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doi:10.2204/iodp.proc.339.108.2013

Downhole measurements

Logging operations

After the last core from Hole U1390A arrived on deck, the hole was prepared for logging. The wiper trip found no fill. A go-devil was pumped to lock open the lockable float valve, and the hole was flushed with 20 bbl of sepiolite mud. The hole was then displaced with 124 bbl of barite-weighted heavy mud (10.5 ppg), and the pipe was set at 96.6 mbsf. Two tool strings were deployed in Hole U1390A, the triple combo and FMS-sonic (Fig. F36; see also “Downhole measurements” and Table T6 in the “Methods” chapter [Expedition 339 Scientists, 2013b] for tool acronym definitions).

The triple combo tool string started downhole at 0000 h on 5 January 2012. The Hostile Environment Natural Gamma Ray Sonde (HNGS), High-Resolution Laterolog Array (HRLA), and Hostile Environment Litho-Density Sonde (HLDS) were included; the Accelerator Porosity Sonde (APS) was omitted because it allowed the HNGS to be placed at the bottom of the tool string to record the sand-rich beds at the base of the hole. The tool string reached the base of the hole at 350 mbsf, and a 70 m repeat pass was taken before the main pass up to seafloor. FMS-sonic tool string rig-up started at 0430 h. Sonic logs were taken on the descent, and two upward passes were made from the bottom of the hole (350 mbsf).

Maximum ship heave during logging was higher than at previous sites, ~1.5 m maximum peak-to-peak for the triple combo run and 1.7 m for the FMS-sonic run. The tide was falling during the triple combo run (+0.5 to –0.1 m) and rising during the FMS-sonic run (–0.5 to +0.1 m) (Fig. F37). The wireline heave compensator was used to counter ship heave during logging.

Log data quality

In Hole U1390A, the quality of the log data is good because of the generally narrow and smooth borehole. In the upper 270 mbsf, the borehole diameter generally ranges from 9.5 to 12 inches and varies smoothly except for some thin washouts, which generally do not exceed 14 inches (Fig. F38). Below 270 mbsf, the borehole quality decreases but is still reasonable, with more washouts and an increase in borehole diameter to >12 inches, exceeding 16 inches in very few places.

Resistivity, sonic velocity, and NGR logs are robust to the moderate changes in hole diameter. As for previous sites, the NGR log (HSGR) correlates well with the NGR values measured on cores (Fig. F39). The density log values match the MAD bulk density values (Fig. F38), and the downhole patterns correlate well with the GRA bulk density values, except for small offsets in depth and value (GRA underestimates density slightly). The photoelectric effect log has anomalously high values (>10 b/e^–) because of barium in the logging mud. FMS resistivity images are of good quality because of good contact with the borehole wall in most of the borehole.

Logging units

The logged intervals at Site U1390 were assigned to one logging unit (Fig. F38) because no major base-level changes in log data are apparent. In this unit, density and sonic velocity have a steadily increasing downhole trend reflecting downhole compaction. Resistivity has a curious curve, declining to low values downhole rather than showing a normal downhole compaction trend (see below). The unit is divided into two subunits mostly on the basis of an increasing proportion of low NGR values below 293 mbsf (Figs. F38, F40).

Logging Subunit 1A: base of drill pipe to 293 mbsf

Logging Subunit 1A is characterized by medium-amplitude alternations in bulk density, NGR (and its uranium, thorium, and potassium components), density, and sonic velocity values (Fig. F38). Several orders of cycles are observed in the NGR curves, varying from one to several tens of meters in thickness (Fig. F40). Potassium and thorium concentrations co-vary closely, suggesting that clay content controls these logs. The uranium concentrations correlate to potassium and thorium at some depths or vary independently at others. As expected from downhole compaction, the density and sonic velocity logs have an increasing downhole trend and are generally well correlated with each other. These logs also correlate well with the resistivity curve at a medium scale (several tens of meters). At the borehole scale, the resistivity curves show an opposite trend, decreasing downhole, that we relate to the increase in interstitial water salinity (see “Geochemistry”), because the amount of ions in the interstitial water solution lowers the apparent formation resistivity. Below ~160 mbsf, the resistivity curves display clearer medium-scale alternations than in the upper part of logging Subunit IA (Fig. F38).

Logging Subunit 1A at Site U1390 resembles logging Subunit 1A observed at Site U1389 (see “Downhole measurements” in the “Site U1389” chapter [Expedition 339 Scientists, 2013g]). Both of the subunits are Pleistocene in age, although their lower boundaries are diachronous (see “Biostratigraphy” in the “Site U1389” chapter [Expedition 339 Scientists, 2013g]).

Logging Subunit 1B: 293–350 mbsf

Logging Subunit 1B is distinguished from the subunit above by its higher amplitude variability and by the presence of lower NGR values (Figs. F38, F40). The potassium and thorium logs contain drops to below 0.6% and 4 ppm, respectively. This suggests that Subunit 1B contains some several-meter thick sandy intervals, which is in agreement with the observations on cores (e.g., see “Lithostratigraphy” and thick sandy packages in Cores 339-U1390A-33X, 34X, 36X, and 37X). In Subunit 1B, uranium is closely correlated to the potassium and thorium curves. The NGR and P-wave velocity curves are anticorrelated in the upper part of this subunit, downhole to ~315 mbsf.

