Subsurface Logging Methods 



421 



Instrumentation 

 The object of radioactivity well logging is to measure the radiations 

 emitted by radioactive substances in the rock formations adjacent to the 

 walls of the drill hole and to plot the intensity of the radiations in the 



INSTRUMENT TRUCK^ 



AMPUFIERS *UIOM»TIC RECORDtK 



Figure 191. Radioactive logging apparatus. (Lane-Wells.) 



form of a graph versus depth. The order of magnitude of the radio- 

 activity to be expected in everyday practice is exceedingly small. Average 

 midcontinent sedimentary rocks contain from two to twenty micromicro- 

 grams of radium per gram of rock. This concentration of radium is so 



