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Making the mesh

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Choose make mesh from the Explorer window. The make mesh workspace is presented. Ultimately a series of frames will be available, however you step through these one at a time, and step backwards through them to change a parameter in an earlier frame. This is done because the order of choices is critical.


Make Mesh Explorer

 

The first frames presented present the option to load topography.
You may choose a flat grid, which will not incorporate the topography, or a grid with topography in which case the elements will be distorted to accomodate the topographic surface.
The first way of incorporating topography is loading a file.
Another way to incorporate topographic information is to use the ZTerrain data from the electrodes. Select Add electrodes Z terrain to topography to use this method.

If  topography has been entered but was not available during data collection, then the electrodes ZTerrain may not be correct. Use Update elevation for Surface-marked electrodes to fix this problem (note that XY coordinates for the surface electrodes must be in the same coordinate system as the topographic information).

Finally the switch Adapt grid to electrodes works whether or not topography is present. If this switch is selected (recommended) then, once a regular mesh has been built the software will move the closest mesh node to any electrode not already on a node to that electrode location. If this option is not selected and some electrodes do not fall on nodes then the electrode locations will be moved, which is less accurate.

Make Mesh First Frames


 

The second frame (right) presents the dimensions of the foreground and background regions. The foreground  dimensions   will be automatically chosen  from the dataset but may need to be updated  to produce a reasonable  sized mesh. This is especially true when electrode spacings are significantly different in one direction. The spacing chosen automatically is 0.5 X the smallest electrode spacing (i.e. 2 elements per electrode pair).  Accuracy drops dramatically if fewer than 2 elements are used per electrode spacing. Working in 3D it is not unlikely that the electrode spacing in one direction (e.g. down the hole or down the line) is smaller than it is in another direction. For this reason changing the electrode spacing to be larger in the cross-line and depth (for surface data), or X and Y directions (for borehole data) should be considered. However, this must be done carefully. It is recommended that nominal element sizes not be different in one direction by more than a factor of 2 from any other direction. It is also worth keeping in mind that if the the Adapt grid to electrodes switch is on, then the finite element mesh will be adapted to electrode locations. Therefore it is best to use an element size that is half the average down-the-line electrode separation, rather than half the smallest electrode separation. Sizes and limits are in meters.

The background region is specified in terms of the nominal foreground element size. The configuration of these parameters is described here and in the command reference. Each number present represents the location of a pad node. The number 1 means 1 X  the  foreground element size,  2 means  2 X the foreground element size, 4 means  4 X the foreground  element size, etc. The actual pad element size is determined by subtracting the previous  node location. For example if the node list is 1, 2, 4, 8, 16 then the element sizes are 1, 1, 2, 4, and 8 X the foreground element size. It is recommended that pad cells not exceed 10 X the foreground element size. The extent of the pad region should be similar to the size of the foreground region, or larger. For example if there are 40 foreground nodes in the X direction, then the minimum recommended X background node location is 40. In the Z direction, for borehole work, the background extent should be 3 X the foreground region. If the foreground Z contains 20 elements, then the background nodes should extend to 60 X the foreground element size. For surface data it is probably possible to reduce this ratio.

The basic aspects of mesh design described above are critical to success. Accuracy will be reduced for any of these reasons:

1) If the number of elements per basic electrode spacing is too small
2) if the number of pad elements is too few
3) if the size of pad elements is too large
4) if the element size in one direction is creater than 2 X the element size in any other direction.

On the other hand run time will be increased dramatically if the mesh is too large. For each factor of 2 increase in number of elements, run-time increases by a factor of 8. Therefore it is very important to spend some time thinking about the mesh design. Typically mesh size should be kept under 100,000 nodes.

 

After choosing apply settings the next frame shows the node locations. Use the buttons to choose between X, Y and Z directions. Nodes may be edited by double-clicking the value. The add Node box may be used to add a node. The values presented are node locations in meters. The smallest values are in the -X (Y or Z) background pads. Scroll down the list to see all the -X background pads. The top of the Foreground node list continues from the bottom of the -X background pad list. Scroll down the Foreground node list to see the extend of the foreground. The top of the +X (Y or Z) background pad list continues from the bottom of the foreground node list.

Make Mesh second frames

Finally the make mesh frame is presented. Check the Nodes and blocks (elements). These numbers should be reasonable for your computer (typically something between 20,000 and 150,000 is reasonable. Large meshes, especially with a correspondingly large number of electrodes will take a long time (many hours to days) to compute. Choose View Mesh to preview the mesh in the 3D viewer to ensure that electrodes and mesh settings are as anticipated. This step is strongly recommended, especially for large meshes.

 



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