The upper part of logging Subunit 1B is in accord with the upper part of lithologic Subunit IB, recognized in Hole U1390A at 293.78 mbsf and characterized by thicker sand-rich beds than the lithologic unit above (see “Lithostratigraphy”). This boundary is also seen in petrophysical properties (upper boundary of physical properties Unit III) as a decrease in magnetic susceptibility and NGR and also by a change in the frequency and amplitude of the observed cycles (see “Physical properties”). The transition between logging Subunits 1A and 1B also corresponds to a minor hiatus recognized between 292.08 and 292.43 mbsf based on micropaleontological analysis, and covering an interval between 0.9 and 1.2 Ma (see “Biostratigraphy”).

Formation MicroScanner images

Because of the good borehole conditions in Hole U1390A, the quality of the FMS resistivity data allows the resistivity of the borehole formation to be interpreted at several scales. At the scale of the borehole, a downhole trend of increasing conductivity is observed, which correlates well with the resistivity logs. The interval above ~160 mbsf is characterized by relatively high resistivity (light-colored upper interval in the FMS image in Fig. F41), becoming less resistive downhole to ~220 mbsf and reaching minimum values between 220 and 320 mbsf (dark color in the FMS image). We relate this general downhole increase in conductivity to the progressive downhole increase in pore water salinity (see “Geochemistry”). At a finer scale, the FMS images reveal numerous resistive and conductive intervals, whose thickness ranges from several tens of centimeters to a few meters. Both gradual and sharp transitions between alternations are observed. In some places, most of these alternations correlate well with other logs. As an example, the conductive interval extending from 220 to 228 mbsf on the FMS images correlates with lower values in NGR, bulk density, P-wave velocity, and resistivity logs (Fig. F41) and corresponds to coarse-grained intervals (sands and silts). These intervals contrast with muddy intervals appearing as less conductive on the FMS images and correlating with higher values in the NGR, bulk density, P-wave, and resistivity logs (see for example the 10 m thick, light-colored interval extending from 228 to 238 mbsf in Fig. F41). The above relation between lithologies and FMS image resistivity is, however, not observed throughout the borehole, especially in Subunit 1B (e.g., see the clay interval located at ~300 mbsf [NGR peak] correlating with a conductive interval on the FMS image in Fig. F41). FMS resistivity images also reveal stratigraphic information at a finer spatial resolution than the standard logs. In Figure F42 (240–253 mbsf), a series of resistive (light colored) layers, with thicknesses ranging from 0.2 to 1 m, correlates with lower values in the NGR log and higher values in the high-resolution bulk density log. We correlate some of these resistive layers with silty sand layers in cores. Some other layers may be related to more subtle changes in clay content or in sediment hardness that have not been visually recognized in cores. Inclined bedding (appearing as sinusoids) at slight angles is observed at some depths. It will be possible to map their occurrence and measure dip directions and angle.

Sonic velocity and two-way traveltime

Sonic velocity logs in Hole U1390A help to establish depth in two-way traveltime (TWT) conversions and link the borehole logs to the seismic section, in the absence of a vertical seismic profile at this site. Sonic velocity repeats well from Pass 1 to Pass 2, and data from the uppermost 20 mbsf are provided by core P-wave logger results (Fig. F43). Sonic traveltimes were calculated from sonic log velocities and give TWTs for the base of the hole (350 mbsf) of ~395 ms below seafloor and ~1705 ms below sea level, assuming a velocity of 1.58 km/s just below the seafloor (Fig. F43). The seafloor reflection peak is at 1310 ms in the seismic section, and precision depth recorder measurements put the seafloor at 1314 ms.

Heat flow

Nine APCT-3 downhole temperature measurements in Holes U1390A–U1390C ranged from 13.6°C at 18.6 mbsf to 16.5°C at 108.9 mbsf (Table T19), giving a geothermal gradient of 32.0°C/km (Fig. F44). The measurements increase linearly with depth, and the trend line intersects the seafloor at 13.03°C. Some scatter and minor differences between the three holes is evident. These differences are probably caused by (1) uncertainty in fitting to the APCT-3 temperature equilibration curves to obtain the formation temperature and (2) lateral heterogeneity in heat flow leading to small temperature differences between the holes. Readings at shallower depths have more uncertainty because the APC barrel has more freedom of movement in the hole where the sediment is soft. Similar to previous sites, the bottom water temperature was difficult to determine accurately from the APCT-3 temperature profiles (Fig. F45). The intersection of the linear temperature trend with the seafloor probably gives the best estimate here.

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 1.5% higher than the measured laboratory values. Thermal resistance was then calculated by integrating the inverse of the in situ thermal conductivity over depth (Fig. F44). A heat flow of 42.5 mW/m² was obtained from the linear fit between temperature and thermal resistance (Pribnow et al., 2000). This value is mid-range for the Gulf of Cádiz area (Grevemeyer et al., 2009).

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