g).
For 0 to 344 mbsf,The LWD density (RHOB) log shows a good fit to the core bulk density (Fig. F22), generally overpredicting the core values by as much as ~0.1 g/cm3. The density log (Fig. F22) shows a large deviation from core values between 0 and 55 mbsf corresponding to the zone of poor LWD data quality (see "Quality of LWD Logs"). Below 55 mbsf the density log shows two overall trends, near constant density from 55 to 321 mbsf and a gradual increase in density from 321 to 713.46 mbsf. In detail (Table T6) density is nearly constant at an average of 1.69 g/cm3 from 55 to 122 mbsf (log Subunit 1b), decreasing to ~1.64 g/cm3 at 130 mbsf. Density shows only a small deviation of ±0.06 around a mean of 1.64 g/cm3 from 130 to 321 mbsf. The interval from 321 to 340 mbsf exhibits large-amplitude fluctuations in density, with oscillation between two peak values of 1.64 and 1.9 g/cm3, associated with the transition from upper to lower Shikoku Basin facies. Density increases steadily from 1.84 g/cm3 at 340 mbsf to 2.05 g/cm3 at 468 mbsf, below which there is a step decrease to 1.99 g/cm3. Density increases rapidly to 2.15 g/cm3 at 503 mbsf and decreases to 2.06 g/cm3 at 558 mbsf. Following a step increase to 2.11 g/cm3 at 565 mbsf, density increases less steeply than between 340 and 468 mbsf to 2.2 g/cm3 at 698 mbsf, where a large negative peak marks the transition from the base of the lower Shikoku Basin facies to the underlying volcaniclastic sequence, in which the density log terminates.
Core grain density
measurements from Hole 1173A (Fig. F23)
indicate changes with depth associated with lithologic variations. The grain
density data were used to calibrate a density to porosity transform for the LWD
density logs. Although small-scale trends are observed in the grain density with
depth, the data follow two general trends divided by an abrupt change in grain
and bulk density corresponding to the transition from upper to lower Shikoku
Basin facies at ~344 mbsf. Least-squares regression, after manual removal of low
and high density spikes (Fig. F23),
was used to evaluate these trends in grain density (g).
For 0 to 344 mbsf,
g
= 2.6882 + (4.7753 x
10-5 x z),
and for 344 to 687 mbsf,
g
= 2.7901 + (5.5909 x
10-5 x
z),
where z is the depth in meters.
Porosity (
)
was calculated from the LWD density log
= (b
- g)
/ (w
- g),
where b
is the log value (bulk density), and assuming a water density (w)
of 1.035 g/cm3. This porosity (Fig. F24)
shows an overall improved fit to core porosity data between 60 and 687 mbsf
compared to the use of constant grain densities, with the LWD porosity
underpredicting the core data by ~0.02 g/cm3 below 340 mbsf. Porosity
is quite well predicted in the interval 315-344 mbsf, which corresponds to a
region of high-amplitude variations in core porosity above the transition from
upper to lower Shikoku Basin facies. Laboratory measurements tend to
overestimate porosity (Brown and Ransom, 1996) due to clay-bound water and the
removal of samples from in situ temperature and pressure conditions (Hamilton,
1971).
As indicated in "Quality of LWD Logs" a differential caliper (DCAL) value of <1 in indicates good borehole conditions, for which the density log values are considered to be accurate to ±0.015 g/cm3. The uppermost 70 m of Hole 1173C (Fig. F25) is characterized by highly variable differential caliper values between 0.2 and 3 in. Differential caliper values average 1.8 in between 13 and 35 mbsf and 1.6 in between 35 and 55 mbsf. Between 55 and 70 mbsf, differential caliper values are reduced to 1.1 in. A closer examination of density and differential caliper values over these intervals shows that the highest differential caliper values correspond to low bulk density values. A good example of this inverse correlation can be seen in two peaks at 31 and 33 mbsf. Accordingly, most bulk density values down to ~55 mbsf appear to be anomalously low compared to those observed in similar depositional environments (Brückmann, 1989).
As reliable bulk density data are an essential prerequisite for modeling purposes and generating synthetic seismic profiles, Hole 1173A core-derived data were used to define a bulk density profile for the uppermost 60 m. A logarithmic curve that provides a more realistic approximation of bulk density was fitted to the Hole 1173C core bulk densities (Fig. F26):
b(z)
= 1.384 + 0.158 x
log (z).
Using this density vs.
depth function, a corrected porosity profile was calculated which yields a 0
value of 87.8% (Fig. F26).
