| 摘要 |
A method for determining the thermal neutron capture cross-section of an earth formation penetrated by a wellbore from counts of capture gamma rays. The capture gamma rays are detected at spaced apart locations after bombarding the formations with high energy neutrons. The method includes determining an apparent capture cross-section at each one of the spaced apart locations from the counts of capture gamma rays, determining a difference in the apparent capture cross-sections determined between pairs of the spaced apart locations, and calculating the capture cross-section by combining the apparent capture cross-sections and the differences into an empirical relationship of known values of the capture cross-section with respect to the differences and the apparent capture cross-sections. In the preferred embodiment, the coefficients of the empirical relationship are determined by minimizing a value of an error function including as components: the differences between simulated values of the capture cross-section calculated from simulated gamma ray counts at each one of the spaced apart locations corresponding to known values of the capture cross-section used to perform the simulation, and the known values of the capture cross-section; and the statistical error in the simulated values of capture cross-section.
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