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doi:10.2204/iodp.proc.343343T.103.2013

Core-log-seismic integration

Comparison of continuous LWD data and physical properties measurements from core

The MSCL-W NGR and noncontact resistivity (NCR) and discrete sample resistivity measurements made on cores recovered from Hole C0019E were compared to the LWD measurements from Hole C0019B. The distance between the tops of the two holes is thought to be ~5 m from the Chikyu sea surface coordinates. NGR and LWD gamma ray values cannot be quantitatively compared because they are recorded in different units (counts per second and gAPI, respectively). NGR values were overlain on the LWD values with a scale that allowed for comparison of trends in the data (Fig. F22). Because of the limited time available for processing core data, a unit conversion was not performed on the gamma ray data; also, NCR core measurements were not corrected for temperature variations downhole.

NGR values in Core 343-C0019E-1R (~176.5–185 mbsf) have more scatter than the LWD values and show a slight increase with depth that is not apparent in the LWD data (Fig. F22). The NCR values are overlain on the average deep, medium, and shallow button resistivity LWD curves in Figure F22. The reliable NCR values are generally lower than the LWD resistivity with the difference between the two measurements decreasing toward the base of Core 1R. Note, however, that within the NCR data, scatter toward higher values is a result of air in gaps and fractures in the measured whole-round core sections. The discrete resistivity measurements have a similar relationship to LWD resistivity; values taken on the core are generally lower and the difference between the two measurements decreases toward the bottom of the core.

Recovery of material in Cores 343-C0019E-2R and 3R (~648–660 mbsf) was highly fragmented and sorted, therefore MCSL-W values are likely not representative of intact formation; however, we compare them here. NGR values are more variable than LWD gamma ray measurements; they also show a stepwise decrease at the boundary between the cores (~650 mbsf), whereas in the gamma ray log an increase with depth trend is observed. NCR resistivity values of Cores 2R and 3R do not match LWD resistivity; they are lower by ~0.4 Ωm. There were no discrete resistivity measurements made on Cores 2R and 3R to compare to the LWD data.

Cores 343-C0019E-4R through 9R were recovered from ~688.5 to 725 mbsf. NGR measurements on the core are scattered but have a relatively consistent value range. The LWD gamma ray curve exhibits lows at 700 and 720 mbsf that are not observed in the NGR. The NCR correlates fairly well with the LWD resistivity from 688.5 to 720 mbsf with some scatter into higher values. From 720 to 725 mbsf, NCR values are still scattered and slightly higher than LWD resistivity. The discrete resistivity measurements in this zone agree well with the LWD data.

Cores 343-C0019E-10R through 21R were recovered from ~770 to 837 mbsf. Within this zone, NGR values match LWD gamma ray values well with some scatter. The zone of increased gamma ray values on the LWD logs corresponding to log Unit III (~820–835 mbsf) is seen in the NGR measurements but is ~3 m shallower. NCR values are scattered but match LWD data well except from ~818 to 826 mbsf. Within this zone NCR measurements show larger variation than LWD resistivity. The discrete resistivity measurements match the LWD data well. The spike in LWD resistivity (~836 mbsf) marking the top of log Unit IV was also observed on Core 343-C0019E-21R. Recovery was poor in this core, and consisted of pebbles. Therefore, no conclusions can be drawn regarding a depth shift between the holes.

Overall, the data from LWD and core measurements are in general agreement. Comparison of the two gamma ray data sets indicates the cored hole may be ~3 m shallower. The shift is observed in the lower section, where log Unit III is characterized by increased gamma radiation. In the shallower section, a lack of significant gamma ray excursions makes it difficult to correlate the data sets. The comparison of resistivity data sets does not clearly indicate any shift between holes; this may be partly due to the relatively low quality of NCR measurements and limited number of discrete sample measurements.

Seismic integration with LWD data

Seismic characteristics on the regional scale

Interpretations of regional seismic imaging generally identify three major seismic units in the lower trench slope where Site C0019 was drilled (Fig. F19; e.g., Tsuru et al., 2002). The uppermost unit (seismic Unit A) is wedge shaped with an acoustically chaotic character, without continuous reflectors, and corresponds to the frontal prism. The second unit (seismic Unit B) consists of continuous/semicontinuous subhorizontal reflectors interpreted as underthrust bedded layers or décollement surfaces. The lowermost unit (seismic Unit C) is the acoustic basement, probably corresponding to oceanic igneous crust.

