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doi:10.2204/iodp.proc.319.104.2010

Log-Seismic integration

Seismic velocity structure and well tie

No check shot data were acquired, and there were no sonic logs run at Site C0010. We examined the correlation between borehole data and seismic data at IODP Sites C0003 and C0004 (Tables T3, T4), where check shots were taken to correlate the seismic data to the borehole logs. These sites are located near Site C0010 and penetrated similar formations (Tobin et al., 2009b) (Figs. F24). We then used check shot data from Site C0004 to correlate borehole data with seismic data at Site C0010 (Table T3).

The one-way traveltime of the check shot data for a given depth below seafloor is greater, and consequently the interval velocity is lower, at Site C0004 than at Site C0003 (Figs. F25, F26). Site C0003 is located northwest of Site C0004 and penetrates the thrust sheet in a more landward location (Fig. F24). Higher velocity at Site C0003 may reflect greater compaction in this region. Sites C0004 and C0003 have relatively thin sediment carapaces (~80 m thick) above the thrust wedge (Figs. F19, F27), whereas at Site C0010, ~180 m of sediment overlies the thrust wedge (Fig. F28).

At Site C0004, we map the top of the thrust wedge as a positive polarity seismic reflection (blue) that overlies the relatively transparent thrust wedge (Fig. F19); we call this the "top wedge" seismic surface. We also map the base of the thrust wedge as a positive polarity seismic reflection (blue) and call it the "base wedge" seismic surface (Fig. F19). These reflections match the top and base of the thrust wedge mapped on seismic data by Expedition 314 scientists (see figs. F34 and F43 in Expedition 314 Scientists, 2009b). Based on the Site C0004 check shot, we find that the top wedge surface at Site C0004 overlies the top of lithologic Subunit IIA by a few meters (Expedition 316 Scientists, 2009), and it lies at the base of the logging Unit I boundary (Expedition 314 Scientists, 2009b) (Fig. F19). The base wedge seismic surface at Site C0004 lies at the boundary of lithologic Units III and IV and just above the logging Unit II/III boundary (Fig. F19).

At Site C0004, resistivity increases at the top of the thrust wedge but does not change substantially beneath the thrust wedge. Velocity increases with depth at the top of the thrust wedge and increases abruptly just above its base (Fig. F19). A synthetic seismogram produced by Expedition 314 scientists produced many, but not all, of the seismic characteristics at Site C0004 (Expedition 314 Scientists, 2009b).

We also correlated Site C0003 log data with the seismic data (Fig. F27; Table T4). As at Site C0004, a strong positive (blue) reflection is present at the top of the thrust wedge. In this location, Expedition 314 scientists also correlated the top of the thrust wedge to this strong positive reflection (Expedition 314 Scientists, 2009b). They interpreted the top of the thrust wedge (logging Unit II) as a very sandy interval with extreme borehole washout.

We used the Site C0004 check shot to tie the seismic data to the well data (logging and units) at Site C0010 (Fig. F28; Table T3). We mapped a weak positive polarity (blue) reflection at the top of the thrust wedge that we called the top wedge. We mapped a negative polarity (red) reflection at the base of the thrust wedge that we called the base wedge (Fig. F28). The top wedge surface lies ~15 m below the Subunit IB/Unit II boundary (Fig. F28), and the base wedge surface lies ~4 m below the Unit II/III boundary (Fig. F28) (see "Lithology"). We believe that our top wedge surface appears to underlie the Subunit IB/Unit II boundary because the time-depth correlation applied from Site C0004 is not correct. At Site C0010, the thrust wedge is buried by ~100 m more sediment than at Site C0004, and these sediments have lower velocities than those within the thrust wedge (Fig. F19). Therefore, between 100 and 200 mbsf, the velocities applied from Site C0004 are higher than those at Site C0010; the result is that our mapped seismic surface at the top of the thrust wedge appears deeper than reality. We suggest that the velocity difference between Site C0004 and Site C0010 is reduced at the depth of the base of the thrust wedge, and therefore the correlation is improved at this depth.

Comparison of Sites C0010 and C0004

There was considerable discussion on the ship about the quality of the LWD data during Run 1 at Site C0010 (see "Physical properties"). During this initial LWD drilling run, there was significant ship heave and stick-slip, particularly in zones where we observe lower gamma ray and resistivity values (Fig. F5). As a result, a section of the hole was relogged around the fault zone target, and data from both logging runs were compared (see "Logging and data quality"). After extensive discussions between the shipboard party and the Schlumberger engineers, we concluded that the resistivity and gamma ray data acquired in the initial logging run (Run 1) most accurately record in situ properties. We compare these data with those from Site C0004 below.

At Site C0010, gamma ray values in the thrust wedge increase from 65 gAPI at the top, to 120 gAPI in the center of the wedge, and back to 80 gAPI at the base (Fig. F29). Superimposed on this trend are a number of ~7–15 m thick cycles where gamma ray drops to between 60 and 80 gAPI. Resistivity parallels gamma ray throughout the borehole (Fig. F29). At the top of the thrust wedge, there is an abrupt increase in resistivity with depth from 0.75 to 1.25 Ωm. Resistivity increases with depth between the top and center of the thrust wedge to a value of ~2.5 Ωm at ~2860 mbsl (Fig. F29) and then decreases to the wedge base.

At Site C0004, gamma ray and resistivity values vary much less downhole than at Site C0010. Gamma ray values range from 60 gAPI above the thrust wedge, to 70–80 gAPI within it, and back to 60–70 gAPI in the underlying section (Fig. F29). Resistivity ranges from 1.0 Ωm above the wedge to 1.5 Ωm in the wedge. Resistivity declines very slightly to values of ~1.25 Ωm below the wedge.

Data from Sites C0004 and C0010 are overlain in the center of Figure F29, and the difference is striking. Gamma ray values are higher at virtually all depths at Site C0010 relative to Site C0004; gamma ray values differ by 40 gAPI within the thrust wedge between the two sites (Fig. F29). Resistivity within the lower half of the thrust wedge is also higher at Site C0010 than at Site C0004 by ~1 Ωm. Above and below the thrust wedge, resistivity at Site C0010 is slightly less than at Site C0004.

From the higher gamma ray response at Site C0010, we interpret that the thrust wedge at Site C0010 has a greater clay fraction and is finer grained than the thrust wedge at Site C0004. We suggest that resistivity within the wedge is higher at Site C0010 for two reasons: (1) the rock is finer grained, pore throats are more tortuous, and electrical resistivity is greater; and (2) the rock is more consolidated (see "Physical properties"). The cyclic decreases in resistivity and gamma ray at Site C0010 record the presence of coarser grained material (more silt- or sand-sized quartz) that is less consolidated. These intervals appear similar in composition to the material throughout the thrust wedge at Site C0004 (i.e., gamma ray and resistivity values are similar) (Fig. F29).

The wedge at Site C0010 is characterized by greater seismic reflection amplitudes than at Site C0004 (Fig. F30). We infer that the compositional cycles within the thrust wedge at Site C0010 drive differences in velocity and density (impedance) that generate seismic reflections. These results suggest significant compositional variation in the thrust wedge over a scale of only a few kilometers along the strike of Nankai Trough. A notable feature of seismic profiles across this thrust wedge is that the negative polarity reflector at the base of the wedge is weak at the tip of the thrust wedge and increases in amplitude where it is more deeply buried (Figs. F31, F32). Increasing consolidation in the thrust wedge relative to the underlying material with distance downdip explains this observation.

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