EMIX MT2D is written in FORTRAN 90, using a protected mode compiler and is now a single executable module. Previous limitations on size of data sets and grids no longer exist, because the space needed to store synthetic results and the finite element grid are dynamically allocated to accommodate the data to be interpreted. In addition, the virtual memory allows the user’s hard disk to be used for storage if actual RAM is not available. The result is that a larger number of stations and grids with a large number of nodes can be accommodated.

EMIX MT 2D can also be used to generate synthetic data from 2-D models without having field data.

EMIX MT 2D uses Inman-style ridge regression inversion of polygon-based 2-D models to best fit the 2-D MT data in a least squares sense. Up to 200 model parameters can be selected from body resistivity and from the x- and z-position of each vertex. In addition, groups of vertices can be locked together to form a single unit whose x- and/or z-position can be used as an inversion parameter. This inversion is different from other methods currently being used in that it requires that the user construct a geological model using the mouse by creating polygons of constant resistivity value. The inversion is used to automatically improve the fit of the model by automatically adjusting some model parameters. This inversion is a genuine 2-D inversion of 2-D MT data that allows the user to select which parameters to vary and which parameters to freeze. By default, all parameters are frozen; the user interactively selects those parameters to be used in the inversion using the mouse during model construction. The 2-D models can be constructed as a combination of up to 100 bodies and layers and up to 1000 vertices per model. Model creation is done by drawing the geological cross-section as layers or bodies. Bodies are closed polygons while layers are entered from left to right and automatically closed at infinity. EMIX MT 2D automatically generates the finite element mesh using the resistivity of the bodies and layers and parameters editable by the user. The resulting grid can be edited by the user, either numerically or graphically. On screen interactive model editing allows the user to change, delete, insert bodies and to zoom the display. The resistivity values and topography can easily be edited. The program calculates results for the TE mode, TM mode or both.

EMIX MT2D determines the starting dimensions of the finite element grid automatically from the number of stations and the station spacing. The package automatically creates a near optimum grid based on the topographic relief, skin depths, and horizontal resistivity contrasts. It also assigns a very high resistivity value for air. Another new feature is that the number of stations is only limited by the computer’s resources. As usual, stations may be placed at any location along the profile line. Importation of field data in EMIX MT format can generate station locations automatically or assign field stations to existing EMIX MT2D stations.

Calculations are carried out using a finite element method similar to that described by Wannamaker and Stodt (Geophysical Journal of the Royal Astronomical Society 88, 1987).

Several options allow for field data or 1-D inversion results (from EMIX MT) to be displayed during 2-D model construction and editing. TE, TM field data and/or 2-D model results can be displayed by simply pointing at a station on the 2-D cross-section display. The local 1-D model results can also be displayed. Alternatively, the results of 1-D modeling for TE, TM and/or Occam’s inversion (Smooth Model Results) can be displayed over the 2-D model as an aid in model construction or editing. The user has control over which mode and which data/models are displayed.

EMIX MT2D provides several options to display the results of modeling. Single stations can be displayed with field curves and local 1-D results as desired.

Modeling results can be displayed in pseudosection or profile form, along with annotated model cross section, if desired. Multiple soundings can be presented together with the model cross section on a single large plot. Field data can be shown with the 2-D model results.