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doi:10.2204/iodp.proc.314315316.113.2009

Physical properties

This section presents physical property measurements acquired at Site C0001. Because the adnVISION tool failed, the image-derived density data (IDRO), and neutron porosity data (TNPH) could only be retrieved from the uppermost 506 m. Resistivity and P-wave velocity were successfully retrieved for the full borehole depth.

Density

The IDRO data for Hole C0001D are plotted in Figure F30. In the 0–44 m LSF interval, the density values in Hole C0001D are highly scattered between 1.1 and 1.7 g/cm³, with a somewhat decreasing trend with depth. However, these data may not be very reliable, partly because the LWD tools were jetted in for the first 70 m. Below 44 m LSF, Hole C0001D density values are scattered between 1.6 and 1.8 g/cm³ until reaching ~160 m LSF. Note that the density between 80 and 160 m LSF remains almost constant at 1.7 g/cm³.

Density increases from ~1.7 g/cm³ at 160 m LSF to ~1.8 g/cm³ at 196.6 m LSF. Within logging Subunit IB, the log shows a sudden drop in density value to ~1.1 g/cm³. There is a significant density contrast between formations immediately above and below the logging Unit I/II boundary. Density of the lower formation is ~0.1 g/cm³ lower than that of the upper formation. Below 200 m LSF the scatter in the density log gradually increases. The span of the scattering is ~1.5–1.8 g/cm³ between 280 and 340 m LSF and increases to ~1.5–1.9 g/cm³ between 340 and 470 m LSF. The density decreases between 470 and 490 m LSF and increases finally to ~1.8 g/cm³ at the bottom of the hole.

Neutron porosity

TNPH data are highly scattered (Fig. F31). The porosity data were smoothed using a 4.5 m running average to reduce scattering.

The overall trend of the Hole C0001D neutron porosity curve is characterized by a slightly decreasing trend with depth. The neutron porosity log steadily decreases from the seafloor to reach ~60% at 190 m LSF (base of logging Unit I). A marked increase in porosity occurs at the boundary between logging Units I and II, where it increases from 60% to as much as 68% across the 30 m interval from 190 to 220 m LSF. Then in logging Unit II, porosity decreases gradually from ~68% at 220 m LSF to ~59% at 465 m LSF. In the interval between 465 and 485 m LSF, porosity increases again to 75% then decreases to 62% at 490 m LSF. Below 490 m LSF the porosity log increases again to 66% at the bottom of the hole.

The overall TNPH porosity values appear to be high compared to those normally seen at other sites in the Nankai accretionary prism (e.g., sites drilled during ODP Legs 190 and 196 at similar depths).

Resistivity and estimated porosity

Resistivity logs

Figure F32 shows the smoothed logs of the five different resistivity measurements: ring; bit; and shallow, medium, and deep button resistivity. A moving average using a 21 point (~3 m interval) window was applied to smooth the resistivity values. It highlights the differences among different resistivity measurements. Superposition of the deep, medium, and shallow button resistivity measurements shows generally very good agreement. The bit resistivity measurement integrates a larger volume of formation than the button and ring resistivities, thus producing a smoother signal.

More systematic comparisons between different resistivity logs were made through cross correlations between them. Figure F33 shows two resistivity cross-plots: bit and ring resistivity and shallow and deep button resistivity. The ring resistivity is systematically higher than the bit resistivity. Although their difference is ~0.05 Ωm at low resistivity (<1.5 Ωm), it increases to as much as 0.3 Ωm as the formation resistivity increases (>1.5 Ωm). These highest values of resistivity correspond to the deepest part of the hole and the highest temperatures. The bit resistivity provides a measurement in a volume less affected by borehole freshening (caused by cold drilling fluid) than the ring measurement because of its position in the tool assembly and its greater depth of investigation. This may explain the systematic lower resistivity of the ring measurement compared to the bit measurement. The cross-plot between shallow and deep button measurements (Fig. F34) also indicates that in the range of medium resistivity (1.5–2.5 Ωm) the difference between the two measurements is negligible. This difference increases below 1.5 Ωm such that the deep resistivity is 0.2 Ωm higher than the shallow resistivity and decreases above 2.5 Ωm such that deep resistivity tends to become lower than shallow resistivity. This difference, however, still remains within the scatter.

A comparison of the five available resistivity logs is shown in Figure F35. High noise levels in the upper 70 m LSF may be artifacts of jetting in without rotation. The downhole trends seem to be identical for the five logs. Because the bit resistivity log shows the least scatter and can represent the formation resistivity better than the others because of the greatest depth of investigation, the following description is based on the bit resistivity log.

Resistivity values generally increase with depth. In logging Unit I, resistivity values gradually increase from 0.9 to 1.1 Ωm. At the base of this logging unit (173–191 m), resistivity slightly increases by ~0.1 Ωm. This evolution is followed by the logging Unit I/II boundary, where the resistivity values fluctuate between ~0.3 and 1.7 Ωm. Logging Unit II is characterized by a nearly constant resistivity value overall, with a slightly increasing trend of resistivity from 1.02 to 1.22 Ωm over logging Subunits IIA and IIB and a virtually constant resistivity (average ~1.1 Ωm) in logging Subunit IIC. This logging unit is followed by the transition zone (Subunit IIIA), which is characterized by a higher degree of variation in the resistivity signal. Logging Unit III is generally characterized by a clear increasing trend of resistivity from 0.9 to 2.4 Ωm. Only in the interval between 815 and 855 m LSF was a marked decrease in resistivity value observed.

