For each strata in the drillhole, this routine can generate 3D grid files of the strata thickness, top & bottom elevation, and attributes such as calcium, moisture, sulfur, and BTU. The grid files make up the geologic model in StrataCalc. A 3D grid file is a rectangular mesh of grid cells where each grid cell is the same size rectangle. The elevation or Z value of the four grid corners equals the value at those points. For example, consider a grid cell for a 3D grid mesh of strata thickness. The X and Y coordinates of a grid cell could be (0,0), (10,0), (10,15), and (0,15). The z values, which might be 4.5, 4.7, 4.8, and 4.6, represent the strata thickness at the four X and Y coordinates.
Make Strata Grid Files reads the strata data from the drillholes.
The strata data is correlated
and processed for beds, pinch out and conformance as specified in the
Configure -Mining Module. The strata data points are then used with the
selected modeling method
to calculate the grid. There are 5 methods for modeling: Triangulation,
Inverse Distance, Polynomial, Kriging and Least Squares. There is an
option to use a parameter file (made with Define Parameters) to filter
the strata data points.
For example, when creating a thickness grid for the overburden of
6_COAL, you could have
a parameter filter of THICKNESS 6_COAL > 1.0. This filter would make
the program use
only drillholes with 6_COAL greater than 1.0 when calculating the
6_COAL overburden grid. See
the Define Parameter File command for more description on parameters.
The routine starts by prompting for the location of the grid files
to create. The location can
be specified by picking the lower left and upper right corners or by
selecting an existing grid file
which sets the new grid location and resolution to match the selected
existing grid file. Next there is the Make Strata Grid File dialog for
choosing the grid cell resolution, modeling method and other
parameters.
After the grid options dialog, the program prompts you to select the drillholes and fault lines. After reading in the selected drillholes, there is a Choose a Strata to Process dialog to choose the strata to process from the list. If Create Composite Grids was turned on, then multiple strata may be selected here. The All, Key Only and Non-Key Only are just viewing options to reduce the number of strata in the window if there are a lot, for ease of selection.
Once the strata is selected, another dialog shows the strata values and attributes that are available to grid. These will be thickness, elevations, equations, and any quality attributes that are in the drillholes. Choose a value to grid from the dialog.
Then the program will prompt for the grid file name to create (.GRD). Next the program will process the strata data points to create the grid and the results are stored in a user-specified file name. The grids created by Make Strata Grid File can be used as the geologic model for the PreCalculated Grids Definition file. Also the grids can be used in the grid application routines, in the Civil Design module, such as Plot 3D Grid File, Grid File Utilities and Elevation Difference.
Making Strata Grids with Fault Lines
There is an option to select fault lines in addition to
the drillholes. The fault lines should be drawn as
3D polylines with elevations that equal the fault
differential. The program will grid with all modelling methods using
the fault lines for
making strata elevation grids. The
3D
fault polylines should be drawn such that the left side of the
polyline, relative to the direction of
the polyline, is the low side of the fault and the right side is the
high side. As each grid corner
elevation is calculated, the program checks each drillhole. If the
drillhole is on the same side of the
fault polyline as the grid corner, then no adjustment is made to the
drillhole elevation data.
Otherwise the drillhole is projected onto the fault polyline and the
polyline value at this point on the
polyline is used to adjust the drillhole elevation. For example, if the
fault polyline value was 5.0 and the
grid corner was on the high side of fault while the drillhole was on
the low side, then 5.0 would be
added to the drillhole elevation for modeling at that grid
corner. If the grid corner was on the low side
and the drillhole was on the high side, then 5.0 would be
subtracted from the drillhole elevation. Reverse Polyline is a
good way to reverse the fault line if it is drawn in the wrong
direction.
Triangulation Modeling Method
This method is straight triangulation between the drillholes.
Triangulation calculates these values by interpolating on the plane
defined by the three points in the triangle that encloses the point.
Since triangulation only interpolates, it can only calculate values
within the area of the data. Afterwards, an extrapolation routine can
then fill in the rest of the grid. This extrapolation uses a safe
method that tends to average out the data. There is an option to
extrapolation to apply the global trend. This option finds the average
slope and direction of the existing data and applies this slope to
extrapolating.
Inverse Distance Modeling Method
Inverse distance calculates the grid values by assigning weights to the
existing data. The grid values calculated by inverse distance are a
weighted average of the existing data. Inverse distance will not carry
trends and the calculated grid values will never be higher than the
highest existing data point. Likewise the calculated grid values will
never be lower than the lowest existing data point. The weights are
proportional to the inverse of the distance between the point to be
estimated and the existing data point. Closer points are weighted more
than points farther away. The inverse distance can be calculated to
first, second, or third power which are (1/d), (1/d^2), and (1/d^3)
respectively. The power can also be any user-specified number such as
2.5. The inverse distance estimate is a weighted average with the
individual weights computed as an inverse power of distance as follows:
where Wi is the weight computed for each sample i, each di is the
distance between the location being estimated and sample I , and –power
is the inverse distance weighting power.
