| ERTLab | > | 2. Working Environment | > Making the mesh |
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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.
The first frames presented present the option to load
topography. 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.
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 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.
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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