Leg 176 drilling operations were quite different from most ODP legs. We conducted all drilling operations at a single site in a single hole. Hole 735B was first established during Leg 118. On that leg, a hard rock base (HRB) was set in 731 m of water and a 9-7/8" hole was cored to a depth of 500 meters below seafloor (mbsf). Another 0.7 m of penetration was made on that leg using the 3 3/4" Navidril core barrel (NCB), establishing a "cored" total depth of 500.7 mbsf. During Leg 176, we set out to reoccupy Hole 735B and deepen it to 1500 mbsf or as deep as possible.
By the end of the leg, Hole 735B was deepened from 1235.8 meters below rig floor (mbrf) or 504.8 mbsf to a total depth (TD) of 2239.0 mbrf or 1508.0 mbsf. We have not established why the Leg 176 tag depth was 4.8 m deeper than the total depth documented at the end of Leg 118.
The rotary core barrel (RCB) wireline coring system was used exclusively during the leg. A total of 10 RCB C-7 core bits resulted in the recovery of 122 cores. We cored a total of 1003.2 m of new hole recovering 865.99 m of core or 86.3% of the section. Penetration rates varied from 1.3 to 6.1 m/hr in the gabbro and olivine gabbro formations. Coring was faster in coarse-grained rocks and highly fractured intervals. The slower rates were experienced in the more massive, finer grained, foliated rocks. The average rate of penetration (ROP) for the leg was 2.7 m/hr. We took drift measurements at eight stations between 500 and 1400 mbsf. Hole deviation based on the Tensor electronic multishot data was moderate, varying between 4.3° and 4.8°. A total of 31 reentries were made into Hole 735B during the course of the leg at an average of 0.3 hr per reentry.
Our progress in the hole ended when a 5" drill pipe connection failed leaving 43-2/3 stands of 5" drill pipe and a coring bottom-hole assembly (BHA) in the hole (1403 m). The failure occurred when the pipe was 97 m off bottom during a routine wiper trip to replace wear-knotted drill pipe with standard 5-1/2" drill pipe. We engaged the fish on our first attempt with a 9-1/2" overshot dressed with a 6-7/8" basket grapple and mill control. It was being lifted to its total weight of 130,000 lb when the fish parted in the tube of the fished 5" drill pipe. The failure point coincided with the point at which the pipe had buckled upon impact with the bottom of the hole. We ultimately recovered 497 m of 5" drill pipe. The fish remaining in the hole (906 m) consists of 26 stands of 5" drill pipe (734 m) plus the coring BHA (172 m). We made seven unsuccessful fishing attempts over a period of 7.4 days, before abandoning attempts to clear the hole. Allowing for the junk in the hole, the current TD is 1337 mbrf or 606 mbsf.
The drill string was not the only thing broken during Leg 176. Several historical records were broken as well. Leg 176 recovered more hard rock (866 m) than any other hole in the history of ODP or the predecessor Deep Sea Drilling Project (DSDP), nearly doubling the previous Leg 118 record of 434 m. We also cored more than twice the hard rock (1003 m) of any other single leg, shattering the previous Leg 118 record of 501 m. Hole 735B is now the fifth deepest hole in ODP history. In addition, Hole 735B now qualifies as the third longest single-hole continuously cored interval (mud or rock) in ODP history.
We conducted logging operations in two phases. The first phase (to 492 mbsf) was done after the initial reentry into the hole before commencing coring operations. It included the triple-combo (natural gamma sonde [NGS], accelerator porosity sonde [APS], hostile environment lithodensity sonde [HLDS], and the dual laterolog [DLL]) plus temperature log. The second string included the NGS, the dipole shear sonic imager (DSI), and the formation microscanner (FMS). We aborted this logging run, however, due to data acquisition difficulties.
We conducted the second phase of logging plus VSP experiments after abandoning attempts to fish the hole. The first tool string consisted of the HLDS, caliper, APS, and the hostile environment spectral natural gamma sonde (HNGS). The second logging run consisted of the natural gamma tool (NGT), DSI, general purpose inclinometry tool (GPIT), and the FMS. The third tool string was composed of the NGT, GPIT, and DLL probes.
During the first logging run, we obtained good density, porosity, and gamma-ray measurements from 49 mbsf to 595 mbsf or 11 m above the hole obstruction at 606 mbsf. The second logging run (DSI-FMS) resulted in good data recorded by the DSI. Cross-dipole and P and S wave mode data were recorded during the first pass and cross-dipole, upper dipole, and Stoneley modes during the second pass. The FMS, however, did not fare as well. After spending some time testing different gain options to improve the data quality from the FMS, we opted to recover the tool and repeat the logging run with a replacement FMS. Unfortunately, the results from the second tool run were similar to those recorded with the first tool. A possible explanation for the poor response is that the tools could not respond quickly enough to the extreme resistivity contrasts between the Fe-Ti oxide gabbros and the olivine gabbros. The deployment of the third tool string resulted in very good resistivity data with the DLL.
The fourth and final logging run consisted of the Schlumberger Seismos Prakla VSP tool (borehole geometry kit tool [BGKT]). The objectives of the VSP were to acquire data over the section of hole not covered by the Leg 118 VSP and to acquire better VSP data using the new tool. We hoped that better quality data would perhaps resolve sub-bottom reflectors below 1500 mbsf and perhaps give better observations of the seismic attenuation at the site. The VSP operation took about 16 hr from rig floor to rig floor. We shot both air gun and water gun sources to the sonde clamped at 23 depths in the hole. The tool did not give appreciably better data than was acquired on Leg 118, but we did fill in the coverage between 500 and 600 mbsf.
After concluding the wireline logging and VSP exercises, we round tripped the drill string one last time for a final fishing attempt. The fish was contacted ~2 m deeper than before at 1339 mbrf or 608 mbsf. During subsequent working of the pipe we contacted the fish at the shallower depth; however, all attempts at engagement proved futile. We abandoned our last fishing attempt at 1400 hr 1 December 1997 and began preparations to get under way for Cape Town, South Africa. We departed Hole 735B at 1900 hr 1 December 1997.
To 176 Port Call - Cape Town
To 176 Table of Contents