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doi:10.2204/iodp.proc.341.103.2014

Core-log-seismic integration

For the purposes of shipboard data correlation for Site U1417, we compared data displayed in the following two depth scales: WMSF (see “Downhole logging”) and CCSF-B (a compressed composite depth scale; see “Stratigraphic correlation”) for logging and core data, respectively. Logging data were depth-matched between different tool strings using the gamma ray logs recorded on each logging run and then shifted to the WMSF depth scale based on the step increase in gamma ray across the seafloor (see “Downhole logging” in the “Methods” chapter). For Site U1417, the maximum observed depth shift was ~4 m. Adding this potential error in depth to uncertainties in the depth of a given feature in CCSF-B results in an ~10 m potential depth error when comparing core-based measurements or lithologic boundaries to downhole logs. Core physical properties were measured in Holes U1417A–U1417E (see “Physical properties”), whereas logging data were recorded only in Hole U1417E (see “Downhole logging”).

To begin correlating lithostratigraphic and logging units with features observed in the seismic data, we converted lithostratigraphic and logging unit boundaries from depth in meters (CCSF-B/WMSF) to two-way traveltime (TWT) using the average velocity of each unit. Average P-wave velocity was derived from physical properties measurements using data from the WRMSL PWL at depths shallower than ~220 m CCSF-B and from the downhole sonic logs deeper in the hole (see “Physical properties” and “Downhole logging”).

Lithostratigraphy–downhole logging data integration

We combined sediment core observations and physical properties data with downhole logging data from Site U1417 to (1) evaluate how representative the recovered cores are relative to the portion of the sedimentary section that was logged, (2) determine the nature and extent of sediment not recovered in the XCB/RCB drilling process, and (3) examine whether observed sedimentary units and features can be correlated to borehole data and ultimately be described at higher vertical resolution at Site U1417. We first compare magnetic susceptibility measurements made on whole-round and split cores from the shipboard core logging systems (WRMSL and SHMSL; see “Physical properties”) with temperature-corrected downhole data from the deep-reading magnetic susceptibility sensor on the triple combo tool string. We then evaluate whether similar lithostratigraphic units can be characterized by downhole resistivity and natural gamma ray measurements also recorded with the triple combo tool string (see “Downhole logging”).

In Figure F58, we compare lithostratigraphic unit boundaries, core recovery, the distribution of diamict intervals, sand-rich units, and magnetic susceptibility measurements recorded in the borehole and on cores. In general, logged and whole-core magnetic susceptibility exhibit similar trends and variability when compared over the same measured interval. We observe that transitions between intervals of high and low magnetic susceptibility in the log data correspond to lithostratigraphic boundaries in the core. These changes are particularly evident at the transitions from lithostratigraphic Units II to III, Units III to IV, and Subunits VC to VD. In areas of lower core recovery, the exact lithostratigraphic unit boundaries may have an error range of a few meters due to recovered intervals being assumed to reside at the top of each cored interval. We also observe an association between relatively high downhole log magnetic susceptibility values and occurrences of sand. Another primary observation is that the highest magnetic susceptibility measured in the downhole logs between 350 and 440 m CCSF-B corresponds to the occurrence of diamict intervals in lithostratigraphic Subunit VA and at the lithostratigraphic Subunit VB–VC transition (Fig. F58). A photograph of a representative core section (341-U1417D-53X-1, 0–75 cm) shows two intervals of diamict with high magnetic susceptibility values separated by an interval of mud with low magnetic susceptibility values (Fig. F58). The image also shows that the diamict intervals consist of fractured and “biscuited” blocks. We speculate that these diamict intervals are responsible for the high magnetic susceptibility values observed in the log data and that poor core recovery (21% in Core 53X) prevented full representation in the cored record. Lithostratigraphic Unit III is interpreted to be an ice-rafted diamict (see “Lithostratigraphy”), and poor borehole conditions across this interval (270–300 m CCSF-B) may have reduced the amplitude of the logged magnetic susceptibility signal. The base of the ice-rafted diamict interval of lithostratigraphic Unit III correlates with an improvement in borehole conditions for logging (logging Unit 1/2 boundary; see “Downhole logging”) to within meters. Overall, the logging data provide evidence for the occurrence of diamict intervals at Site U1417. Based on these initial observations, we speculate that the deeper diamicts, possibly representing sediment gravity flow deposits, are a primary part of the sedimentary section between 350 and 400 m CCSF-B and between 450 and 500 m CCSF-B and the thickness of these lithologies are underestimated in the cored intervals.

