Using Grid Math in Surfer to Create Geological Models

Transcription

1 Using Grid Math in Surfer to Create Geological Models When gridding data in Surfer, the grid file created is based on the input data provided. However, the data provided may not be complete, or may not actually be in the final format required. Grid Math is a powerful function in Surfer that allows users to apply a mathematical equations and functions to the Z values in the grid file, compare multiple grid files, change Z values from one value to another, or fill in regions with other data. Below are two user cases where Grid Math was used in an applied setting. Truncate One Grid at the Intersection of Another Grid In this use case, a user wanted to display a 3D surface elevation of their map overlaid with another 3D surface of their orebody. The goal was to truncate a 3D surface of the orebody at the intersection of the ground surface, so no part of the orebody was in the air. This can be done easily in Surfer. As long as the grids have the same geometry (minimum, maximum and spacing), you can use Grid Math to blank the grid nodes in the orebody grid file when they go above the elevation grid. When there are two intersecting grid files, Grid Math can be used to truncate one grid file at the intersection of another grid file. To truncate a grid file to another grid, follow these steps: 1. Click Grid Math. 2. In the Grid Math dialog, click the Add Grids button, select both the surface elevation grid and the orebody grid and click Open. Let s say the orebody grid is grid A (i.e. there is an A under the Variable column for that grid file). 3. Enter the function IF(A>B,B-1,A). This tells Surfer that if the orebody grid is greater than the surface elevation grid, then replace that value with the ground surface elevation value minus a small number (1 in this case, this can be smaller or eliminated, as desired), otherwise use the data in grid A (the orebody grid value).

2 4. Give the file a new Output Grid File name and click OK. Create the 3D surface map of the orebody using this grid. Truncate the orebody grid when it goes above the surface elevation grid using the Grid Math function IF(A>B,B-1,A). Create Data in One Grid a Certain Distance Above or Below Another Grid In this use case, a user had two grid files; one for ground surface elevation and one for bedrock. There were ground surface elevation points everywhere in the map area, but bedrock elevations only in scattered locations. In the areas where there was no bedrock elevation, the user wanted to simply create bedrock data a specified distance (e.g. 50 meters) below the ground surface. To do this: 1. Grid the ground surface elevation data. a. Click Grid Data, select the ground surface elevation data and click Open. b. In the Grid Data dialog, select the gridding parameters desired. c. Note the Grid Line Geometry values (i.e. Minimum, Maximum and Spacing for the X Direction and Y Direction). d. Click OK. 2. Grid the bedrock data a. Click Grid Data, select the bedrock data and click Open. b. In the Grid Data dialog, select the gridding parameters desired. c. Enter the exact same Grid Line Geometry value as noted above for the elevation grid. d. Click OK. This creates a grid file that is completely filled with data. 3. Blank the areas that should be assigned as no data in the bedrock grid. The first step is to create a BLN file of the areas in the grid that have bedrock data. a. Click Map New Post Map, select the bedrock data file and click Open. A post map of the bedrock data is created to see where the spatial distribution of the points. b. Click Map Add Empty Base Layer to add an empty Base layer, as seen in the Object Manager. c. Right click over the Base layer in the Object Manager and click Enter Group. d. Click Draw Polygon and draw polygons around the point groups (the bedrock outcrops). Inside the polygons is where the original bedrock data should be interpolated, and outside the polygons is where the bedrock grid should be assigned a certain distance below the ground surface elevation grid.

3 e. After all the polygons are drawn, select them all by clicking Edit Select All. f. Click Geoprocessing Combine Islands/Lakes to combine them into a single complex polygon. g. Right click over the Base layer in the Object Manager and click Exit Group. Draw polygons around the areas that should use the interpolated bedrock data in the grid. h. Turn off all four axes and the post layer by unchecking them in the Object Manager, so just the polygon on the map in visible. i. Click File Export and export the polygon to a BLN file. 4. Click Grid Blank, select the bedrock grid and click Open, select the BLN file exported above and click Open, give the new blanked grid file a name and click Save. The blanked grid is created. 5. Now replace the blanking value in the blanked grid with another unique value. a. Click Grid Math. b. In the Grid Math dialog, click the Add Grids button, select the blanked bedrock grid and click Open. c. Change the Blank Handling option from Blank output to Remap to. d. Enter a unique Remap Value (e.g ). e. For the function, simply enter A. f. Give the file a new Output Grid File name and click OK. The grid is created, and all the blanked areas in the original grid file were assigned a value of The finishing step is to create the final grid. a. Click Grid Math. b. In the Grid Math dialog, click the Add Grids button, select both the surface elevation grid and the bedrock grid where the blanked values were converted to -9999, and click Open. Let s say the bedrock grid is grid A (i.e. there is an A under the Variable column for that grid file). c. Enter the function IF (A= -9999, B-50, A). This tells Surfer that if the bedrock grid value is (i.e. in the blanked area), then replace that value with the ground surface elevation value minus 50 (this value can be changed to whatever distance is desired), otherwise use the data in grid A (the original bedrock grid value).

4 d. Give the file a new Output Grid File name and click OK. This grid has the bedrock data in the areas inside the polygons in the BLN file, and data the specified distance below the ground surface in all other areas. Create the bedrock map from this grid file. Use Grid Math to generate a grid file that has data from one data set within a polygon, and calculated data from another grid file outside the polygon. 7. If the border between the bedrock interpolated data and the surface data is too abrupt, there are some steps to smooth it out. a. Click Grid Convert, select the bedrock grid file created above and click Open. b. Select to save as in DAT XYZ (*.dat) format and click Save. c. Click Grid Data, select the DAT file saved above and click Open. d. In the Grid Data dialog, select the gridding parameters desired. e. Click the Filter Data button. In the Filter Data dialog, under Duplicate Data change the To Keep to Average and increase the X Tolerance and Y Tolerance values twice that of the grid spacing. Under the Data Exclusion Filter, enter Z>1e+030. Click OK. f. Change the Output Grid File name if desired, and click OK. This map will have a smoother transition between the bedrock data and the calculated data from the ground surface elevation.

5 If the generated grid has an abrupt transition between the gridded data and the calculated data, converting the grid to a DAT file and regridding will help smooth the boundary. With the ability to apply mathematical functions with Boolean expressions and comparison operators to single or multiple grid files, Surfer offers all the tools necessary to present information to its fullest.