Optimisation of spectrometric gamma-gamma probe configuration using very low radioactivity sources for lead and zinc grade determination in borehole logging

摘要

The suitability of spectrometric backscattered gamma-gamma well logging measurements to predict lead and zinc metal equivalent content is demonstrated. A centralised tool employing a gamma-ray source of very low radioactivity (1.8 MB_(q)) is used. The logging tool is tested using ~(133)Ba and ~(137)Cs sources with a 37 mm (diameter) x 75 mm NaI (TI) scintillation detector. Five source-to-detector configurations were analysed for 18 geophysical models, 13 of which had a borehole diameter of 130 mm and the other 5 had a borehole diameter of 160 mm. Regression analysis on the laboratory logging data for each configuration in order to establish the calibration equation for a lead (Pb) and zinc metal equivalent (ZME) prediction is carried out. The optimum configuration for the logging probe using a ~(133)Ba source was determined to be 52 mm source-to-detector spacing. This configuration gives the best results for both Pb and ZME grade. The rms deviations for Pb and ZME were 0.33 and 2.3%, respectively. The optimum configuration for the logging probe using a ~(137)Cs source was determined to be 64 mm source-to-detector spacing. This configuration gives the best results for both Pb and ZME grade. The rms deviations for Pb and ZME were 0. 3 6 and 2.2%, respectively.