[en] Logging tools that record the entire natural gamma ray spectrum are designed to measure concentrations of the naturally occurring radioactive elements Potassium (K), Uranium (U), and Thorium (Th). A sophisticated mathematical method is used to compute the calibration spectrum for each of the tracer elements in a range of environments. The Boltzmann transport equation is solved for the gamma ray flux at the detector surface and convolved with a detector response function determined by Monte Carlo simulation to obtain the spectra. Once properly calibrated, a spectral stripping technique computes quantitative concentrations for all radioactive isotopes in the formation. Techniques for utilization of these concentrations is discussed. Several log interpretation examples illustrate the discussion