Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



879 



erties, or (2) by determining the activity of formations in situ with portable 

 ionization chambers. The first exploration procedure is identical with the 

 methods described in the preceding section ; the second group of procedures 

 includes: (a) measurement of the entire radiation with open-bottom ioniza- 

 tion chambers, (b) measurement of the soil-air emanation (mostly radon), 

 and (c) measurement of the penetrating (gamma) radiation. 



(a) An open-bottom ionization chamber is illustrated in Fig. 12-3. It 

 consists of a metal box with insulated dispersion rod and double-leaf 

 electroscope. The latter is detachable (as in most other chamber-electro- 

 scope combinations) . The dispersion rod may be charged by a rubber rod 

 or small dry-cell battery. The charge is so dimensioned that approxi- 

 mately the same leaf deflection is ob- ^ 

 tained for every measurement. The 

 instrument is first tested in a room 

 where no radioactive substances are 

 present, and the normal dispersion rate 

 is determined. When the chamber 

 is placed on the ground (after the 

 surface vegetation and top soil have 

 been cleared off), the radioactivity of 

 the surface formations and soil air 

 will ionize the air in the chamber 

 (mostly by alpha radiation) and pro- 

 duce a corresponding decay of the 

 charge. In a calibrated electroscope 

 the potential difference corresponding 

 to a scale division is known, and there- 

 fore the decay or dispersion dE/dt may 

 be expressed in volts (or millivolts) 



per second. The latter is proportional to the conductivity of, and there- 

 fore to the (saturation) current, I, in the chamber: 



mm. 



r 



o 



Fig. 12-3. Open-bottom ionization 

 chamber (schematic, after Hummel). 

 (1) Case, (2) electrometer, (3) leaf, 

 (4) insulator, (5) dispersion rod, (6) 

 charging rod. 



-f ^ dE , 



1 = C'—r- e.s.u. s. 



dt 



(12-1) 



C'is the capacity of the chamber (in centimeters) and dE/dt is the dis- 

 persion (in statvolts = (volts/300) see" ). Current in amperes is obtained 

 by multiplication by 1/9 X 10~" instead of by 1/300. The current is 

 proportional to the number (n) of ions formed, so that 



n = 



dE 



300-6.7 dt ' 



(12-2) 



•' By a dry battery and precision high-resistance voltmeter. 



