After reaching a depth of 158 mbsf on 9 May, preparations for logging began immediately (Table T16). The hole was conditioned and filled with freshwater gel mud with a density of 8.8 lb/gal, then the bit was released at the bottom of the hole. The pipe was pulled to 35.7 mbsf, and four tool strings were deployed on 10 May. The first tool string deployed was the sonic digital tool (SDT), dual induction tool (DIT), natural gamma-ray tool (NGT), and the temperature-logging tool (TLT). The second logging run consisted of the Formation MicroScanner (FMS) and NGT. The third tool string contained the hostile lithodensity sonde (HLDS), accelerator porosity sonde (APS), and the hostile natural gamma sonde (HNGS). The fourth and final tool string consisted of the borehole compensated sonic (BHC) and NGT. During each logging run, the wireline heave compensator (WHC) was turned on following the exit from pipe and used continuously while the tool strings were in open hole. Additionally, the drill pipe was pulled to a depth of 19.9 mbsf before tool string reentry into pipe during each run. Following data acquisition, log data was transmitted to the Lamont-Doherty Earth Observatory (LDEO) for depth and environmental correction processing and returned to the ship for use in this site chapter report.
The tool string was assembled and lowered through the drill pipe at 10,000 ft/hr until the seafloor was reached at 714 mbrf. Following a 2-min interval of temperature tool equilibration, the tool string was lowered through the end of the drill pipe. The tool string was lowered to the total depth (TD) of 157 mbsf without difficulty and then pulled upward at a constant rate of 1800 ft/hr. During the first pass, the DIT and NGT yielded good data, whereas the SDT produced noisy data likely resulting from the excessive energy generated as the tool slid on the hard formation. In an attempt to improve the SDT signal, a second pass was made with the digital transmitter turned off and only the analog transmitter firing. The tool string was lowered to TD, logged upward to the seafloor depth, pulled from the hole, and dissembled. The SDT results were not improved during the second pass.
The NGT and FMS tool string was assembled and at the bottom of the hole in <3 hr after logging operations began. The tool string was pulled to the surface at the logging speed of 900 ft/hr for the first pass and 1800 ft/hr for the second pass. Maximum FMS gain settings of three were selected in anticipation of extremely resistive rocks. It was evident during both passes that excellent data were being acquired as many boundaries, fractures, and borehole features were clearly imaged. Furthermore, the logging speed of 1800 ft/hr did not have any affect on the FMS data, although the NGT data resolution may have been slightly degraded. Following the conclusion of the second pass, the tool string was pulled to the surface and disassembled.
The gamma, porosity, and density tool string was run in the hole to TD and pulled upward at 900 and 1800 ft/hr during the first and second runs. During the first run, the gamma and porosity data appeared to be good, but the HLDS developed an intermittent data writing error. To correct the problem, the tool was lowered to the bottom of the hole, and a second pass was started with the APS powered down and a new surface telemetry control panel installed. The density tool response was normal for the duration of the logging run and good data were acquired.
A fourth run was attempted as a result of additional rig time becoming available. The BHC and NGT tool string was selected to acquire additional sonic data in light of the poor sonic data acquired by the SDT during run 1. One pass was made with the tool string from 157.6 to 11.3 mbsf and logged upward at 1200 ft/hr. Log data appeared to be of good quality during data acquisition. The high frequency noise and cycle skipping observed in the SDT measurement during run 1 were absent.
A selection of logs acquired during Leg 179 is presented in Figures F77, F78, and F79. Their uppermost limit corresponds to the pipe depth and is indicated by the dashed horizontal line. Their lowermost limit represents the deepest depth achieved by the various tools (Fig. F13 in the "Explanatory Notes" chapter). The diameter of Hole 1105A, as measured by the HLDS mechanical caliper (LCAL log, Fig. F77), varies between 10.0 and 14.2 in, with the largest diameters at 82.4 and 99.8 mbsf. The two caliper logs from the FMS illustrate two orthogonal dimensions of the borehole with depth. At 40.4, 80.6, and 98.6 mbsf, the borehole appears to be slightly elliptical in cross section with the difference between the two orthogonal measurements (logs C1 and C2) remaining under 1.6 in.
