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doi:10.2204/iodp.proc.314315316.118.2009

Log-seismic correlation

Seismic reflection interpretation

The upper part of Hole C0006A penetrated a section of accreted trench strata at the prism toe. The section is cut by several northwest-dipping thrust faults that offset the sedimentary reflections. At least two reflections, shown in yellow and blue on Figure F27, are repeated at the borehole by thrust faults. The deepest fault, which continues to the southeast, is inferred to be the main frontal thrust between the overriding thrust sheet and the underthrusting trench sediment section. The complex pattern of reflections below the main frontal thrust is believed to represent channels within the upper part of the trench sediment section that are being overridden by the thrust sheet above.

Check shot survey data

Check shot data were acquired at 44 depths in Hole C0006B. Two stations were unusable because of excessive noise levels and two others were above the seafloor, but the remaining 40 stations yielded excellent quality waveforms (Fig. F28). These data sample depths from the seafloor to 863.09 m LSF. Approximately 15 air gun array shots were fired at each station during LWD drilling, and eight shots were fired at each station during the pipe trip out of the hole. We note that before tripping out of the hole, only one 9.5 m joint was removed, rather than the planned two, so a nonuniform station spacing resulted.

Noisy traces and traces with poor first arrival waveforms were deleted. 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 F28. The first arrival wavelet is unambiguous on all traces.

The first arrival time was picked manually to yield a correlation between seismic traveltime and depth (Table T8). Raw interval and average velocities were determined for each interval. 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 overfitting and underfitting the data. The improvement in estimated interval velocities, indicated by the smoothness of the curve and the general downward increase of velocity (Fig. F29), is dramatic. The accompanying changes in arrival times are very small. We used smoothed arrival picks and tool depths as the check shot curve, which we then used for correcting the seismic depth section through the drill site (Fig. F27).

We were not able to construct a meaningful vertical seismic profile (VSP) 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.

Overall log unit correlation

To visualize the correlation of log and seismic reflection data at this site, we present a series of figures (Figs. F28, F29, F30, F31, F32, F33, F34) in which we have superimposed logs over a portion of the check shot–corrected prestack depth-migrated seismic reflection profile. Logging units do not correlate well with the seismic reflection data, probably because the section is strongly faulted (Fig. F30). Several features in the LWD logs do, however, correlate with features in the seismic data. For example, the sandy layers (low gamma ray values and low resistivity) at ~220, 240, 300, and 335 m LSF correlate with strong reflections and appear to be parts of the same unit that have been repeated as a result of thrusting (Figs. F27, F31, F32, F33). Several seismically defined thrust faults also correlate with features in the resistivity images, such as the conductive fractures at 360 and 381 m LSF (see “Structural geology and geomechanics”). The main frontal thrust correlates with a dramatic increase in hole size (Fig. F34) and may correlate with a fracture at 657 m LSF (Fig. F29).

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