Orthographically determine the true dip and strike given two apparent dips Two apparent dips () of a plane (e.g., dike, vein) are measured in the field on two surfaces (e.g., joints). Determine the true dip () and strike of the plane. Example: A dike has an apparent dip of 1 = 20 along the 296 direction and 2 = 30 along the 046 directions. What is the attitude of the dike? Procedure: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Pick a point, O, at the center of your graph paper. Draw the direction of the apparent dips (i.e., 296 and 046) from point O. Call these FL1 and FL2, respectively. Subtend the angle 1 from FL1, and angle 2 from FL2. Use a straight edge and choose an arbitrary depth d perpendicular to FL1. Find point A where d intersects the trend of the apparent dip (i.e., the 296 line) Use the same d for FL2, and repeat steps 5 and 6 for the second apparent dip, except that the intersection point is marked as B. Connect A to B. This is the strike of the dike. Draw a perpendicular line to AB from point O. Find its intersection with the strike. Mark it point C. Line OC is the dip direction. Measure the same d distance along AB from point C. Determine point C’. Connect C’ to O, and then read the angle between OC and OC’. This is the true dip amount (). Read the strike from North, and using a protractor A Strike: 090 B d d A’ True dip direction Example: A dike has an apparent dip of 1 = 20 along the 296 direction and 2 = 30 along the 046 directions. What is the attitude of the dike? =40N 2=30 1=20 O Answer: Strike: 090, True dip:40N B’

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# Example 3: Determine true dip of a plane given two apparent dips.