A VSP experiment was carried out during the logging run at Site 1118. The goals of the VSP experiment were to give accurate depth estimations of reflectors identified in the multichannel seismic (MCS) data, allowing correlation with lithostratigraphic units, and to provide parameters with which to improve 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 "Vertical Seismic Profiling" in the "Explanatory Notes" chapter).
As part of the preparation for recording the VSP, the end of the drill pipe was positioned at 110 mbsf. A restriction in the hole prevented the WST from being lowered past 690 mbsf. The WST was raised to 686.1 mbsf, at which point the clamping arm was engaged, pushing the instrument against the wall of the hole. A total of 20 stations were occupied. The clamping interval was 30 m, except in the case of the last station, which was 25 m above the previous station (Table T17). At each station, the air gun was triggered several times. Each good shot was stacked, with firing continuing until the stack contained seven shots. Clamping quality was excellent at all stations.
The Schlumberger MAXIS system was used for preliminary shipboard processing of the VSP. P-wave transit times, used to derive interval velocities and check-shot information, were picked as the first arrival in the downgoing wavefield at the WST (Table T20 in the "Site 1109" chapter). Velocity filtering, wave shape deconvolution, a zero-phase 10- to 60-Hz bandpass filter, and corridor stacking were applied to the data (see "Vertical Seismic Profiling" in the "Explanatory Notes" chapter). A five-level velocity filter was used to separate the upgoing and downgoing wavefields. The resulting traces were stacked into a single corridor stack. Reflection events in the migrated MCS data at this location correlate well with those of the VSP, although relative amplitudes need some adjustment postcruise to account for the automatic gain control applied to the MCS data only (Fig. F89).
Interval velocities were calculated between all stations (Table T17; Fig. F90). Fitting a straight line by linear regression gives an average interval velocity of 1934 m·s-1 between 121.0 and 686.1 mbsf (Fig. F90).
To convert between seismic traveltime and depth below seafloor, 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, the more densely sampled velocities (DTCO; see "Downhole Measurements") measured downhole were used in preference to values determined from laboratory measurements. Laboratory measurements of velocity were used below 875 mbsf, where log sonic velocities were absent. Above 100 mbsf, neither laboratory nor logging measurements of velocities were taken. To estimate the velocity over this interval, the first arrival time of the VSP station at 121.1 mbsf was converted to two-way traveltime (TWT) (Table T18) relative to the seafloor. Using this information and the velocity profile between 100 and 121.1 mbsf, we derived an average interval velocity of 1690 m·s-1 for the interval from 0 to 100 mbsf.
The resultant velocity-depth function of the borehole (Fig. F91) was passed through a 2-m Gaussian filter and resampled to 1 m before being used to convert from depth to TWT. To confirm the viability of the depth conversion, it was compared with VSP check-shot information. The P-wave transit times to the WST geophone at depth (Table T17) were 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. This traveltime can then be directly correlated to the MCS data. The VSP experiment covered a wide depth range, so that we can accurately tie depth to time over a significant portion of the borehole. Table T18 shows that the difference between the VSP depth and that calculated for an equivalent time from the velocity-depth function ranges from 0.05 to 15.24 m, with an average of 5.98 m. Additional work using the VSP results as a check-shot survey will allow us to reduce these errors.
Figure F92 shows the correlation between the MCS data in time, the depth-converted MCS traces, and the lithostratigraphy column (see "Lithostratigraphy"). Lines are drawn at 30-m intervals connecting the depth-converted MCS data and the lithostratigraphy column to the MCS data in time. A prominent reflector at 3.33-s two-way traveltime, or 220 mbsf, corresponds to a prominent regional reflector that was interpreted as a sandy horizon at approximately the same depth at Site 1109 (see "Downhole Measurements" in the "Site 1109" chapter). At this site and at Site 1109, this was a zone of very low recovery. A reflector at 3.97-s TWT falls at 855 mbsf at Site 1118. It is not clear if this reflector is from the top of lithostratigraphic Unit VI, VII, or maybe even VIII. The depth-converted VSP corridor stack generally correlates well with the depth-converted MCS data. The strong reflector at 855 mbsf discussed above is clearly visible at the same depth in the VSP trace. Refinements in the depth conversion, further processing of the VSP data, and production of synthetic seismic traces will aid in the correlation between this site and other sites.