EM1DTM introduction: 
1D inversion of time-domain EM data

The name, EM1DTM, refers to electromagnetics (“EM”), one-dimensional models (“1D”), time-domain observations (“T”), and magnetic sources and receivers (“M”).

This inversion and modelling program is designed to construct 1D models composed of layers of uniform conductivity with fixed interface depths. The value of the conductivity in each layer is sought by the inversion. Any type of geophysical time domain loop-loop data can be inverted, using one of four variations of the inversion algorithm.  Many permutations of inversion objective function type, data type and algorithm choice are possible. Importantly, there are more flexible ways of modifying the model objective function than earlier ubc-gif codes. For these reasons 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 (sources and receivers) can be inverted?

Single soundings can be inverted, or groups of soundings can be inverted together. The transmitter loop can have any number of sides (greater than 2), and can be at any height above the ground surface. It is assumed to be horizontal. The transmitter current waveform can be a step off, a linear ramp turn-off, or a general waveform that is provided in discretized form.

The observations (data) are values of voltage (i.e., dB/dt) or magnetic field. Receiver coils can be oriented in the x-, y- or z-directions, and they can be at any position relative to the transmitter loop.

Observations can be for any time after the step or ramp turn off, or any time after or during a discretized waveform. All the observations (in any combination) that are provided for a particular transmitter loop constitute a “sounding”, and are used to construct the one-dimensional model for that sounding. Measurement uncertainties can be in the same units as the observations or as relative uncertainties in percent.

A command line utility called is included which produces an EM1DTM data file based upon a columns-like data file. The input is an XYZ-columns-like data file formatted with the data for each sounding location on a single line of the file. Each column is a datum at a different delay time or receiver orientation/location.

EM1DTM output for multiple soundings is a set of 1D models. These can be gathered and formatted for viewing in three dimensions using MeshTools3D.

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

1D (layered earth) electrical conductivity models are produced by the inversion. The Earth models are composed of layers of uniform conductivity with fixed interface depths. The value of the conductivity in each layer is sought by the inversion. Multiple soundings can be handled in a single run of the program. Each sounding is inverted independently for a one-dimensional model under the sounding location, with the sequence of one-dimensional models written out. These can be viewed directly as a composite two-dimensional image using the graphical user interface, or converted to a format which is suitable for viewing as a three-dimensional image using MeshTools3D.

What inversion algorithm is employed?

Fundamentally the inversion methodology is similar to all ubc-gif inversion codes. However EM1DTM introduces some new, more powerful capabilities not yet seen in ubc-gif inversion schemes:

1. A general measure for the data misfit allows for a range of variations, from the traditional sum-of-squares measure of misfit, to more robust measures which can ignore outliers in the observations.
2. A general measure for the amount of model structure allows for a range of model types previously not available. Model types can range from smooth to piecewise-constant. Also, as in all ubc-gif inversion schemes, the balance between "flattest" and "smallest" parts of the model components is adjustable.

In addition, there are four variations of the inversion algorithm.

• 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.

Also, 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. The program is the most complex inversion code yet delivered to the geophysics community by the UBC-GIF (February 2007).

Documentation is provided in five parts.

1. Background theory (PDF) summarizes the detailed mathematical basis of the forward modelling and inversion algorithms.
2. The complete User's Manual (PDF) contains detailed descriptions of the main input file, the observations (data) file, model files, and output files and messages. There are also brief descriptions of one utility program and the independant forward modelling program called EM1DTMFWD.
3. 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
4. Three examples are explained in detail between pages 13 and 25 of the User's manual, and results from one are illustrated on an interactive web page.
   These examples are discussed in more detail on a separate page.
5. A workflow outlining all necessary and recommended steps for running EH3DTDinv

Licensing:

There is NO constrained educational version of EM1DTM

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.

Quick Start

1. Copy the entire CD-ROM into new folder on your hard drive, keeping all files and subdirectories intact.
2. There are five executable files in the "root", documentation is in the documents folder, and then results of many test cases are in the large folder called Tests.
3. NOTE: To use the example files, you may have to remove their read-only attributes.
   1. Do this by clicking on the folder containing example files that you will work with.
   2. Select Edit | Select all (or type CTRL-A).
   3. Select File | Properties and un-check the "Attributes; Read-only" check box.
   4. This is necessary for writting over files that were copied off a CD-ROM.
4. Double-click on the EM1DTMgui.exe file to start the interface for EM1DTM and read the information in the opening banner window.
5. In the menu, choose File menu, Open option. Then work your way through the "Tests" folder to find the example you want to work with. The file \documents\examples.htm contains pointers to information about the examples, but there are many versions of the examples (especially example #1). You could consider starting with the example file in \Tests\Example1\Invalg1\tB_ramp_out\L1L1\em1dtm.in
   Open the "em1dtm.in" file. The fields in the interface will then be filled in with the parameters from that input file.
   
   NOTE THERE MUST BE A TRANSMITTER WAVEFORM FILE PRESENT IN THE CORRECT PLACE. See section 2.1.3 in the user manual document, and the parameter "tcwfn_a(is)" (pg 5 of the manual) which specifies the filename for transmitter information in the observations file. This transmitter waveform file name including it's path are relative to the working directory where you saved the input parameter file.
     
6. To start the inversion, click on the "save" button to save the parameters, PREFERABLY TO A NEW FOLDER. Note the NOTE above, regarding the transmitter waveform file. Then click the "run" button. A black command line window will appear on the screen which displays the progress of the inversion.
7. While the inversion is running, use the View menu data/model option (or the equivalent button on the toolbar). A window will apear which diplays the recovered models and predicted data. This window will be updated as the inversion proceeds.
8. When the inversion finishes you can try changing some of the parameters and rerunning the inversion to see what affect it has on the final models. Or, if you are running a line of data, then "right-click" at any location of the 2D model in the results window. That brings up a line plot of the recovered 1D model(s) at that location.
9. While viewing a line of inversion results, click the Options menu Invert Sounding option, and a new interface window will appear. You can adjust the
   parameters, save the file, and re-invert that individual sounding.
10. You can work without the GUI (i.e. at the command line). Run the program by typing "EM1DTM". There are no parameters; all instructions to the program are contained in the parameter file, and the program expects that file, with the correct name, to be in the working directory.

Please refer to the documentation for . . .

• Complete background material,
• for details on input, output, data and model file structures, and
• using the GUI interface.