The Monte Carlo-library least squares analysis principle for borehole nuclear well logging elemental analyzers

Monte Carlo simulation is an ideal way to generate the spectral response of well logging tools which are based on the (n, γ) technique. The primary use of Monte Carlo models in the past has been in the calibration of tools and log interpretation. Recently, it has been recognized that Monte Carlo simulation can also be used directly for the analysis of elemental composition. This new analysis principle combines the power of Monte Carlo for accurate simulation of the spectrum of each of the elements that are present in the formation with the linear library least squares method for the determination of elemental amounts. The methodology of Monte Carlo-library least squares (MC-LLS) analysis is based on the following procedure: (1) for an assumed composition of the formation, generate the complete spectral response by Monte Carlo, (2) within the program, keep track of the individual spectral responses for each element, (3) using the library least squares (LLS) analysis method, calculate the elemental amounts in the formation using the measured spectrum of that formation and the individual elemental spectra from the Monte Carlo simulation as the library standards and (4) if the elemental amounts calculated in step 3 are not close enough to those assumed in step 1, repeat steps 1-3 using an assumed composition that is closer to that of the formation. Preliminary results from the application of this method to X-ray and neutron capture γ-ray spectra show considerable promise and can be adapted for elemental composition analysis using spectral nuclear logging tools. (author)