EM1DFM introduction: 
1D inversion of frequency-domain EM data

This inversion program is designed to construct one of four types of 1D models, using any type of geophysical frequency domain loop-loop data using one of four variations of the inversion algorithm.  Many permutations of model type, data type and algorithm choice are possible, therefore the program is significantly more complicated to manage than previous UBC-GIF inversion codes. Also, input file structures are complicated in order to facilitate this flexibility.

What data types can be inverted?

The observations are the inphase and/or quadrature components of the secondary H-field normalized by the primary (i.e., free-space) field in ppm, or the secondary H-field normalized by the primary field in %, or the secondary H-field in A/m, or the total H-field in A/m. Receiver coils can be oriented in x-, y- or z-directions, and they can be at any position relative to their respective magnetic dipole transmitter. Transmitters can be at any height, can be oriented in the x-, y- or z- directions, and any frequency or set of frequencies may be involved. All the observations (in any combination) to be used to construct the one-dimensional model at a particular horizontal location are grouped together as one "sounding". Measurement uncertainties can be in the same units as the observations or as relative uncertainties in percent. Command-line Utilities: A utility code is available for converting DIGHEM-like data files into files appropriate for input to program "EM1DFM". There is also a program for taking EM1DFM results at stations spread out over a 2D area, and converting the output file into a 3D volume, with physical properties interpolated into the volume from the 1D results. The volume is compatible for viewing in MeshTools3D. Click here to see instructions for these small utilities. Please contact us at ubcgif@eos.ubc.ca if you would like a free copy of these utilities.

Screen shot from EM1DFM with data (top) and 29 recovered 1D models of conductivity and susceptibility concatenated into a 2D cross section under the survey line.

What types of Earth model does this program strive to recover?

The inversion program constructs one-dimensional (1D) models of the Earth under each sounding.  There will be many layers, and depths to each interface must be pre-defined. The inversion program seeks values of conductivity and magnetic susceptibility for each layer. Models that can be sought are:

• An electrical conductivity model (with magnetic susceptibility fixed), or
• a strictly-positive magnetic susceptibility model (with conductivity fixed), or
• both conductivity and strictly-positive susceptibility models, or
• both conductivity and susceptibility (with no positivity constraint) models.

Multiple soundings can be handled in a single run of the program. Each sounding is interpreted independantly with a 1D model produced under the sounding location. When all soundings have been inverted, a composite two-dimensional model is written out to facilitate interpretation of a line of soundings.

What inversion algorithm is employed?

• The program can work using a constant (user-supplied) trade-off parameter in the objective function being minimized, or
• the trade-off parameter can be automatically chosen to achieve a user-supplied target misfit, or
• the trade-off parameter can be automatically chosen using the generalized cross validation (GCV) criterion, or
• the trade-off parameter can be automatically chosen using the L-curve criterion.

Full flexibility of the sum-of-squares measure of model structure is provided. The balance between conductivity and susceptibility components is adjustable (if both are active in the inversion). The balance between "flattest" and "smallest" parts of both conductivity and susceptibility components is also adjustable. Reference models can be included in either or both the "smallest" and "flattest" parts, and additional user-supplied weighting of the layers in the model can be incorporated.

Clearly, many permutations of model type, data type and algorithm choice are possible, therefore the program is significantly more complexe to manage than previous UBC-GIF inversion codes. Also, input file structures are complicated in order to facilitate this flexibility.

Documentation is provided in four parts.

1. Background theory is provided in an Acrobat PDF document. It summarizes the detailed mathematical basis of the forward modelling and inversion algorithms.
2. There is a complete manual for the program EM1DFM. It contains detailed descriptions of the main input file, the observations file, model files, and output files and messages. There are also brief descriptions of two utility programs and the independant forward modelling program.
3. Examples are given showing application of the inversion algorithm to synthetic and real field data sets.
4. The graphical user interface (GUI) has two components, and their use is described using two frames-based HTML documents.
   1. EM1DFM GUI
   2. 1D Model viewing utility
5. There is also a short "Quick Start" page with minimal instructions for running an inversion using example data provided.

---

Licensing:

There is NO constrained educational version of EM1DFM

The licensing policy document (on the UBC-GIF website) explains about academic research and commercial use licenses, and includes links to licensing agreements. 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.