Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 140

Fig. 11.2 Gamma ray log
presentation
Origin of Natural Gamma Rays
Gamma rays originate from three sources in nature: the radioactive elements in the
Uranium and Thorium groups, and Potassium. Uranium 235 , Uranium 238 , and
Thorium 232 all decay, via a long chain of daughter products, to stable lead isotopes.
An isotope of Potassium, K 40 , decays to Argon, giving off a gamma ray as shown in
Fig. 11.3 .
Note that each type of decay is characterized by a gamma ray of a specific energy
(wave length) and that the frequency of occurrence for each specific energy level is
different. Figure 11.4 shows this relationship between gamma ray energy and fre-
quency of occurrence. This is an important concept, since it is used as the basis for
measurement in the natural gamma spectroscopy tools.