DOWNHOLE MEASUREMENTS

Before logging at Hole 1186A, we flushed the hole with sepiolite (Table T15). The usual "wiper trip" with the drill string was not conducted because of the softness of much of the sedimentary column. We retracted the drill string to 123 mbsf for logging. Throughout logging operations, sea conditions were extremely calm with a swell of 1-2 m, and the wireline heave compensator was working normally. All logging runs were made at 274 m/hr, the standard rate used for high-resolution logs.

We logged Hole 1186A using two tool strings: the geophysical (resistivity, neutron porosity, density, and natural gamma) and the FMS/sonic tool string. During the first logging run, the geophysical tool string descended to the bottom of the hole without difficulty. Logging proceeded upward from 1034 to 679 mbsf, slightly above the top of the cored interval. Logging was then stopped, and the tool string was pulled up to just below the base of the pipe. Gamma ray logging was resumed from 162.6 mbsf to 16 m above the seafloor, to provide a log for depth matching of wireline depth and drillers' depth, for which both the bottom of the pipe and the seafloor are used as reference points. We then recovered the tool string.

On the second logging run, the FMS/sonic tool string descended to within 1 m of the bottom of the hole without difficulty. Logging proceeded upward from 1033 mbsf, with the FMS gain setting at 3 and the sonic tool operating in compressional and shear wave (P&S) modes. At 948 mbsf we abruptly lost communication with the tool string. The tool string was shut down, reinitialized, and lowered back to the bottom of the hole to restart the logging run. Noting that the FMS data were close to detector saturation, we lowered the FMS gain to a setting of 2. The sonic tool was set to lower dipole as well as P&S mode, with four-waveform stacking.

The second FMS/sonic run proceeded smoothly from 1032.7 to 690 mbsf, near the top of the cored interval, after which the tool string was lowered back to the bottom of the hole for a second complete pass. The FMS gain was set at 2, and the sonic tool was set to lower dipole and P&S modes. The third FMS/sonic pass covered the interval from 1033 to 975 mbsf, following which the tool was pulled back to the ship.

The caliper log shows that the borehole was very regular and 30 to 36 cm (12 to 14 in) in diameter in basaltic basement (Fig. F56). The borehole widened slightly in the Subunit IIIB Aptian-Albian limestone, was washed out to beyond caliper range (48 cm or 19 in) throughout the nannofossil chalk of Campanian-Maastrichtian Subunit IIIA, and was ridged or corrugated but stayed within caliper range in the interbedded Paleocene-Eocene limestone and chert. Because the FMS and nuclear tools must be in contact with the borehole wall to acquire reliable data, borehole diameter and smoothness are the primary factors influencing data quality. Accordingly, data quality of all logs is very good in basement and the lower sedimentary subunit, extremely poor throughout the nannofossil chalk, and good in the upper limestone and chert. The resistivity, porosity, and density log responses correlate closely with lithologies observed in the cores, and FMS images in the basement section are excellent. Despite changes of gain settings, FMS data from all passes are good, showing many features seen in the cores. However, the FMS pads appear to have followed almost identical tracks in the different logging passes, so the increased coverage of the borehole wall usually obtained by conducting several passes was not realized.

The match between wireline depths and drillers' depths is uncertain, in part because low gamma ray levels made a precise pick of the mudline difficult. For the geophysical tool string run, we applied a 4-m downward depth shift to wireline depths. This value was the median between the offsets observed at the mudline, the bottom of the pipe, and the basement interface. The FMS logs were all shifted downward by 0.6 m, corresponding to the offset between the transition to basement observed on the third FMS pass and the drillers' basement depth. Throughout the logged interval, the offset between different FMS passes shifted up and down by as much as 1 m, reflecting incomplete compensation for sticking and tool string accelerations in the shipboard image processing. Notably, this effect results in an offset of 1 m in basement depth between the second and third FMS passes, although depths between these passes generally agree through other parts of the logs. More sophisticated processing at Lamont-Doherty Earth Observatory should reduce the effects of this problem.

Sedimentary Unit II (Paleocene-Eocene) is characterized by alternating layers of limestone and chert (see "Lithostratigraphy"). The most notable feature in this interval is a thick accumulation of chert between 724 and 728.3 mbsf, which is clearly visible on FMS images (Fig. F12) and shows a strong signature on the sonic velocity, electrical resistivity, density log, and porosity log (Fig. F56). Thinner interbedded cherts throughout the remainder of Unit II are evident on FMS images (Fig. F13) and on porosity and density logs, causing variations between ~40% and 70% in the former and between 1.8 and 2.0 g/cm³ in the latter. Variations in electrical resistivity and sonic velocity caused by these layers are minor, as these logs represent the integrated response of physical properties over an interval much larger than chert layer thicknesses. Bulk resistivity is generally <1 m, and sonic velocity is in the 2300-2500 m/s range. Gamma ray levels remain uniformly low (<10 gAPI) and show no distinctive patterns throughout this unit.

