DOWNHOLE MEASUREMENTS

Operations

Four logging strings were run in Hole 1115C: one triple combo that logged from a total depth of 784 mbsf to above mudline, one FMS-sonic run that logged from total depth to the bottom of the pipe, an ultrasonic borehole imager string that failed downhole and did not record any useful data, and a VSP run (Table T15). The VSP data and operations are described in the section, "Vertical Seismic Profile and Depth Conversion" and this section concentrates on the other runs.

For the first run, the pipe was raised to 99 mbsf, and the triple combo with the dual induction tool (see Table T7  and Fig. F15,  both in the "Explanatory Notes" chapter) was lowered downhole. Natural gamma ray logging (HNGS) data were monitored during the descent in the pipe to locate the mudline, where the string was stopped for 3 min to provide a depth reference for the temperature-logging tool (TLT) temperature log. Upon reaching open hole, HNGS and dual induction tool (DIT) data were logged during the descent to near total depth at 784 mbsf (Table T15). An upward log was then recorded at 300 m/hr. The pipe was raised to 79 mbsf, and the tool string was stopped at the mudline for a few minutes both to run calibrations and to give the TLT data a depth reference, after which logging continued up to 38 m above seafloor.

The caliper log from the lithodensity tool (Fig. F59) revealed a narrow interval at 567 mbsf where the borehole diameter was 13 cm (5 in) and smaller than drill-bit size (25.1 cm [9 in]). Log quality is degraded in areas of enlarged diameter and rapidly changing hole diameter (see "Downhole Measurements"  in the "Explanatory Notes" chapter). In Hole 1115C, the caliper from the triple combo run reached maximum extension between 618 and 624 mbsf and intermittently between 508 and 566 mbsf (Fig. F59).

For the second run the pipe was lowered again to 99 mbsf, and the FMS-sonic string (see Table T7  and Fig. F15,  both in the "Explanatory Notes" chapter) was lowered downhole. The string reached near total depth at 787 mbsf (Table T15), and a first pass was recorded up at 300 m/hr. The real-time FMS images seemed to indicate an irregular tool motion, and it was thought that this could be caused by poor synchronization of the heave compensator. The pass was therefore aborted at 726 mbsf, the tool brought back down to total depth, and the heave compensator stopped and restarted for a second pass. In fact the image quality of this second pass was similar to that of the first pass, and the log was recorded up to 152.4 mbsf. The tool was then lowered to total depth again for a third pass to increase the borehole coverage of the images. The imaging log ended at the bottom of the pipe, which was raised to 79 mbsf (Table T15) where the pads were closed, but NGR continued to be recorded up to 42 mbsf.

The ultrasonic borehole imager string was then sent downhole, but it could detect neither hole fluid sonic velocity while going down in the pipe nor borehole wall reflections upon reaching open hole. The log was then aborted and the tool string brought back to the rig floor.

Depth Shifts

The mudline wireline depth (Table T15), which defines the depth shift from mbrf to mbsf, was located by its associated gamma-ray decay for the triple combo run (Fig. F60). The depth shift for the two FMS-sonic passes was derived by correlating the NGR with that of the triple combo run within the 534-578 mbsf interval, which was chosen for its large characteristic variations (Fig. F61). It appears that differential cable stretching can result in depth mismatch up to 0.5 m in a few places (such as at 555 mbsf; Fig. F61).

Lithologic Analysis

The interpretation, based mainly on NGR, the relationship between the neutron (APLC) and density porosity (DPHI), and the PEFL (see "Downhole Measurements"  in the "Explanatory Notes" chapter), and the results of core analysis (see "Lithostratigraphy"), defines nine logging units (Fig. F62).

The neutron porosity is larger than the density porosity, which indicates a significant clay content. The logs show little indication of coarser grained beds.

