1. EM1DTM package overview

1.1. Description

EM1DTM is a Fortran-based, multi-platform program library for carrying out the one-dimensional inversion of time-domain, small loop, electromagnetic data acquired to determine the spatial variation of the electrical conductivity and/or magnetic susceptibility of the subsurface. The name “EM1DTM” derives from: electromagnetics (“EM”), one-dimensional models (“1D”), time-domain observations (“T”), and magnetic (dipole) sources and receivers (“M”).

The observations 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. 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. 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.

../_images/domain.png

Fig. 1.1 Layered 1D model describing the Earth for each sounding.

The product: an electrical conductivity model. 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. For any sounding location, the mathematical representation that EM1DTM uses to model the Earth varies only with depth. In particular, the representation comprises many uniform “infinite” horizontal layers. For a complete mathematical treatment of the forward and inversion problem, see Background Theory.

General measures for both the measure of data misfit and the measure of the amount of model structure:

  • Huber M-measure for data misfit, and
  • Ekblom p-measure for model structure, allow for a whole suite of variations, from the traditional sum- of-squares measures, to more robust measures which can ignore outliers in the observations and which can generate piecewise-constant models.

The initial research underlying this program library was funded principally by the “IMAGE” consortium, of which the following companies were participants: AGIP, Anglo American, Billiton, Cominco, Falconbridge, INCO, MIM, Muskox Minerals, Newmont, Placer Dome and Rio Tinto, and from the Natural Sciences and Engineering Research Council of Canada (NSERC).

1.2. Program library content

This package consists of two programs:

  • EM1DTM: carries out the inversion of small loop, time-domain EM data assuming a layered Earth model
  • EM1DTMFWD: a stand-alone program for forward modeling the time-domain response assuming a layered Earth model

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 the . 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
  2. Add this directory as new path to your environment variables.

Two additional notes about installation:

  • 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.