While many naturally occurring elements have radioactive isotopes, only potassium, and the uranium and thorium decay series, have radioisotopes that produce gamma rays of sufficient energy and intensity to be measured by gamma ray spectrometry. This is because they are relatively abundant in the natural environment. Average crustal abundances of these elements quoted in the literature are in the range 2.0-2.5 % K, 2-3 ppm U and 8-12 ppm Th.
⁴⁰K is the radioactive isotope of potassium, and occurs as 0.012% of natural potassium. This isotope decays to ⁴⁰Ar with the emission of gamma rays with energy 1.46 MeV. Since ⁴⁰K occurs as a fixed proportion of K in the natural environment, these gamma rays can be used to estimate the total amount of K present. The half-life of ⁴⁰K is 1.3x10⁹ years.
Uranium occurs naturally as the radioisotopes ²³⁸U and ²³⁵U which give rise to decay series that terminate in the stable isotopes ²⁰⁶Pb and ²⁰⁷Pb respectively. The half-lives of ²³⁸U and ²³⁵U are 4.46x10⁹ and 7.13x10⁸ years, respectively. Thorium occurs naturally as the radioisotope ²³²Th which gives rise to a decay series that terminates in the stable isotope ²⁰⁸Pb. The half life of  ²³²Th is 1.39X10¹⁰ years. Neither ²³⁸U, nor ²³²Th emit gamma rays, and gamma ray emissions from their radioactive daughter products are used to estimate their concentrations (IAEA, 2003). Therefore, distinct emission peaks associated with ²⁰⁸Tl and ²¹⁴Bi, are used to calculate the abundance of Th and U, respectively (Minty, 1997). Consequently, U and Th are usually expressed in equivalent parts per million (eU and eTh), which indicates that their concentrations are inferred from daughter elements in their decay chain, whereas, because of its higher crustal abundance, K is typically expressed as a percentage (K%) (Wilford et al. 1997).
However, the estimation of U and Th in this manner assumes that the daughter products in the Th and U decay series are in equilibrium. In some cases the measured isotopes in the U and Th decay series (i.e. ²⁰⁸Tl and ²¹⁴Bi) may not perfectly quantify the parent elements, because of disequilibrium in the decay chain. Disequilibrium occurs when one or more of the daughter products in the decay chain is removed or concentrated. For example, in some salt lakes and groundwater discharge sites, high eU and eTh values may be due to the accumulation of radium isotopes that are mobile in acid saline solutions (Dickson, 1985). Disequilibrium should therefore be considered when interpreting gamma ray data.

Application of gamma-ray spectrometry in soil and regolith mapping
Radiometric surveying is a non-invasive, fast method for identifying and quantifying radionuclide concentration and distribution in the landscape. The technique does not require extensive laboratory analysis, and hence the cost is lower than equivalent standard field surveying and mapping methods (Pracilio et al., 2003).
Radiometric response is closely associated with soil texture (Talibudeen, 1964), and can be used to identify and/or differentiate landscape processes. The influence of other soil characteristics, such as soil pH, Eh and structure, on radionuclide distribution is more difficult to identify unless they have direct influence on the radiometric response (Beckett, 2007).