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doi:10.2204/iodp.proc.314315316.118.2009

Physical properties

The available data relevant to physical properties include five different sets of resistivity logs (bit; ring; and shallow, medium, and deep button) and sonic P-wave velocity. No neutron porosity (TNPH) or density (RHOB) data were recorded.

As at previous sites, additional analyses were conducted to produce porosity estimates derived from resistivity. In the process, the estimation of temperatures and formation factors were carried out.

Resistivity and estimated porosity

Resistivity logs

Figure F16 shows ring and bit resistivity measurements side by side and smoothed logs of shallow, medium, and deep button resistivity. A moving average using a 21-point (~3 m interval) window was used to smooth the resistivity values. Superposition of deep, medium, and shallow button resistivity measurements shows very good agreement between medium and deep button resistivities. Shallow button resistivity is significantly lower than the two other button resistivities in logging Units I and IV (Figs. F16, F17).

Based on bit resistivity, logging Unit I resistivity values generally increase from 1.1 to 2.0 Ωm and show an important excursion to higher resistivity values from 50 to 100 m LSF that includes a peak of resistivity of 2.6 Ωm at 85 m LSF. The trend of resistivity becomes fairly constant in logging Unit II where it varies slightly around 1.9 Ωm. Four excursions to low resistivity value are noticeable in this unit.

At the top of logging Subunit IIIA (from 429 to 460 m LSF), the trend is a decreasing one from 1.8 to 1.5 Ωm and increases again to 1.7 Ωm at 530 m LSF. The bottom of this subunit is marked by a sharp minimum of resistivity at 1.3 Ωm. The underlying logging subunit (IIIB) is characterized by alternating zones of higher and lower resistivity values. From 545 to 590 m LSF, resistivity values fluctuate around 1.7 Ωm with a minimum in resistivity (1.2 Ωm) at 560 m LSF. This zone ends on a decreasing step of resistivity (from 1.8 to 1.4 Ωm) and is followed by a zone of constant resistivity values (1.4 Ωm) from 590 to 615 m LSF. Below this zone, the trend of resistivity increases again to 2.1 Ωm at 640 m LSF, where a second decreasing step is observed (from 2.1 to 1.4 Ωm). This decreasing step is followed by a decreasing trend of resistivity (from 1.4 Ωm at 655 m LSF to 1.2 Ωm at 690 m LSF) and an increasing trend (from 1.2 Ωm at 690 m LSF to 1.7 Ωm at 710 m LSF). The boundary between logging Units III and IV is marked by a decreasing step of resistivity from 1.7 to 1.2 Ωm.

The trend of resistivity stays fairly constant (1.2 Ωm) in the uppermost 30 m of logging Unit IV. At 740 m LSF another decreasing step of resistivity is observed (from 1.2 to 0.9 Ωm), followed by a nearly constant trend of resistivity (0.9 Ωm). At 840 m LSF a final increasing step of resistivity is observed from 0.8 to 1.2 Ωm followed by a decreasing trend of resistivity reaching 0.75 Ωm at the bottom of the hole.

Estimation of temperature profile

The downhole temperature profile was estimated from a regional surface heat flow of 80 mW/m² (Kinoshita et al., 2003) and 2°C surface temperature. Thermal conductivity (k) was estimated from Ocean Drilling Program Leg 131 Site 808 data as k = 1 + (z/800) for z < 400 m and k = 1.5 + (z – 400)/2000 for z > 400 m. The estimated temperature is 52°C at 886 m LSF.

Estimation of porosity from resistivity

Bit and ring formation factors have been calculated from resistivity logs and temperature-corrected seawater electrical resistivity. They were converted to porosity using Archie’s law. In the absence of neutron porosity data to calibrate Archie’s law parameters, the parameter values a = 1 and m = 2.4 were used. The values are the same as those at previously drilled Expedition 314 sites where these values were calibrated from comparison with log porosity data (neutron and density porosity). However, sand content at Site C0006, apparently much greater than that found at other sites, means that porosity and density estimates may be strongly affected by lithology, which is not accounted for in these calculations.

In logging Unit I, bit resistivity–derived porosity decreases from 85% at 0 m LSF to 51% at 50 m LSF (Fig. F18) and 41% at 86 m LSF. Resistivity-derived porosity then increases to 49% at 116 m LSF and decreases back to 40% at 197.8 m LSF at the boundary between logging Units I and II. In Unit II, resistivity-derived porosity decreases slowly to 38% at 428 m LSF, the transition with Unit III. In the uppermost 34 m of Unit III, resistivity-derived porosity increases to 41% at 462 m LSF and then decreases to 36% at 590 m LSF. At the base of Unit III, resistivity-derived porosity is 36% and jumps to 41% over 6 m at the transition with Unit IV. In Unit IV, resistivity-derived porosity begins by increasing to 48% at 762 m LSF. Resistivity-derived porosity then fluctuates around an average value of 46%. At 850 m LSF resistivity-derived porosity decreases to a low value of 39% before increasing back to 47% at 884 m LSF at the base of the hole. Again, caution is required in interpreting these values, as Unit IV is interpreted as being a much sandier lithology than the basis of calibration (see “Log characterization and lithologic interpretation”).

Estimation of density

Because of the absence of bulk density measurements, we made an estimation of bulk density from resistivity-derived porosity (Fig. F18). Resistivity-derived porosity was converted to density using standard methods (see “Physical properties” in the “Expedition 314 methods” chapter).

P-wave velocity

Because of a malfunction of the sonic tool at Site C0006, P-wave velocity data were measured only to 160 m LSF, which corresponds to sediments in most of logging Unit I (Fig. F19). In the uppermost 40 m, the measured P-wave velocity is indistinguishable from the mud arrival (~1500 m/s). At ~40 m LSF, velocity jumps to ~1750 m/s. Below this level, velocity appears to be nearly constant or slightly increasing. At the base of measurements (160 m LSF), velocity is measured at ~1860 m/s. Some velocity peaks are noted at 70, 94, 101, and 130–140 m LSF, which may be interpreted as clay-rich intervals where gamma ray values are relatively high (see “Log characterization and lithologic interpretation”).

Velocity and resistivity are generally in good agreement; high velocity zones correspond to high resistivity zones. This is clearly seen in the cross-plot between velocity and bit resistivity, in which their relationship is approximately linear (Fig. F20). The only exception is high-resistivity data (>2 m) in the interval between 80 and 90 m LSF. In this low–gamma ray sandy zone, velocities are low (~1700 m/s).

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