A general purpose inclinometry tool (GPIT) is routinely deployed with the FMS to provide accurate positioning of the string and to allow subsequent depth and azimuthal corrections on the acquired data. The GPIT utilizes a three-axis inclinometer and a three-axis magnetometer for the orientation and acceleration measurements. The borehole vertical deviation, as measured by the GPIT, remains under 3.5º (DEVI, Fig. F79), and the absolute value of the magnetic field inclination fluctuates between 57.7º and 63.3º with a mean value of 60.6º. The field intensity is 0.37 A/m. Unlike the vertical hole deviation and magnetic field inclination curves, the hole azimuth logs (HAZI) do not overlap because both the acceleration and the magnetic field are used for the computation. HAZI fluctuates between N7E and N21E. The abundance of magnetic minerals in Hole 1105A likely has an influence on the azimuth measurements obtained with the GPIT magnetometer. Acute variations in the HAZI log correlate with small azimuthal shifts of the FMS image (<3º) and are within intervals <0.5 m long. However, considered on a 2-m scale and larger, the FMS orientation remains nearly constant and does not appear significantly affected by layers containing magnetite. Locally high abundances of magnetite induce only a slight fluctuation in the orientation of the local magnetic field, and the effect on the FMS pad orientation is likely to be limited. At this time, it is not possible to certify that the tool maintained true north as its reference when entering the borehole. Additional studies will be made postcruise to determine the true extent of mineralogic influence on the FMS azimuthal reference.
Electrical resistivity measurements
and images were obtained with the DIT and FMS, which record three different electrical
logs and one type of formation image. The shallow (SFLU), medium (IMPH), and deep (IDPH)
measurements given by the DIT are the only quantitative assessments of the formation's
resistivity. Resistivities exhibit saturation where formation resistivity values are
>1950 
m for IMPH and IDPH, and
>9700 m for SFLU. Minimum
values for SFLU, IMPH, and IDPH are 4.1, 5.4, and 7.6 m, respectively. Further deconvolution was performed by the
Schlumberger engineer to enhance the logs' quality and vertical resolution. This
additional phasor processing did not improve the abrupt variations and apparent saturation
of the logs. Because the DIT was intended for use in lower resistivity rocks, the logs for
this tool should be used with caution.
The quality of the FMS images from both passes is very good. As suggested, flushing the hole with resistive fresh water mud yielded an improved FMS response compared to the one observed during Leg 176 logging operations. The poor quality FMS images acquired during Leg 176 were attributed to the excessive resistivity contrast between the borehole fluid and the resistive rocks. If the resistivity contrast between the formations and the mud is too large, the FMS current will tend to flow into the borehole fluid rather than into the formation. Some vertical streaking in the FMS image occasionally is present at certain depths (e.g., pads 2 and 3 at 87 mbsf or pads 1 through 3 at 68 mbsf). This may be caused by mild fouling of some buttons by sticking mud. Additionally, a preferential path followed by the FMS appears to guide the pads. Below 83 mbsf, pad 1 of the second pass overlaps the measurement recorded by pad 3 of the first pass exactly. Then, between 80 and 83 mbsf, the FMS (second pass) rotated 90º clockwise (the caliper shows an increase of the diameter at this depth). Above 80 mbsf, pad 1 of the second pass overlaps the measurement recorded by pad 4 of the first pass (Fig. F79, track 5). Also note that the azimuth reference kept by the tool between 80 and 85 mbsf is the same within a few degrees (HAZI 1 and HAZI 2 logs). This reference azimuth should not be confused with the rotation of the tool itself, given by pad 1 azimuth (P1AZ).
FMS images of Hole 1105A show layers of resistive rock with 1- to 6-m-thick conductive material at irregular intervals. Furthermore, resistive zones are intersected by a number of thin conductive features whose dip and approximated azimuth can be determined (e.g., Fig. F80). The conductive areas correlate quite well with the oxide and olivine oxide gabbro lithologic units defined in the core description. Conversely, the resistive intervals correspond to gabbro and olivine-bearing gabbro.
The spectral gamma-ray logs were measured with both the NGT and HNGS tools. These tools have been used with the four logging strings for lithologic and depth correlation between the acquired logs. Despite the low overall radioactivity of the rocks encountered at Site 1105, the correlation between the natural gamma-ray logs is very good. The profiles obtained during the first run (Fig. F77) are likely to be more reliable as they were acquired at the low logging speed of 900 ft/hr and, hence, provided an improved statistical measurement. The total spectral gamma-ray (HSGR) varies from 0.35 to 10.7 API units with a mean value of 4.1 API units.
The uranium component of the spectral gamma measurement is observed to be negative at certain depths in the log. The negative uranium values can be correlated to intervals in the spectral log where the computed gamma-ray log, which is a measure of thorium and potassium contribution to the natural radioactivity, contains values which are greater than the HSGR values. This response can likely be attributed to tool design criteria where extremely low gamma counts associated with igneous rocks were not anticipated. The most significant variation observed on the HSGR log is at 102.6 mbsf and correlates with significant tool responses in velocity, porosity, and density measurements.
