The Profile-Area and Fracture-Frequency Methods: Two Quantitative Procedures for Evaluating Fracture Spacing in Drill Core

Abstract

The profile-area method (PA) of evaluating fracture spacing in drill core provides a convenient and rapid means of quantifying a fundamental rock mass property. A profile is drawn by plotting core segment lengths against depth. The PA method supplements the rock quality designation (RQD) values as only those segments of core greater than 0.3 ft (4 in.) are plotted, thereby giving a quantitative measure of fracture spacing not available by the RQD technique alone. The area thus formed under the profile line is determined by use of a planimeter. Small PA values indicate closer spaced fractures than do large PA values. The values determined by this method are scale dependent. Two useful parameters may be developed from PA values: the J factor and the K factor. The J factor is defined as the PA value divided by the length of the interval being investigated. The K factor is defined as the PA value divided by the number of core segments of the interval being examined. These two factors are useful in comparing fracture density of zones within a drill hole as well as between drill holes at the site. Core length data can also be used as basis for statistical analyses by the fracture-frequency method (FT). Core segment lengths are transformed into logarithmic values by the H equation. When such H values are plotted against cumulative percent on probability paper and a straight line or linear relationship results, the data are log-normal distributed. The H equation is thus a useful tool for normalization as five out of the six samples plotted linear. The H values were also used in computation of the median, mean and standard deviation. Small means or medians along with small standard deviation indicate poor rock. The smaller the standard deviation, the more uniform the fracture spacing.