ODP Site 1179 is located in ~5500 m water depth (Kanazawa, Sager, Escutia, et al., 2001). Five holes were drilled at this site during Leg 191 to characterize the stratigraphy and to achieve the primary objective of installing a borehole seismometer at this location. The sediment stratigraphy consists largely of siliceous oozes and clays with numerous volcanic ash layers and some interbedded cherts in the deeper section (Kanazawa, Sager, Escutia, et al., 2001). During Leg 191, Hole 1179D was logged using the MGT as well as the standard Schlumberger HNGS logging tool used by ODP (Kanazawa, Sager, Escutia, et al., 2001). This was the first at-sea deployment of the MGT in marine sediments, and high-resolution gamma ray logging data were successfully recorded.
The logged interval consisted of the shallow upper Miocene clay-rich sediments and ashes above 300 m below seafloor (mbsf). The average core recovery was nearly 99%, allowing for high-resolution core sampling to be conducted (Kanazawa, Sager, Escutia, et al., 2001). The borehole conditions during logging in Hole 1179D were difficult, however. Extensive enlargement of the borehole (>42 cm) occurs above 256 mbsf and constrictions of the borehole diameter (<20 cm) occur below (Kanazawa, Sager, Escutia, et al., 2001). The effect of the borehole enlargement on these measurements is discussed below. Two passes of each tool, first the HNGS and then the MGT, plus short repeat runs of each for data quality assurance, were planned in Hole 1179D. After the first HNGS pass upward from 300 to 203 mbsf, however, the constriction at 256 mbsf precluded further logging below. Two passes of the MGT and the repeat HNGS pass were recorded above this depth. Hence, this paper compares results from the MGT and HNGS tools only over the interval where both logs were recorded, from ~160–230 mbsf.