A comparison among caliper-log, gamma-ray-log, and other diamond-drillhole data

Abstract

To obtain comparative data on the variation in gamma-ray intensity accompanying possible variation in the diameter of small-diameter diamond-drill holes, six drill holes in the Jo Dandy area, Montrose County, Colo., were caliper logged using a well-bore caliper developed by the II. S. Bureau of Mines. The caliper logs show that within radioactive-ore zones the variation in drill-hole diameter is insufficient to cause significant variation in the gamma-ray logging measurement, that with increasing particle size in the ore-bearing sandstone the drillhole diameter tends to increase slightly, and that with increase in hole diameter the core recovery tends to decrease slightly. INTRODUCTION In the investigation of gamma-ray logs, particular emphasis has been placed on the calibration of the logs in terms of the thickness and grade of radioactive ores. One factor in such calibration is variation in drill-hole diameter. Controlled experimental gammaray logging measurements have shown that an increase in the diameter of a drill hole might either increase or decrease the measured radiation intensity, depending upon the geometry of a particular measurement. Thus, sufficient variation in the hole diameter where overbreaking, sloughing, or swelling has occurred in the drill hole might lead to erroneous estimation from the gamma-ray log of the thickness and grade of ore. Although few or no data existed on the variation of hole diameter in actual exploratory drilling for carnotite deposits in the Colorado Plateau region before the present study, overbreaking or sloughing of drill-hole walls was suspected to occur in the higher grade ore zones. The need for caliper logs of drill holes has long been obvious, but the small diameter of the customary diamond-drill holes in the Colorado Plateau region most are AX and BX size has heretofore prevented such logging. Recently, a caliper designed to pass through a 2-inch ID pipe was developed by the U. S. Bureau of Mines. By cooperative agreement between the U. S. Bureau of Mines and the *TJ. S. Bureau of Mines. 241 242 EXPERIMENTAL AND THEORETICAL GEOPHYSICS U. S. Geological Survey, six selected drill holes were caliper logged in October 1953. This work was done on behalf of the Division of Raw Materials of the U. S. Atomic Energy Commission. LOCATION The drill holes (JD-327, JD-329, JD-339, JD-341, A, and B) are part of the Jo Dandy drilling project in Montrose County, Colo, the general area of which is shown on the index map (fig. 66). The first Grand Junction^ Whitewg Thompson GRAND MESA