Synthetic seismogram and time-depth calibration

To aid integration of LWD and seismic data, a synthetic seismic trace was created for Hole C0019B. The density and velocity logs used to determine the reflectivity series were calculated from resistivity-based porosity (see “Logging while drilling” in the “Methods” chapter [Expedition 343/343T Scientists, 2013]) and a wavelet extracted from Line HD33B. The wavelet and seismic data are Society of Exploration Geophysicists (SEG) reverse polarity (increases in acoustic impedance are troughs) and have a peak frequency of 48–50 Hz. The synthetic trace shows five possible significant reflectors at the seafloor and 70, 269, 368, and 839 mbsf.

An initial time-depth relationship was calculated at the LWD sampling interval (0.1524 m) for the borehole using the velocity log calculated from resistivity. The water column velocity was calculated from mudline depth on logs and the two-way traveltime of the seafloor reflector from seismic data. To preserve the character of the seafloor reflection, the calculated velocity log was spliced in at 3 m above the seafloor. The spliced log was used to determine Δt at the log sample interval, which was then integrated to get one-way traveltimes for each depth.

In the initial comparison of the synthetic seismic trace with the seismic data, the timing of the seafloor lined up well but the deeper section appeared slightly shifted. The three reflectors on the synthetic trace at 70, 269, and 358 mbsf were located within seismic Unit A. Observed coherent reflectors are rare in seismic Unit A and, where present, are often overprinted by steeply dipping noise, so confident correlations within the unit are difficult. The lower strong reflector on the synthetic trace was located within seismic Unit B but did not correlate with a specific observed seismic reflector using the initial time-depth relationship. To aid in better correlating the lower section, cross-correlation of the seismic and the lower 87 m of the synthetic was computed. The result indicates the synthetic reflector at 839 mbsf correlates best with the first strong reflector of bedded unit (9994 ms). To facilitate the fit of the synthetic and observed seismic traces, the rock velocities calculated from resistivity were increased uniformly by 2.5% and the time-depth relationship was recalculated. The fit of the synthetic seismic trace for Hole C0019B to the observed seismic data using the final time-depth relationship is shown in Figure F70.

Comparison of log and seismic units

The seismic section calibrated with the final time-depth relationship and the synthetic seismogram is compared to the four log units (Fig. F71; see “Log characterization and lithologic interpretation”). Log Unit I and most of log Unit II correspond to seismic Unit A. At the log Unit I/II boundary the reflection coefficient does not peak but reflectivity generally decreases across the boundary. Within log Units I and II, the reflectivity variations are coincident with resistivity excursions and likely represent structural or bedding features within the units.

The base of log Unit II and log Units III and IV are located within seismic Unit B. The top of log Unit III is coincident with a negative reflection coefficient, but because of the limited bandwidth of the synthetic and seismic data, it does not align with a peak on the seismic data at the location of Hole C0019B. The top of log Unit IV is coincident with a spike in resistivity and a strong positive-negative reflection coefficient pair. At 5 m below the top of the unit is a stepwise increase in resistivity and a strong positive reflection coefficient. The result in the synthetic seismogram is a tuned waveform with an apparent strong reflection just below the top of log Unit IV that matches well with the seismic section.

The seismic Unit A/B boundary is a continuous but relatively weak reflector on the seismic section. The boundary coincides with a slight stepwise increase in LWD resistivity and a small positive reflection coefficient at 785 mbsf. On the synthetic seismogram the boundary is represented by a relatively weak reflector consistent with the seismic character.

Geology and geophysics of Site C0019

Core, log, and seismic data are integrated, and the identified log units and structural domains, seismic units, lithologic units, and major structural features are compared in order to develop a unified interpretation of the geology and geophysics of the drill site. The log units are based on LWD gamma ray and resistivity responses and compare favorably with lithologic units identified through core analysis, particularly at the base of the borehole where contrasts in lithology and geophysical response are most dramatic (Fig. F72). The log Unit II/III, structural Domain 2/3, and lithologic Unit 3/4 boundaries correlate exactly. The changes observed across this boundary, specifically the abrupt change in bedding dip seen in RAB image logs and core, the increase in clay content of core samples as reflected by the concomitant increase in gamma radiation, K₂O, Al₂O, and MnO, and the presence of the scaly clay fault-rock that make up lithologic Unit 4, point to this boundary as a significant fault contact. The similarity of the lithologic units seen in core below the boundary with strata deposited on the Pacific plate at DSDP Site 436 (Shipboard Scientific Party, 1980) suggests this boundary is the plate interface (i.e., the décollement). Furthermore, the fact that observations of both the core samples and RAB image logs indicate that the entire sediment section above the boundary (structural Domains 1 and 2 and lithologic Units 1–3) is variable, often steeply dipping and fractured, which means it is consistent with a section comprising a shortened and accreted sequence of strata making up the frontal prism.