The bulk density of the formation and the photoelectric factor (PEFL) were measured using the HLDS and are shown in Figure F78. Density values range from 2.6 to 3.4 g/cm3 with a mean value for the entire logged section of 3.0 g/cm3. The PEFL, which varies from 2.7 to 11.5 barns/e-, is often a good indicator of lithologic variations. For example, a surge of the PEFL at 61 mbsf corroborates a change in lithology from gabbro to oxide gabbro (see "Igneous and Metamorphic Petrology and Geochemistry"). Peaks in the PEFL correlate well with changes between gabbro and oxide gabbro lithologies throughout the log, but not all lithologic boundaries are detected by the photoelectric index. Density values from discrete laboratory measurements show a good correlation with log measurements except between 43 and 75 mbsf, where they deviate.
The APS records a near/array porosity (APLC) and a far/near porosity (FPLC). The APLC is the primary porosity measurement because it is less influenced by changes in the density of the formation. The APLC in the logged section of the hole is 3.7%. Two distinct maxima are reached at 102.3 and 112.6 mbsf (Fig. F78). Respective porosities are 20.3% and 18.1%. High values may correspond to borehole washouts or fractures in igneous rocks and generally correlate with low peaks in the density log. Local variations of the porosity log do not correlate well with discrete laboratory measurements. This may be caused by sampling bias because fractured intervals commonly result in poor recovery, and the rocks recovered have veins and fractures that are not often sampled for physical property measurements. The porosity log shows lower values and less variability at the depth intervals 48-62, 87-96, and 128-148 mbsf. The mean porosities and variances are 2.0% and 0.47, 1.6% and 0.13, and 1.5% and 0.84, respectively. They significantly vary in contrast with the mean porosities and variances at 19-48 mbsf (5.9% and 8.2), 62-87 mbsf (3.2% and 2.9), and 96-128 mbsf (4.7% and 10.0).
The APS sigma formation log (SIGF) measures the formation capture cross section of thermal neutrons. This measurement can be expressed in capture units and values for elements, minerals and rock types can be found in Schlumberger (1994; e.g., 7.0 for plagioclase feldspar, 21.5 for gabbro, 31.7 for olivine, and 112.1 for magnetite). The capture cross section is ~10 to 30 for most silicate minerals compared to >100 for Ti-Fe oxides. As expected, the correlation between SIGF peaks, and the conductive oxide-rich intervals of the FMS image are excellent. Figure F81 compares SIGF directly to the geochemical data to confirm this pattern.
A second sonic tool deployment was made at this site to compensate for the poor quality logs provided by the SDT. The BHC was run and fortunately gave much better results (Fig. F78). The BHC gives only the interval transit time, or Delta T (DT) required for a compressional sound wave to travel through 1 ft of formation between two transmitters and four receivers. The mean DT value in Hole 1105A is 49 µs/ft and the corresponding velocities range from 4.2 to 6.9 km/s. The mean value for Vp is 6.0 km/s. In the 20 to 30 mbsf range, the pipe interferes with the tool response and produces high amplitude variations of Vp. The Vp processed for borehole effects exhibits slight trends, which can be described by linear best fit curves on the depth intervals 47-97 and 105-150 mbsf. These curves have respective slopes of 10.6/s and 15.8/s, and the respective average velocity differences between the beginning and the end of the intervals are 0.53 km/s and 0.71 km/s. Microcrack propagation caused by lithostatic pressure released as the cores equilibrate to sea-level pressures should lead to lower values of Vp measured on board. This is not the case, but as for Hole 735B, the constant offset observed between logs and core data allows the logs to be used as proxies of the velocity through the formation.
Temperature measurements obtained with the TLT at Hole 1105A indicate an increasing hydrothermal gradient of 1ºC/100 m. Perturbations in the hydrothermal gradient, seen as rapid increases in borehole fluid temperature, are clearly observed at 46-49, 68-70, 88, 96, 102-104, and 136 mbsf (Fig. F77; Note: the temperature log was not reprocessed or depth-shifted at LDEO). These easily distinguishable perturbations likely are a result of pumping the homogenous freshwater gel mud into the borehole and the subsequent borehole equilibration. As the borehole equilibrated to hydrostatic pressure and normalized temperature, water flowing from zones of secondary porosity may have altered the temperature of the borehole fluid. The most notable example is at 102-104 mbsf, which represents a 0.6ºC increase in borehole fluid temperature. Other in situ measurements at this interval confirm the existence of an enlarged borehole, increased porosity, and lower velocity zone, and the FMS log indicates features that may be interpreted as fractures. Furthermore, core recovery in this interval was low, only pebble- or gravel-sized material was recovered. Drilling notes indicate that the rate of penetration (ROP) increased and the seismic while drilling (SWD) signal dropped sharply as well. A similar response by all indicators may be seen at 96 m.
