Geological compass

Classic geological compass (Brunton), sideview
Classic geological compass (Brunton), topview

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 is the analogue compass.

Classic geological compasses

Classic geological 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.

Modern geological compasses

The concept of modern geological compass was developed by Eberhard Clar of the University of Vienna during his work as structural geologist. He published it in 1954.[3] An advantage of his concept is that strike and dip is measured in one step, using the vertical circle for dip angle and the compass for the strike direction. The first implementation was done by the VEB Freiberger Präzisionsmechanik in Freiberg, Germany. The details of the design were made in a close cooperation with the Freiberg University of Mining and Technology.[4]

Usage

Strike line and dip of a plane describing attitude relative to a horizontal plane and a vertical plane perpendicular to the strike line

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 smartphone, geological compass programs based on the 3-axis teslameter and the 3-axis accelerometer have also begun to appear. These compass programs use vector algebra to compute plane and lineation orientations from the accelerometer and magnetometer data, and permit rapid collection of many measurements. However, some problems are potentially present. Smartphones produce a strong magnetic field of their own which must be compensated by software; as well, because the Earth's magnetic field fluctuates rapidly, measurements made by smartphone geological compasses can potentially be susceptible to considerable noise. Users of a smartphone compass should carefully calibrate their devices and run several tests against traditional magnetic compasses in order to understand the limitations of their chosen program.

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. ↑ Clar, E.: A dual-circle geologist’s and miner’s compass for the measurement of areal and linear geological elements Separate print from the negotiations of the Federal Institute of Geology Vienna, 1954, vol. 4
  4. ↑ http://www.fpm.de/downloads/GeologistCompass_eng.pdf

External links

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