876 



MISCELLANEOUS GEOPHYSICAL METHODS 



[Chap. 12 



plant remains. In some localities carnotite is associated with fossil wood. 

 An increased radioactivity of oil sands has been observed when the drill 

 cores were tested by the alpha-ray method. However, gamma-ray logging 

 has failed to indicate an increase in this radiation.^ 



Radioactivities of minerals and rocks may be determined in the labora- 

 tory in three ways: (1) by measuring the total radiation of a given weight 

 of substance in an ionization chamber, (2) by preparing a solution and 

 measuring the radiation of the radon contained in it, (3) by measuring the 



Fig. 12-L Measurement of rock radioactivity by emanation method (Mache and 

 Bamberger, after Kirsch). (a) Motor, (b) shaking table, (c) drying tube, (d) ion 

 trap, (e) dry-cell battery, (/) ionization chamber, (g) electroscope, (h) manometer, 

 (i) guard ring, (k) battery. 



penetrating (gamma) radiation of a given weight of substance with a 

 Geiger counter or gamma-ray electroscope. 



Measurements under (1) are made as follows: The specimen is ground, 

 dried, and weighed; and a definite amount (say 2 g) is placed in the tray 

 of the ionization chamber of an electroscope whose scale value (volts per 

 scale division) and normal dispersion rate are known. Assume the former 

 to be 51 volts per scale division and the latter 3.5 millivolts per second. 

 Then, if a 2-g specimen produces a total decay of 10 scale divisions in 

 8 minutes, 50 seconds, the reduced decay is 96.2 — 3.5 = 92.7 millivolts 

 per second, and the decay per unit weight is proportional to 46.4-10" 



« See p. 864. 



