Brunton compass

A standard Brunton Geo, used commonly by geologists

A Brunton compass, properly known as the Brunton Pocket Transit, is a precision compass made by Brunton, Inc. of Riverton, Wyoming. The instrument was patented in 1894 by Canadian-born geologist David W. Brunton.^[1] Unlike most modern compasses, the Brunton Pocket Transit utilizes magnetic induction damping rather than fluid to damp needle oscillation. Although Brunton, Inc. makes many other types of magnetic compasses, the Brunton Pocket Transit is a specialized instrument used widely by those needing to make accurate navigational and slope-angle measurements in the field. Users are primarily geologists, but archaeologists, environmental engineers, mining engineers and surveyors also make use of the Brunton's capabilities. The United States Army has adopted the Pocket Transit as the M2 Compass for use by crew-served artillery.

Overview

The Pocket Transit may be adjusted for declination angle according to one's location on the Earth. It is used to get directional degree measurements (azimuth) through use of the Earth's magnetic field. Holding the compass at waist-height, the user looks down into the mirror and lines up the target, needle, and guide line that is on the mirror. Once all three are lined up and the compass is level, the reading for that azimuth can be made. Arguably the most frequent use for the Brunton in the field is the calculation of the strike and dip of geological features (faults, contacts, foliation, sedimentary strata, etc.). Strike is measured by leveling (with the bull's eye level) the compass along the plane being measured. Dip is taken by laying the side of the compass perpendicular to the strike measurement and rotating horizontal level until the bubble is stable and the reading has been made. If field conditions allow, additional features of the compass allow users to measure such geological attributes from a distance.

As with most traditional compasses, directional measurements are made in reference to the Earth's magnetic field. Thus, measurements are sensitive to magnetic interference. For example, if the user is near an outcrop that contains magnetite or some other iron-bearing material, compass readings can be affected anywhere from several inches from the outcrop to tens of yards away (depending on the strength of the magnetic field). Since they are measured with a rotating level, dip measurements are unaffected by magnetic interference.

Other compasses

There are numerous other compasses used by geologists: The Breithaupt compass is one example. Portable electronic devices are changing the way geologists conduct field work. Unlike analogue compasses, a digital compass relies on an accelerometer and a magnetometer, and may provide information about the reliability of a measurement (e.g. by repeating the same measurement and performing statistical analysis).

Uses

Taking a bearing
Measuring vertical angles with clinometer
Defining points of the same elevation
Measuring strike and dip.^[2]

References

^ Hodgson, Michael, Compass and Map Navigator, Guilford, CT: The Globe Pequot Press, (1997) ISBN 0-7627-0488-0, p. 4
^ Assaad, Fakhry A.; LaMoreaux, James W.; Hughes, Travis, eds. (2013). Field Methods for Geologists and Hydrogeologists. Springer Science & Business Media. pp. 38–39. ISBN 978-3-662-05438-3. Retrieved 10 June 2015.

External links

[1] Hodgson, Michael, Compass and Map Navigator, Guilford, CT: The Globe Pequot Press, (1997) ISBN 0-7627-0488-0, p. 4

[2] Assaad, Fakhry A.; LaMoreaux, James W.; Hughes, Travis, eds. (2013). Field Methods for Geologists and Hydrogeologists. Springer Science & Business Media. pp. 38–39. ISBN 978-3-662-05438-3. Retrieved 10 June 2015.

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v t e Structural geology
Underlying theory	Deformation mechanism Lamé's stress ellipsoid Mohr–Coulomb theory Mohr's circle Overburden pressure Rock mechanics Strain Strain partitioning Stress Stress field Tension
Measurement conventions	Brunton compass Inclinometer Orthographic projection Rake Stereographic projection Strike and dip
Large-scale tectonics	Accretionary wedge Allochthon Autochthon Back-arc basin Continental collision Convergent boundary Décollement Divergent boundary Extensional tectonics Fault block Fenster Fold and thrust belt Fold mountains Foreland basin Graben Half-graben Horse Horst Horst and graben Intra-arc basin Inversion Klippe List of tectonic plate interactions Mélange Mountain formation Nappe Obduction Orogeny Passive margin Plate tectonics Pull-apart basin Rift valley Rift Saddle Strike-slip tectonics Structural basin Suture Syneclise Terrane Thick-skinned deformation Thin-skinned deformation Thrust tectonics
Fracturing	Columnar jointing Dike Exfoliation joint Fissure Joint Vein
Faulting	Asperity Cataclasite Cataclastic rock Detachment fault Disturbance Fault friction Fault mechanics Fault scarp Fault trace Thrust fault Transfer zone Transform fault
Foliation and lineation	Cleavage Compaction Crenulation Fissility Oblique foliation Pressure solution Rock microstructure Slickenside Stylolite Tectonic phase Tectonite
Folding	Anticline Chevron Detachment fold Dome Homocline Monocline Syncline Vergence
Boudinage	Competence
Kinematic analysis	3D fold evolution Paleostress inversion Paleostress Section restoration
Shear zone	Mylonite Pure shear Shear
Category