Geological Mapping Stereographic projections Part C 3 point

Geological Mapping Stereographic projections (Part C) + 3 -point problem with wells Instructor: S. Kokkalas Assoc. Prof.

Stereonets-Rotations about horizontal axis • Imagine rotation about a horizontal axis as a divider sweeps out a cone when one of the legs rotates as the other rotates along its axis. • In stereographic projections such a cone projects to give small circles. Therefore, a given line rotating about a fixed axis moves to take up different positions on the same small circle on the stereonet. Main Applications • • • Restoration of tilt of beds Restoring paleocurrents in tilted or folded data Paleomagnetic rotations

Stereonets-Rotations about horizontal axis To rotate a line: • Plot lineation L. R line is the axis of rotation and occupies the centre of the small circle defined by the line which rotates. • Rotate net beneath transparent paper until centre of all small circles is positioned beneath R. • New orientation of L is found by moving point around a small circle by an amount as the required angle of rotation. To rotate a plane: • Plot plane as a pole and follow same procedure as with a line.

Stereonets-Rotations about horizontal axis Example: Restoration of tilted beds.

a. Three-point problem If the height of a bed is known at three or more points (not in straight line), it is possible to find the direction of strike and calculate the dip of the bed, provided dip is uniform. Applications in mining, opencast and borehole problems. b. Prediction of surface exposure of a plane