In Configure under Mining there are several options for
controlling inverse distance. The Inverse Distance Search Radius is
used for calculating a value at a point such that only drillholes that
are within this Search Radius will be used in the calculations. The
Inverse Distance Max Samples value limits calculations to the nearest
specified number of drillholes to the point. For example, the program
will use the nearest 10 drillholes. Inverse distance can also be
controlled by quadrants which are divided into northeast, southeast,
southwest and northwest. The Min Quadrants setting will use at least
this specified number of drillholes from each quadrant as long as there
are drillholes in the quadrant within the Search Radius. For instance,
a setting of Min Quadrants of one would make the program look for at
least one drillhole from each quadrant. The Max Quadrants value limits
the number of drillholes used from each quadrant. For example if Max
Samples was set to 25 and Max Quadrants was 10, then the total samples
would be 25 with no more than 10 of the closest ones from each
quadrant.
Elliptical inverse distance modeling method is an option that appears any time the Inverse distance modeling method is chosen. The prompt will appear:
Use inverse distance to which power
[First/<Second>/Third/Other]? Second
Use elliptical inverse distance
[Yes/<No>]? Yes
Enter azimuth of anisotrophy: 45
Enter anisotropic factor: 1
Calculating grid by inverse distances
33880...
This will produce oval shaped inverse distance "bulls eyes" that will use the 2nd weighting power, with the 1st weighting power applied to an azimuth of 45. This option will appear anywhere the Inverse Distance modeling method is selected, such as isopaching.
Kriging Modeling Method
Kriging estimates the grid values by figuring the relationship between
all the existing data points and then assigning weights to this data.
Kriging finds the best fit linear unbiased estimates for the given data
and model. Kriging can carry trends within and beyond the limits of the
data and can find new high and low values. You must supply a model that
defines the spatial relationship of the data which can be difficult. In
fact, Kriging is a very complicated subject and you will need to
reference an outside source for a detailed description such as An
Introduction to Applied Geostatistics by Isaaks and Srivastava.
Carlson uses Ordinary Kriging. All the parameters for this Kriging are
specified in the dialog shown.
Polynomial Modeling Method
The polynomial method is based off of triangulation. The difference is
that instead of directly interpolating within each triangle, the
polynomial method creates smooth transitions by using a fifth degree
polynomial function that accounts for neighboring triangles. Since
polynomial needs adjoining triangles, when there are fewer than five
data points, there will be fewer than four triangles and the polynomial
method will revert to straight triangulation. The same extrapolation
logic for triangulation applies to the polynomial method.
Linear Least Squares Modeling Method
The linear least squares method finds the least squares best fit plane
at each grid corner. The least squares routine weights each data point
by inverse distance so that closer points are weighted more than points
farther away. So the best fit plane varies at different points on the
surface. The linear least squares method extrapolates trends very well.
A lower inverse distance factor (i.e. 1.0) will weight the data points
more equally which models the trends more globally (sometimes called
"global dip"). Likewise a higher inverse distance factor (i.e. 3.0) will
weight the closer data points more heavily which models local trends
strongly (sometimes called "local dip"). Least squares will trend and
allows for data points that are new highs and lows, that don’t appear
in the original drillhole/point data. It does produce very nice, smooth
contours that honor the data points.
Use position from another file or pick grid position
(File/<Pick)? press Enter Using the position of an existing file
copies the grid resolution and corner point locations to the new grid
files.
This is useful if you need to have grid files match exactly. Most of
the time, grids should match position in the geologic model.
Pick Lower Left grid corner: enter or pick a point
Pick Upper Right grid corner: enter or pick the second
point to define the grid position
Make Strata Grid File dialog box
Set the grid resolution and other options. A higher grid resolution
increases the processing
time. Also choose the Modeling Method in this dialog.
Select drillholes, channel samples and strata polylines.
Select objects: select the drillhole symbols
Select fault lines or Enter for none.
Select objects: press Enter for none, or select the fault
lines
Choose Strata to Process dialog
Choose Attribute to Process dialog
Pulldown Menu Location: StrataCalc in Advanced Mining
Keyboard Command: chgrid
File Names: \lsp\dtmmkgrd.lsp,\lsp\makegrid.arx,
\lsp\makegrid.dcl