Field and theoretical investigations of fractured crystalline rock near Oracle, Arizona

Abstract

A combination of geophysical and hydraulic testing has been conducted in granite near Oracle, Arizona. The purpose of the work is to determine relationships, if any, among (1) fracture distribution, (2) geophysical properties, and (3) hydraulic properties of fractured rock of low hydraulic conductivity. To date, eight vertical borings spaced 20 to 50 feet apart, ranging from 250 to 300 feet in depth, have been drilled. The data obtained from neutron, gamma, acoustic-velocity, electrical-resistivity, and acoustic-televiewer logs, with the results of over 100 single-hole, straddle-packer injection tests make possible a detailed description of the fracture system. Geophysical logs readily detect fractures and are sensitive to subtle lithologic variations of the granite. Orientation and distribution of individual fractures were determined from interpretation of the acoustic-televiewer data, and from the analysis of core obtained from one borehole. Fracture densities over the 13-foot long straddle-packer test intervals did not correlate with measured hydraulic conductivity for each interval. A strong correlation between the neutron-log response and measured hydraulic conductivity does exist; it was used to supplant conductivity mesurements. The geostatistical technique of kriging provided a three-dimensional map of hydraulic conductivity that can be compared with subsurface interpretations of the geophysical logs. 52 refs., 42more » figs., 8 tabs.« less