RADIOACTIVITY METHODS 



1009 



Russellf showed that even in some "areas of deeply weathered residual 

 soils the stratigraphy of the underlying formations may be mapped by 

 plotting the area variations in the radioactivity of the weathered products," 

 and Ridlandl clearly recorded the No. 3 ore zone at LaBine Point, N.W.T., 

 where a vein of pitchblende comes within 60 feet of the surface, by detect- 

 ing the radiation from radon gas carried along the otherwise barren fissure 

 above the vein. (See Figure 627.) Radon-carrying waters are found occa- 

 sionally along fault zones and fissures in regions where no uranium ore is 

 known. The radioactivity profile may show a pronounced peak over a 

 structure of this sort if it is not buried too deeply. On the other hand, very 

 misleading measurements may be made in regions covered by glacial drift 

 and in the uranium mining districts, where much high-grade ore is frequent- 

 ly scattered about the countryside by blasting and spillage from ore trucks. 



Gamma-Ray Logging 



The radioactivity of sedimentary formations varies within wide limits, 

 and a log of radioactivity vs. depth in a drill hole can be of great value 

 for stratigraphic correlation, § and for tracing radioactive cement. Gamma 

 and neutron logging are described in Chapter XI. 



The gamma-ray log is made by- 

 lowering a sensitive probe to the bot- 

 tom of the well and recording the ac- 

 tivity at the surface while the probe is 

 being withdrawn. Relative radioactiv- 

 ity data are all that is required where 

 the logs are used primarily for strati- 

 graphic correlation, but in exploration 

 for uranium absolute calibration is of 

 the utmost importance. It is essential 

 to know as much as possible about the 

 actual concentration of radioactive 

 matter in the immediate vicinity of the 

 hole. The theoretical approach is not 

 simple but it is possible to calibrate a 

 probe empirically by surrounding it 

 with radioactive material of known 

 concentrations arranged in layers of 

 different thickness to form a "synthetic 

 drillhole". The material must be chosen 

 so as to have a gamma-ray absorption 

 coefficient similar to that of the actual rocks expected in the field. 



Generally, with an 8-inch G-M tube in sandstone the total indication of the count- 

 ing rate meter is proportional to the absolute concentration of radioactive matter 

 around the tube only when the radioactive layer is over 2 feet thick and laterally con- 

 tinuous for more than a foot or two. In thin layers of "ore" the factors of thickness 

 and grade cannot be separated by analysis of readings taken with a single G-M tube, 



t W L. Russell, "The Total Gamma-Ray Activity of Sedimentary Rocks as Indicated by Geiger 

 Counter Determinations," Geophysics IX, lSO-216 (1944). 



t G. C. Ridland, "Use of the G-M Counter in the Search for Pitchblende-Bearing Veins at 

 Great Bear Lake, Canada," Trans. A.I.M.E. 164. 117 (1945). 



. § W. J. Jackson and J. L. P. Campbell, "Some Practical Aspects of Radioactivity Well Log- 

 ging," Trans. A.I.M.E. 164. 295-321, and 165, 241-67 (1945-6). 



Fig. 628. — Maximum meter deflection plotted 

 as function of thickness of a uniform horizontal 

 bed of radioactive rock drilled vertically and 

 logged with an 8-inch Geiger-Miiller counter. 



