Prompt neutron‐gamma logging for coal ash in water‐filled boreholes

Abstract

Use of the prompt‐neutron gamma technique to determine the ash content, depth, and thickness of coal strata was successfully demonstrated in water‐filled boreholes at six deposits in eastern Australia. The diameter of the boreholes varied from 96 mm (nominal diameter) to around 175 mm. The root‐mean‐square (rms) deviation between ash determinations based on regression analysis of logging data and laboratory assays for ash content varied between 1.4 and 2.8 weight percent. This variation is partially due to errors in sampling and chemical analysis (the errors are generally unknown). The optimum configuration for the logging probe consists of a 0.7 μg [Formula: see text] neutron source, a 51 × 51 mm BGO detector, and a source‐to‐detector spacing of 15 cm. This configuration gives the best vertical resolution as well as the smallest rms deviation for ash determination. Probe diameter was 70 mm and wall thickness was 3.2 mm. Regression analysis of the logging data was performed for each coal deposit to establish a calibration equation for the deposit. The calibration equations obtained in one of the deposits (Hunter Valley Deposit B) were applied to new boreholes in the same deposit. The rms deviation between the ash content predicted by the technique and the chemical analysis was ≃2 percent, which was consistent with the results for the calibration holes.