A VSP experiment was carried out during the logging run at Site 1109. The goals of the VSP experiment were to give accurate depth estimations of reflectors identified in the multichannel seismic (MCS) data, thereby allowing correlation of lithostratigraphic units, and to provide parameters with which to improve velocity processing of existing MCS data. A 300-in3 air gun was used to generate a source signal, which was received in the hole by the WST. The source signal was also recorded on a hydrophone close to the air gun (see "Acquisition Hardware" in "Vertical Seismic Profiling" the "Explanatory Notes" chapter).
As part of the preparation for recording the VSP, the end of the drill pipe was lowered to 380 mbsf to allow logging instruments to pass through sections of the hole that had become severely restricted (see "Downhole Measurements"). Restrictions deeper in the hole prevented the WST from being lowered past 462.4 mbsf. The WST was raised to 459.5 mbsf, at which point the clamping arm was engaged, pushing the instrument against the wall of the hole. By raising the WST in ~10-m intervals, a total of nine clamping stations were occupied. Variations in the clamping interval were a result of the WST not being seated properly or of a better location being found from the caliper log. Before the last two stations, the drill pipe was raised by 20 m. The final station occupied was at 378.1 mbsf. At each station, the air gun was triggered a number of times. Shots that did not have a clear first arrival and showed high noise levels were flagged. Each good shot was stacked, with firing continuing until the stack contained seven shots. Movement of the WST after it had been clamped against the side of the hole was noted at half of the clamping stations. This accounted for a significant percentage of the noise in the signal arriving at the WST.
The Schlumberger MAXIS system was used for preliminary shipboard processing of the VSP. The P-wave transit times used to derive interval velocities were picked as the first arrival in the downgoing wavefield at the WST (Table T20). Velocity filtering, wave shape deconvolution, a zero-phase 10- to 60-Hz bandpass filter, and corridor stacking (see "Vertical Seismic Profiling" in the "Explanatory Notes" chapter), were applied to the data. Velocity filtering was tried for four stations (3, 5, 7, 9). A five-level velocity filter sufficiently separated the upgoing and downgoing wavefields, leaving five traces that were stacked into a single corridor stack. This stack was found to match very poorly with the migrated MCS data at this location (Fig. F114). Because of the position of the WST low in Hole 1109D, little is seen of the sedimentary section on top of the Miocene forearc basin sequence. A reflector, possibly associated with the top of lithostratigraphic Unit XI (see "Lithostratigraphic Unit XI"), is seen at ~3.83 s two-way traveltime (TWT), but beyond this, correlations with the MCS data are limited. Further shore-based processing may prove fruitful.
Interval velocities calculated over the depth extent of the VSP experiment are variable (Table T20). Fitting a straight line to the data gives an interval velocity of 1777 m·s-1 (Fig. F115). This compares with 1800-1830 m·s-1 velocities obtained by logging (see "Downhole Measurements") and laboratory measurements (see "Physical Properties").
The short depth extent of the VSP makes it unsuitable for the determination of depths to the primary reflectors identified in the MCS data. In order to correlate between seismic traveltime and hole depths, we examined the velocity information available from laboratory measurements and sonic logs to develop a model of the variation of velocity with depth. In this model, sonic velocities measured downhole were used rather than values determined from laboratory measurements, except for 0-84, 336-374, and 707-783 mbsf. In these intervals, log sonic velocities were absent or of poor quality. Because of an offset between the laboratory velocities measured using the PWS3 transducer and those of the PWS1 and PWS2 transducers (see "Compressional-Wave Velocity"), the consistently 40 m·s-1 higher PWS3 (transverse) measurement was used. Where velocities measured using the PWS3 transducer were absent, the vertical velocity measured using the PWS1 transducer was substituted with a +40 m·s-1 offset. The resultant velocity-depth function (Fig. F116) was used to create a TWT-to-depth conversion. To confirm the validity of the depth conversion, it was compared with VSP check-shot information. The P-wave transit time to the WST geophone and depth (Table T20) were properly corrected for the geometry of the experiment (see "Vertical Seismic Profiling" in the "Explanatory Notes" chapter) to give a direct and absolute tie between TWT and depth. Comparing this to the MCS data, correlates the horizons from TWT to depth. The VSP is limited to depths from 378.1 to 459.5 mbsf; therefore, this comparison was conducted only over a short depth interval. Table T21 shows that the difference between the VSP depth and that calculated for an equivalent time from the velocity-depth function ranges from -4.13 to 0.85 m, with an average of 2.12 m. Thus, we can predict that depth conversions using our velocity-depth relationship are accurate to ~3 m above 459.5 mbsf. Below this depth, we estimate that the depth conversion has a similar accuracy.
A synthetic seismic trace was generated using velocity and density data from Site 1109. Density data for Site 1109 were compiled from downhole logging measurements and laboratory index measurements. Similar to the compilation of velocity measurements, in situ logging measurements were preferred to laboratory measurements, except where they were absent or of poor quality (0-95 and 680-800 mbsf; Fig. F117). A number of synthetic seismic traces were computed following the application of different filtering parameters to the velocity and density data. The preferred synthetic trace was generated by applying to both data sets a 3-m median filter to remove spikes, a 2-m gaussian filter to smooth the data, and an Akima spline to resample the data to 1 m. An Akima spline was used in preference to a simple linear extrapolation because of the variation in measurement interval between the laboratory (meter scale) and downhole (centimeter scale) measurements. A reflection coefficient was subsequently calculated from the two data sets. Using the depth-time relationship derived above, the reflection coefficient was converted from depth to time (Fig. F118). A source signal, comprising the first 500 ms of the direct wave from the migrated MCS coinciding with Site 1109, was convolved with the reflection coefficient to give the synthetic seismic trace. Using the depth-time function derived above, both the synthetic trace and a number of migrated MCS traces close to the site were converted to depth (Fig. F118).
The match between the synthetic seismic and the MCS data is good above 350 mbsf, with the exception that some amplitude estimates are not accurate. Below this depth, the match becomes poor (Fig. F118). Given that above, we have shown our conversion between time and depth is accurate, this indicates that the problem is with the parameters used to generate the synthetic seismic trace. Further work on this problem will include adjusting the filtering parameters of the velocity and density data, as well as experimenting with the source signal. A reflector at 3.8 s TWT, directly above the reflector interpreted as the top of the Miocene forearc sequence (Fig. F119), is estimated to be at 740 mbsf at Site 1109. At this depth, there is a 40-m-thick conglomerate unit, underlain by dolerite (see "Lithostratigraphic Unit X"). Both these units are associated with high P-wave velocities and densities (see "Density and Porosity" and "Compressional-Wave Velocity"). At 3.275 s TWT, a prominent doublet reflection is seen (Fig. F119). Depth conversion predicts that the top of this reflector occurs at a depth of 243 mbsf. This coincides with a 30-m-thick unit in the logging data that is associated with high velocity and density and interpreted to be a sand layer. We had poor core recovery in this unit, hence, it was poorly represented in laboratory physical properties measurements and lithostratigraphic descriptions. However, this unit is well reproduced in the synthetic seismic trace. At 3.6 s TWT, a negative polarity event is seen in the MCS data. In the depth-converted section, this event is at ~563 mbsf and coincides with the upper boundary of lithostratigraphic Unit VII, which is characterized by high calcite content (see "Lithostratigraphic Unit VII").