Each of the MGT detectors was individually calibrated at a commercial gamma ray calibration facility by recording in standardized test pits with known concentrations of U, Th, and K as well as total gamma ray levels (see "Appendix A" for details of the calibration tests and results). The pits are 23 cm in diameter and are filled with water. The response of each detector to the known concentration standard in each pit yields a calibration coefficient. Calibration coefficients for the stacked data are derived from statistical processing of individual channel counts. A stacking algorithm uses these coefficients to compute the K, U, and Th concentrations and total gamma ray response in gAPI units from the MGT data.
Each detector module internally records 256-channel natural gamma spectrometry data downhole. For transmission and processing, these channels are compressed into eight synthesized energy windows emulating the conventional five- and three-window systems that are used in commercial logging systems (Table T2). There are advantages and disadvantages to each. The three-window system gives the concentrations of K, U, and Th directly and can be used for real-time computations. The five-window system includes lower energy windows as well as second-order spectral peaks but may also introduce cross-window errors at certain spectral frequencies. The comparison of K, U, and Th concentrations between tools may be affected by the assumptions made in either window processing systems as well as by the detection threshold of the sensors.