Logging operation in Hole 1198B started at 2020 hr on 10 February with rigging up the triple combo tool string (Table T13). At 2140 hr, the tools were lowered into the hole but only reached a depth of 235.5 mbsf. When the triple combo tried to pass this tight spot, the hole diameter narrowed to ~4 in and trapped the tool string. The tools were pulled free, and logging started at 231 mbsf. Because of the bad hole condition, no further logging was attempted, ending the operation at 0145 hr on 11 February.
The hole was dramatically enlarged above 192 mbsf, thus negatively affecting the quality of the porosity and density data and, to a lesser extent, resistivity data (Fig. F27). Even though the log density values are slightly lower than the shipboard GRA density values measured on corresponding cores, the data profiles show good agreement in local trends and corroborate a good depth match (Fig. F28). The MST natural gamma ray data are highly scattered but follow the general log trend.
The total spectral gamma ray (HSGR) log is the most reliable log because it is corrected up to a borehole size of 17 in. Consequently, the HSGR log is used to distinguish two log units in the logged 135-m interval.
Logging Unit 1 is characterized by low HSGR values of 20-30 gAPI with moderate variability. An upper interval (74-110.5 mbsf) has constantly low values of ~20 gAPI. At 110.5 mbsf, HSGR values increase to 30 gAPI with small-scaled variations superimposed on a nearly linear downhole trend. Logging Unit 1 corresponds to the skeletal packstones of the drift deposits of lithologic Subunit IB. In the cores, a gradual downhole increase in clay content can be observed from 138 mbsf to the bottom of the unit (see "Lithostratigraphy and Sedimentology"). The top of the clay interval is thus 28 m deeper than the change in the logs to higher HSGR values. The clay content in the sediments is reflected by a positive correlation of potassium and thorium logs with average values of 0.5 wt% for potassium and 2.5 ppm for thorium. At the bottom of logging Unit 1, a peak in the uranium log occurs. It corresponds to a hardground with rounded phosphatic nodules, developed on top of Subunit IIA (see "Lithostratigraphy and Sedimentology").
A sharp decrease in the HSGR values down to ~12 gAPI at 199 mbsf marks the top of logging Unit 2. Only ~10 m of this unit were logged by the tool string. This interval coincides with the top of lithologic Unit IIA and seismic Megasequence C. Lithologic Subunit IIA consists of coarse-grained carbonate-rich rudstone and floatstone, capped by the dark hardground with phosphatic nodules.