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doi:10.2204/iodp.proc.341.107.2014

Core-log-seismic integration

For the purposes of shipboard data correlation at Site U1421, we compared data displayed in the following two depth scales: WMSF (see “Downhole logging”) and CSF-A for logging and core data, respectively. Logging data were depth-matched between different tool strings using the gamma ray and resistivity logs recorded on each logging run and then shifted to the WMSF depth scale (see “Downhole logging” in the “Methods” chapter [Jaeger et al., 2014]). Logging data were recorded in Hole U1421A (see “Downhole logging”). Core physical properties were measured on cores from Holes U1421A–U1421C (see “Physical properties”), but because of the shallow total depths of Holes U1421B and U1421C (shallower than 40 m CSF-A), the majority of the data compared here were measured in the deepest hole, Hole U1421A.

For preliminary correlation between Site U1421 lithostratigraphic and logging units with features observed in seismic data, we converted lithostratigraphic and logging unit boundaries from depth in meters CSF-A/WMSF to two-way traveltime using the average velocity of each unit. In the shallowest ~95 m of the hole, average P-wave velocity was derived from core physical properties measurements using data from the PWC within lithostratigraphic Unit I and the core PWL in the shallowest section of lithostratigraphic Unit II (see “Physical properties”). Deeper than ~95 m CSF-A/WMSF, average P-wave velocity was derived from downhole sonic logging data (see “Downhole logging”). Detailed correlations for this site using fully integrated velocity measurements will require postcruise research.

Lithostratigraphy–downhole logging data correlation

Sediment core descriptions and downhole logging data obtained from Site U1421 were combined in order to examine the similarities and differences between the different data sets. We compared the lithostratigraphic units and recovered sediment with logging data recorded with the Sonic-induction tool string between ~95 and 700 m WMSF (see “Downhole logging”). Because of poor core recovery throughout the interval, identifying clear correlations is difficult. Nevertheless, we observed three areas of very low gamma ray and resistivity data that likely correspond to three prominent bioturbated diatom ooze intervals at 300–310, 490–510, and 600–610 m CSF-A in the core (Fig. F35). Another interval characterized by low gamma ray and resistivity data between 430 and 460 m WMSF could similarly correspond to a diatom ooze lithology. These low gamma ray features also correspond to washed-out intervals in the borehole at ~300, ~495, and ~590 m WMSF. Similar observations were made at Site U1418, which supports the interpretation that borehole size is influenced by the lithology. Based on these observations, the interpreted vertical offset between these features in cores and logs is ~13 m and is the same for all correlated biogenic units.

Physical properties–downhole logging data correlation

Although core-log comparison is limited by poor core recovery through most of the borehole, there is a similarity in general trends and features between core physical properties and log data where they overlap (Fig. F36). Core recovery was most complete in the upper ~100 m at this site, but this depth interval is typically not logged because of the placement of the drill pipe during logging operations (see “Downhole logging”). However, gamma ray data recorded through the drill pipe in Hole U1421A allow a relative comparison of logging data with core NGR data. The log gamma ray signal, though highly attenuated by the pipe, shows elevated values, increasing with depth, between the seafloor and ~18 m WMSF. There is a similar increase in the core NGR values; these data have been corrected for volume using GRA density. Gamma ray values drop sharply deeper than ~20 m in both log (WMSF) and core (CSF-A) depth scales. It is difficult to establish whether there is a depth offset between core and log data in this shallow interval, but detailed comparison of the in-pipe gamma ray log with NGR suggests a potential offset of 0 to 2 m (see “Downhole logging;” Fig. F33). One potential explanation for a reduced offset shallow in the borehole (<2 m) versus greater offset deeper in the hole (perhaps 13 m based on correlated biogenic units) is that there is some cumulative offset with depth. Below ~100 m CSF-A/WMSF, where core recovery drops to an average value of ≤10%, there are no distinct features in the core NGR data to compare with logging data.

The P-wave velocity log indicates generally higher formation velocities than core-based velocity measurements from Site U1421 (Fig. F36). In the interval between ~60 and 95 m CSF-A (the deepest PWL measurements), the typical P-wave velocity measured on the core with the WRMSL PWL is fairly consistent with the P-wave velocity log in the shallower section of the logged interval of the borehole (Fig. F37). Discrete PWC measurements on cores, collected through the entire cored interval where recovery permitted, generally show lower velocity values than core PWL and the velocity log. This discrepancy is likely related to poor core recovery, leading to discrete measurements being biased toward the properties of easier to recover lithologies. In addition, discrete PWC measurements are point samples, whereas the downhole velocity log integrates potentially lower velocity matrix and potentially higher velocity clasts. Discrete P-wave data show similar trends to the velocity log, in particular capturing a lower velocity interval between 490 and 500 m CSF-A/WMSF and an increasing trend in velocity with depth between ~595 and 670 m CSF-A/WMSF. The high quality of the sonic logs acquired in Hole U1421A (see “Downhole logging”) suggests that these data will be valuable for more detailed postcruise correlations between borehole data and seismic images.

Seismic sequences and correlation with lithostratigraphy and downhole logs

Two seismic profiles cross Site U1421: GOA2503 (Fig. F3), acquired in 2004 aboard the R/V Maurice Ewing, and STEEP07 (Fig. F1), acquired in 2008 aboard the R/V Marcus Langseth. In preparation for core-log-seismic integration, we interpreted key seismic horizons that mark a change in acoustic facies or a reflector truncation surface. Horizons H1, H2, and H3 were previously interpreted by Worthington et al. (2008, 2010). Here, we name subhorizons using the Worthington et al. (2008, 2010) naming convention. Additional internal packages are broken out, defined either by a high-amplitude, continuous reflector or a minor change in seismic character.

The seismic profiles capture the correlative slope facies of the sequences imaged on the shelf near Site U1420 (Figs. F37, F38). At Site U1421, Horizon H1 denotes the correlative conformity associated with the shelf unconformity seen at Site U1420. The lithostratigraphic Unit I/II boundary (57 m CSF-A) and the logging Unit 1/2 boundary (202 m WMSF) correlate to traveltime positions shallower than Horizon H1. Lithostratigraphic Unit I is characterized by a massive clast-rich diamict. The lithostratigraphic Unit I/II boundary is therefore likely located within a package of semichaotic, high-amplitude reflectors (Fig. F37). In lithostratigraphic Unit II, there are multiple intervals in which core recovery is <10% and the recovered material consists primarily of washed pebbles and drilled clasts of varying lithologies (see “Lithostratigraphy”). The logging Unit 1/2 boundary maps to the bottom of a semiparallel, high-amplitude package. Below this package is a section of acoustically semitransparent and semichaotic character (Fig. F37).

Using the sonic log for two-way traveltime–depth conversions, we correlated the remaining logging unit boundaries with the seismic data (Fig. F37). Further investigation, including developing synthetic seismograms, is required for more definitive correlation. The logging Unit 2/3 boundary, defined by significant decreases in velocity and resistivity (see “Downhole logging”), may coincide with a horizon within a high-amplitude package above Horizon H2A (Fig. F37). The logging Unit 3/4 boundary, defined by increases in velocity and resistivity and a change in log character (see “Downhole logging”), appears to correlate within the semitransparent facies between Horizons H2A and H2B. The base of logging Unit 4 and the deepest drilled depth in Hole U1421A coincide with the top of Horizon H2B, which is a high-amplitude reflector that separates two semitransparent facies with internal parallel structure.

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