The Monte Carlo Multiply Scattered Components (MCMSC) approach for natural gamma-ray log interpretation

A new approach called the Monte Carlo Multiply Scattered Component (MCMSC) approach has been developed for natural gamma-ray logging tool interpretation. It consists of first determining individual scatter component library gamma-ray spectra for each of the three sources of natural gamma radiation (⁴⁰K, U, and Th) for a reference sample by Monte Carlo simulation. Then these scatter component library spectra, normalized to unit area, are used to obtain coefficients by the library least-squares (LLS) method with the natural radioisotope library spectra for a range of sample parameters such as borehole fluid density and composition as well as formation density and composition. Finally, empirical relationships are developed between the individual scatter component library coefficients and the sample parameters. It is found that the normalized scatter component libraries are insensitive to the formation lithology, and the range of allowable parameter variations covers most of the sedimentary formations. This approach allows one to construct the appropriate natural radioisotope libraries for ⁴⁰ K, uranium, and thorium for a sample with known characteristics in a very efficient and accurate manner. This, in turn, allows the calculation (again by the LLS method) of amounts of the three sources of natural gamma rays for any sample spectrum