A variety of core- and logging-based physical property data were collected at Sites 1173, 1174, and 808. As both velocity and porosity data are available from cores for all three sites, we used the shipboard core measurements to develop an empirical velocity-porosity relationship for the Shikoku Basin underthrust sediments. The only logging data (wireline and LWD) collected across the underthrust section were at Site 1173. The available logging data for this site were used in conjunction with the core data to determine corrections to be applied for unloading from in situ conditions.
Shipboard laboratory measurements of compressional wave velocity were made on core samples from Sites 1173, 1174, and 808 (Fig. F4) (Shipboard Scientific Party, 1991, 2001c, 2001d). For unconsolidated sediments, probes were inserted into the split cores to measure transverse and longitudinal velocity. For consolidated sediments, measurements were made on samples using a Hamilton frame velocimeter contact probe system (Boyce, 1976; Hamilton, 1965). Velocity was measured in the x-, y-, and z-directions (where the z-direction is parallel to the long axis of the whole-round core section). Measurements were taken from cores collected at ~3-m intervals for Site 1173, 4-m intervals for Site 1174, and 1.5-m intervals for Site 808.
Wireline velocity data were collected at Site 1173 using the Dipole Shear Sonic Imager (DSI) tool. The DSI velocity data were reprocessed by Goldberg (this volume) using a phase-picking method to improve the quality of the logs.
Porosity values for sediments from Sites 1173, 1174, and 808 (Fig. F4) (Shipboard Scientific Party, 1991, 2001c, 2001d) were obtained from shipboard laboratory measurements of moisture and density made on core samples. Wet mass, dry mass, and dry volume were measured on split core specimens. Moisture content, grain density, bulk density, and porosity were then calculated from the measured wet mass, dry mass, and dry volume values as described by Blum (1997). Measurements were taken from cores collected at ~1-m intervals for Sites 1173 and 808 and 2-m intervals for Site 1174.
Bulk density data were collected for Site 1173 (Figs. F5, F6) using the wireline Hostile Environment Litho-Density Sonde (HLDS) and the LWD Azimuthal Density Neutron (ADN) tool (Shipboard Scientific Party 2001a, 2002a). Bulk density was converted to porosity using values for mean grain density and pore water density. Measurements for both the wireline and LWD logs were collected approximately every 15 cm.
Corrections were applied to the raw core data to account for unloading from in situ conditions. As samples are unloaded from pressure at depth, the samples undergo a decrease in grain contact stress, which causes both an increase in porosity and a decrease in seismic velocity. Corrections to velocity and porosity values were made by comparing the core data to available wireline and LWD data at corresponding depths. Because the most complete combination of core and logging data are available for Site 1173, this site was used as a standard to calculate the in situ corrections applied to core measurements from all three sites.
The Site 1173 core porosity values were compared to both wireline and LWD porosity values. As the core and logging measurements were not sampled at the same depths, a data set with matching depths was needed. Using the depths where core measurements were available, logging (wireline and LWD) data were selected for depths located ±10 cm from each core sample depth. The difference between the core and logging data sets was then calculated by subtracting the values at the matching depths. The difference between the core and logging data appears to be constant with depth when comparing the core data to both the wireline (Fig. F5) and LWD (Fig. F6) data. For the core-wireline comparison, the mean fractional porosity offset is 0.0277 ± 0.0200 (core porosity is greater than logging porosity). For the core-LWD comparison, the mean difference is 0.0307 ± 0.0249. Taking the average of these two values, the in situ correction we have chosen to apply to the core data is a constant value of 0.0289, which has an error of ±0.0224. This correction was applied to the core porosity data for Sites 1173, 1174, and 808.
The wireline DSI data from Site 1173, reprocessed by Goldberg (this volume), was used to determine the in situ velocity correction. The wireline velocities deviate from a general trend of increasing velocity and begin to decrease at ~345 meters below seafloor (mbsf) (Fig. F7), which is the boundary between the upper and lower Shikoku Basin facies. Because of this change in velocity, the in situ correction was calculated based only on the data from above ~340 mbsf.
As for the porosity data sets, core velocity and logging velocity measurements were not taken at the same depths. Using the same method described for the porosity correction, logging data were selected for depths located ±20 cm from each core depth. The difference between the core and logging data was calculated by subtracting the data sets at the matching depths.
The difference between the core and wireline velocities is expected to increase with depth (e.g., Hamilton, 1979); however, the standard deviation of the mean difference between the data sets is too large for any depth-dependent trend to be apparent. Instead, we have chosen to use the average difference between the core and wireline velocity data of 43.39 m/s (with an error of ±29.05 m/s) as a constant in situ correction. This correction was subtracted from the core velocity data for Sites 1173, 1174, and 808.