Figure F27
shows the five resistivity logs (see "Logging while Drilling"
in the "Explanatory
Notes" chapter for details about differences in acquisition,
resolution, and depth of investigation). The borehole fluid was seawater
although periodic mud sweeps (sepiolite) were used. The five logs show a similar
overall resistivity trend. The bit resistivity log is smoother than the others
because the bit electrode spans a larger vertical interval (2.7 m) (see "Logging
while Drilling" in the "Explanatory
Notes" chapter). The previously defined log units are well
expressed in the resistivity logs. Following is a description of each unit as
observed on the ring resistivity log. Unit 1 has an average resistivity of ~0.6 
m.
However, the quality of the logs in this unit may be degraded as suggested by
the high value of the differential caliper signal (see "Quality
of LWD Logs"). This unit is divided into two parts: Subunit
1a, where the resistivity increases from 0.4 to 0.7 m
with depth, and Subunit 1b, in which resistivity decreases from 0.6 to 0.4 m
with depth. The rate of decrease changes sharply at 122 mbsf, corresponding to
the boundary between Subunit 1b and Unit 2. In Unit 2, resistivity decreases
from 0.5 m
at 122 mbsf to 0.3 m
at the Unit 2/3 boundary (~340 mbsf). Two peaks, both ~10 m in thickness, mark
the base of this unit. Unit 3 is characterized by a higher average resistivity
(~0.55 m)
and a greater variability superimposed on a gradual increase with depth. The
Unit 3/4 boundary (698 mbsf) is characterized by a peak of low resistivity (~0.3
m).
Resistivity in Unit 4, which corresponds to the volcaniclastic facies, increases
rapidly with depth, reaching 0.75 m
at the boundary with Unit 5 (735 mbsf). Resistivities in Unit 5 are shown in
detail in Figure F28.
This unit is characterized by resistivities between 2.5 and 11 m,
and probably corresponds to basaltic basement.
A comparison of the five logs (Fig. F29) shows differences between resistivity values obtained from different measurement methods. The deep- and medium-focused resistivities give similar values, which can be seen in a correlation diagram where both resistivities fit very well (Fig. F30A). There are two intervals where the curves differ (550-554 and 612-630 mbsf). An unexplained feature is the inverse correlation of the different resistivity measurements. The shallow resistivity is systematically higher than both the deep and medium resistivity, which is opposite to the trend expected for this environment. This causes the correlation cloud to plot above the line of unit slope on the correlation diagram (Fig. F30B). The ring and bit resistivity logs similarly show differences in some regions of the log: bit resistivity is systematically higher in nearly all of Unit 2, whereas ring resistivity is higher in some zones of Unit 3. The correlation diagram (Fig. F30C) of these two sets of measurements accordingly presents an asymmetrical cloud. Finally, Figure F30D is a correlation diagram between deep and ring resistivity. It shows good agreement between those two measurements, with slightly lower values of deep resistivity.
The wireline resistivity
log (dual induction tool [DIT]) measured during Leg 190 is plotted on Figure F29
for comparison. The correlation of signal peaks between Leg 190 and 196
measurements is very good, but the Leg 196 LWD resistivity values are ~0.1 m
lower than the wireline resistivity values. It should be noted that the DIT is
an induction tool better adapted to measure low resistivity formations than the
RAB tool used in Hole 1173B.
The neutron porosity profile broadly mimics trends observed in other LWD logs (Fig. F31A), but with a larger degree of scatter. In log Unit 1 neutron porosity and core-derived porosity are in good agreement, whereas RHOB-derived porosity is consistently higher in log Unit 2 and increasingly lower in Units 3 and 4. Although the difference between neutron and RHOB-derived porosity in Unit 2 (differential porosity) is constant within ±10%, in Unit 3 the difference progressively increases downhole by about 5% to 15% (Fig. F31B). The increased separation between the two types of porosity is attributed to the lithologic change from upper to lower Shikoku Basin sediments that have higher clay mineral content. The neutron porosity is biased toward higher values by the bound water in clay minerals.
The ISONIC data returned to the ship after Schlumberger processing consisted of P-wave slowness at ~10-cm depth intervals and coherence of the stacked waveforms as a function of slowness and traveltime at the points where formation P-wave arrivals were picked (see "Quality of LWD Logs"). Identification and picking of the P-wave arrival from coherence analysis in ISONIC data is not straightforward. The preliminary ISONIC P-wave velocity values (Fig. F32) deviate substantially from core and wireline values, which correlate well with each other. Therefore we have low confidence in the quality of the preliminary waveform correlation picking of the formation P-wave phase and resulting calculated velocities. Further postcruise processing and analysis of ISONIC waveform data will be required to produce a log of formation P-wave velocity in which we have a high level of confidence.
