One logging run was made at Site 1225 with the triple combo tool string (see "Downhole Measurements" in the "Explanatory Notes" chapter). After the last core was recovered at 1930 hr on 10 February, the hole was conditioned for logging. It was swept with 20 bbl of sepiolite mud; a wiper trip determined that there was 2 m of fill at the bottom of the hole. The hole was then displaced with 130 bbl of sepiolite, and the bottom of the drill string was positioned at 80 mbsf. Logging rig-up started at 0030 hr on 11 February. The 35-m-long logging string started downhole at 0300 hr, and two passes were made without difficulty, both times reaching the bottom of the hole at the wireline depth 323 mbsf. Logging operations and rig-down were completed by 1045 hr on 11 February (see Table T9 for a complete summary of the operations).
The caliper log (Fig. F32A) shows that the borehole diameter was generally large and irregular. However, except in a few intervals (above 103 mbsf and from 200 to 230 mbsf), the caliper arm remained in good contact with the formation, suggesting that the data are of good quality. Even in the interval with poor contact, the density and porosity data, which are the most dependent on borehole wall contact, do not seem to have been significantly affected. Both logs agree very well for most of the logged section with the MAD measurements made on cores (shown as discrete circles in Figs. F32E and F32F). The only significant excursion in the density log that could be attributed to borehole effect is present between 100 and 103 mbsf. The lower density values in this interval were not observed in the cores. Above 100 mbsf, the density log is again in agreement with the MAD measurements. The porosity log seems more affected by the enlarged hole above 100 mbsf. Above this depth, both porosity logs (near- and far-source/receiver spacing) have values higher than the core measurements. The near-array porosity reading is the highest because the shorter distance between the neutron source and the receiver makes it more sensitive to borehole enlargements (see "Downhole Measurements" in the "Explanatory Notes" chapter). The far-array porosity is less affected, but its significant differences from core measurements suggest that it is unreliable above 90 mbsf. Below 100 mbsf, both porosity logs are in good agreement with the MAD measurements. As neutrons penetrate deeper into the nondisturbed formation to reach the far receiver, the far porosity readings are generally slightly lower than the near porosity.
Figure F33 compares the logs from Hole 1225A (solid red line) with the data recorded in Hole 851B during Leg 138 (green dashed line). The comparison between the two calipers in Figure F33A shows that hole conditions were better in Hole 851B. However, Figure F33G shows that, despite the different hole qualities, the density logs are almost identical, confirming that except for the 100- to 103-mbsf interval, the density log in Hole 1225A did not suffer from the large borehole diameter. The few differences between the two density logs in the deeper part of the hole could indicate differences in stratigraphic thickness between the two sites. When comparing the two curves, one can note that similar features between the two logs have a larger vertical extent in Hole 851B. This apparent greater thickness of the deeper sequences might be a result of the tool sticking to the borehole wall and of the wireline stretching. It might also indicate a higher sedimentation rate at Site 851 in the early stages of sediment deposition on the oceanic crust. The differences in sequence thickness between the two sites are also apparent in the two resistivity logs in Figure F33F, which are almost identical above 230 mbsf.
The main differences between the two data sets are in the gamma ray log and in the element concentrations derived from this log. In particular, the thorium concentration in Figure F33C seems to increase with depth in Hole 1225A, whereas it was uniformly very low in Hole 851B. The differences cannot be attributed to the larger borehole diameter in Hole 1225A. If any effect was expected, the larger hole should induce decreased gamma ray counts, but gamma ray values are generally higher in Hole 1225A. The main reason for the differences lies in the different tools used and in the very low natural radioactivity of the calcareous ooze that constitutes the major component of the sediments at the two sites. Both tools (the Natural Gamma Tool [NGT] used during Leg 138 and the Hostile Environment Gamma Ray Sonde (HNGS) used in Hole 1225A, see "Downhole Measurements" in the "Explanatory Notes" chapter) operate here in the lower range of their sensitivity. Because the HNGS was developed more recently and employs a larger and improved scintillation detector than the NGT, the readings in Hole 1225A should be more accurate. Consequently, the downhole increase in thorium detected at Site 1225 should be real, although it could not be detected during Leg 138.
