Using Stereonets in Geology

by

Andy Sutton
The Open University

The module comprises four main sections accessible from this main menu

Many students have difficulty with stereonets although they are a fundamental tool for the geologist with applications in several branches of the subject. A difficulty stems from the fact that many students fail to grasp the basic principles of stereographic projections, and are unable to visualise what is happening in three dimensions. Another difficulty arises because the principles are often taught early on in degree courses yet students are not required to apply them until a year or so later. When students do need to use them in earnest they often attempt to apply rules rather than think about each problem and work out what has to be done.

This interactive courseware module introduces the basics of the stereographic projection and its use in geology. It makes extensive use of computer graphics and animations in an attempt to overcome the difficulties students encounter with traditional teaching methods. They are used, for example, to help show how 2-D stereograms relate to planes, points and hemispheres in 3-D, and to provide step-by-step sequences showing how a range of common stereographic plotting procedures are undertaken.

The ability to visualise structures represented by stereograms is tested in this click-and-drag exercise

Description of the module

The module occupies about 10Mb and consists of four main sections. Users are free to work through as many sections as they want, and in any order. The module is intended to be equivalent to about four hours of conventional teaching. The sections are as follows:

Introduction to stereographic projection

An explanation is given of why the stereographic projection is needed, how the stereographic projection works, visualising stereograms, and stereonets.

What use is stereographic projection? The principle of using the stereographic projection to display linear and planar geological features in 2-D and to perform geometrical analysis of such 3-D features.

Who uses stereographic projection? The main uses in structural geology and crystallography are discussed.

Projecting to a sphere. The general principles of projecting points and planes onto a reference sphere.

Flattening the sphere. Projecting points and circles on to the equatorial plane to give a stereogram.

Visualising stereograms. Developing the skill of visualising the appearance of a stereogram before plotting it.

Stereonets. What a stereonet is and how to recognise Wulff and Schmidt stereonets.

To check the user understands how to measure from the stereogram, a numerical answer has to be entered

Using stereographic projection in structural geology

After demonstrating basic plotting procedures for lines and planes this section looks at common situtations in which stereographic projection is useful. The convention used to define the orientation of planes is strike/dip dip direction, e.g. 090/21 N. For lines it is angle of plunge followed by direction of plunge, e.g. 30 068.

Basic plotting procedures for lines and planes. Examples are given of linear and planar features as they appear in the field and how they are recorded. Step-by-step instructions are then given for plotting lines from their pitch and from their plunge, and for plotting planes as great circles and poles.

Common uses of the stereographic projection. Simple geometrical problems covered include intersecting planes and the treatment of true and apparent dip.

Analysis of folds. Finding the fold axis from bedding dip and strike readings. Determining the interlimb angle from limb orientations. The shapes of folds from the distribution of poles.

Patterns in orientation data. The properties and use of Wulff and Schmidt stereographic nets are described and examples given.

Restoring original orientations by rotation. Two examples are given; restoring the tilt of beds, and restoring palaeocurrent direction indicators.

Using stereographic projection in crystallography

In addition to a brief introduction and summary, the following topics are covered.

Spherical projection of crystals. The way crystal faces project as points onto a sphere and the relationship between zones and great circles.

Stereographic projection of crystals. The generation of a stereogram by projection onto the equatorial plane.

Stereographic projection case study. Users are required to help generate a stereogram of a distorted crystal by defining pole positions for selected faces.

Measuring interfacial angles. How to measure the angle between two faces in a zone using a stereogram.

Using small circles. An example where small circles need to be used is given.

Three types of user interaction are employed here, including a multiple choice question

Stereoplot Quiz

Five exercises designed to give the user practice in estimating the orientation of linear and planar features from their stereographic representations, and in plotting lines, planes and poles to planes on stereograms. Each exercise involves either clicking the correct position or typing in an answer to a question. As this is done the correct answer is displayed and a score registered. Each question is posed repeatedly with randomly generated numerical values. From the feedback users can see how well they are doing and have a chance to improve their score.

To learn how stereograms for crystals are produced, users participate in plotting poles

Running the software

The different parts of the module are accessed via a main menu and each section has a sub-menu. Individual sections comprise up to seven pages. Navigation around the module is by means of buttons along the base of the screen. A brief explanation of the way these work is given in a separate 'About this module' section accessible from the main menu.

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