Above the plate boundary décollement and within the frontal prism, the log unit boundaries, structural domain boundaries, and lithologic boundaries do not correlate well. Boundaries demarcated on the basis of changes in stress indicated by borehole breakout patterns also do not correlate. Moreover, the entire prism (seismic Unit A) at Site C0019 lacks coherent or continuous reflectors and appears relatively transparent in seismic profiles. This likely reflects both the presence of inclined and faulted bedding and the relatively uniform properties of the sediment. Although the spot cores taken at 176.5–186.0 and 648.0–660.5 mbsf are considered to represent different lithologic units, both of these sections, as well as core from deeper intervals in the prism (688.5–729.0 and 770–821.5 mbsf), are mudstones composed predominantly of terrigenous silt and clay with varying amounts of vitric ash and biogenic silica. The subtle cyclicity in gamma ray and resistivity log response with depth in the prism may result from stacked and folded packages of sediment by reverse faulting. The apparent gradual decrease in porosity with depth from progressive consolidation interrupted by stepwise increases in porosity is consistent with such a structural interpretation. Given the variable but dominantly steep eastward dip of the sediment throughout most of the prism, even relatively small displacement on contractional faults would lead to significant repetition of strata as seen in a vertical borehole.

Although log, structural, and lithologic boundaries do not correlate well in the prism, correlations are noted between abrupt changes or discontinuities in different signals at several different locations in the prism. For example, at ~140 mbsf, an abrupt reversal in the progressive change in borehole breakout orientation correlates with a sharp change in bedding from moderate dips to very shallow dips, which likely reflects a fault contact. Another marked change in borehole breakout distribution occurs at ~550 mbsf, and this appears to correlate with a local low gamma ray response. The fault and bounding fractured zone identified on the basis of RAB image logs and resistivity curves around 720 mbsf correlates with the fault zone at 719–725 mbsf and the geochemical anomaly at 700 mbsf identified in core samples. In addition, this fault coincides with an abrupt change in bedding dip documented in both logs and core.

The final time-depth relationship determined from the synthetic seismic trace and log-based density and seismic velocity model, allows the identification of the plate boundary décollement in the seismic profile. The lowermost strong reflector in the synthetic seismic trace falls within the bedded seismic Unit B. After cross-correlation–based alignment, this strong synthetic reflector at 839 mbsf correlates best with the first strong seismic reflector of the bedded unit at 9994 ms two-way traveltime. In the borehole, the plate boundary décollement occurs at ~820 mbsf, ~20 m shallower than the prominent reflector. Because the first strong reflector is fairly continuous in the seismic profile, one can infer the position of the décollement both landward and seaward in the seismic profile. As shown by the in-line seismic profile that passes through the drill site, this reflector can be traced from the site ~1.2 km to the east-southeast and several kilometers to the west-northwest above the horst. The cross-line seismic profile that passes through the drill site indicates the reflectors are continuous several kilometers parallel to the trench as well. Approximately 1.2 km in-line to the east-southeast, the basement is down-dropped along a normal fault to form a prominent sediment-filled graben that spans the axis of the trench. A distinct reflector continues from the top of the horst to the east and into the sediment fill of the graben (Fig. F19). The seismic character along this reflector is consistent with the general seismic structure of the décollement documented at the drill site (i.e., a hanging wall characterized as chaotic and seismically transparent [seismic Unit A] and a footwall consisting of subhorizontal seismic reflectors representing the bedded sediments [seismic Unit B] conformable with the underlying igneous basement of the subducting plate [seismic Unit C]). Thus, the reflector that continues from the horst into the graben and cuts some sediment layers in the footwall defines the décollement at the base of the displaced and thickened prism. The seismic data support the simple interpretation that the prism extends some 5 km east from the drill site to the axis of the trench (also see Kodaira et al., 2012). A first-order palinspastic reconstruction of the prism, assuming constant area balancing, implies the displacement on the décollement at the drill site is on the order of 3 km. Although the deformation associated with the décollement is much more localized than that seen at other subduction décollements (e.g., Nankai, Barbados), the structure and fabric of the décollement at the drill site is compatible with displacements of this magnitude. Thus it is expected that the décollement at the drill site is likely continuous with the deeper portions of the plate boundary interface tens of kilometers downdip. A plate boundary décollement of this size and position is hypothesized as the locus of tectonic displacement of the subducting plate in the geologic past, as well as during the recent rupture that propagated to the trench during the Tohoku-oki event.

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