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EXPLORATION GEOPHYSICS 



shown in Figures 612 and 626. A skillful operator can learn to pick up 

 moderate changes in the counting rate without actually stopping to count 

 the clicks, and the light weight and low cost of the "snooping" counters 

 make them attractive to the prospector. 



Traversing 



Having found a geologically interesting area, it may be advisable to 

 examine its surface radioactivity systematically. The best procedure usually 

 is to measure the activity at a number of points, which later are surveyed 

 and plotted to scale on a map. Radioactivity contours then may be drawn, 

 or the data can be plotted in profiles showing radiation intensity versus 

 traverse distance. Considerable geological information usually can be 

 obtained from plots showing only relative radiation intensity, expressed in 

 some arbitrary unit (e.g., counts per minute), but it is often desirable to 

 calibrate such a map to show the actual concentration of the radioactive 



Fig. 637. — Profile of surface radioactivity across two ore zones at LaBine Point, N.W.T. 

 (After Ridland, Trans. A.I.M.E., 164, 117, 1945.) 



material in the surface layer of rock. The most frequently used method of 

 calibrationf involves sampling at selected points and laboratory analysis of 

 these samples. The relative-intensity map is then used to interpolate 

 between the sampled points, giving consideration to the surface conditions 

 which may afifect the results. 



Although it is obvious that quantitative results are best obtained in 

 areas of bare rock, traversing may be useful in regions covered with over- 

 burden or in areas where radioactive bodies may exist at shallow depths. 



Szalay and E. Csongor, "Determination of Radioactive Content of Rocks by Means of 

 Geiger-Muller Counters," Science 109, 146-7 (1949). 



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