
9. The depth weighting function and sensitivity are computed separately.

### 1.5.1. Notes on computation speed¶

• For large problems, MAG3D is significantly faster than the previous single processor inversion because of the parallelization for computing the sensitivity matrix computation and inversion calculations. Using multiple threads for running the parallelized version resulted in sensitivity matrix calculation speedup proportional to the number of threads. The increase in speed for the inversion was less pronounced, but still substantial.
• It is strongly recommended to use multi-core processors for running the weighting and sensitivity calculation. The calculation of the sensitivity matrix (G) is directly proportional to the number of data. The parallelized calculation of the $$n$$ rows of $$\mathbf{G}$$ is split between $$p$$ processors. By default, all available processors are used. There is a feature to limit $$p$$ to a user-defined number of processors.
• In the parallelized inversion calculation, $$\mathbf{G}^T \mathbf{G}$$ is multiplied by a vector, therefore each parallel process uses only a sub-matrix of $$\mathbf{G}$$ and then the calculations are summed. Since there is significant communication between the CPUs, the speedup is less than a direct proportionality to the number of processors. However when running the same inversion under MPI environment on multiple computers the advantage is that a single computer does not have to store the entire sensitivity matrix.
• For incorporating bound information, the implementation of the projected gradient algorithm in version MAG3D is primarily that it results in a significantly faster solution than the logarithmic barrier technique used in earlier versions. An added benefit is the ability to reach the bounds given by the user.