A narrative description of VSP operations during Leg 204, in the order in which they were attempted, is included here in case they are of interest to others when planning similar experiments in the future.
After completion of the conventional wireline logging (CWL) operations, rig-up of the three-node VSI for the VSP survey started on 15 August 2002 at 0830 hr local time. The plan was to conduct three types of survey during this run: vertical, constant-offset, and walkaway. At this time, the Maurice Ewing was navigating nearby to provide support for the constant-offset and the walkaway surveys.
Despite the generally good hole conditions that were observed during the standard logging runs, it became clear after a few attempts that it would be difficult to achieve simultaneous clamping of all three sensor packages. In fact, very poor signal-to-noise ratios when shots were fired from either the JOIDES Resolution or the Maurice Ewing indicated that none of the sensor packages were well coupled to the borehole. After some discussion with the Schlumberger engineer responsible for the VSI downhole seismometer, the reason for this problem became apparent. The average hole size, which ranged from 12 to 16 in based on the caliper log (see "Downhole Logging" in Shipboard Scientific Party, 2003b), was equal to or larger than the maximum operational hole size for the VSI (12 in).
Shots were fired from the JOIDES Resolution and from the Maurice Ewing for several hours without recording a consistent signal at any of the attempted depths. At 1700 hr, it was decided to abort this run and try to use the WST-1 to conduct at least a vertical VSP. When the VSI reached the rig floor it appeared that the arms of the three shuttles had been damaged during our numerous clamping attempts. The WST was lowered by 1945 hr. After several unsuccessful attempts, it was clear that the geophone was not working properly and the tool was brought back to replace the geophone. During the following lowering, and despite the proper behavior of the WST, it was clear that the hole was too large and the formation to soft to get a good clamping. The survey was aborted, and the final rig-down was complete at 0200 hr on 16 August 2002.
After completion of CWL operations in Hole 1251H, rig-up for the VSP survey started at 0345 hr on 18 August 2002. The triple combination and Formation MicroScanner-sonic calipers had shown that the hole was irregular, which suggested that the three-component WST-3 would be the most likely to provide data. This tool had been flown in since the previous failure of the VSI at Site 1245. The WST-3 was lowered at 0445 hr, and it proved again difficult to get a consistently good clamping; however, the signal appeared of good enough quality to proceed with a systematic vertical and constant-offset VSP. Stations were made over the entire hole every 7.5 m, starting at 1405 meters below rig floor (mbrf). Considering the overall good quality of the data, it was then decided to attempt a walkaway VSP. It proved impossible to get good clamping at the initial target depths, but a good station was found at 1320 mbrf and the Maurice Ewing shot two perpendicular lines, each taking ~2 hr. Final rig-down was completed at 2030 hr on 18 August 2002.
After completion of CWL operations in Hole 1244E, VSP rig-up started at 0130 hr local time on 21 August 2002. Both CWL logging runs had indicated very good hole conditions, and the possibility to use a modified VSI was considered until it was pointed out that the modifications that were made to extend its arms would not allow the tool to exit the advanced piston corer/extended core barrel bit. The WST-3 was lowered instead, and the vertical/constant-offset VSP started from the bottom of the hole (~1155 mbrf) at 0400 hr. Stations were made every 5 m and the survey was completed at 0830 hr without any problem in excellent hole and sea conditions. Two walkaway stations, including two orthogonal lines each, were made at 1045 and 1020 mbrf, respectively. The final rig-down ended at 0100 hr local time, 22 August 2002.
After completion of CWL operations in Hole 1247B, rig-up of the WST-3 started at 0415 hr local time, 24 August 2002. Considering the good hole conditions, the plan was to conduct a complete survey including two walkaway stations. Initial tests before lowering the assembled tool string indicated serious problems, including the impossibility to properly close and open the arm, and it was decided to use the WST-1 to perform only a vertical VSP. After lowering the tool, several unsuccessful attempts to record a decent signal indicated that the geophone was damaged. Because no spare geophone was available and considering the extremely good hole and sea conditions, we decided to try using one of the shuttles of the VSI, which had been restored to its original configuration. Lowering of the VSI started at 0945 hr, and preliminary tests showed that the tool was working properly. The tool reached the bottom of the hole (1066 mbrf) at 1015 hr, and vertical/constant-offset VSPs were acquired by alternating shots between the JOIDES Resolution and Maurice Ewing as the tool was raised from 1060 and 930 mbrf with stations every 5 m. After completion of this survey, it proved impossible to get a satisfactory station that would allow a walkaway. Considering the poor quality of the signal, it was suspected that the tool had been damaged and it was brought back to the rig floor for inspection. This confirmed that the arm had been damaged, possibly while the tool was on station too close to the bottom of the pipe. It was decided to abort any further attempt in order to preserve the remaining shuttles for the final VSP survey at Site 1250. Final rig-down was complete at 1730 hr, 24 August 2002.
After completion of CWL operations, rig-up of a the single-shuttle VSI started at 0610 hr, 26 August 2002. The tool was lowered at 0715 hr and the vertical/constant-offset survey started from the bottom of the hole (980 mbrf) at 0800 hr. Stations were recorded every 5 m up to 890 mbrf. Coupling between the arm and the formation was poor in the upper part of the hole, and no reliable shots were recorded above 930 mbrf. Because of the generally poor signal, it was suspected that the tool had been damaged, and it was brought back to the surface. Inspection showed that it was in working order, and it was lowered again at 1230 hr to find possible walkaway stations. After systematic attempts along most of the hole, three walkaway surveys were recorded at 945, 898, and 979 mbrf. Final rig-down was completed at 0630 hr on 27 August 2002, a few hours before the Schlumberger VSP engineer was to leave the ship.