The MGT was calibrated at a commercial test facility at the Halliburton Energy Services campus in Houston, Texas (USA) (Gadeken et al., 2000). These tests enable the comparison of the MGT detector responses to known concentrations of K, U, and Th and gamma ray levels in separate, 150-cm-thick test zones (Table AT1). Background measurements were also made for all detectors. The three water-filled pits contain 23-cm-diameter cased holes as the test zones. Gamma ray counts were measured independently for each of the four MGT sodium iodide detectors over an acquisition time of 10–20 min in each zone. Preliminary MGT calibration tests were also completed at the American Petroleum Institute/University of Houston, Texas (USA), facility and reported by Goldberg et al. (2001).
Based on these standards, we compute coefficients normalizing the response of each detector to the appropriate K, U, and Th and total gamma ray levels. Table AT2 provides the calibration coefficients for processing raw MGT counts and computing the log responses. Calibration coefficients for the stacked data are also derived from statistical processing of individual channel counts.