Previous   |    Next

doi:10.2204/iodp.proc.339.104.2013

Downhole measurements

Logging operations

Downhole logging measurements in Hole U1386C were made after completion of RCB coring to a total depth of 526 m drillers depth below seafloor (DSF). In preparation for logging, the hole was flushed with a 30 bbl sweep of high-viscosity mud and the bit was released at the bottom of the hole. The hole was then displaced with 172 bbl of barite-weighted mud (10.5 ppg), and the pipe was pulled up to 102.4 m DSF. Three tool strings were deployed in Hole U1386C: a modified triple combo, the FMS-sonic, and the VSI (Fig. F47; see also “Downhole measurements” and Table T6 in the “Methods” chapter [Expedition 339 Scientists, 2013b] for tool definitions). The triple combo was modified to include both the old phasor dual induction–spherically focused resistivity tool (DIT-SFL) and the new High-Resolution Laterolog Array (HRLA) in order to test the HRLA in low-resistivity formations (see Table T6 in the “Methods” chapter [Expedition 339 Scientists, 2013b] for tool definitions).

At 1935 h on 7 December 2011, the triple combo tool string (resistivity, density, and NGR tools) descended from the rig floor into the pipe (Fig. F48). When the tool string was just below the end of the pipe, the wireline heave compensator (WHC) settings were optimized to minimize downhole tool motion. The downlog proceeded at ~1000 m/h and reached the base of the hole at ~526 mbsf. The hole was logged up to seafloor at 275 m/h. The tool string returned for a short uphole repeat section from 227 to 103 m wireline depth below seafloor.

Rig-up of the FMS-sonic tool string started at 0245 h on 8 December. A downlog was taken at 730 m/h, from which we established the best settings for source frequency and automated P-wave velocity picking from the sonic waveforms. The tool string was blocked from further downhole progress by a bridge at 375 mbsf that had been observed in the triple combo run but had now fully closed up. Two uphole passes of the FMS-sonic tool string were run, Pass 1 to the seafloor and Pass 2 to the base of the pipe, both at 550 m/h. Rig-down was completed at 0930 h.

Marine mammal watch for the VSP started at 0800 h. The soft-start of the Sercel G. Gun parallel cluster (two 250 in³ air guns separated by 1 m) started at 0930 h. The air gun cluster was positioned on the port side of the JOIDES Resolution at a water depth of ~7 mbsl with a borehole offset of ~45 m. Before reaching the bottom of the hole, a short uplog was taken from 334 to 290 mbsf to ensure depth accuracy by comparing the shape of the VSI gamma curve to those of the other logging runs. The VSI tool string reached the blocked zone at 369 mbsf, a few meters shallower than the previous tool string. Obtaining a good clamp with the VSI caliper arm was difficult, probably because of the rugose borehole and soft formation, and consequently the received sonic waveforms were noisy. The initial plan was to make check shot stations at 25 m intervals, but because of the variability in borehole diameter, station depths were relocated to where the borehole was smooth and in gauge based on examination of the triple combo caliper curve. Only a fraction of the shots produced clean first arrivals, but enough first arrivals were taken at most stations to stack. A good first arrival pick in the upper part of the hole was especially difficult to obtain.

The log data quality was reduced by washouts and bridging of the borehole wall. Although the borehole had a baseline diameter (10–11 inches) close to the bit size, small- to large-scale washouts were pervasive throughout the logged section (Fig. F49). The 406–412 mbsf interval was washed out beyond the limit of the Hostile Environment Litho-Density Sonde (HLDS) caliper, and the narrow section at 376–383 mbsf found during the triple combo run subsequently blocked the hole for the next two tool strings. Resistivity, sonic velocity, and NGR logs were generally robust to changes in hole diameter, although there was some reduction in values in the larger washouts. Density was more affected, with lower density values than core measurements in washouts (Fig. F49). The photoelectric effect log has anomalously high values, especially in the washouts, because barium in the logging mud swamped the signal. However, barite-weighted mud was a necessity to keep the borehole open. The FMS resistivity images were also dominated by poor contact with the borehole wall in the washed out areas.

