The effect of enlarged borehole size on the MGT and HNGS logs can be significant. In smaller-diameter holes, the reduced volume of borehole fluid attenuates fewer gamma rays transiting from the formation to the crystal detector; larger-diameter holes contain a greater volume of fluid that attenuates more gamma rays. Each detector's counting effectiveness depends on the actual borehole diameter, the position of the tool in the hole, and mud weight. Thus, under field conditions, appropriate corrections are necessary. The borehole diameter is difficult to control, in particular for ODP holes, and empirical corrections based on the caliper log are commonly used to correct for environmental effects to the extent possible. The HNGS total gamma ray log is corrected in real time based on the single-axis caliper reading provided by the Schlumberger tool string. Unfortunately, the maximum caliper reading is ~42 cm, and in the interval of interest above 250 mbsf, Hole 1179D is enlarged beyond this maximum dimension (Kanazawa, Sager, Escutia, et al., 2001).
For the MGT total gamma ray log, we apply corrections for borehole size in Hole 1179D based on the maximum caliper log extension. This assumption provides a partial correction for borehole size, which is reasonable given the reliable response of other logging tools in this hole. The correction was applied to the MGT data in post-processing using an algorithm adapted from experimental data by Mathis et al. (1984) and from Schlumberger charts (Schlumberger, 1994), taking both hole size and tool geometry into account. The equations describing these corrections are given in "Appendix B". For more precise estimates of these parameters, calibration of the tool under varying environmental conditions and an accurate caliper log must be acquired. For our purposes, the environmental corrections to the MGT and HNGS total gamma ray logs are comparable.