Technical Note 25

SCIENTISTS' PERCEPTION OF LEG 153 OPERATIONS

Despite not meeting the deep drilling objective set forth in the scientific prospectus for Leg 153, the lateral and potential temporal variability preserved in the cores recovered during this leg represent a major contribution to our understanding of ocean crust and upper mantle processes. Extensive documentation of this variability is presented in the Preliminary Report and the Initial Report volumes for this leg. The following represents the perceptions of the scientific party regarding the operational aspects of drilling on Leg 153.

Given what we understood to be the operational limits of the guide base (30°-35° slope), the precruise submersible surveys identified several potential sites within these limits. We considered the precruise surveys to be adequate in light of these operational limits. Our experience with attempts at guide base deployment, however, demonstrated that the limits of operation we envisioned were theoretical limits, and not true operational reality. Obviously, this misconception may have been prevented with more thorough communication between the scientists planning the cruise and the engineers charged with development of operational techniques. One ancillary benefit of this publication is to ensure that communication at this level is more common in planning future legs.

Once we understood the true operational limits of the guide base (20°), the precruise site survey data were no longer adequate for establishing a guide base location. Necessarily, we were forced to reevaluate our drilling strategy, and undertake extensive video surveys to find appropriate sites. As a consequence of the amount of time dedicated to seafloor surveys designed to locate drill sites within the true operational limits of the guide base, drilling time was significantly reduced. Had these surveys been designed with the intention of locating sites for single bit holes, several more sites could have been drilled, since the precruise site survey data were adequate for this type of operation.

Sites within the operational limits of the guide base are well below the angle of repose for talus and pelagic sediments. Bare rock outcrops are, therefore, very difficult to identify, particularly given the low resolution of the current shipboard video survey system. Additionally, the hard rock guide base was not designed to be emplaced on sediment, which is a general characteristic of areas on the seafloor with such gentle slopes. However, during the course of our surveys we successfully located outcrops within these operational limits. Pilot holes at these sites demonstrated that downhole instability precluded establishing a multiple reentry hole with the available drilling and casing tools and techniques.

Operations on Leg 153 proved invaluable in two principal aspects. We now recognize that to establish a multiple reentry site, many potential sites must be surveyed because downhole conditions are not predictable at this stage. Even if a survey identifies a site with all the criteria necessary to allow establishing a hole, downhole formation characteristics not perceptible to current survey methods may prohibit deep penetration. Second, we recognize that existing drilling, casing, and surveying tools need to be improved, and new methods and tools must be investigated and developed if we are to meet the deep penetration objectives of offset drilling.

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