In Figure F59, we compare downhole resistivity; total gamma ray; and K, Th, and U in standard deviation units to the lithostratigraphy, as well as the distribution of sand, ash, and volcaniclastic lithologies. Resistivity and natural gamma ray profiles are generally low in the shallowest 150 m of the logged interval (~80–230 m WMSF) and exhibit a step increase at ~250 m WMSF. Resistivity increases from 250 m WMSF to the base of the logged interval but continues to exhibit a high degree of variability. Low resistivity values in the shallowest 250 m of the sedimentary section may be related to the occurrence of mud-dominated lithologies. However, borehole dimensions are highly variable within this interval and it is difficult to attribute these variations solely to lithology. In general, we observe multiple examples where volcaniclastic sand/silt corresponds with increases in standardized downhole profiles of total gamma radiation and K. Sand layers generally have high magnetic susceptibility and low gamma ray counts—a good example of this relationship occurs at 450 m CCSF-B/WMSF. The highly resolved NGR profiles provide an opportunity to map the distribution of facies throughout the logged interval, particularly where there is poor recovery in the drilling process.

Physical properties–downhole logging data correlation

In general, there seems to be a good correspondence between the physical properties and logging data, with a vertical offset on the order of a few meters. The focus of shipboard correlation was the depth interval from ~305 to 615 m CCSF-B/WMSF within logging Subunit 2B, where reasonable borehole conditions resulted in logging data of higher quality (see “Downhole logging;” Fig. F60).

The natural gamma ray log shows good agreement with core NGR to within a few meters depth (Fig. F60); core NGR has been corrected for volume using GRA density (see “Physical properties”). The high degree of correspondence indicates both that the downhole gamma ray log is not compromised by the variability in borehole diameter within this part of the hole and that the volume correction significantly improves the fit between the downhole log and core-based NGR data. The increasing trend with depth in both data sets supports the interpretation of higher natural radioactivity inputs likely associated with relatively muddier lithologies deeper at Site U1417.

The bulk density data also show variable agreement between downhole log, core logger, and discrete core samples. There is more scatter in the GRA density from the core logger; however, the downhole density log values generally correspond to the higher end in the range of GRA density values (Fig. F60). Discrete MAD measurements overlap with the downhole density log, showing a similar range in values. Because core measurements are necessarily limited to recovered intervals and lithologies, comparison of the log and core data suggests that recovery in this part of the hole is biased toward high-density lithologies and that lower density lithologies may not be fully recovered in cores. Although the MAD data are interpreted as showing a change in trend to increasing density below ~470 m CCSF-B (see “Physical properties”), the density log suggests that the change in trend may occur at a shallower depth (~420 m WMSF). There are corresponding changes to increased values in the resistivity and P-wave velocity logs at the same depth, suggesting that this depth may represent a significant transition.

Discrete P-wave velocity measurements on core show good correspondence with the P-wave velocity log to ~430 m CCSF-B/WMSF (Fig. F60). Deeper in the borehole, the P-wave velocity log shows generally higher velocities than the discrete core data. The lower velocity for the discrete measurements at these deeper intervals could be due to biased core recovery, choice of lithology selected for sampling, or fracturing of the split-core samples during measurements given the increasing induration downcore. However, below ~420 m CCSF-B, isolated elevated velocities (>4000 m/s) are measured on specific discrete samples (not displayed at the scale of Fig. F60) and correlated to cemented intervals in the recovered core, whereas lower velocities are correlated to diatom ooze (see “Lithostratigraphy” and “Physical properties”). The P-wave velocity log, with a sampling interval of ~15 cm and a vertical resolution of ~100 cm, may show intermediate velocities as it averages across centimeter-scale variations between faster and slower lithologies.

Magnetic susceptibility data show reasonable agreement between log and core measurements (Fig. F60). The magnetic susceptibility log, from the deep-reading sensor of the MSS, has been corrected for temperature (see “Downhole logging”). Core magnetic susceptibility data from the loop have been corrected for volume using GRA density (see “Physical properties”). Although there is considerable scatter in the core magnetic susceptibility, there is a clear distinction in the range and amplitude of values across the logging Subunit 2A/2B boundary at ~470–480 m WMSF. Both log and core data show higher susceptibility values above ~470 m CCSF-B/WMSF.

Higher resolution comparison of core and downhole logging data will require detailed correlation of these two data sets. Although core recovery is lower in the XCB and RCB cored intervals (deeper than ~220 m CCSF-B), which could limit the success of core-log integration, Figure F60 shows distinctive features in both data sets that can likely be matched more precisely. These efforts can be undertaken postcruise, utilizing the total gamma radiation and magnetic susceptibility data and more detailed comparison to both visual core description and seismic images.