The clay content is still large, but there are many short intervals where the two porosity logs converge, indicating thin "clean" beds. In two such beds at 246 and 248 mbsf, high PEFL and high density suggest high calcium carbonate content, which is confirmed by high calcium carbonate in core samples (see "Organic Geochemistry"). A third thin bed at 292 mbsf exhibits very high density and PEFL of about 4 barns/e-, which suggests dolomite. The presence of dolomite is confirmed by XRD analysis (see "Lithostratigraphy"). High calcium carbonate content is indicated by chemical analysis (see "Organic Geochemistry"). The thin bed at 292 mbsf corresponds to a high sonic velocity of 5.2 km·s^-1 measured on core samples (see "Physical Properties") but not observed on the sonic log, and to a hole diameter decrease. It is also located at the depth where it became necessary to abandon XCB coring for RCB coring.

A PEFL increase indicates higher calcium carbonate content than in log Unit L2, but the unit is generally not clean as shown by the porosity divergence. However, several coarser grained intervals exist where the porosity curves converge. Bulk density also increases slightly at the boundary between log Units L2 and L3.

This unit does not wash out and shows slightly elevated PEFL because of calcium carbonate content. The relationship between neutron and density porosities is variable and suggests a thin-bedded variable lithology that includes sands, clays, and mixtures of both. Uranium content is the highest of any unit, indicating reducing conditions at the time of deposition.

Like log Unit L4, this unit has a high uranium content (a consistent 5 ppm), which indicates a reducing environment, but it shows a series of washouts that affect the porosity measurements. Where the data quality is good, the convergence of neutron and density porosities, coupled with low PEFL, indicate a primarily sandy formation. The consistent total gamma-ray character suggests that the washed out intervals are similarly sandy.

This unit shows a series of washouts, some correlating with gamma-ray lows. Only one small section from 553 to 557 mbsf yields good porosity data. The difference between the neutron and density porosities suggests that it is a clay. It has a moderate gamma-ray magnitude. The gamma-ray lows are up to 2 m thick and are unusual in that they have low resistivity and low sonic velocity. Thin streaks of lignite were observed in the core (see "Lithostratigraphy"); one can speculate that these gamma-ray lows are large beds of ligneous coal or peat. The upper part of this unit shows an increase in uranium, which is likely to indicate increasingly reducing conditions upsection. The washed-out nature of the upper section, combined with generally increasing total gamma-ray magnitude, suggests a predominantly sandy formation. It is noteworthy also that the Th/K ratio is ~4 ppm/%, a value commonly observed in sands at Site 1109 (see "Downhole Measurements" in the "Site 1109" chapter).

The FMS image clearly shows this unit to be a conglomerate with resistive cobbles up to a few centimeters in size. The high bulk density and low natural gamma-ray magnitude suggests that the clasts are intermediate or mafic igneous rocks, and the response of the logs bears similarity to the dolerite conglomerate observed in Hole 1109D (see "Downhole Measurements" in the "Site 1109" chapter). In this 2-m interval, a large increase in porosity, along with a swollen formation evidenced by the caliper, suggest that the matrix has changed from a coarser grained matrix to predominantly clay in this interval. A conglomerate with apparently basaltic clasts and predominantly sandy matrix was observed in the cores at this depth (see "Lithostratigraphy").

There are two similar sequences within this unit, the lower boundary of which is most easily recognized by an increase in bulk density. The lower sequence begins with a few meters containing thin streaks that are resistive and have a high PEFL, suggesting that they are calcium carbonate dominated. Above this, improved convergence between neutron and density porosities, decreasing PEFL, and increasing NGR magnitude indicate that sand content increases upward. The top of the first sequence at 619 mbsf is almost pure sand. The second sequence is much thicker, with a more homogeneous massive carbonate at the bottom. Again, the formation grades upward into a thick radioactive sand. It is interesting to note that in these sands, the Th/K ratio is about 2 ppm/%, which is lower than that of any other sands observed during this leg. The presence of potassium minerals, possibly muscovite or K-feldspar, is therefore suggested.