Differences were observed in the seafloor depth given by the step in the gamma radiation logs. The triple combo main pass found seafloor at 575 mbrf and FMS-sonic Pass 2 found it at 570 mbrf, compared to the drillers mudline at 573.2 mbrf (Hole U1386A). The differences were partly due to tides; sea level was at +0.8 m for the triple combo main pass, –0.8 m for FMS-sonic Pass 2, and –0.9 m for the drillers mudline in Hole U1386A (Fig. F48). Seas were calm (maximum peak-to-peak heave = 40–50 cm), giving little contribution to the offset, and the hole was relatively shallow, so cable stretch was not an issue either. Some variability is typical in choosing the exact point at which the gamma ray log steps up at the seafloor because it can appear as a gradual change. The remaining difference between the triple combo and FMS-sonic tool string seafloor depths can be attributed to the WHC. A ~43 cm difference exists between the upper and lower extents of the “home” window of the WHC flying head, which translates to a 2.6 m difference in cable length because of the six-wheel pulley system of the compensator. However, a reasonably good depth match between the open-hole NGR logs between logging runs and the core data was achieved using a seafloor depth of 573 mbrf for the triple combo tool string main pass and 570 mbrf for the second pass of the FMS-sonic tool string.

Logging units

Hole U1386C downhole logs have moderately high amplitude variability on a several-meter to submeter scale, and the character of the logs changes gradually downhole with no major steps in the base levels. Therefore, the entire logged interval was assigned to one logging unit (Fig. F49). At the scale of this unit, the NGR signal typically ranges from 50 to 70 gAPI and is dominated by the radioactivity of potassium and thorium, with uranium contributing a relatively minor component (Fig. F50). Potassium and thorium curves are closely correlated. Both elements are found in clay minerals. Quartz, calcite, and illite comprise as much as >80% of the sediment at Site U1386 based on bulk XRD analyses (see “Lithostratigraphy”), so the NGR signal is primarily tracking clay content (quartz and calcite contain no radioactive elements). Minerals like K-feldspar and mica also contribute to the NGR signal. Density and sonic velocity logs increase downhole, reflecting sediment compaction with depth. Low density values generally correspond to intervals with borehole enlargements and lower NGR values.

Logging Unit 1 was divided into three subunits based on changes in mineralogy inferred from the potassium log, a change in character of the density log, and the onset of lithification.

Logging Subunit 1A: base of drill pipe to 346 mbsf

The upper logging subunit is characterized by medium-amplitude cyclic swings in bulk density, NGR (including uranium, thorium, and potassium components), resistivity, and sonic velocity values. Several orders of cycles are observed, varying from 1 m to several tens of meters in thickness (Fig. F49). The potassium and thorium concentrations co-vary closely, suggesting that clay content controls these logs. The uranium concentrations generally vary independently or are anticorrelated. As expected from downhole compaction, the resistivity, density, and sonic velocity logs have an increasing downhole trend and are generally well correlated.

Logging Subunit 1B: 346–464 mbsf

Logging Subunit 1B is distinguished from the subunit above by its higher potassium content, which reaches a maximum of 2.2% at 382.5 mbsf (Fig. F50). The thorium content is relatively comparable to Subunit 1A, suggesting that overall clay content remains similar but additional potassium-bearing minerals are present (see “Lithostratigraphy”) (e.g., K-feldspar, mica, and glauconite, which contains potassium but only low levels of thorium compared to other clay minerals). Subunit 1B is also characterized by a reduction in the number of thin washouts, and consequently the short wavelength variability disappears from the density log, but the longer wavelength swings remain. NGR continues to show cyclic variability. A large decrease to anomalously low values in all logs between 406 and 412 mbsf is due to a 6 m thick washed out level, wider than the 18 inch limit of the HLDS caliper arm, that possibly reflects a loose coarse-grained interval (pebbly sand was recovered at the same depth in Section 339-U1386B-45X-CC). An interval of higher resistivity is observed from 422 to 425 mbsf and probably correlates to cores with a cemented sandy interval (Cores 339-U1386B-6R through 8R). Resistivity returns to lower values downhole and corresponds to unconsolidated intervals. The transition between logging Subunits 1A and 1B does not correlate with any major changes in sediment deposits or faunal assemblages (see “Lithostratigraphy” and “Biostratigraphy”). It is close to the top of the termination of the Jaramillo Subchron (C1r.1n), identified in Hole U1386A at ~344.6 mbsf (Section 339-U1386A-39X-4) and in Hole U1386B at ~347.9 mbsf (Section 339-U1386B-39X-6) (see “Paleomagnetism”), providing an age of ~0.988 Ma.

