Many natural isotopes are radioactive. Only three of them play a notable role in the natural radioactivity of minerals and rocks. The others are extremely unstable or extremely rare.

The three isotopes which interest the geophysicist are:

• Uranium ₂₃₈U : It emits a gamma radiation of high energy, and accounts for on average 2 to 3 ppm of the earth's crust. One gram of uranium with its daughter products emits 33400 photons gammas per second.
• Thorium ₂₃₂Th : It is less active than Uranium, one gram of thorium and its products daughter emit 17400 gammas per second.
• Potassium ₄₀K : Potassium is a very widespread element in the rocks, its abundance is about 2,5 % of all elements. Isotope 40 is radioactive; it accounts for approximately 0,012% of total potassium so that we can say that radioactive potassium 40 constitutes some 2,8 ppm of the earth's crust. The energy of the gamma radiation due to potassium is relatively weak. Its activity is of 3.31 photons gammas per second for one gram of total potassium, that is to say 27583 photons gammas per second for one gram of potassium 40 radioactive.