The sediments at Site 1225 can be generally described by low natural radioactivity and physical attributes (density and resistivity) typical of high porosity. Both porosity logs and MAD measurements show that the porosity is consistently at ~70%, explaining the generally low density and resistivity. The low gamma ray values are typical of the calcareous sediments present at Site 1225. Because of the fairly uniform lithology indicated by consistently low gamma ray values, our characterization of the formation from the logs is based on the physical measurements that show the clearest distinction, specifically the resistivity and the density. Because of the limited variations in all data, we consider that the sediments penetrated by Hole 1225A correspond to only one logging unit; however, minor covariation differences allow us to distinguish five subunits (see Fig. F32).
Logging Subunit 1A (80-150 mbsf) is characterized by generally higher gamma ray and uranium readings and by lower resistivity and density than the underlying sediments.
Logging Subunit 1B (150-205 mbsf) displays generally higher resistivity and density values than the surrounding formations. Its top and bottom are defined, respectively, by a sharp increase in resistivity and density at 150 mbsf and by a similar decrease at 205 mbsf. The bottom of this subunit corresponds to the bottom of lithostratigraphic Subunit IB, which is characterized by an increase in diatom ooze (see "Description of Lithostratigraphic Units" in "Lithostratigraphy").
In logging Subunit 1C (205-255 mbsf), resistivity and density decrease steadily with depth, whereas logging Subunit 1D (255-272 mbsf) consists of a distinct ~20-m-thick layer with higher resistivity and density indicative of a more cohesive and possibly more lithified unit. Core description showed that logging Subunit 1D has a higher proportion of nannofossil ooze to diatom ooze in Core 201-1225A-29P and Cores 201-1225C-28H and 29H. Core recovery in both Holes 1225A and 1225C was also slightly lower over this interval, which is consistent with a higher induration.
The top of logging Subunit 1E at 272 mbsf coincides with the top of lithostratigraphic Subunit ID, which is defined by an increase in diatoms (See "Description of Lithostratigraphic Units" in "Lithostratigraphy"). This subunit is characterized by a steady increase in resistivity and density, with the highest values in Hole 1225A recorded below 285 mbsf.
Temperatures were recorded with the Lamont-Doherty Earth Observatory (LDEO) Temperature/Acceleration/Pressure (TAP) memory tool attached at the bottom of the triple combo string (Fig. F34). Because only a few hours had passed since the end of drilling operations and hole conditioning, the borehole temperature is not representative of the actual equilibrium temperature distribution of the formation. In the case of Hole 1225A, the warm (~25°C) surface seawater used during drilling generated borehole fluid temperatures higher than the formation temperatures. Discrete measurements made with the DVTP indicate a maximum formation temperature of 6.56°C at 320 mbsf (see "In Situ Temperature Measurements" in "Downhole Tools"), whereas the maximum temperature recorded by the TAP tool at the same depth is 7.90°C. Temperature variations in this record could, in principle, indicate possible fluid conduits intersected by the borehole. However, despite its coincidence with a relatively lower resistivity interval at ~180 mbsf, the ~1°C temperature increase at this depth is more likely the result of an incomplete "hole displacement" during hole conditioning before logging. Based on the bit diameter size, 130 bbl of mud was used to stabilize the hole for logging. Because of the enlarged hole conditions, with an average diameter of 42 cm, this volume filled only the deepest ~140 m of the hole. The observed temperature increase at ~180 mbsf is likely the result of the denser and warmer drilling mud cooling more slowly than the overlying borehole seawater. Because the temperature sensor is in an open compartment located at the bottom of the 35-m-long tool string, it is in the turbulence following the tool string on its way up. As a result, the temperature profiles recorded while logging uphole are almost uniform, as the string carried some of the downhole fluid and heat in its wake. By the time of the second pass down an hour later, the borehole fluids had recovered their gravitational equilibrium and the temperature increase at 180 mbsf is apparent again.