Geological compass

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There are a number of different (specialised) magnetic compasses used by geologists to measure orientation of geological structures, as they map in the field, to analyse (and document) the geometry of bedding planes, joints, and/or metamorphic foliations and lineations.[1][2] In this aspect the most common device used to date its the analogue compass.

Analogue compasses

Analogue compasses that are of practical use combine two functions, direction finding and navigation (especially in remote areas), and the ability to measure strike and dip of bedding surfaces and/or metamorphic foliation planes. Structural geologists (i.e. those concerned with geometry and the pattern of relative movement) also have a need to measure the plunge and plunge direction of lineations.

Compasses in common use include the Brunton compass and the Silva compass. More specialised structural geologists also use the Breithaupt compass because of its ease of use in measuring foliations and lineations.

The "Freiberg compass"[3] is similar, but it suffers from the disadvantage that the needle is not damped. In consequence much time can be lost waiting for the compass to stabilise.

The Breithaupt compass [4] is one of the most specialised magnetic compasses available, used in structural geology.[5] At first sight it appears confusing to the novice user, for the numbers on the compass dial ascend in an anticlockwise direction. This is because the compass is used to determine dip and dip-direction of surfaces (foliations), and plunge and plunge-direction of lines (lineations). To use the compass one aligns the lid of the compass with the orientation of the surface to be measured (to obtain dip and dip direction), or the edge of the lid of the compass with the orientation of the line (to obtain plunge and plunge direction). The compass must be twisted so that the base of the compass becomes horizontal, as accomplished using the spirit level incorporated in it. The needle of the compass is then freed by using the side button, and allowed to spin until the damping action slows its movement, and then stabilises. The side button is released and the needle is then firmly held in place, allowing the user thereafter to conveniently read the orientation measured. One first reads the scale that shows the angle subtended by the lid of the compass, and then depending on the colour shown (red or black) the end of the compass needle with the corresponding colour. Data are then recorded as (for example) 25°->333° (dip and dip-direction) or (plunge and plunge-direction).

This compass has the most use by structural geologists, measuring foliation and lineation in metamorphic rocks, or faults and joints in mining areas.

Digital compasses

With the advent of the iPhone compasses based on the 3-axis teslameter and the 3-axis accelerometer have also begun to appear. The listerCompass[6] uses vector algebra to determine the orientation of the compass in two frameworks: 1) strike-dip-rake (using the convention adopted by seismology to describe the strike and dip of the top surface of the compass, and the rake of the long axis of the compass in that plane); 2) yaw-pitch-roll (as used by aircraft to describe the heading, plunge, and the roll). The second method is best for lineations (lines that appear on rock surfaces) while the former is more appropriate for planes. The advantage of this method is that one measurement is sufficient to describe both a foliation and a lineation.

An iPhone used as a geology compass [1]

References

  1. ↑ The Mapping of Geological Structures (Geological Society of London Handbook Series) [Paperback] K. R. McClay
  2. ↑ Statistics of Earth Science Data: Their Distribution in Time, Space and Orientation [Paperback] Graham J. Borradaile (Author)
  3. ↑ http://www.prospectors.com.au/p-327-freiberg-geological-360-compass.aspx
  4. ↑ http://www.breithaupt.de"/
  5. ↑ Evert Hoek; Peter K. Kaiser; W. F. Bawden (1995). Support of Underground Excavations in Hard Rock. Taylor & Francis. pp. 22–23. ISBN 978-90-5410-186-4. 
  6. ↑ http://geologycompass.com)

External links

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