As noted above, the quality of logs throughout the nannofossil chalk of Subunit IIIA (Campanian-Maastrichtian) is very poor as a result of the enlargement of the borehole. The only logs that we consider at least qualitatively reliable in Subunit IIIA are the electrical resistivity and sonic logs, both of which respond to formation properties well beyond the borehole walls, and are therefore less sensitive to borehole enlargement. These logs indicate a remarkably homogeneous subunit without distinguishing features, with bulk electrical and sonic properties differing little from those of Unit II. A peak in gamma ray emission is notable between 840 and 850 mbsf, and, at greater depths, gamma ray levels are generally higher than they are above this level (Fig. F56). Assessing whether these features are real or not is difficult. Nuclear measurements are particularly susceptible to degraded borehole conditions, and the features shown in Fig. F56 are not reproduced on the gamma logs made by the natural gamma spectrometry tool (NGT) on the FMS/sonic tool string. However, the peak between 840 and 850 mbsf is not easily explained on the basis of noise or tool motion, and the hostile-environment natural gamma ray sonde (HNGS) detector on the geophysical tool string with which this measurement was made is much more sensitive than that used on the FMS/sonic tool string. If the peak is real, it probably represents a layer rich in organic matter.

Physical properties change markedly in the limestone of Subunit IIIB (Aptian-Albian). The top of this subunit is marked by a peak in gamma ray emission at 933 mbsf, which may indicate an ash or volcaniclastic deposit at the subunit boundary. Another such peak is observed in the middle of this unit at 953 mbsf. The unit is characterized by an overall increase in sonic velocity to ~3300 m/s, in electrical resistivity to between 2 and 3 m, and in density to between 2.2 and 2.3 g/cm³. Porosity decreases to ~20%, and the porosity and density curves cross. FMS images show tightly banded layers throughout this unit (Fig. F24).

The sediment-basement contact is clearly visible on FMS images (Fig. F57). The thin conductive layer immediately overlying basement correlates with the brown claystone cored in this interval (see "Lithostratigraphy"). The upper meter of basement appears to be pillows with some interbedded sediments, but there is a rapid transition to what may be either a more massive interval or a very large pillow. Microresistivity in the upper 2 m of basalt is the highest observed in the entire basement section. This may reflect alteration and cementation of pores as a result of weathering of the seafloor. As expected, the transition from sediment to basalt is marked by abrupt increases in sonic velocity, electrical resistivity, and density and a decrease in porosity (Fig. F58).

The broad features of basement are most apparent from a small-scale plot of the FMS log (Fig. F59). Pillows can be seen between 971 and 974 mbsf and between 976 and 981 mbsf (Fig. F60), characterized by their rounded edges and by the presence of large interstitial zones of low-resistivity fill. Distinguishing on FMS images whether the intervals between 969.5 and 971 mbsf and between 974 and 976 mbsf are large pillows, pillow lobes, or thin massive intervals is difficult, mirroring the same uncertainty over these intervals in core observations (see "Igneous Petrology"). A general increase in gamma ray emission from the top of basement to 983 mbsf corresponds to variability in sonic, resistivity, porosity, and density logs throughout this interval, which may suggest that it consists of pillow lava. Interpillow material is apparent in several thin intervals of low resistivity, which have resistivities comparable to those of sediment immediately overlying basement. This interpillow material is particularly notable at 972.8 mbsf, which may imply an eruptive hiatus at this depth. The next such break is observed at 979.5 mbsf and appears to correspond to the carbonate, clay, and glass breccia observed in the core at 980 mbsf (see "Igneous Petrology"). The transition from pillowed to massive basalt, observed in the core at 981 mbsf, is also apparent in the FMS images. Note that the standard logs suggest a gradation of properties at the top of the unit because the sonic, resistivity, porosity, and density logs all flatten out at 984 mbsf.

Figure F61 shows the interval from 979 to 990 mbsf. A massive flow seems to be present between 981 and 988 mbsf, with pillows at its base (988-992 mbsf). Between 992 and 997 mbsf, the images suggest alternating layers of pillows and interbedded sediments (Fig. F62). On the conventional logs, these alternations have a strong signature on electrical conductivity, density, and porosity; gamma ray emissions associated with this interval are elevated. Erosion of the borehole in intervals containing sedimentary interbeds is evident from the caliper track, which also indicates the softness of such material.

Another unit appears at 997 mbsf on the FMS log. Apparently, in both FMS data and on the resistivity, porosity, and density logs, the top of this unit is fractured and it is only from 1003 to 1006 mbsf that its properties appear relatively homogeneous (Fig. F63). Below 1006 mbsf, there is a transition from massive basalt to pillows with interpillow material. At 1009 mbsf is a conductive layer which appears to be another sedimentary interbed. From 1009.5 to 1011 mbsf are pillows, beneath which is a massive interval that extends to the bottom of the cored section (Fig. F64).

Distinct electrical resistivity variations are evident within the lower massive basalt. Resistivity is high from 1011 to 1013 mbsf, decreases from 1013 to 1016 mbsf, and increases again from 1016 to 1019.5 mbsf. From this point to the bottom of the cored interval, resistivity generally decreases. These observations closely match alternations of grain size observed in core (see "Igneous Petrology"). The high-resistivity sections correspond to the mottled basalt observed to result from varying proportions of fine-grained and aphanitic patches. Lower resistivities correspond to an increase in grain size. Finer-grained material will have a more tightly packed matrix with less, and less connected, pore space for ion transport via pore fluids. The upward-trending porosity between 1017 and 1028 mbsf (see "Physical Properties") is accordingly reflected in the decreasing microresistivity evident in the FMS data over this interval. Note that as FMS measurements are sensitive to microresistivity variations, they are less influenced by large-scale fracturing than are conventional electrical measurements.