This unit appears to be rich in clay, but also has significant calcium carbonate content inferred from the moderately high PEFL. The relatively low total NGR may result from this calcium carbonate content. Several short intervals (<10 cm to 4 m) are found where density and neutron porosities converge, suggesting a "clean" lithology. Some of the thinner instances show a slight PEFL increase, indicating a possible calcium carbonate dominated bed rather than sand or silt, which appears more commonly.

Borehole Geometry, Magnetic Field, and FMS Dynamics

The accelerometer data of the FMS show that the hole deviation remains below a half degree (Fig. F63). They also show acceleration magnitudes (Fig. F64) that deviate by 0.2 m·s^-2 from the average, which is twice the amount of a good quality run (such as the lower run in Hole 1109D; see "Downhole Measurements" in the "Site 1109" chapter). This translates into irregular tool movements and lower quality raw images, as observed on those displayed during logging.

The FMS caliper data reveal three washout intervals where the natural tool rotation slows down, at 120-290, 500-565, and 590-670 mbsf (Fig. F65). The steady tool orientations cluster around the north-south and east-west directions, suggesting an oriented hole ellipticity. To further investigate this trend, the difference between the two caliper measurement (C1 and C2) and the orientation of the largest of the two diameters are displayed in Figure F66. The orientations are displayed only if the caliper difference is above a threshold of either 2 or 5 cm.

Between 120 and 290 mbsf, washouts are probably caused by the circulation that is maintained when the drill bit progression halts during core retrieval, because the spacing between the washouts corresponds to the core length of ~10 m. Because both caliper readings are above drill-bit size, these elongations correspond to a preferential orientation of the washouts. The small elongations in the 135-250 mbsf interval strike northwest, and their orientation becomes erratic in the 230-290 mbsf interval.

The thick washed-out intervals between 500 and 565 mbsf (Fig. F67) are sufficiently large so that one or even both of the caliper measurements are saturated. The orientation of the elongation generally trends north-south, but with a significant deviation. Further detailed analysis will need to separate measurements where both calipers are saturated and the orientation is, therefore, meaningless, from those where only one caliper is saturated and the orientation is meaningful.

The thinner elongations observed between 590 and 675 mbsf (Fig. F68) are present in intervals where the smallest caliper reads close to drill-bit size (25 cm [9 in]). One of these elongations, seen at 595 mbsf, must be restricted in azimuth because it is almost missed by the second pass. It also prevents rotation during the third pass. The thicker elongation at 618-625 mbsf also locks the tool rotations. In all cases the elongations are oriented north-south. These elongations, therefore, satisfy most of the breakout criteria (see "Borehole Geometry" in "Downhole Measurements" in the "Explanatory Notes" chapter). They could be related to structural directions (faulting or bedding directions), or to a north-south minimum horizontal principal stress direction (i.e., to a north-south active extension). The main goal of the failed ultrasonic borehole imager logging run was to further investigate the shape of these elongations.

The magnetometer measurements yield an inclination around -30° (Fig. F69A). The scatter at the bottom of the hole might partly be caused by poorer tool centralization in the washouts. Alternatively, it may correspond to a real change because it correlates with both the L8/L9 log unit boundary and an intensity increase (Fig. F69B).

FMS Images

The two FMS images acquired from Hole 1115C are rotated by as much as 45° with respect to each other for about one-fifth of the two passes (Fig. F65), which provides improved coverage of the borehole in these intervals. The FMS processing steps for this site included speed correction, depth shifting to seafloor (Table T15), and static and dynamic normalization using a 1-m moving window. A smaller cable confidence factor of 4, instead of the more commonly used value of 5, was applied during speed correction to allow for the large amount of tool sticking that occurred during the two passes. (The cable confidence factor is used in the design of relative weights in a Kalman filter, which is applied to find the best estimates of tool speed and depth that are consistent with cable depth and the three-axis magnetometer-inclinometer logging tool (GPIT) z-axis accelerometer data.) Tool sticking resulted in often large, variable vertical depth shifts between the two passes after speed correction, and thus, the data require additional postcruise processing.

