A PORTABLE GAMMA RAY LOGGER AND THE ESTIMATION OF GRADE AND ORE RESERVES IN RADIOACTIVE DEPOSITS WITH REFERENCES TO LOGGING AT OUCHI DISTRICT MIYAGI PREFECTURE

Variations of the distribution of the gamma ray intensity in drill holes, with the length of the detector, thickness of radioactive beds and diameter of the holes, are discussed. A portable gamma ray logger, with a special scintillation detector having a diameter of only 30 millimeters, was constructed for the purpose of prospecting in mountain regions. A method of estimating the thickness of the horizontal ore beds and the grades of ores was obtained from a theoretical calculation, utilizing a constant determined by experiments in drill holes. Moreover, the area between the counting rate curve and the base line on a radioactivity log is proportional to the product of grade and thickness, independent of drill-hole diameter and logging speed. Therefore, the ore reserves may be estimated rather easily, and the mean grade is calculated when a proper value for thickness is given. Using this portable radioactivity logger, radioactivity in the Ouchi district of Miyagi prefecture was measured. A slight anomaly was recognized, and the mean grade of ore was estimated at about 0.002% eU. The anomalous radioactivity was detected in a mudstone containing carbonaceous material, and also in a fine-grained sandstone. Electric logging was also carried out in many of the drill holes there. If all the drill holes were to be examined by electric logging, important information for interpretation of beds showing anomalous radioactivity could be obtained.--Author.