Figure F5, displays the comparison between core sample and downhole log porosity, density, and velocity at Site 1095. With the exception of the lower part of the section (below ~400 mbsf), downhole log porosity is systematically larger than core sample porosity. Excessively high neutron log porosity is normally caused by bound water in clay minerals (Broglia and Ellis, 1990; Goldberg, 1997). However, we used porosity data conventionally corrected for this effect, so that the error induced is surely attenuated, if not eliminated (see "Downhole Measurements" in Shipboard Scientific Party, 1999a). RHOM density, on the other hand, is in fairly good agreement between downhole and core sample data, with the exception of the occurrence of unrealistically low values in the downhole log set, induced by the excessively large size of the hole. This is demonstrated in Figure F5 where a comparison with the caliper log shows that the negative deviations in density occur preferentially when the caliper goes out of range. The effect of hole diameter on the density log is dramatic, whereas it produces a lower effect, but smeared over almost the entire section, on the porosity log. As expected in normally consolidated sediments, porosity decreases downhole, although with variable gradients, from 70%-75% at the sediment surface to ~45% at 550 mbsf. A large reduction in porosity occurs at ~480 mbsf. As expected, the density log obtained from core samples mirrors the porosity log, ranging from ~1.6 g/cm3 at the top (core data) to ~2.1 g/cm3 at the bottom of the hole.
Figure F5 also illustrates the acoustic velocity values for Site 1095 obtained from core data. Velocity generally increases downhole, from ~1500 m/s at the top to ~2000 m/s at the bottom of the hole. There is a fairly steady increase from 0 to 400 mbsf and a sharp increase below 480 mbsf, in parallel with the decreasing porosity and increasing density (see above).
At Site 1095 (Fig. F6), additional information on acoustic velocity comes from the velocity check shots. Eleven interval velocities were calculated, in total agreement with those obtained on board the JOIDES Resolution and presented in the Leg 178 Initial Reports volume (Shipboard Scientific Party, 1999b). Here, we compare these interval velocities with the nearest available stacking velocities obtained from MCS line 1095 (SP 1210, 1250 m away from the location of Site 1095) and with a reduction to the same intervals of the velocities obtained from core samples. Evidently, the in situ velocity check shots provide more reliable velocity information than the stacking velocity, indicating consistently lower velocity throughout the section. The anomalously high check shot velocity obtained at the base of the hole is interpreted as being produced by an error in the positioning of the geophone. The velocity measured on core samples is consistent with the in situ check shots.
The data plotted in Figure F5 are provided in Table T2.
Figure F7 displays the comparison between core sample and downhole porosity, density, and velocity at Site 1096. With the exception of the lowermost part of the section, the porosity distribution with depth varies significantly between the two data sets; downhole logging porosity is systematically higher than core sample porosity. The discrepancy is as large as 25% in the middle part of the section. The downhole log data contain much higher internal variability than the core sample data. According to the core sample data, porosity at the top of the section is in the range of 65%-70%. Downhole log values are not available at the top, but unrealistic values in the range of 70%-80% are present at ~100 mbsf. Both sets converge toward ~60% porosity at the bottom of the hole (580 mbsf). As for neutron porosity data at Site 1095, we think that the clay bound water effect is at least partly attenuated by the standard correction applied to the APLC data set. We consider the data set from downhole logging unrealistic because the logging of Hole 1096 suffered from various problems. Principally, the hole was unstable and large cavities were encountered. Secondarily, and more importantly, there was a problem in the range calibration of the caliper so that the hole diameter values used in the correction of the data were probably too small (see "Downhole Measurements" in Shipboard Scientific Party, 1999a). A characteristic of the porosity variation with depth at Site 1096 is that there is a porosity reduction in the uppermost 100 mbsf followed by a slight increase downhole. As a consequence, the lowest porosity values (~50%) are found at 100-150 mbsf. We think that this anomalous trend is real because it is preserved even if the effect of rebound after sampling (normally 2%-3% in this range of overburden pressure, according to Hamilton [1976]) is considered. The density distribution with depth is consistent with the porosity trend. Core sample density increases from ~1.6 g/cm3 at the surface to ~1.9 g/cm3 at 100-150 mbsf, decreasing steadily downward to 1.7 g/cm3 at the bottom of the hole. The downhole logging data are limited to the lower part of the hole and are in fairly good agreement with the core sample data. Such anomalous behavior of bulk density and porosity at Site 1096 can be explained by the relatively high sedimentation rate (up to 18 cm/k.y.) or by the presence of a large component of biogenic silica (diatom and radiolarian skeletons) throughout the section (Shipboard Scientific Party, 1999c).
Figure F7 also illustrates acoustic velocity values for Site 1096, obtained from core data. Velocity increases rather steadily downhole, with narrow positive and negative excursions from the general trend. The highest gradient is in the upper 100 mbsf, a smaller gradient between 100 and 480 mbsf, and almost no gradient below 480 mbsf. Values range from ~1500 m/s at the sediment surface to ~1700 m/s at the bottom of the hole.
At Site 1096, additional velocity information comes from the acoustic tomographic inversion of traveltimes. Figure F8 shows the distribution with depth of the interval velocity along MCS profile IT90-109. Note that the section is plotted with a vertical scale in kilometers. Velocity increases downward everywhere in this section. Figure F9 shows a plot of the acoustic tomographic velocity with depth at Site 1096 (extracted from the section in Figure F8). The profile is compared with the nearest available stacking velocity (SP 1917, 650 m away from Site 1096) and with velocities obtained from core samples reduced to the same intervals as the tomographic velocities. At this site, there is a significant difference between the three profiles. The tomographic inversion produces values consistently ~200 m/s in excess of the PWS3 velocities, whereas the stacking velocity is higher than the laboratory measurements, as was observed at Site 1095. We think that the tomographic method here provides a better estimate of interval velocities than the stacking procedure. However, because of the limited offset compared to the depth of the objectives, tomography is sensitive to imperfections of the geometrical scheme used for inversion, thus introducing a systematic error. Therefore, we consider the PWS3 core profile to be the most reliable one for any application.
The data plotted in Figure F7 are provided in Table T3.
Only data from core measurements are available at Site 1101. Figure F10 displays the porosity, density, and velocity distributions with depth. Density and porosity display mirrored trends. Porosity decreases from ~70% at the seafloor to ~50% at 100 mbsf, showing a higher degree of compaction than at the other two sites. An increase of porosity downhole occurs below 100 mbsf, which brings the value to ~60% at the base of the hole (215 mbsf). The velocity distribution with depth is apparently not affected by the porosity and density distribution. Velocity increases steadily from ~1500 m/s at the seafloor to a little over 1600 m/s at the bottom of the hole.
All data plotted in Figure F10 are provided in Table T4.