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doi:10.2204/iodp.proc.314315316.116.2009

Log-seismic correlation

Overall logging unit correlation

Logging Unit I/II boundary and the base of slope sediments

The base of the slope sediment section corresponds to the logging Unit I/II boundary (Figs. F34, F35; Table T7). Sonic velocity increases at the boundary from little more than drilling fluid velocity to ~1600 m/s (Fig. F36). Gamma ray and resistivity logs similarly increase (Figs. F37, F38). The change in gamma ray value suggests that Unit II is somewhat more clay rich than Unit I. The caliper log shows that hole size drops to nearly in-gauge at the boundary and continues nearly in-gauge in the uppermost ~20 m of Unit II (Fig. F39). The transition in each of the logs is gradual rather than abrupt. These gradual changes likely account for the relatively low frequency character of the reflection at the base of the slope sediments.

Logging Subunit IID: fault zone between thrust sheet and underthrust sediments

Logging Subunit IID corresponds to a thick zone of roughly parallel, northwest-dipping reflections caused by a system of faults along which older accreted rocks have been thrust over slope deposits (Figs. F34, F40; Table T7). In Hole C0004B the zone of parallel reflections form a peak, wide-trough, peak pattern from ~252 to 323 m SSF. However, the seismic reflections on both the inline and cross-line (Fig. F2) adjacent to the hole show considerable 3-D variation in this pattern on a scale of 50–100 m. Therefore, it is not reasonable to expect an exact correlation between log values in a single hole and the seismic data, which smear the image laterally on a scale of 20–40 m.

The velocity in the upper half of the dipping reflection package decreases from ~1900 to ~1800 m/s over the range from 243 to 291 m LSF (Fig. F40). This corresponds to the upper peak and about half of the broad low-amplitude trough. The sonic log begins a significant increase in interval velocity, from ~1900 to 2100 m/s, at 291 m LSF. Velocity remains high to ~313 m LSF before decreasing to ~2000 m/s. This high velocity corresponds to the base of the broad trough and top of the basal peak of the dipping reflection section. A thin layer with dramatically lower velocity at ~306 m LSF is within the basal bright peak of the dipping sequence. The nearly flat horizons below have velocities varying between 2000 and 2100 m/s and form a series of bright peaks and troughs.

Check shot survey data

Check shot data were acquired at 23 depths in Hole C0004B. Because of excessive noise levels, two of the stations were not useable, but the remaining 21 stations yielded excellent quality waveforms (Fig. F41; Table T7). These data sample depths from the seafloor to 376 m LSF. Approximately 15 air gun array shots were fired at each station during the LWD drilling and 8 shots were fired at each station during the pipe trip out of the hole. Noisy traces and traces with poor first arrival waveforms were deleted. The remaining traces were filtered (trapezoidal, minimum phase, and 30-40-150-200 Hz band-pass) and stacked to produce the traces shown in Figure F41. The first arrival wavelet is unambiguous on all traces.

The first arrival time was 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 their uncertainty. We estimated the uncertainty of the arrivals to be ~0.3 ms. We used an inversion damping coefficient of 0.5 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. The improvement in estimated interval velocities, indicated by the smoothness of the curve and the general downward increase of velocity (Fig. F42), is dramatic. The accompanying changes in arrival times are very small. We used the smoothed arrival picks and the tool depths as the check shot curve, which we then used for synthetic seismogram preparation.

Beyond the general increase of interval velocity from 1500 m/s at the seafloor to 2100 m/s at 400 m LSF, there is not an exact match between the check shot velocity curve and the sonic log values (Fig. F42). Between the seafloor and ~60 m LSF, the sonic velocity is low. This is the region where it was not possible to distinguish the sonic arrival through the drilling fluid (mud arrival) and the formation. Below 60 m LSF, the sonic velocity merges with the check shot velocity. From ~100 to 250 m LSF, the check shot velocity matches the lower envelope of the sonic velocities. The difference between the trend of the sonic velocity and the check shot velocity is ~600 m/s. The check shot curve fails to resolve the zones of low velocity above, and high velocity below, ~290 m LSF. This is partly due to sparse sampling, with data points spaced at an average of 39 m. It is probably also affected by the smoothing applied to the check shot velocities. This technique treats interval velocity variations from station to station as noise and smoothes them out. Whereas this method is necessary to get rid of real noise, it may need to be applied differently to avoid misinterpreting abrupt geological velocity variation with depth.

We were not able to construct a meaningful vertical seismic profile using these data. We tried a number of filtering and gain combinations but could not identify coherent upward-traveling reflections. We attribute this to noise from the banging of the drill pipe in the current and other sources of downhole noise.

Synthetic seismogram

In order to construct a synthetic seismogram for Site C0004, we used the high-quality DTCO log, but because of the absence of an azimuthal density neutron log, we used a pseudodensity log calculated from resistivity (see “Physical properties”). The resulting synthetic seismogram correlates well with the seismic reflection data at several depths, but there are also several discrepancies.

Good to excellent correlations were made for the following regions:

1. The base of the slope sediments (logging Unit I/II boundary),
2. Some individual reflections within the thrust sheet (hanging wall),
3. The top of the main fault zone (logging Subunit IIC/IID boundary), and
4. Nearly every reflection within the footwall underthrust sediments (Fig. F43).

Two locations that match poorly include a zone from 93 to 99 m LSF, which exhibits low then high sonic velocity values and produces an antisymmetric high-amplitude reflection, and a zone from 236 to 248 m LSF, which is a high and then low zone of sonic velocity values that results in an anomalous negative polarity reflection.

The reflection at the base of the slope sediments in the synthetic seismogram is a positive polarity reflection with a complicated base in the form of a double positive peak. This reflection in the seismic data locally is also a positive polarity reflection with a complicated base, but usually the base is in the form of a small negative peak.

The synthetic seismogram matches the top of the main fault zone well but has a strong long-wavelength positive polarity reflection within the zone that obscures any reflection at the base of the zone.

No reflection seems to correspond exactly to the gradual but significant increase in sonic log velocity within the fault zone at 291 m LSF.

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