Reading a stereonet

A stereonet shows orientations as points on a circle. These are the ideas behind the charts and statistics.

  1. Poles and great circles: A plane is shown by its pole (the line at right angles to it) or by its great circle (the line where it cuts the sphere). Poles are used for counting and statistics; great circles are easier to picture.
  2. Lower or upper hemisphere: Geologists project the lower half of a sphere; the upper half is used in some other fields. Choose it under What to show (Advanced).
  3. Equal area and equal angle: Equal area (Schmidt) keeps the density of points true, so it is used for contours and statistics. Equal angle (Wulff) keeps angles true, so it is used for measuring and constructions.
  4. Cluster: Poles that bunch together: one consistent orientation, such as undisturbed bedding. The mean and its cone describe it. Kappa is how tight it is (above 10 is reliable); the cone is the 95 % range the true mean lies in.
  5. Girdle: Poles that spread along a great circle: folded layers. The pole to that circle is the fold axis (the girdle’s best fit). A single mean would be meaningless here.
  6. K and C: Woodcock’s shape: K above 1 is more cluster-like, below 1 more girdle-like. C is the strength of the fabric. Both come from the eigenvalues of the orientation matrix.
  7. Contours: Shaded density of poles. Kamb contours are in sigma above what a uniform scatter would give: 2 sigma and more is conventionally significant. Schmidt and Fisher-kernel contours are in multiples of a uniform density.
  8. Rose diagram: How many readings point each way. A plane has two ends, so strikes are drawn both ways. The grey wedge is the confidence range of the mean direction.
  9. Orientation sets: A joint set is a group of joints with similar orientation. The finder splits the readings into the number of sets the data support (chosen with the BIC), allowing background scatter.
  10. Sampling bias: A drillhole meets planes across it far more often than planes along it, so steep joints are under-counted in a vertical hole. The Terzaghi correction weights each reading to allow for it.
  11. Rake: The angle of a line (a slickenline, say) within a plane, measured from the strike direction. The app converts a plane and a rake to the line’s plunge and trend.