Hole 1115C is characterized by generally uniform, flat-lying (<5°) clayey beds with thin (5-10 cm) resistive sandy or calcium carbonate-rich layers overlying more thinly layered beds that correlate to the late Miocene forearc sequence (see "Lithostratigraphy" and "Biostratigraphy"). Many of the log units defined on the basis of the triple combo logs are not very distinctive with respect to each other in the FMS images; therefore, several of the units are grouped together in the discussion below.

These units are characterized by uniformly low resistivity with minor occurrences of thin (<5 cm) more resistive layers that correspond to calcareous silty clays and volcanic ash layers observed in core samples (Fig. F70; see "Lithostratigraphy"). An increase in frequency of presence of thicker (5-10 cm), resistive beds is found near the top of log Unit L2, ~153 mbsf, and continues downward throughout log Unit L2. The resistive beds commonly have sharp boundaries against the surrounding, more clayey material.

Large-scale resistivity varies within log Units L3 and L4 and increases with depth. Few resistive beds are present, and those that do occur show a consistent thickness of ~10 cm. The unit displays a grainy texture in the FMS images (Fig. F71) that is commonly observed within calcareous units at this site, as well as in Hole 1109D (see "Downhole Measurements" in the "Site 1109" chapter). Bedforms are deformed to nonexistent within this interval, which may be caused by bioturbation (see "Lithostratigraphy"). Thin, resistive layering is more frequent toward the base of log Unit L4, suggesting less bioturbation.

The FMS image quality within log Units L5 and L6 was degraded by poor pad contact with the borehole because of numerous washouts as shown in the caliper log (Fig. F59A). The few well-imaged intervals appear similar to the clayey units above, contain thin resistive sandy or calcium carbonate-rich layers, and possess a grainy texture characteristic of calcium carbonate-rich sediments in this area.

The top 2 m of log Unit L7 are composed of coarse-grained, low-resistivity material overlying highly resistive, wavy layers at 565.5 mbsf. The resistive layers are interbedded with conglomeratic material composed of a moderately resistive matrix (Fig. F72). Unit L8 consists of north-northwestward-dipping (5°-10°), thinly layered (<0.5 cm) beds, the top of which are interpreted to approximately correlate with the late Miocene forearc sequence at 573.5 mbsf (see "Depositional History" and "Biostratigraphy"). Beds within log Unit L8 occasionally show cross lamination. At 605 mbsf, layering abruptly terminates, and the sediments appear structureless toward the base of Unit L8.

The top of log Unit L9 is cut by steep (~70°), south-southwest-dipping fractures between 626.5 and 631 mbsf (Fig. F73), and corresponds to a zone of low core recovery (Cores 180-1115C-36R through 39R). Below 631 mbsf, the unit appears structureless and only occasional resistive beds show continuity from pad to pad. However, flat-lying beds are better defined toward the base of log Unit L9, particularly where bright, resistive beds are clearly imaged across all four (or eight) FMS pads. Thin, near-vertical, low-resistivity lines commonly are present in the images within log Unit L9 and appear to be artificial gouges in the formation caused by the logging tools during previous passes or may represent material (e.g., mud, pebbles) that was dragged along by the FMS pads (Fig. F74).

Temperature Data

The temperature profiles within the pipe and in open hole are shown in Figure F75. The temperature at the bottom of the hole (785 mbsf) reaches 12°C, but this is not representative of in situ temperatures. No evidence for influx of warm fluids can be seen in the profile. Mudline temperature was measured to be 4.3°C. This value is lower than those measured during the Adara temperature tool (Adara) and Davis-Villinger temperature probe (DVTP) runs (see "In Situ Temperature Measurements"), which were suspected of being erroneously high.