1096 EXPLORATION GEOPHYSICS 



of large errors, both random and systematic. Improved instrumentation 

 has corrected this difficulty. 



Physics of Modern Radioactivity Logs 



The gamma-ray logf recognizes the radioactive families occurring in 

 nature and emitting gamma rays. For the most part, the families may be 

 considered to be in radioactive equilibrium, in rocks which are old enough 

 and have remained undisturbed long enough. A discussion of radioactive 

 families and their radiations has been given in Chapter X. Recent evi- 

 dencej points to a much greater significance of potassium in gamma-ray 

 logs than had hitherto been recognized. 



The size of the sample observed during gamma-ray logging depends, 

 in each particular case, on the" nature of the radiation emitter which is 

 permanently present. § Speed of logging, required accuracy of the meas- 

 urement, and length of the section sampled at each point of the log are 

 interrelated in a manner which depends on the statistics of the arrival of 

 the radiation. This problem has been extensively discussed in Chapter X 

 and in earlier publications. ff 



Kubitschek and Dancofif|$ have made extensive studies of the gamma 

 rays of several elements, which are emitted by these elements upon absorb- 

 ing a slow neutron. Their experiments show a typically rather large emis- 

 sion of energy by an element-capturing neutron. They find, in general, that 

 the energy so emitted is more than twice the energy of the quanta radiated 

 by the most energetic gamma-emitting radioelements found in the rocks. 

 Fearon§§ and Pontecorvoftf have published descriptions of the use made 

 of these facts in logging. :i::j::j: 



It is interesting to note that whereas the gamma-ray curve is funda- 

 mentally limited in regard to logging speed, no such limitation is essential 

 to the neutron curve, which, in principle, can always be speeded up by 

 using stronger radiation sources. Sampling of the rock by the neutron 

 process is extensive and representative. The sample is egg-shaped§§§ rather 

 than cylindrical, as in the case with the gamma-ray log. Examples showing 

 the desirable qualities of the sampling obtained in neutron well-logging are 

 given in the reference cited. As discussed in Chapter X, hydrogen is the 

 element chiefly recognized by a neutron curve. 



The neutron curve therefore may be expected to show a minimum 

 opposite petroleum. There are many other factors which can cause a mini- 

 mum, as will be seen in the section on interpretation. It is interesting to 

 note that the neutron curve is an atomic detector of hydrogen, being sensi- 



t See list of patents on gamma-ray logging at end of chapter. 

 t Physics Review. 72, 640 (1947). 



§ R. E. Fearon, "Gamma Ray Well Logging," Nucleonics 4, 70 (1949). 

 tt R. E. Fearon, "Gamma Ray Well Logging," Nucleonics 4, 71 (1949). 



R. E. Fearon, "Gamma Ray Well Logging," Oil Weekly 118, 36 (1945). 

 tt Data released at April meeting, A.P.S., 1948 (to be published). 



§§ R. E. Fearon, "Well Logging Method and Device," U. S. Patent 2,349,712, May 23, 1944. 

 ttt Loc. cit. 



t%% See end of chapter for list of patents on neutron logging. 

 §§§ R. E. Fearon, "Neutron Well Logging," Nucleonics, June, 1949. 



