Slide 1 STEREONET BASICS

Pages 692-704

(The figures in this section of your text are especially important)

Slide 2 ### Stereonets

- Stereonets are used for plotting and analyzing 3-D orientations of lines and planes in 2-D space
- It is MUCH more convenient than using Cartesian space (x-y-z coordinates) for graphically representing and analyzing 3-D data

Slide 3 ### Stereonets: Why bother?

Stereonets are used in:

- Landslide hazard/slope failure studies
- Earthquake studies
- Fracture analyses used in hydrogeology and/or groundwater pollution potentials
- Mining industry (fossil fuels included)
- Engineering
- Practically anything that deals with relative orientations of planes and lines

Slide 4 ### Mysteries of Stereographic projection (691-694)

- Any line or plane can be assumed to pass through the center of a reference sphere
- Planes intersect the lower hemisphere as GREAT CIRCLES
- Lines intersect the lower hemisphere as POINTS
- The great circles or points are projected on the horizontal plane to create STEREOGRAPHIC PROJECTIONS or stereograms

Slide 5 Small circles

(Look like LATITUDES)

Great circles

(Look like LONGITUDES)

Slide 6 ### Mysteries of Stereographic projection (691-694)

- The horizontal plane or the plane of reference (the EQUATORIAL PLANE, Page 692) is represented by the outer circle of the stereogram
- A vertical plane shows up as a straight line on the stereogram
- Inclined planes (0<dip angle<90º) are represented by projections of the great circles (show up as curved lines)

Slide 7 Dip angles

Equatorial circle = horizontal plane

Straight lines = vertical planes

40

0

20

20

60

60

0

80

80

40

Great circles = inclined planes

Slide 8 ### Mysteries of Stereographic projection (691-694)

- The projection of a gently dipping plane (dip angle <45º) will be more curved than that of a steeply dipping plane (dip angle > 45º)
- A line is represented as a point on the stereogram
- A horizontal line will project as a point on the equatorial plane
- Vertical line???

Slide 9 Small circles = Paths of inclined lines around the N-S axis

N

20

40

60

80

W

E

S

Slide 10 ### Lines and planes are plotted as stereograms by combining the great and the small circles on the stereonets

Slide 11 ### Plotting the orientation of a line using a stereonet (694-697)Lab 2

Slide 12 ### Plotting a plane by its dip and dip direction on a stereonet (also known as DIP VECTOR)

- Dip = inclination of the line of greatest slope on an inclined plane
- Refers to TRUE DIP as opposed to APPARENT DIP of a plane
- 0 ≤ apparent dip <true dip
- Dip direction is ALWAYS perpendicular to strike direction
- The dip and dip direction of an inclined plane completely defines its attitude
- Plotted the same way as lines

Slide 13 ### Defining a plane by its POLE (page 698)

- POLE of a plane = line perpendicular to the plane
- A plane can have ONLY ONE pole
- The orientation of the pole of a plane completely defines the orientation of the plane
- This is the MOST common way planes are represented on a stereogram

Slide 14 ### Plotting the pole of a plane (page 698)

Slide 15 ### Measuring the angle between two lines

Angle between two lines is measured on the plane containing both lines

- Plot the points representing the lines
- Rotate your tracing paper so both points lie on the same great circle. This great circle represents the plane containing both lines
- Count the small circles between those two points along the great circle to determine the angle between the lines.

Slide 16 ### Measuring the angle between two planes

- Angle between two planes is the same as the angle between their poles (this is yet another reason for plotting poles instead of great circles for planes)
- Plot the poles for the planes
- Rotate your tracing paper so both poles lie on the same great circle.
- Count the small circles between those two poles along the great circle to determine the angle between the two planes.

Slide 17 ### Measuring angle between two planes on stereonet (lab 3)

Measure the angles between the pairs of planes with the given attitudes

- Strike
- 342
- S27W
- N35W
- 278
- 132
- N25E

Dip/dip direction

38NE

43SE

57SW

23N

65SW

71NW

Pair #1

Pair #2

Pair #3

Slide 18 ### Plotting a plane using trend and plunge (or apparent dip/dip direction) data of two lines lying on that plane

- Plot the points representing the lines
- Rotate your tracing paper so those two points lie on the same great circle
- Trace and label that great circle

Slide 19 ### Plotting a plane from trend/plunge data of two lines (lab 3)

Identify the plane containing the following pairs of lines with the given attitudes

- Trend
- 357.5
- 112.5
- 17.5
- 282.5
- 77.5
- 330.5

Plunge

67

26

58

59

90

58

Pair #1

Pair #2

Pair #3