Once the basic structural characteristics of a diamond drilling core have been determined, it’s time to make some detailed measurements. A statistically robust quantification of the structural characteristics of a core allows for the development of 3d geologic models of the region being explored.
The methods used for making measurements of structural characteristics will depend on the nature of the core, and, most importantly, whether or not the core has been oriented.
When cores are removed from the borehole, they are usually rotated from their original orientation. Fortunately, tools have been developed to mark or preserve the original orientation of the core. This is important because without knowledge of original orientation, it is impossible to determine the true orientation of geologic structures measured along the core.
Measurements made on a single oriented core can be used to describe the overall orientations of structural features. In contrast, if a core is not oriented, the true orientation of geologic features such as faults and folds can only be determined by combining data from three or more cores.
Measurements of oriented cores are often taken with the assistance of a core frame – an adjustable core holder that can be oriented in any direction. When a core section is removed from a core tray, it is placed in the frame so that it is oriented in the same direction as it was originally oriented underground. Geologists can then take measurements of the core using a standard geologic compass.
Although this method is intellectually straightforward, adjusting a core frame and removing a core from the core box all take time and risk damage to the core.
An alternative method involves measurement of the orientation of structures while a core is safely stored in a core tray. This allows both oriented and non-oriented cores to be measured more quickly.
In this alternative method, orientations of geologic structures are measured relative to the core axis. Since the orientation of the diamond core borehole is known, a computer program or stereonet can be used to correct the apparent angle measurements from the core to true angle measurements without the difficulties of placing the core in a core frame.
This method is much preferred, especially if a large number of measurements are taken.
One final note: the optimum frequency of measurement of structures on a core depends on how quickly the orientation of structures is changing with depth. If the structures are all oriented the same way, measurements can be widely spaced. In contrast, if the orientations fluctuate frequently with depth, a closer spacing of measurements may be required to describe the structures.