Coal Evaluation Case Study Using the RockWorks Playlist 11/30/20/JPR
Introduction
The purpose of this paper is to show how the RockWorks Playlist tool can be used to evaluate geology based solely on data within the RockWorks Datasheet as opposed to the Borehole Manager database. The study area is a small-scale hypothetical coal field comprised of three semi-continuous seams (Figure 1). The endgame is to determine where to extract coal that meets a set of user-defined criteria including overburden thickness, interburden thickness, BTUs, sulfur content, ash content, and areal extent (Figure 2). This is a highly-selective methodology referred to as “surgical mining” or “high-grading.”
Figure 1. Fence Diagram Depicting Semi-Continuous Interburden & Coal Seams (Vertical Exaggeration = 4X)
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Figure 2. Preliminary Extraction Plan Based on Multiple Optimization Parameters (Vertical Exaggeration = 4X)
Please note that this approach could readily be applied to other types of stratigraphic deposits (sand, gravel, clay, trona, gypsum, etc.). The only caveat is that this approach uses grid-based modeling, in which the parameters for each borehole are considered to be vertically constant. For example, for any given borehole there is only one BTU value for Coal #1 which means that the BTU values from the top to the base of Coal #1 at that particular point do not vary. If these types of assumptions are unacceptable (e.g. grade varies vertically within stratigraphic intervals), the data must be entered into the RockWorks Borehole Manager database which will use true 3D block modeling methods, thereby allowing for vertical variations within any of the individual unit properties.
The Datasheet
The data for 583 boreholes is contained within a datasheet titled “Coal_Data.RwDat”, which is included within a RockWorks sample folder titled “Coal” (Figure 3).
Figure 3. Datasheet Containing Data for 583 Boreholes
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The terminology used within this datasheet is depicted within Figure 4
Figure 4. Stratigraphic Terminology Index The Coal Evaluation Playlist A screen capture of the Coal Evaluation Playlist is shown within Figure 5 and described within the following pages. Please make note of the following considerations: 1. Parameters associated with each step may be examined or changed by double-clicking on the item title. For example, the gridding algorithm used to interpolate the thickness grids may be changed such that it is different from the algorithm used to model the analytical parameters (e.g. BTU).
2. The order of the Playlist items is important because many of the items are processing data that is created withing preceding steps. For example, creating the Acceptable Overburden Thickness grid is only possible if the Overburden Thickness grid has been interpolated.
3. The checkboxes adjacent to each Playlist item determine if that step should be included in the bulk processing that occurs when the Process Playlist button is double-clicked. This provides a means to selectively process the steps and avoid re-processing the entire list of commandsa. For example, consider a scenario in which the entire Playlist has been processed but a change to the final open-pit bench height is desired. The following steps will re-generate the report accordingly:
a. Use the Disable button to uncheck all of the items within the Playlist.
b. Change the bench height by double-clicking the Create Extraction Plan item and adjusting the bench height setting.
c. Check the boxes for steps 19, 20, and 21.
d. Double-click the Process Playlist button.
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Figure 5. Coal Evaluation Playlist Page 4 of 60
The Coal Evaluation Playlist shown within Figure 5 requires approximately 11 minutes to perform the following operations: 1. A datasheet titled “Coal_Data.RwDat” is loaded into the RockWorks Datasheet.
2. A ground surface elevation model is interpolated. This step uses the XYZ -> Grid program.
3. Thickness grid models are interpolated for the overburden, the two interburden units, and the three coal seams. These steps use the XYZ -> Grid program.
4. Grids are interpolated for the analytical data (BTU, sulfur, and ash) for each of the three coal seams. These steps use the XYZ -> Grid program.
5. In order to create a structural grid model representing the top of Coal 1, the overburden thickness grid model is subtracted from the ground surface grid model. The Coal 1 thickness model is then subtracted from the grid representing the top of Coal 1 in order to create a grid that represents the base of Coal 1. This same strategy is applied to the each successively lower unit. These steps use the Grid & Grid Math -> Grid program.
6. A file titled “Grid_List_1.rwdat” (Figure 6) is loaded into the RockWorks Datasheet to facilitate the next step.
Figure 6. Grid_List_1 Datasheet
7. A stratigrahic block model is created based on the grid surfaces and g-values defined within the Grid_List_1 datasheet. This step uses the Grids -> Stratigraphic Solid program.
8. The stratigrahic model is rendered as a 3D fence diagram (Figure 1). This step uses the Solid -> Fence program.
9. Histograms are generated for the thickness and analytical grids. These steps use the Grid -> Histogram program.
10. The thickness and analytical grids are converted to “acceptable” Boolean (true/false) grid models based on the cutoff values shown within Figure 7. These steps use the Grid Model -> Boolean Grid program.
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Figure 7. Cutoff Values Used When Creating "Acceptable" Grids
11. A file titled “Grid_List_2.rwdat” (Figure 8) is loaded into the RockWorks Datasheet to facilitate the next step.
Figure 8. Grid_List_2 Datasheet.rwdat
12. The thickness grids for Coal 1, 2, and 3 are added together to create a total coal thickness grid. This step uses the Add Multiple Grids program.
13. The total thickness grid is then filtered to exclude cells in which the thickness value is less than 10’. This step uses the Grid Model -> Boolean Grid program.
