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doi:10.2204/iodp.proc.314315316.114.2009

Log-seismic correlation

Kumano Basin seismic stratigraphy

A series of seismic units lie within the southwestern Kumano Basin in the area imaged by the Kumano 3-D seismic volume (Gulick et al., 2007; Moore et al., 2007). These seismic units (Fig. F14; Table T6) show the evolving uplift within the southwestern basin and in places can be correlated to specific features within the logs. To aid correlation of bedding strike and dip observed in the image logs, we report the dip and dip directions for each of the sequence boundaries that separate our defined seismic units (Table T6).

Seismic Unit Kumano 2/3/4 (undifferentiated) appears to dip to the southeast based on the dip of the underlying horizon, Top of Kumano 5. In contrast, the underlying seismic Units Kumano 5–11 all dip to the northwest, based on their sequence boundaries. As the Kumano Basin sequences were deposited, there appears to have been a gradual shift from more northward dip direction in the oldest sediments to more northwestward dips in the younger sequences, ending with the deposition of Kumano 5. Kumano 5 shows lower dip than the underlying sequences. Lastly, at the base of the sequence is an angular unconformity between the Kumano Basin units and a seismic unit called Lower Sediments 1. The unconformity dips to the southeast, whereas the underlying horizon, Top of the Accretionary Prism, dips to the north (Table T6); therefore, the dips of individual strata within seismic Unit Lower Sediments 1 may vary (Fig. F14).

Overall log unit correlation

Logging Unit I (Fig. F14) corresponds to seismic Unit Kumano 2/3/4 (undifferentiated) (Gulick et al., 2007). Depths to the boundaries of the seismic units and their structural orientations are shown in Table T6. The structural orientations of the faults near the borehole are shown in Table T6. Logging Unit I appears to consist of hemipelagic sediments that dip <1° to the southeast (Fig. F14). The base of logging Unit I correlates with the positive peak below an unconformity in the seismic data. We mapped this unconformity using the positive peak at the top of the unconformity and named it horizon Top of Kumano 5. The interval between the base of logging Unit I and horizon Top of Kumano 5 is ~30 m; however, the difference may arise from which peak or trough is selected to trace the top of seismic Unit Kumano 5. This boundary is subjective enough to allow for a 30 m difference.

Logging Unit II (Fig. F14) corresponds to the majority of seismic Unit Kumano 5, all of Units Kumano 6–10, and a portion of Unit Kumano 11. These are older forearc basin sediments, now dipping gently to the northwest, as will be discussed below in greater detail (Table T6). Included within logging Unit II are a prominent BSR, a north-northwest–dipping normal fault, an interval of interpreted gas hydrate, an interval of negative polarity reflection suggestive of trace amounts of free gas at the base of the BSR, and a dipping negative polarity reflection deeper than the BSR that also suggests the presence of free gas.

Although there is an obvious unconformity in the seismic data between seismic Units Kumano 11 and Lower Sediments 1 that suggests a hiatus across the boundary, there appears to be little lithologic or physical property contrast. Therefore, logging Unit III (Fig. F14) corresponds to the middle part of the northwest-dipping seismic Unit Kumano 11, the onlap of Kumano 11 onto the top of southeast-dipping seismic Unit Lower Sediments 1, and the Lower Sediments 1 strata. Logging Unit IV (Fig. F14) corresponds to the drilled portion of the old accretionary prism. The seismic data show that the top of the prism is a rough surface that is difficult to pick definitively everywhere but locally produces a bright low-frequency reflection. The abrupt changes in most of the logs at ~947 m LSF seem likely to accurately locate the top of the accretionary prism.

To facilitate discussions involving the correlation of log and seismic reflection data at this site, we present a series of figures (Figs. F39, F40, F41, F42, F43, F44, F45) in which we have superimposed logs over a portion of the check shot–corrected prestack depth migrated (PSDM) seismic reflection profile. The seismic reflection profile was produced by converting the PSDM profile from depth to time using the PSDM velocity model, and converting the PSDM time section from time to depth using the velocity depth function obtained from the check shot survey at this site. This process should yield the best seismic profile to compare to logs and to estimate the depths to significant stratigraphic and structural features (Table T6).

