Source: Laboratory of Alan Lester - University of Colorado Boulder
Most rock units exhibit some form of planar surfaces or linear features. Examples include bedding-, fault-, fracture-, and joint-surfaces, and various forms of foliation and mineral alignment. The spatial orientation of these features form the critical raw data used to constrain models addressing the origin and subsequent deformation of rock units.
Although now over 100 years since its invention and introduction, the Brunton compass (Figure 1) remains a central tool in the modern geologist’s arsenal of field equipment. It is still the primary tool used to generate field data regarding the geometric orientation of planar rock surfaces or linear rock features. These orientation measurements are referred to as strike and dip, and provide the fundamental data for making geologic maps. Furthermore, the Brunton Compass can also function as a traditional compass for location exercises and triangulation. Finally, it can also serve as a pocket transit for measuring angular elevations.
Figure 1. The Brunton compass.
Most rock layers (either sedimentary layering, igneous layering, or metamorphic banding/foliation) can be described as a planar surface in space. As such, the surface has an angular deviation from horizontal of anywhere between 0° and 90°. This angular deviation is known as “dip” (Figure 2). All rock surfaces that have greater than 0° dip have a linear intersection with an imaginary horizontal plane, and the compass direction of that linear intersection (the line formed by the intersection of the rock layer and a horizontal plane) is referred to as “strike” (Figure 3).
To determine the strike and dip of a rock surface, the Brunton compass must be properly prepared and then aligned with the surface being evaluated.
Figure 2. The dip, or deviation from the horizontal, of a geologic feature.
Figure 3. The strike, or deviation from North, of a geologic feature.
- Check for free needle motion. Verify that the needle is unimpeded when held in the horizontal plane. Some compasses have restrictor buttons that hold the needle in place, and if present, check to see that pushing the restrictor does not move the needle.
- Check the “bull’s eye bubble” centering and continuity. This bubble is one of two leveling bubbles and is used to determine horizontality of the compass. The other bubble is used for inclination measurements.
- Check for correct magnetic declination setting. Since earth’s magnetic and geographic poles are not coincident, in order to accurately assess compass directions (relative to true North) the declination pin must be set to the correct magnetic declination for the location of usage.
2. Establish Suitable Representative Surface for Measurement
- In the field, a geologist must establish suitable representative surfaces for measurement. The idea is to approximate the overall orientation of the feature being evaluated (bedding, jointing, foliation, etc.) at this particular location. One of the simplest ways to do this is to place a notebook or clipboard onto the rock in this average and representative orientation.
In a lab demonstration, any flat surface can be used as a representative surface (a board/model on a desk, or an architectural element of a building).
3. Set the Compass on the Surface
- Next, the lower edge of the Brunton Compass is set upon the surface, such that the entire edge is flush with the surface.
4. Center the “Bull’s Eye Bubble”
- Without taking any part of this edge off the surface (a common mistake) the Brunton Compass is rotated until the “bull’s eye bubble” is centered.
5. Read the Azimith Orientation, or Measure Strike
- By centering the bull’s eye bubble, the Brunton Compass becomes aligned in the horizontal plane, and this allows for reading the azimith orientation of the line formed by the intersection of the rock surface and the horizontal—i.e. the definition of “strike”.
Note: By convention strike is measured in the northern quadrant. For example a direction of S30degE (30° to the east of due South) would be reported as N30W.
6. Measure Dip
- The final step is to measure dip. This is measured perpendicularly to the strike direction and is defined as the angular deviation of the surface from horizontal. For example, a nearly vertically oriented rock layer might have a dip magnitude of 85SE, indicating that the surface is dipping 85° from the horizontal, in a southeasterly direction.
Note: Dip direction is given in a general sense (NE, SE, SW, NW) because its exact direction is always 90° from strike.
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!
A set of strike and dip data for a non-dipping rock layer has a range of values. The precision of a single measurement is, of course, linked to mechanical compass-errors and the experience of the compass-user. The accuracy of the final analysis is dependent on the uniformity of the natural surface (many nominally “flat-lying” rock layers have some degree of inherent surface undulations) and the number of total measurements taken.
Strike and dip data are initially recorded in field notebooks, and then transferred to tabulated form, and ultimately onto geologic maps (Figure 4). All geologic maps show the boundaries between rock units, and the strike and dip data (bar and stick symbols) provides the three-dimensional component, describing the spatial orientation of each rock unit.
Strike and dip of bedding, the most common kind of rock orientation data, is shown at a specific location with symbols like the ones below.
Besides strike and dip of bedding, there are many other sorts of planar and/or linear rock features that have strike and dip and some of these are shown in Figure 5.
Figure 4. Strike and dip of bedding on a map. Strike and dip of bedding, the most common kind of rock orientation data, is shown at a specific location with symbols like the ones below.
Figure 5. Strike and dip map key. Map key for planar and/or rock features demonstrating strike and dip.
Applications and Summary
Geologists strive to understand the earth in four-dimensions. The goal is to interpret the structure of rocks on the surface, in the subsurface, and through time. Strike and dip information generated by the Brunton Compass is the starting point with which geologists make geologic maps, and then those maps can be used to make cross sectional diagrams, showing the structures in the subsurface (Figure 6).
Understanding rock structures in the three spatial dimensions and also through time provides a window on the physical evolution of our planet. In addition, this kind of knowledge is central to many industrial and economic applications. One example is the identification of rock up-warps, where layers have been bent in domes or fold structures called anticlines— and it is at the apex of these structures that oil and gas often collect.
Figure 6. Geologic cross section. Geologic cross sections are representations of underground geology. The line (D-D’) on the map is the line along which the cross section was drawn. Anticlines, synclines, and faults can be seen in cross sections.