Intensity (joint surface area per unit volume of rock) of a multiset system of joints can be calculated from measurements of perpendicular spacings between adjacent joints in the same sets. Several estimators of intensity are easily calculated. The estimator that uses the trimean of spacings is least distorted by irregular spacing data that are observed and expected in joint systems.

Such a measure of joint intensity can be used in academic, engineering, environmental, and economic geology, and in mine design. We apply it to evaluating an exposed analogue of probable fractured gas reservoirs. A field test shows that a major cross-strike structural discontinuity in the Valley and Ridge province of the central Appalachians crosses into the easternmost Appalachian Plateau province as a zone of about twice-normal intensity of systematic jointing.

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