Log-seismic correlation in zones of interest

Logging Unit I/II boundary

The strata of logging Unit I, which in general correlates with seismic Units Kumano 2/3/4 (undifferentiated) (Fig. F14), show local evidence of erosion and normal faulting. The boundary between logging Units I and II likely correlates with the top of seismic Unit Kumano 5 despite the depth difference, as discussed above (Fig. F46A). P-wave velocity increases at the boundary (Fig. F46B), suggesting a transition from higher to lower porosity hemipelagic sediments (possibly the result of compaction and consolidation). The gamma ray value abruptly increases to a higher value in logging Unit II than that in Unit I (Fig. F46C), suggesting that the top of seismic Unit Kumano 5 is an abrupt boundary.

Zones A and B in logging Unit II

The base of Zone A matches exactly with the BSR in the seismic section (Fig. F47A). This boundary is clearly indicated by high resistivity above the BSR in Zone A and lower resistivity below (Fig. F47B), suggesting the existence of gas hydrate in Zone A. P-wave velocity beneath the BSR is lower than that in Zone A (Fig. F47C), which is consistent with the presence of trace amounts of free gas.

The upper boundary of Zone B corresponds to a bright reflection with negative polarity that dips to the northwest (Fig. F47A). Regionally it is a high-amplitude, negative polarity reflection only below the depth of the BSR, and the seismic image is poorly focused in the same region (see Fig. F4 in the “Expedition 314 summary” chapter). Zone B correlates with a low P-wave velocity zone beneath a zone of higher velocity (Fig. F47C), suggesting that Zone B may be a zone of permeable coarser sediments where small amounts of free gas may be migrating updip.

Logging Unit III/IV boundary

The boundary between logging Units III and IV corresponds to the seismic reflection Top of Accretionary Prism (Figs. F14, F48A). This major boundary lies between the basin sediments and the apparent older accreted sequence. The PEF log curve shows a significant change at the boundary, with a larger value above and lower value below (Fig. F48B). The gamma ray curve exhibits a zone of low values at the top of logging Unit IV (Fig. F48C), suggesting that a sandy interval may lie near the top of accretionary prism, aiding in the generation of a strong seismic reflection.

Structural orientations from seismic observations

In order to allow accurate correlation between the regional seismic observations of faults and stratigraphy and the observations in the logs, we computed orientations for features of interest. For these calculations, we used the prestack time-migrated seismic volume combined with the velocities provided by the check shot data. We used the horizontal plane (time slice) at 2888 ms, the depth where Hole C0002A intersects a normal fault, to determine the strikes of nearby faults, and we generated seismic sections from the 3-D volume oriented in the direction of maximum fault dip to determine their respective dips. The accuracy of the reported strikes and dips for the faults near Site C0002 is estimated at ±5° based on the velocity-based errors inherent in using the prestack time-migrated volume and the human error of determining the exact strike along a curved fault plane. Dip directions for horizons were determined by digitizing a 100–400 m long transect centered on the borehole orthogonal to the two-way traveltime contours of each horizon. The dips were then calculated from the end points of these digitized transects with time to depth conversion computed using the check shot velocities. The estimated error for the reported dips for the seismic horizons close to the borehole is ±1°, whereas the estimated error in the dip direction is ±5°. The dips within the prism are less reliable because of the reflections being laterally discontinuous and affected by migration algorithms; therefore, the error is estimated at ±5°.

The southern boundary of the Kumano Basin is cut by numerous normal faults (Fig. F38), and Site C0002 penetrated one of these faults at 253 m SSF (Table T6). The trend of these faults changes along strike. However, near Site C0002 three of four faults show remarkably consistent strikes and dips (Table T6). These faults all strike east-northeast, whereas a potentially older fault strikes more northeast (Table T7; Fig. F38). Regionally there are distinct fault populations, including a dominant population of normal faults striking east-northeast, a smaller population striking northeast, a few faults related to the seafloor depression to the south (Fig. F38) that strike east, and a few faults striking northwest. This last population may be responsible for accommodating some of the 3-D complexities of the basin. The east–west striking faults close to the seafloor depression are either near vertical or dipping south (Martin et al., 2007).

