To compute K, U, and Th concentrations from the MGT, data from the four detectors are stacked in post-processing using the three-window processing system (see Table T2). The influence of enlarged borehole size is greater on K, U, and Th computations than on the total gamma ray measurements because of their lower count levels. The corrections given in "Appendix B" should be computed using accurate caliper logs, and detector calibration coefficients should be measured under similar large-hole conditions. Neither of these corrections are possible for the data collected in Hole 1179D. However, environmental corrections were not applied in real time to the HNGS or in post-processing to the MGT data, so the field results from both tools may be compared with confidence. The absolute values of elemental concentrations cannot be precisely estimated, however, and very low concentrations may be below the detection threshold in these tools. Comparison of natural K, U, and Th concentration logs from the MGT and HNGS are shown in Figures F5, F6, and F7, respectively, over their common depth interval from 160 to 230 mbsf in ODP Hole 1179D. Their correlation and differences are discussed below.
Figure F5 shows the comparison of K concentrations. The MGT and HNGS logs are in excellent overall agreement. The processing differences for K concentration with either windowing system are minimal (Mathis et al., 1984). Concentrations range from 1 to 2.5 wt% in this interval and appear to increase slightly with depth. Higher vertical resolution of the MGT highlights some offset in gamma values from the HNGS log across thin layers.
In Figure F6, the comparison of U concentrations indicates that the MGT estimates are ~1–2 ppm greater than the HNGS on average and that differences reach 35%–40% at particular depths. Good peak-to-peak correlation is apparent only for HNGS values of ~2 ppm or more, and the MGT log shows a slightly increasing trend with depth. Some HNGS values are negative, which indicates that the low U concentration in these rocks may be below its detection threshold or that processing may introduce an estimation error.
Figure F7 shows the comparison of Th concentrations with large differences between the MGT and HNGS estimates. The HNGS log increases slightly with depth, and, in general, peak-to-peak correlation with the MGT log is poor. The MGT estimates are 26 ppm (70%–80%) lower than the HNGS, on average. The low Th concentrations in these sediments may be below the detection threshold of the MGT at this logging speed, which is further suppressed by the effects of borehole enlargement. A comparison in another environment with higher Th concentration would provide more reliable estimates for both tools and a more valuable comparison.