DRILLING & GEOLOGY
True Thickness Calculator
Estimate the true thickness of a geological or mineralized zone from a drillhole intercept and the angle at which the drillhole intersects the zone.
DRILLHOLE GEOMETRY
Calculate True Thickness
Enter the measured drillhole intercept length and the intersection angle to estimate the true thickness of the geological interval.
INTERVAL GEOMETRY
True Thickness Calculation
Enter the measured drillhole intercept length and the intersection angle between the drillhole and geological plane.
ESTIMATED TRUE THICKNESS
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Enter an intercept length and intersection angle.
FORMULA
True Thickness Formula
True thickness is the shortest distance across a geological layer, mineralized zone, vein, or other approximately planar feature. A drillhole intercept may be longer than the true thickness when the drillhole crosses the feature obliquely.
In this calculation, the intersection angle is the acute angle between the drillhole trajectory and the geological plane. An intersection angle of 90° represents a perpendicular intersection and therefore produces a true thickness equal to the measured drillhole intercept.
WORKED EXAMPLE
True Thickness Example
Consider a drillhole that intersects a mineralized zone over a measured length of 20 meters at an intersection angle of 30°.
Drillhole intercept: 20 m
Intersection angle: 30°
Calculation: 20 m × sin(30°)
Estimated true thickness: 10 m
Because the drillhole crosses the zone at an oblique angle, the measured intercept is longer than the estimated true thickness.
INTERSECTION GEOMETRY
Understanding the Intersection Angle
The intersection angle used by this calculator is not automatically the drillhole inclination or the dip of the geological plane. It represents the angle between the drillhole trajectory and the plane being intersected.
When a drillhole crosses a planar feature at 90°, the measured intercept represents true thickness. As the intersection becomes more oblique, the measured intercept becomes increasingly longer than the true thickness.
90° intersection: 100 m intercept → 100 m true thickness
60° intersection: 100 m intercept → 86.60 m true thickness
30° intersection: 100 m intercept → 50.00 m true thickness
10° intersection: 100 m intercept → 17.36 m true thickness
GEOLOGICAL USE
Why True Thickness Matters
Drillhole intercept length describes the distance measured along the drillhole, whereas true thickness describes the thickness of the geological feature measured perpendicular to its boundaries. Distinguishing between the two is important when interpreting veins, stratiform mineralization, tabular ore zones, sedimentary units, and other planar geological features.
True-thickness estimates can provide more meaningful comparisons between drillholes that intersect the same geological feature at different angles. They can also help prevent long, oblique intercepts from being mistaken for unusually thick geological or mineralized zones.
LIMITATIONS
Important Considerations
The simple relationship used by this calculator assumes that the geological feature can be reasonably approximated as planar and that the intersection angle is known. Natural geological bodies may curve, fold, pinch and swell, branch, or have irregular boundaries.
Determining the intersection angle may also require reliable information about drillhole orientation and the orientation of the geological plane. Drillhole inclination and geological dip alone are not always sufficient when the drillhole and geological feature have different azimuths.
The calculated value should therefore be treated as a geometric estimate based on the supplied intersection angle rather than as a substitute for full three-dimensional geological interpretation.