RESULTS AND DISCUSSION

Depth and traveltime correlations calculated from the synthetic seismogram, MCS velocity analyses, and velocity-depth data on cores are compared and used to estimate a range of depths to volcanic basement. TWTs defining the range of seismic units, including the top of volcanic basement, are picked from the MCS data at the Site 999 location. These traveltimes are fixed and corresponding depths are calculated from the various velocity data. Depths of the top and bottom of the sonic/density log and total hole depth are also fixed, and corresponding TWT and average velocities are calculated. The results are reviewed below and summarized in Figure 2.

1. The CB1/CB4 boundary at 4.066 s TWT corresponds to 239 mbsf and results in an average velocity of 1.593 km/s for this interval using laboratory velocity data. This velocity is just below the range determined by MCS velocity analyses. In unlithified sediment and ooze, sonic velocities measured in the laboratory are typically less than those measured under in situ conditions. The depth to this boundary and average velocity for this interval are 260 mbsf and 1.733 km/s respectively, using velocities corrected to in situ stress by the empirical formula of Urmos et al. (1993) (Shipboard Scientific Party, 1997, Site 999). These velocities, referred to as "corrected velocities," appear to better match MCS semblance velocities for this depth interval.
2. Correlation of synthetic and observed seismograms indicates the start time of the synthetic seismogram is 4.335 s TWT, which corresponds to the start depth of the sonic/density log at 525.8 mbsf. The CB4A/CB4B boundary at 4.400 s TWT lies only 65 ms below this and corresponds to 599 mbsf, resulting in a calculated average velocity of 1.890 km/s for the interval from seafloor to the CB4A/CB4B boundary.
3. Laboratory velocity-depth data indicate that the start of the sonic log at 525.8 mbsf corresponds to 4.381 s TWT or 4.310 s TWT, using uncorrected or corrected velocities respectively. These values bound that of 4.335 s TWT predicted by the synthetic seismogram correlation to observed seismic data.
4. Correlations with the synthetic seismogram indicate that the CB4B/CB5 boundary at 4.556 s TWT corresponds to 823 mbsf and results in an average velocity of 2.872 km/s for the CB4B interval. This average velocity is within the range determined by MCS velocity analyses (2.6 ± 0.4 km/s) and comparable to that predicted from laboratory measurements (3.046 km/s).
5. Correlations with the synthetic seismogram indicate that the end of the sonic log at 1041.65 mbsf corresponds to 4.695 s TWT, resulting in an average velocity of 3.146 km/s in this portion of CB5. Assuming this average velocity for the entire CB5 seismic unit indicates that the total depth at Site 999 of 1066.4 mbsf corresponds to 4.711 s TWT and that the top of volcanic basement at 4.936 s TWT corresponds to 1420 mbsf.
6. Laboratory data indicate that the total depth of 1066.4 corresponds to 4.711 s TWT, a fortuitous exact match with that predicted from correlations with the synthetic seismogram, and results in an average velocity of 3.617 km/s for this portion of CB5. Assuming this velocity for the entire CB5 unit results in a volcanic basement depth of 1473 mbsf.