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doi:10.2204/iodp.proc.314315316.122.2009

Core-log-seismic integration

No logging was done during Expedition 315. The integration of LWD data with seismic reflection data at sites drilled during Expedition 315 was done during Expedition 314. Reference seismic profiles are prestack depth-migrated lines from the Center for Deep Earth Exploration (CDEX) 3-D block. Depths in the seismic line were fitted to LWD check shot data provided by the Expedition 314 Scientists, and we will rely on their work.

Integration of cores with LWD data relies primarily on lithologic boundaries, magnetic susceptibility, and NGR MSCL data. Details about MSCL measurements are described in “Physical properties.” Magnetic susceptibility data were used for correlation between cores from different holes at the same site. NGR data were used for correlation between the cores and the log. The LWD geoVISION resistivity tool was used at Sites C0001 and C0002 and comprises NGR and resistivity measurement and imagery. Sonic log slowness data, gamma ray density, and neutron porosity are available from the other tools used and may also be compared with MSCL measurements on cores or with measurements on discrete samples (MAD and P-wave velocity measurements; see “Physical properties” for details). Core low-field magnetic susceptibility and NGR emissions were measured with the MSCL on whole-round sections every 4 cm and every 15 or 16 cm, respectively. The magnetic susceptibility core log was of good quality overall and was filtered for voids and disturbed intervals according to the core density from gamma ray attenuation MSCL-W measurements. On HPCS and ESCS cores, magnetic susceptibility measurements made at intervals with measured density <1500 kg/m³ (70% porosity) were discarded. On RCB cores, the threshold was set to 1600 kg/m³ (65% porosity).

Mechanical difficulties prevented systematic NGR measurements on whole-round cores. Measurements on the archive half after splitting yield data with a higher noise level, but these data are still usable as a correlation tool. The unit of the MSCL-W gamma emission data is counts per second, and the only calibration is an ambient noise measurement (~40 cps for MSCL-W). Values below 15 cps (or 55 cps before noise correction) were discarded because they often correspond to voids. Assuming the liner has negligible radioactivity, the ambient noise level should be the same when measuring the whole-round core or the archive half. Considering the symmetry of the gamma ray counting system, which comprises four detectors at 90° angles, the number of counts from the half core should be half the number of counts from the whole-round cores. Comparison of data acquired on the same cores shows that applying a 38 cps ambient noise correction and a corrective factor of two to the archive half counts yields results consistent with measurements on whole-round cores (Fig. F22).

LSF is the depth reference. There are two definitions of core depth. In the standard definition of CSF (IODP Method A), the depth in the core is added to the depth of the drill bit at the beginning of coring. This definition leads to overlaps between expanded HPCS cores and, sometimes, between RCB cores when drilling fluid and/or cuttings enter the core barrel. The compressed core depth below seafloor (CSF-B; IODP Method B) is also given by J-CORES and corrects the discrepancy between the length of expanded cores and the advance of the drill bit by applying an arbitrary linear correction over the whole length of core. The correlation between core depth below seafloor and LWD depth below seafloor is based on this depth (rather than the Method A depth). Depth transfer functions are defined by linear regression between correlated peaks. Cores with <5 m of recovery do not generally provide useful constraints. On intervals with better recovery, several peak-to-peak correlations can sometimes be done within a single RCB core, and the Method B core depth log appears condensed by 5% to 30% with respect to LWD data. This indicates that partial recovery can spread over most of the cored interval, in contrast to the assumptions of the Method A depth definition.

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