The DLIS-format vertical seismic profile (VSP) data were imported into Landmark's MCS ProMAX processing package to perform quality control and first-break picks on the surface hydrophone and downhole receiver data. Quality control included the inspection of pre- and post-stack data for excessive noise from external sources or downhole tool motion after the calipers had been clamped at a station in the hole. Transit times were corrected for the depth of the air gun, separation of the air gun and the surface hydrophone, elevation of rig floor above sea level, and horizontal separation of the downhole hydrophone and the air gun. The resultant one-way traveltimes are zero offset with respect to sea level. Check-shot corrections were applied to the velocity profiles derived above by dividing the one-way traveltime between two check-shot depths (T´) by the one-way traveltime derived from the composite velocity profile for the same depth interval (T). Multiplying the result (C) by each velocity value in our composite profile for that interval yielded velocities (V) that gave a depth-to-time conversion consistent with the check-shot information:
where
Specific information for each site is given below. No attempt was made to correct velocities below the deepest check shot at any of the three sites.
Twenty levels between 142.3 and 682.3 mbsf were occupied during the VSP survey at Site 1118. A combination of optimizing signal-to-noise ratio and distributing check shots throughout the borehole resulted in the selection of check shots at 265.3, 415.3, and 685.3 mbsf, corresponding to two-way traveltimes of 318.32, 474.36, and 739.36 ms, respectively. Check-shot corrections increased velocity by 3.14% between 0 and 265.3 mbsf, decreased it by 2.41% between 265.3 and 415.3 mbsf, and decreased it by 0.85% between 415.3 and 685.3 mbsf.
The VSP recorded at Site 1109 occupied nine levels in the borehole between 378.9 and 460.3 mbsf. Of these, only the records from 398.9 and 408.9 mbsf had a high enough signal-to-noise ratio to allow a reliable first-break pick. As a very minor error in first-break time can lead to a large error in interval velocity over a short depth interval, only the check shot recorded at 398.9 mbsf was used. The two-way traveltime for this depth interval was 474.33 ms. The resultant check-shot correction increased the velocities over the interval of 0 to 398.9 mbsf by 2.08%.
The VSP recorded at Site 1115 occupied 16 levels in the borehole between 315.2 and 552.2 mbsf. Problems with the heave compensator and an unidentified source of noise resulted in only two closely spaced depth intervals with a high enough signal-to-noise ratio. The check shot at 552.2 mbsf yielded a two-way traveltime of 645.16 ms and resulted in velocities between 0 and 552.2 mbsf increasing by 0.4%.