Check shot survey and vertical seismic profile data

A very complete check shot survey was acquired at Site C0002. One-way traveltime seismic records were obtained at 72 depths from the seafloor to 1355 m LSF in Hole C0002A (Fig. F49). Of these observations, 63 were of adequate signal strength to be used in the check shot analysis (Table T8). Data were acquired during each pipe connection as the pipe was lowered and as it was pulled out of the hole. Each depth observation involved acquiring and stacking multiple shots, typically 15 per observation on the way down and 8 per observation on the way up. Noisy traces and traces with poor first arrival waveforms were deleted or edited. The remaining traces were filtered (trapezoidal, zero phase, and 30-40-150-200 Hz band-pass) and stacked to produce the traces shown in Figure F48. The first arrival wavelet is unambiguous on all traces, although noise makes identifying the true first break difficult on a few of the traces.

The first arrival times were picked manually. These are the “raw first arrival” times in Table T8. We applied a damped least-squares inversion to the observed depth/time data (Lizarralde and Swift, 1999). This inversion determines a smooth velocity depth curve by varying the arrival times by amounts that are within the data uncertainty. We estimated the uncertainty of the arrivals to be ~1 ms. We used an inversion damping coefficient of 0.05 because it produced a χ² value consistent with the optimal balance between over- and underfitting the data. The smoothed interval velocities and adjusted arrival times are shown in Table T8. We used the smoothed first arrival times and the observation depths as the check shot function, which we then used for the time-depth correction of seismic reflection profiles and synthetic seismogram preparation.

The smoothed interval velocity curve and sonic log interval velocity data are generally similar (Fig. F50). The most significant difference between the two is between ~550 and 675 m LSF, where the check shot velocity is 100–200 m/s lower than the sonic log velocities. The discrepancy in this interval may be a result of the smoothing method applied to the check shot arrival times. Both curves show a general pattern of steep increases of velocity separated by intervals of approximately constant velocity. There appear to be ~4 such cycles in the hole. The check shot velocities show a low-velocity zone that has a lowest point at ~540 m LSF.

We also obtained a near-zero offset VSP using the check shot data (Fig. F51). In the VSP, we observe clear upgoing reflection arrivals originating from the seafloor to the top of the accretionary prism. The signal-to-noise ratio is not high for these arrivals, but they seem to correlate reasonably well with packages of reflections in the seismic profile at the site.

Synthetic seismogram

In order to construct a synthetic seismogram for Site C0002, we used the IDRO log and the sonic (DTCO) log. The density log was edited to remove densities associated with density-derived caliper values (DCAV) of 9.5 inches or larger and DTCO. It should be noted that we later learned that the ultrasonic caliper (HORD) might have been superior to the density-derived caliper as a log to filter density values, but the change in the result of the synthetic seismogram would likely be inconsequential and not worth the effort of repeating the calculations. Because both the density and sonic logs have gaps in them, we used a 51-point running average over the gaps in the logs to fill them in. Where the gaps were greater than ~50 points, we linearly interpolated between the final running averaged values within each gap. Because of the high caliper values for the seafloor and uppermost sediments, where we were jetting-in, we created a seafloor curve shaped similar to that observed in Hole C0001C with values derived from the highest recorded IDRO densities in Hole C0002A.

The resulting synthetic seismogram correlates with the observed reflections in the seismic data in some intervals such as at the BSR (404 m SSF) (Fig. F52). However, both on the edges of the longer gaps in the logs and at locations where high caliper values correlate with low gamma ray values, the gap-filling process introduced artifacts. In some cases, the artifacts were false reflections produced in the synthetic seismogram. In other cases, expected reflections are not produced. A variety of smoothing, editing, and filtering techniques were tested, but a highly correlated synthetic seismogram was not achieved.

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