Logging Subunit 1C: 464–526 mbsf

Several intervals of high resistivity values are observed in Subunit 1C at 465–475 and 478–481 mbsf (Fig. F49). These high values likely represent the onset of more pervasive lithification and correlate with cemented bioclastic turbidites and debrites (Cores 399-U1386C-12R through 15R; see “Lithostratigraphy”). In addition to the presence of sands, the abundance of shell fragments in this interval may dilute the clay fraction and therefore contribute to the lower gamma content observed in the same interval. Potassium contents return to values similar to Subunit 1A. The transition between logging Subunits 1B and 1C corresponds to a major hiatus of at least 1.4 m.y. identified between Sections 339-U1386C-11R-CC and 12R-1 by biostratigraphic analysis (see “Biostratigraphy”).

Comparison of the HRLA and DIT-SFL resistivity logs

The Schlumberger HRLA resistivity tool (Griffiths et al., 2000) was run in high-resistivity ocean crust on the two most recent JOIDES Resolution IODP expeditions (335 and 336) but until now has not been run in unconsolidated sediments. Previous generations of laterolog, such as the Dual Laterolog, were not designed for low-resistivity unlithified sedimentary formations such as those encountered during IODP expeditions. However, the HRLA measurement range extends to low resistivities and therefore is likely to be suitable for IODP use. In Hole U1386C, we obtained both HRLA and DIT-SFL resistivity logs in order to compare absolute resistivity values, vertical resolutions, and effects of washouts.

Overall, the two sets of logs show the same resistivity trends and fluctuations in Hole U1386C (Figs. F49, F51). The higher vertical resolution of the HRLA is apparent in the deep-reading logs from both tools (deep resistivity [RLA5] from the HRLA and deep induction resistivity [IDHP] from the DIT-SFL; Fig. F49). RLA5 has a ~30 cm vertical resolution, compared to ~240 cm for IDPH, and IDPH follows almost exactly the lower envelope of RLA5 resistivity values. Zooming in on an example interval, 250–300 mbsf (Fig. F51), the shallow-reading logs from both tools have lower resistivity values than the deep-reading logs, as expected, because the shallow-reading logs sample proportionately more seawater and less formation than the deep-reading logs. For the shallowest reading logs (spherically focused resistivity [SFLU] from the DIT-SFL and RLA1 from the HRLA), RLA1 has very slightly higher values and a higher vertical resolution than SFLU. The separation of the shallow and deep HRLA log values is much reduced where the borehole diameter is in gauge (e.g., 285 mbsf) and expanded where the hole is washed out (e.g., 259 mbsf) or rugose (e.g., 268 mbsf), as is also the case for the DIT-SFL. The vertical resolution of the HRLA logs does not appear to drop off for the deeper reading measurements, as is the case for the DIT-SFL logs. The features in the HRLA resistivity logs were cross-checked against the velocity logs. The same features match very well in both (Fig. F51).

Formation MicroScanner images

Despite the rugosity of the borehole wall associated with the presence of washout intervals, FMS resistivity images reveal numerous gradual and sharp transitions between alternations of resistive and conductive beds larger than ~5 cm. Most of these alternations correlate well with the resistivity curves from the triple combo tool string (Fig. F52), but the FMS images also reveal stratigraphic information at a finer spatial resolution than the standard resistivity logs. Where the silty sand layers correlate with lower values in the gamma ray log (see ~341 and 362 mbsf in Fig. F52), the FMS images illustrate smallest scale changes in the sediment electrical properties that may be related to the subtle changes in clay content or in sediment hardness (see for example several-centimeter thick, light–dark alternations from 345 and 354 mbsf in Fig. F52). Inclined bedding (appearing as sinusoids) at slight angle is observed at some depths. It will be possible to measure dip directions and angle.

