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STEREONET BASICS

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STEREONET BASICS

Pages 692-704

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

- 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

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

- 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

Small circles

(Look like LATITUDES)

Great circles

(Look like LONGITUDES)

- 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)

Dip angles

Equatorial circle = horizontal plane

Straight lines = vertical planes

40

0

20

20

60

60

0

80

80

40

Great circles = inclined planes

- 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???

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

N

20

40

60

80

W

E

S

- 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

- 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

- If you have strike/dip/dip direction data, Start the same way you normally would for plotting the great circle for the plane
- Identify the dip line (the line of greatest slope) on the great circle ***
- The POLE is the line perpendicular to the dip line
- To get to the pole of the plane, count 90 from the dip line along the E-W vertical plane, and mark the point
***You don’t need to draw the great circle

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.

- 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.

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

- 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

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