864 



GEOPHYSICAL WELL TESTING 



[Chap. 11 



of well logging through casing/^ A radioactive well logging arrangement 

 is shown in Fig. 11-33. Two Geiger counters*^ (to eliminate chance varia- 

 tions) are connected separately through two low-impedance secondary'- 

 transformers to two A.C. amplifiers feeding into thyratron-controlled fre- 

 quency meters. The latter are provided with a tank circuit (12 micro- 

 farads across 1 megohm) to 

 smooth out the current varia- 

 tions, so that the galvanometer 

 indication is proportional to 

 an average pulse frequency. 

 The measuring cylinder may 

 be lowered and records may 

 be taken at a rate of about 

 1500 feet per hour. As Fig. 

 11-34 indicates, the 7-ray in- 

 dication is markedly parallel 

 to the potential and to the 

 impedance record of the elec- 

 trical log. High porosity sands 

 are indicated by radioactivity 

 lows, shales by radioactivity 

 highs. The sand indications 

 do not appear to be affected 

 by variations in oil content. 

 AVhile a-ray determinations 

 made on oil sand cores show 

 an increase of radioactivity, 

 7-ray logging does not seem to 

 be sensitive to the presence of 

 oil. However, it has been 

 found that gamma rays will 

 pick up readily the presence 

 of unconformities, which is 

 probably due to the concen- 

 tration of radioactive materials 

 on such surfaces. Comparison 

 of the two lower curves in Fig. 11-34 gives an idea of the absorption of 

 gamma radiation in the well casing. 



Oeiger Counter 



Fig. 11-33. Geiger-Mueller tube arrangement 

 for gamma-ray well logging (after Howell and 

 Frosch). 



^^ L. G. Howell and A. Frosch, Geophysics, 4(2), 106-114 (1939). 

 *8 See pp. 881-883. 



*' The high radioactivity of shales is due largely to the ability of colloids to absorb 

 radioactive substances and possibly to the presence of potassium compounds 



(see p. 875). 



