In mineral exploration, a core is a cylindrical sample of rock and other subsurface materials obtained by drilling, which is then brought to the surface for geological examination and analysis.
When drilling in an area of good outcrop, where the rocks have a simple pervasive fabric of constant known attitude, it is generally not necessary to orient drill core. The orientation of the known structures in the core (for example, regular bedding or cleavage surfaces) can be used to orient the core, thus enabling other features whose orientation is not known (for example mineral veins) to be measured.
When drilling in an area of poor or no outcrop, the first few drill holes may be all that is necessary to establish the orientation of a dominant fabric and make orienting subsequent holes unnecessary.
However, where structure is complex and variable, it may be necessary to orient the core from every hole drilled in the area. Core orientation is carried out on a drilled run or barrel of core.
If the recovered rock is relatively unbroken, with little core loss, and the broken ends of the core pieces can be matched and reassembled across several barrels of core, it may only be necessary to orient every second or third run of core.
Where there is broken core, core loss and difficulty in achieving good orientation marks, it is necessary to attempt to orient every barrel of core. However, since the decision to orient a run of core must be made before it is pulled from the ground, and the condition of the core is generally only known after it has been extracted, it is always better to err on the safe side and attempt orientation of as many barrels as possible.
Once a few holes have been drilled, knowledge of the condition of the core should enable a decision to be made on how frequently it will be necessary to attempt orientation of subsequent holes.
