Core angles (most commonly referred to as internal core angles) are internal geometric measurements taken from cylindrical rock samples extracted during diamond core drilling. Geologists use these angles—specifically alpha ( ) and beta (
)—to determine the true real-world orientation (strike and dip) of subsurface geological features like faults, veins, and bedding planes. The Two Primary Core Angles
When a drill cuts through a planar geological structure, it slices it at an angle, creating an ellipse around the cylindrical core sample. Geologists measure two primary angles from this ellipse: Alpha (
) Angle (0° to 90°): This is the acute angle measured between the central core axis and the long axis of the intersection ellipse. It indicates how steeply the structure cuts across the drill hole. Alpha can be measured on any piece of core, even if the core’s original position in the ground is unknown. Beta (
) Angle (0° to 360°): This is a rotational angle measured clockwise around the circumference of the core. It is measured from a known reference line—usually the Bottom-of-Hole (BOH) line—to the furthest or lowest apex of the ellipse. Beta tells you how the geological structure is rotated relative to the drill hole. How They Are Measured
Geologists use specialized logging equipment to quickly capture these angles: Six rules for alpha-beta measurements in drill core
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