12 



MISCELLANEOUS GEOPHYSICAL METHODS' 



I. RADIOACTIVITY MEASUREMENTS 



A. General 



Applications of radioactivity measurements in geophysical exploration 

 are concerned with the location of concentrations of radioactive material 

 and radioactive rocks. Other applications of radioactivity in geology, 

 such as its possible contribution to the earth's temperature and its use 

 in the measurement of ages of rocks, are not discussed in this chapter. 

 As a whole, radioactive substances are fairly uniformly distributed all 

 over the earth and are present in the atmosphere, the water, and the solid 

 earth. Local enrichments occur by association with certain rocks (mostly 

 acidic igneous rocks), absorption in certain liquids (radon in water and 

 oil), and confinement to predetermined transportation channels (faults, 

 crevices, dikes, and the like). 



As is well known, radioactivity is a group of phenomena characteristic of 

 substances with high atomic weight (except potassium and rubidium), of 

 which the best-known examples are uranium, radium, and thorium. 

 Probably tremendous energies were required to build up and hold these 

 atoms together in the very early stages of the earth's history. Since these 

 conditions no longer prevail, they are now in a process of spontaneous 

 decomposition. This process affects almost entirely the nucleus of the 

 atom and is therefore unaffected by ordinary physical and chemical proc- 

 esses such as heat, electrical and magnetic fields, and mechanical pressure. 

 It can be changed and produced, however, by bombardment of the nucleus 

 with particles comparable with, or identical in velocity and nature to, 

 those released by radioactive atoms. If it were possible to produce such 

 nuclear changes by thermal energy, temperatures of the order of 10 

 degrees C. would be necessary (Nemst). While in chemical transforma- 



» The symbols in this chapter are the same as in Chapter 9, except where other- 

 wise noted. 



870 