Cyclicity

Figure F53 illustrates the cyclic nature of the sediment record, which alternates between high and low gamma ray log values at intervals of 5–10 m. As a first rough estimate of the average duration of these alternations, the cycles in Subunit 1A were counted. For the 102–346 mbsf interval, ~35 alternations in the potassium curve (average thickness = ~7 m) occur over an interval of ~700 k.y. (based on a mean sedimentation rate of 0.35 m/k.y., see “Biostratigraphy” and “Paleomagnetism”). This gives an average duration of approximately 20 k.y. for each cycle, which is close to the orbital precession Milankovitch cycle (~19 and 23 k.y. periodicity). Given the uncertainties in the initial age estimates, the probability that all cycles are not recorded equally well in the sediment record, and the subjective nature of counting cycles, this early estimate requires further verification. However, it certainly seems reasonable that precession-band variability is recorded at Site U1386. Nearby in the Gulf of Cádiz, gamma ray and sonic log cyclic patterns have been observed in lower Pliocene sediments, where they were tuned to eccentricity-modulated precession cycles of the 65°N summer insolation curve (Sierro et al., 2000).

Vertical seismic profile and sonic velocity

One objective of the expedition was to establish the age and lithologic origin of the seismic reflections previously identified in seismic sections. The VSP, traveltime derived from the sonic velocity log, and synthetic seismograms provide three ways of making these correlations between the stratigraphy and the seismic sections.

Although many of the sonic waveforms recorded downhole during the VSP were noisy, 12 out of the 14 stations yielded check shot traveltimes, ranging from 0.9224 s two-way traveltime (TWT) at 143.2 mbsf to 1.1604 s TWT at the deepest station at 369.4 mbsf (Fig. F54; Table T22). Sonic traveltimes were also calculated from the sonic log velocities, which give two-way traveltimes within 0.01 s of those from the VSP. Sonic velocities (104–354 mbsf) increase downhole with a linear trend of ~0.1 km/s per 100 m. Extrapolating this trend to the base of the hole, the interval velocity yields an estimate for 526 m of 1.309 s TWT (0.569 s below the seafloor reflection). This is not an ideal method, but in the absence of velocity data from the lowermost 172 m of the hole, it is a simple assumption. The boundary between logging Subunits 1B and 1C at 464 m (a hiatus in the core) is at 1.2485 s TWT, which correlates well with a previously defined unconformity by Llave et al. (2001, 2007, 2011). The similarity of the resistivity logs to the sonic velocity log (Fig. F49) offers the potential for a “pseudosonic” log to be constructed from the resistivity data to the base of the hole and used as input for a synthetic seismogram.

Heat flow

Twelve APCT-3 downhole temperature measurements in Holes U1386A and U1386B ranged from 13.56°C at 16.8 mbsf to 18.88°C at 167.2 mbsf (Table T23), giving a geothermal gradient of 34.3°C/km (Fig. F36). The measurements increase linearly with depth, and the trend line intersects the seafloor at 13.1°C. The linearity over this depth range indicates fairly steady temperature conditions on the century scale, and that MOW bottom water at this location and depth has been historically at a steady temperature of ~13°C. Oceanographic measurements indicate that MOW has warmed and become saltier over the last few decades: deep MOW has warmed by 0.3°C over 20 y (Millot et al., 2006) and by 0.5°C over 50 y (Potter and Lozier, 2004). Unfortunately, at Site U1386 the bottom water temperature was difficult to determine accurately from the 12 APCT-3 temperature profiles. The average of the minimum temperature in the profiles differed significantly between Holes U1386A and U1386B (12.79° and 13.65°C, respectively). The traditional method of estimating seafloor temperature, the average temperature while the APCT-3 is held at the mudline, yields 14.06° and 14.00°C for the two holes. Typically, bottom water will be colder than the measured water in the pipe, so the coldest temperature in the APCT-3 profile is normally more representative because of the time needed for temperature equilibration.

Thermal conductivity under in situ conditions was estimated from laboratory-determined thermal conductivity using the method of Hyndman et al. (1974) (see “Physical properties” in the “Methods” chapter [Expedition 339 Scientists, 2013b]). The calculated in situ values average 0.5% below the measured laboratory values. Thermal resistance was calculated by integrating the inverse of in situ thermal conductivity over depth (Fig. F36). A heat flow of 42.1 mW/m² was obtained from the linear fit between temperature and thermal resistance (Fig. F36) (Pribnow et al., 2000). This value is in the lower half of the normal range for heat flow on the Portuguese margin (Grevemeyer et al., 2009).

Top of page   |    Previous   |    Next