Estimation of temperature profile

The in situ temperature profile was estimated from the regional surface heat flow of 60 mW/m² (Kinoshita et al., 2003), and assuming 1 W/(m·K) thermal conductivity for the upper 200 m LSF and 1.5 W/(m·K) below 200 m LSF and 2°C surface temperature. The resulting temperature reaches 45°C at 973 m LSF.

Estimation of porosity from resistivity

The TNPH and IDRO data only extend to half the borehole depth. Thus, an estimate of porosity from the resistivity data is proposed in this section using Archie's law, as explained in the “Expedition 314 methods” chapter, including the borehole temperature effect on fluid resistivity. This transformation has been calibrated with porosities measured on cores from the Muroto transect of the Nankai prism at Sites 1175, 1176, and 808, the only ones available at that time. The structural position of Site C0001 in the Kumano transect is somewhat intermediate between the positions of Sites 1175 and 1176 and Site 808 in the Muroto transect. The values of the Archie's law parameters necessary to best fit these data are a = 1 and m = 2.4 (Fig. F36). It should be noted again that the resistivity-derived porosity estimate is not intended to provide the "true" porosity but remains a very useful estimate, especially below 500 m where no other data exist.

From the seafloor to the boundary between logging Units I and II, the resistivity-derived porosity decreases gradually to ~52% at 196 m LSF (Fig. F37).

The resistivity-derived porosity log derived from the bit resistivity measurement presents a steep increase from 52% at 196 m LSF to 55% at 200 m LSF with a local peak of 58% at 197.8 m LSF.

Below 200 m LSF, the resistivity-derived porosity decreases with depth to ~46% at 428 m LSF. There is an abrupt increase to 48% at 438 m LSF, with a 51% peak at 434.5 m LSF. Below 438 m LSF, the resistivity-derived porosity decreases slowly again to 45% at 528 m LSF (1 m above the base of logging Unit II).

At 534 m LSF resistivity-derived porosity jumps from 45% to 50% over a 1 m interval. Beneath this interval, there is a zone of rapid decrease, reaching ~40% at 633 m LSF. This zone includes two wide oscillations of ~5% amplitude, each spanning a 20–25 m depth interval, which could be attributed to the disrupted zone identified in the resistivity images but could also be attributed to poor hole conditions.

Below 633 m LSF, the resistivity-derived porosity decreases more gently, reaching 31% at 969 m LSF. A notable deviation from this trend occurs between 810 and 910 m LSF, where it increases from ~34% at 810 m LSF to ~37% at 859 m LSF and then decreases rapidly to rejoin the overall gently decreasing porosity trend.

In summary, the profile presents an abrupt step at 200 m LSF to higher values and two zones significantly departing from the general decreasing trend, which are characterized by a relative increase in value (428–633 and 810–910 m LSF).

A porosity profile was also calculated using IDRO (see the “Expedition 314 methods” chapter). The comparison of the resistivity-derived porosity with TNPH and the porosity derived from IDRO shows that the resistivity-derived porosity only fits IDRO-derived porosity well in logging Unit I. In logging Unit II, the resistivity-derived porosity mimics the evolution of the lowest values of IDRO-derived porosity. The difference between the two logging units may be a consequence of significant lithologic change (Fig. F37).

P-wave velocity

Sonic P-wave velocity was calculated from compressional wave slowness logs (DTCO). P-wave velocity generally increases with depth (Fig. F35). In each logging unit (I, II, and III), the increase in velocity with depth is approximately a linear relationship. The gradients of velocity with depth are 0.76, 0.98, and 1.39 m/s per meter, respectively, for logging Units I, II and III, indicating that velocity increase accelerates with depth.

The formation velocity starts to be identified below 175 m LSF (see “Data and log quality”). Above this depth, velocities are likely to be those of the drilling fluid. Such a transition causes a jump in V_P at this depth and is almost certainly an artifact. The boundary zone between logging Units I and II is characterized by relatively low velocities (1600–1670 m/s) compared to those in formations immediately above and below (1650–1750 m/s). However, the absolute values of velocity in the boundary zone may not represent those of the formation because no clear distinction between formation and mud arrival could be made (see “Data and log quality”).

P-wave velocity in logging Unit II is characterized by a monotonic increase in values with minor fluctuations (with a cycle of ~25 m). This is somewhat in contrast with resistivity logs, which have no clear increasing or decreasing trend with depth.

Sonic velocity in logging Unit III is characterized by a general increase in value with depth, with several major low-velocity zones where velocities decrease with depth (529–600, 660–695, 773–827, 884–894, and 915–965 m LSF) (Fig. F35). Below each decreasing velocity region, the velocities increase again to the main trend line. The shallowest low-velocity zone corresponds to the disrupted zone identified in the resistivity images and could be attributed to poor hole conditions. From ~722 to ~901 m LSF the sonic log shows a mixture of high- and low-frequency components. At the Subunit IIIB/IIIC boundary (905 m LSF), there is a significant increase in velocity (to a value >2600 m/s), followed by the deepest of the decreasing velocity zones.

Comparison of P-wave velocity with other properties

Figure F38 shows cross-plots between P-wave velocity and other properties such as resistivity (bit) and resistivity-derived porosity. Porosities used here are those estimated from the bit resistivity log. Both the velocity versus resistivity and the velocity versus resistivity-derived porosity cross-plots show generally good correlations. In the velocity versus resistivity plot, low resistivity data corresponding to logging Unit II and logging Unit I (corresponding to the deeper parts of the unit) appear to overlap (some Unit I data are hidden beneath Unit II data), mainly because resistivity is not well resolved between the two units, whereas velocity is better resolved. However, in the velocity versus resistivity-derived porosity plot there seems to be a good match between these porosities.

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