14. A file titled “Grid_List_3.rwdat” (Figure 9) is loaded into the RockWorks Datasheet to facilitate the next step.
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Figure 9. Grid_List_3 Datasheet
15. All of the grids listed within Figure 9 are multiplied together to create a final grid (Figure 10) in which all of the criteria specified within Figure 7 are met. This is analogous to the “olden days” when transparent maps were placed on a light table to see where overlaps occur. This step uses the Multiply Multiple Grids program.
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Figure 10. Boolean Grids Multiplied To Determine Where All Criteria Are "True" (1.0)
16. The output from Step 15 is then passed through a minimum-contiguous filter to eliminate targets with a surface area less than 40,000 square feet. This step uses the Grid & Area Threshold -> Minimum Area Grid program.
17. The grid created within Step 16 is converted to a solid. This step uses the Grid -> Solid program.
18. The stratigraphic block model created in Step 7 is multiplied by the solid created in Step 16 to remove ore that was eliminated within Step 16. This step uses the Solid & Solid Math -> Solid program.
19. An excavation surface pit plan is created based on the surface elevation model generated in Step 2 and the ore model generated within Step 17. In addition, a volumetric report is generated (Figure 11). This step uses the Solid & Grid -> Optimized Excavation program. Page 8 of 60
Figure 11. Volumetric Report
20. A file titled “HTML_Builder_Instructions.RwDat” (Figure 12) is loaded into the RockWorks Datasheet. This file contains a list of images and associated captions in the order that they will be shown within the final report. Note that the order is different from the order in which they were created.
Figure 12. HTML_Builder_Instructions.RwDat
21. Finally, the list loaded within Step 20 is used to create a report (Appendix 1) which is automatically loaded into Microsoft Word. This step uses the HTML Builder program. Note: The display option for all of the programs used within this Playlist have been disabled because they are redundant with the images depicted within the final report.
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Conclusion
Hand contouring 17 maps based on 583 boreholes is a daunting proposition. Overlaying these maps on a light table to identify optimal areas is equally daunting. Finally, designing excavations to extract the optimized coal is a formidable task. A gut-feel time estimate is six months to one year. Performing the individual steps and running through each menu would probably require one to two weeks. Creating the Playlist required one day and the processing time required approximately 11 minutes. Re- generating all of the maps and the report if one or more data points are changed requires another 11 minutes.
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Appendix 1 – Report Generated by the Coal Evaluation Playlist
Multi-Seam Coal Evaluation Report
Stratigraphy Fence Showing Three Coal Seams (Vertical Exaggeration = 4X)
Final Extraction Plan
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Resource Extraction Report
Description Results Units ------Pit: Volume ...... 28,766,200 Cubic Feet Mass ...... 1,352,011.4 Tons Depth ...... 41.7 Feet Stripping Ratio ...... 3.893 OB:1 User-Defined Parameters: Maximum Depth ...... n/a Maximum Slope ...... -30.0 Degrees Maximum Bench Height ...... 20.0 Feet Minimum Coal Value ...... 0.5 Maximum Coal Value ...... 1.5 Density Conversion Factor .. 0.047 Tons Per Cubic Feet
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Borehole Location Map
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Ground Surface Model
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Overburden Thickness Model
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Overbuden Thickness Histogram
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Acceptable Overburden Thickness Model (<16')
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Coal #1 Thickness Model
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Coal #1 Thickness Histogram
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Coal #1 BTU Model
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Coal #1 BTU Histogram
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Coal #1 Acceptable BTU Model (>10,000 BTU)
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Coal #1 Sulfur Model
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Coal #1 Sulfur Histogram
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Coal #1 Acceptable Sulfur Model (<5.6%)
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Coal #1 Ash Model
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Coal #1 Ash Histogram
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Coal #1 Acceptable Ash Model (<8%)
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Interburden #1 Thickness Model
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Interburden #1 Thickness Histogram
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Acceptable Interburden #1 Thickness Model (<3')
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Coal #2 Thickness Model
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Coal #2 Thickness Histogram
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Coal #2 BTU Model
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Coal #2 BTU Histogram
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Coal #2 Acceptable BTU Model (>11,000 BTU)
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Coal #2 Sulfur Model
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Coal #2 Sulfur Histogram
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Coal #2 Acceptable Sulfur Model (<6%)
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Coal #2 Ash Model
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Coal #2 Ash Histogram
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Coal #2 Acceptable Ash Model (<7.5%)
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Interburden #2 Thickness Model
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Interburden #2 Thickness Histogram
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Acceptable Interburden #2 Thickness Model (<2.5')
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Coal #3 Thickness Model
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Coal #3 Thickness Histogram
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Coal #3 BTU Model
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Coal #3 BTU Histogram
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Coal #3 Acceptable BTU Model (>12,000 BTU)
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Coal #3 Sulfur Model
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Coal #3 Sulfur Histogram
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Coal #3 Acceptable Sulfur Model (<5.4%)
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Coal #3 Ash Model
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Coal #3 Ash Histogram
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Coal #3 Acceptable Ash Model (<7.0%)
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Total Coal Thickness Model
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Acceptable Total Coal Thickness Model (>10')
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Acceptable Coal Model (All Parameters Acceptable)
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Larger Contiguous Acceptable Coal Model (>40,000 Square Feet)
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