Seismic sequences and correlation with lithostratigraphy and downhole logs

Seismic Lines MGL1109MSC01 (Fig. F61) and MGL1109MSC14 (Fig. F62), acquired in 2011 aboard the R/V Marcus Langseth, cross Site U1417 (Walton et al., submitted). The primary seismic sequences on each profile, Sequences I–III, are interpreted after Reece et al. (2011). In preparation for core-log-seismic integration, Sequence I was divided into three subsections, IA–IC. Subsection IC was further subdivided into Intervals IC1 and IC2. Each of these sequence boundaries defines either a change in dominant seismic facies or a truncation surface.

Seismic Sequence III is characterized by smooth, continuous reflectors and limited seismic transparency. At Site U1417, the Seismic Sequence II/III boundary (~5805 ms TWT) is defined by high-amplitude variation. Below the two prominent reflectors at the top of Sequence II, the section loses amplitude and shows similar seismic characteristics to Sequence III. Based on our estimated depth to TWT conversion, Seismic Sequence III corresponds to lithostratigraphic Unit I: dark gray mud with thin beds of volcanic ash (Subunit IA) and gray mud with thin beds of volcanic ash and diatom ooze (Subunit IB). An increase in lonestones (outsized clasts) in Subunit IB may correspond to a package of higher amplitude reflectors at ~5.71 s TWT (using a velocity of 1518 m/s) bounded above and below by semitransparent facies (Fig. F30).

Seismic Sequence II is characterized by smooth, continuous reflectors that are semitransparent in the seismic profile. Based on our depth to TWT conversion, this sequence corresponds to lithostratigraphic Units II–IV: gray mud with 1–5 cm thick interbeds of fine sand and coarse silt (Unit II), thick beds of diamict interbedded with gray mud (Unit III), and a highly bioturbated gray mud with diatom-bearing intervals (Unit IV). Two check shots close to 211 m WMSF correlate to the seismic data at 5.87 s TWT within Seismic Sequence II. The Unit III/IV boundary appears to map to the lower part of Seismic Sequence II, where a pair of medium-amplitude reflectors lies just above the high-amplitude package that defines the boundary between Seismic Sequences II and I. The boundary between logging Units 1 and 2 may correlate to increased reflectivity in the lower part of Sequence II (Fig. F30). However, establishing the precise position of lithostratigraphic Units III and IV relative to the Sequence II/I boundary will require more core-log-seismic analysis.

The Seismic Sequence II/I boundary (~6000 ms TWT) is located at the top of a prominent grouping of high-amplitude reflectors that are slightly more chaotic and discontinuous than those observed in Sequence II. Sequence I can be divided into three distinct seismic facies packages: Sequence IC, characterized by high-amplitude, semicontinuous reflectors; Sequence IB, a seismically transparent section with faint, semicontinuous reflectors; and Sequence IA, a high-amplitude chaotic layer overlying acoustic basement. Sequence IC is composed of two distinct packages (Sequences IC1 and Sequence IC2) divided by a truncation surface. The base of Sequence I was not penetrated at Site U1417; based on correlation with Site 178, Sequence I is ~400 m thick at Site 178 (Fig. F30). With our estimated velocities, the lithostratigraphic Unit IV/V boundary at ~350 m CCSF-B maps to ~6.05 s TWT, which is at or just below the top of Seismic Sequence I (Fig. F30). A series of thick high-amplitude reflectors comprise this boundary and may be related to the presence of cemented intervals that inhibited core recovery. Lithologically, the boundary between Units IV and V represents a change from to highly bioturbated gray mud with diatom-bearing intervals (Unit IV) to gray mud with diamict, interbedded silt and sand, and diatom ooze (Unit V).

Because of the lack of check shots deeper in the borehole, precise correlation between Seismic Sequence I, lithologic Units IV and V, and logging Unit 2 will need to be undertaken postcruise; however, some comparisons between lithofacies, seismic facies, and log character can be discussed. In general, the increased velocity and density contrasts within logging Subunit 2A and the upper part of Subunit 2B likely correspond to the series of brighter reflectors that define Seismic Sequence IC, and in turn these may correlate with the various lithofacies of lithostratigraphic Unit IV and Subunits VC or VD. The boundary between logging Units 2A and 2B may be associated with a large negative-amplitude reflector that separates seismic Subunits 1B and 1C at ~6.19 s TWT (Fig. F30). If that correlation is validated, then the remaining lithostratigraphic Subunits VE–VJ and the lower part of logging Unit 2B all lie within Seismic Sequences IB and IA. Correlations deeper in the borehole can be checked against results from Site 178 (Shipboard Scientific Party, 1973), located ~1.5 km away, by using the basement depth of 780 m DSF and creating a pinned traveltime/depth boundary at the top of the basement during the creation of a synthetic seismogram.

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