Determination of correction factors for borehole natural gamma-ray measurements by Monte Carlo simulations

The analysis of natural γ-ray spectra measured in boreholes has to take into account borehole parameters such as the presence of casings and borehole diameter. For large, high-efficiency γ-ray detectors, such as BGO-based systems, which employ full-spectrum data analysis, corresponding corrections were not previously determined. In a joint project of the Nuclear Geophysics Division of the Kernfysisch Versneller Instituut (NGD/KVI), Groningen, Medusa Explorations B.V. and the Dutch Institute for Applied Geosciences (TNO-NITG) a catalogue of corrections was constructed. Using the Monte Carlo code MCNP, the influence of steel casings, borehole diameter, central axis probe position and the diameter of the γ-ray detector on the γ-ray spectra has been investigated for nearly 20 geometries. The calculated γ-ray spectra are compared qualitatively and quantitatively. In a case study, γ-ray spectra from a borehole measured in a cased and uncased configuration are analyzed with simulated spectra. When no corrections are used, the activity concentrations deviated by as much as 50% between the two measurements. Taking into account the specific measurement geometry, the activity concentrations were found to be identical within the statistical and systematic uncertainties of the experiment for the same borehole, with and without casing. These results illustrate the need for borehole-specific corrections and this study demonstrates that Monte Carlo methods are a fast and reliable way to calibrate well-logging tools for a wide variety of configurations.