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Resistivity File (.resistivity)

The resistivity iteration file contains pointers to all the other files (data, polygon, penalty, and settings files), a few options for the inversion algorithm, and then lists the resistivity parameters for each region. Each inversion iteration writes out a new resistivity file with that iteration’s results. The name convention is <BaseName>.<iteration# >.resistivity. The starting model should be <BaseName>.0.resistivity. See the example below for further details.

Model File (.poly)

The model file (also referred to as the poly or polygon file) defines the model parameter polygons using nodes and segments. It is assumed that the model has been built sensibly so that no two segments intersect (except where they share a common node), that the model is free of slivers (small angle intersections between segments, less than about 20º), and that each segment bound region has an identifier in the regional attributes listing.

The model file uses the format defined for J. Shewchuk’s mesh generation code Triangle.c, which is the mesh generation code used by MARE2DEM. The file format is:

First line: <# vertices> <dimension (must be 2)> <# attributes> <# boundary markers (0 or 1)>
Following lines: <vertex #> <y> <z> [attributes][boundary marker]
One line: <# of segments> <# of boundary markers (0 or 1)>
Following lines: <segment #> <endpoint> <endpoint>[boundary marker]
One line: <# of holes>
Following lines: <hole #> <y> <z>
Optional line: <# of regional attributes and/or area constraints>
Optional following lines: <region #> <y> <z> <attribute><maximum area>

MARE2DEM uses the vertex locations, the segment endpoints, segment boundary markers, and the region locations and numbers. Holes are ignored. For the regional attributes, the attribute value and maximum area constraints are ignored by MARE2DEM, but the (y,z) location of each regional attribute is used to identify the region number of segment bound regions, and should be located at some point interior to the region. The segment boundary markers are used to store which segments have penalty cuts.

More detailed information on the Poly file format can be found at the Triangle.c website: http://www.cs.cmu.edu/~quake/triangle.html.

Example Resistivity and Model Files

Here’s an example of the resistivity and polygon files for a layer of air over two quarter spaces (so three regions). This is only a toy example since it has only three parameters and uses a few of the special add-on features of the resistivity file (bounds and prejudices).

The file Test.0.resistivity specifies the resistivity (in linear ohm-m) of each region and if it is a free parameter. Free parameters can also have inversion bounds and prejudices. In this example regions 1 and 3 are free parameters so they have non-zero parameter numbers. Each has a starting value of 1 ohm-m and is bound between 0.1 to 1000 ohm-m, with a prejudiced value of 10 (and weight 1). Bounds that are 0 are ignored. Prejudices are ignored when the weights are 0. Region 2 is the overlying air layer, hence it is not a free parameter and has a large resistivity.

Format:                         MARE2DEM_1.1                 ! input
Model File:                     Test.poly                    ! input
Data File:                      Test.emdata                  ! input
Settings File:                  mare2dem.settings            ! input
Penalty File:                   Test.penalty                 ! input (required for inversion)
Maximum Iterations:             100                          ! input (optional for inversion)
Bounds Transform:               bandpass                     ! input (optional for inversion)
Global Bounds:                  1.0000E-01,   1.0000E+05     ! input (optional for inversion)
Print Level:                    1                            ! input (optional for inversion)
Target Misfit:                  1.000                        ! input (required for inversion)
Misfit Decrease Threshold:      0.8500                       ! input (optional for inversion)
Converge Slowly:                no                           ! input (required for inversion)
Lagrange Value:                 -0.1684E+01                  ! input/output (required for inversion)
Model Roughness:                74.95                        ! output from inversion
Model Misfit:                   1.0010                       ! output from inversion
Date/Time:                      01/26/2012 01:05:48          ! output from inversion
Anisotropy:                     isotropic
Number of regions:  3
!#      Rho  Param # Lower/Upper Bounds  Prejudices & Weights
1       1        1        0.1      1000     10       1
2       1e+12    0        0        0        0        0
3       1        2        0.1      1000     10       1

The file Test.poly:

8 2 0 0
1 -100000 -100000
2 -100000 100000
3 100000 100000
4 100000 -100000
5 -100000 0
6 100000 0
7 0 0
8 0 100000
10 0
1 4 1
2 5 1
3 5 2
4 6 3
5 6 4
6 7 5
7 7 6
8 8 2
9 8 3
10 8 7
1 -70710.7 29289.3 1 -1
2 -70710.7 -29289.3 2 -1
3 -29289.3 70710.7 3 -1

There are several other features of the resistivity file that are not described here, and the format expands to handle various types of anisotropy MARE2DEM is capable of modeling.