Student guide

This guide is for students learning structural geology with Structural Analysis. The help pages earlier in this manual say where each control is. This guide says what each control does to your data, what you will see, and why you would use it. It covers three tabs in depth:

Words in bold are the labels you see on screen. Terms such as pole, girdle and kappa are explained in the Glossary and in Reading a stereonet.

How the app is put together

The app works on readings: one orientation per row of your file, either a plane (bedding, a joint, a fault) or a line (a fold axis, a lineation, a slickenline).

  1. The data source (top right: Demo, Upload or Enter data) decides where the readings come from.
  2. The Filters (the band under the tabs) decide which readings are used and how their numbers are read: which columns hold the angles, which structure types and domains to include, the strike convention and the magnetic declination. The Filters apply to every tab.
  3. Quality control runs automatically. Readings with impossible or missing angles are left out, and the reason is recorded. The Data tab shows every reading with its status.
  4. The Charts and Statistics tabs draw and measure the readings that are left.

Everything you do is worked out again straight away. Nothing is saved on a server and nothing changes your file; when you close the browser tab, your data are gone.

A first session (about 30 minutes)

Work through this with the demo data before using your own.

  1. Choose a demo. Top right, click Demo. In the Filters, set Demo dataset to Victoria, Stawell-Ararat zone: bedding (girdle). This is bedding measured across folded rocks in western Victoria.
  2. Look at the readings. Open the Data tab. Each row is one bedding plane. The qc_status column says “ok” for every reading the app could use. Open the DATA HEALTH bar above the table: it tells you how many rows were read, selected and usable.
  3. Plot them. Open the Charts tab. You see a stereonet: each dot is the pole to one bedding plane. The poles spread along a curved band rather than bunching in one place. That band is a girdle, and it is what folded bedding looks like.
  4. Let the app describe the pattern. Read the footer under the chart. It names the pattern (girdle), gives Woodcock’s K, and states the fold axis: the line the beds were folded about.
  5. Add contours. In Statistics on the net, tick Contour fill (density of poles). The shading shows where the poles are densest. In a girdle the two densest patches are usually the two fold limbs.
  6. Compare with a cluster. Change the demo to Victoria, Lachlan Fold Belt: bedding (cluster). Now the poles bunch together, and the footer gives a mean plane, Fisher kappa and a cone of confidence instead of a fold axis.
  7. Read the numbers. Open the Statistics tab and read the Orientation statistics table against the Statistics tab guide.
  8. Find sets you already know are there. Change the demo to Synthetic drillholes: bedding, joints, fault zone (strip log). In the Filters, add all three types to Include categories: bedding, joint and fault zone. (Mixing types is normally a mistake; here it is a test.) On the Statistics tab click Find orientation sets. The app is not told which reading is which type, yet it should find three sets that match the three types. Click a set in the table to see it alone on the chart.

By the end you will have seen the two basic patterns (cluster and girdle), the statistics that go with each, and how sets are separated.

Good habits

  • Analyse one structure type in one domain at a time. Bedding and joints, or two fault blocks, mixed together give a pattern that belongs to neither. The app warns you when you include several.
  • Check the Data tab before you trust a chart. A wrong column, a strike convention set the wrong way, or rounded readings all give plausible-looking but wrong results.
  • Report your settings. The strike convention, the declination, the projection, the contour method and the confidence level all change the results. The report (Charts tab, Export and report) lists them for you.
  • Treat oriented-core conventions as assumptions until checked. Alpha / beta conventions differ between sites. See Oriented core.

When the app changes

The app is still developing. This guide is checked against the app each time the manual is rebuilt, and changes are listed in What’s new. If something on screen does not match this guide, the screen is right: please tell us through the Feedback tab.