Displaying Strike and Dip Measurements on Your Map in Surfer

Transcription

1 Displaying Strike and Dip Measurements on Your Map in Surfer Measuring strike and dip is a fundamental part of geological mapping, and displaying strike and dip information on a map is an effective way to show field data and the orientation of a surface in 3-dimensional space. The measured surface can be bedding, foliation, or a fault, joint or fracture plane. The measurements can aid interpretation of the geological history, and help determine the location of the measured surface at depth. Surfer has the ability to display this information with the simple creation of a post map. Add strike and dip symbols to your map to display geologic field data such as foliation orientation If you do not have existing strike and dip measurements, but have a grid file of the top of the surface (i.e. the top of a bedding unit), you can calculate strike and dip values from the grid file. To do this, please see our Knowledge Base article: How can I calculate the strike and dip values from my grid file? If you have strike and dip measurements already from literature, field measurements, borehole data, or you calculate it as above, you can display the measurements directly on a map using a post map. All you need is a data file with the XY location, the azimuth of the strike line, and the dip amount for each point. NOTE: This article assumes the azimuth measurement conforms to the American right hand rule, which is to always report the strike azimuth so that the dip direction is on the right side of the strike line (i.e. if you make an L shape with the thumb and index finger of your right hand, palm down, then your index finger points down dip and your thumb points to the strike azimuth). This example will walk through displaying strike and dip symbols of foliation on an aerial photo. The strike and dip data used are from an Arizona State University class and can be downloaded from here. The coordinates are

2 in the UTM NAD83 zone 12N coordinate system. To create the post map with the strike and dip symbols in Surfer, follow these steps: 1. First, the strike azimuth data needs to be manipulated slightly to be compatible with Surfer so that the strike/dip symbols are rotated correctly. This manipulation can be done in the Surfer worksheet. Click File Open, select the data file and click Open. The data opens in the Surfer worksheet. 2. To make the strike azimuth be compatible, the data needs to have 90 subtracted from it, and the result multiplied by -1. To do this, click Data Transform. a. Enter the function G=(C-90)*-1, where C is the letter designation of the column containing the strike azimuth data and column G is the column that you want to save the new data in. b. Click OK and the new column of data is created. Now when the symbols in the post map are rotated by the azimuth value, they are rotated to the correct orientation. Transform the strike data by subtracting 90 and multiplying by Click File Save and File Close. 4. In the plot window, click Map New Post Map, select the data file saved above and click Open. 5. Select the Post layer in the Object Manager. 6. In the Property Manager, click the General tab: a. Set the appropriate worksheet columns for the X Coordinates and Y Coordinates. In this example, the correct columns (Column A and Column B) are selected by default. b. If you created the strike and dip data from a grid file, you may want to adjust the Frequency number to reduce the number of symbols. 7. Since the coordinate system for the Easting and Northing coordinates is known, it can be specified for this layer. Click the Coordinate System tab: a. Click the Set button. b. In the Assign Coordinate System dialog, select Predefined Projected Systems UTM North America North America NAD83 UTM zone 12N. c. Click OK.

3 8. To change the symbols to strike/dip symbols, click the Symbol tab: a. Expand the Marker Properties section by clicking the + sign. b. Change the Symbol Set to GSI Geologic Symbols. c. Change the Symbol to a strike/dip symbol. For example, for non-vertical and non-horizontal orientations, you could use Symbol 0 for bedding ( ), Symbol 26 for foliation ( ), or Symbol 14 for joints ( ). In this example, Symbol 26 is selected. d. Under Symbol Angle, set the Angle column to be the column containing the transformed azimuth or strike direction (i.e. Column G). e. Under Symbol Size, change the Symbol size to something a little larger (i.e in). 9. To label the symbols with the dip amount, click the Labels tab: a. Under Label Set 1, change the Worksheet column to the column containing the dip amount (i.e. Column D). b. To adjust the label position relative to the symbol, change the Position relative to symbol from Above to another value (i.e. Right). c. If desired, the font properties can be adjusted by expanding the Font Properties section and changing the options. For example, change the Size (points) to 12 and the Foreground color to Blue. Create a post map of the data and change the symbol properties to show a strike/dip symbol in the correct orientation. 10. For clarity, some users prefer to view maps in latitude and longitude coordinates. To convert the map from UTM to lat/long: a. Select Map in the Object Manager. b. In the Property Manager click the Coordinate System tab. c. Click the Change button.

4 d. Select Predefined Geographic (lat/lon) World Geodetic System e. Click OK. 11. You may also want to fine tune the label positions. There is not a way to automatically place the dip labels in the location they are usually depicted (with the dip label in front of the symbol, regardless of rotation). All labels in Surfer are positioned relative to the symbol XY point location, and not adjusted for any rotation. You can either find the best placement using one of the predefined or custom Position relative to symbols options, or you can manually edit the position of each label. If you choose to manually edit the labels, be sure to do this after any changes to the coordinate system of the map (as in the previous step), as changing the map s coordinate system will eliminate the custom label positions. To manually edit the labels, follow these steps: a. Right click over the Post layer in the Object Manager and click Edit Post Labels. b. Drag and drop the labels to the desired positions. c. Press ESC on the keyboard when you are finished. d. Turn off the leader lines by clicking the Labels tab in the Property Manager, and under the Leader Lines section, unchecking Enable for dragged labels. 12. Add other map layers such as contour layers or base layers of aerial photographs to create an informative map. You may be able to download this information from online sources such as The National Map Viewer. For example, to display contour lines behind the symbols, you can download this ZIP file and extract the grid file and GSR2 reference file to your hard drive. Then to add the contours to the existing map: a. Select the map and click Map Add Contour Layer. b. In the Open Grid dialog, select the grid file and click Open. c. If prompted to adjust the map limits, click No. d. You can adjust any of the contour settings by selecting the Contours layer in the Object Manager and changing properties (such as contour interval or line color) in the Property Manager.

5 Create a post map from the strike/dip data and specify the symbol and label properties. Change the coordinate system or add other layers to create an informative map. A visual representation of the structural attitudes conveys more information about the geology and geological history than simply looking at contours of topography or an aerial photograph alone. In this example, you can see a regional strike orientation to the NE, with almost all dips to the SE. This information can indicate relative age (i.e. in this case younger layers are to the southeast), indicate folding or faulting (i.e. if the strikes or dips had changed direction), or may highlight areas that need more detail (i.e. there is one measurement dipping to the NW, which could indicate a fault, fold, overturned bed, another localized geological process, or simple human error). With the ability to display strike/dip symbols and labels directly on the map, Surfer gives you all the tools you need to present geological information to its fullest.