Determining Spatial Orientation of Rock Layers with the Brunton Compass

The role of geology is to understand the earth in four dimensions: spatial as well as time.

The Brunton compass, while over 100 years old, is still the primary tool for generating geologic field data. There are several key components of the compass, including the sighting arm, magnetic needle, clinometer, index pin, and bubble and clinometer levels. The compass is used to collect field data regarding the geometric orientation of planar rock surfaces, known as strike and dip. This information is the fundamental data for generating geologic maps.

This video will demonstrate the proper way to measure strike and dip with the Brunton compass.

Most rock units exhibit some form of planar surface structure, such as bedding. Rock layers can be described as a planar surface in space. Any angular deviation for the horizontal is known as ?dip?. Dip is reported in degrees, with a range between 0 and 90. The value is followed by the general direction of the dipping.

In addition to the deviation from the horizontal, geologists also measure the deviation of the rock surface from North, or, ?strike?. Strike can be visualized as the linear intersection of the horizontal plane and the surface being studied. Strike is reported in degrees from North.

Now that you understand the principles behind strike and dip, let?s see how it is measured in the field.

Before measurements can be collected with the compass, the functionality of the components must be verified.

First, the needle must be unimpeded when held in the horizontal plane. Second, verify that the lift pin locks the needle in place when depressed.

Third, check that the bull?s eye level can be centered in a smooth, uninterrupted manner. The bubble is used to determine the horizontality of the compass.

Finally, while the geographic North Pole is a static location, the magnetic north pole moves over time. Because of this, a declination pin is used to correct for the difference. Find the declination on a local topographic map, and the adjust the set-screw to the appropriate value.

Because natural surfaces are inherently rough, a representative, flat surface must be established. A way to create the surface is the place a notebook or clipboard onto the rock in a representative orientation.

Place the compass against the surface. Rotate the compass until the bubble is centered in the bull?s eye level.

With the bull?s eye leveled, the compass is now aligned in the horizontal plane. The strike is indicated by the compass needle. The value at either end of the needle is correct, but by convention, the value closer to North is used.

Dip is measured perpendicular to the strike. Set the compass on its side, aligned along the downward slope. Adjust the inclinometer until the bubble is leveled. The dip magnitude is indicated be the inclinometer. In addition, the general direction of the dip is notated.

The process of collecting strike and dip values is continued for all rock units of interest.

When taking measurements, it?s important to practice good technique and verify the compass is working properly. This will ensure good precision for the data.

The accuracy of the data is dependent on the uniformity of the natural surface. Taking multiple measurements of the same surface can increase the accuracy.

Once the strike and dip values have been correctly recorded in the field, they are combined into geologic maps. These maps show the boundaries between rock units, and the strike and dip data provides the spatial orientation of each rock.

Strike and dip data is the starting point to understanding more complicated geological structures.

Once the geologic maps are created, geologic cross-sections can be generated. The information in the geologic map is extrapolated to determine the structure of rocks below the surface. In turn, this can provide information about the physical evolution of the area.

Another use of strike and dip data is to identify anticlines. Anticlines are upward folds in rock strata, caused by compressional stress. When one of the strata in the anticline is impenetrable, buoyant gas and oil are trapped beneath it. Drilling companies can use this information to locate drilling sites.

You have just watched JoVE?s introduction of the Brunton compass. You should now understand the setup of the compass, proper usage, and how to take strike and dip measurements. Thanks for watching!