Ch. 31] RADIOACTIVITY LOGS 565 



a few feet, and the thicknesses of individual beds can be determined 

 with similar accuracy. A formation will exhibit a characteristic pat- 

 tern that is usually recognizable in other wells. As a resiTlt, electric 

 logs are extremely useful for correlation, especially for distances up to 

 5 or 10 miles. They are simpler and more accurate than sample logs 

 for these short distances. Over longer distances the minor lithologic 

 changes are less easily recognized, and such features as rock color and 

 texture and fossils are necessary. 



RADIOACTIVITY LOGS 



All rocks display varying degrees of radioactivity which are due to 

 the presence of one or more unstable mineral components. Radium, 

 uranium, actinium, and thorium are the radioactive elements of such 

 unstable minerals which are continually disintegrating and forming 

 other minerals of slightly lower molecular weight which in themselves 

 are radioactive. Igneous rocks are the source of all other types of 

 rocks, and radioactive minerals are present in very minute amounts in 

 both this primary source and the metamorphic and sedimentary deriva- 

 tives. Thus radioactive minerals occur in rocks of all ages and types. 

 Radioactive minerals are more concentrated in some types of sedimen- 

 tary rocks than in others. In general, shales and clays are more radio- 

 active than sandstones, and "the average radioactivity of limestones 

 is less than that of sandstones" (Bell et at., 1940, pp. 1539-1540). 

 Russell (1941, p. 1769) has observed, "that pure sandstones and lime- 

 stones almost invariably show less radioactivity than the shales." 

 Coal, salt, anhydrite, and other salines also are weakly radioactive. 



In radioactivity logging, the differences in the degree of radioactivity 

 of these different types of sedimentary rocks afford a basis for identify- 

 ing the different formations. Such differences are well illustrated on 

 the radioactivity log of Fig. 2. An instrument has been devised which 

 is sensitive to radioactive gamma-ray radiation. It consists of a steel 

 cylinder, closed at both ends, containing high-molecular-weight gas. 

 The cylinder is fitted with insulated electrodes which are connected 

 with an external source of direct current. There is an amplifier in the 

 upper part of the instrument, as well as batteries which supply the 

 current to the ionization chamber. As the ionization chamber traverses 

 the hole, gamma-ray emanations from the surrounding strata permit 

 current to flow through the gas between the electrodes of the ioniza- 

 tion chamber. The amount of current is directly proportional to the 

 intensity of radioactive emanation from the surrounding stratum. 



