DOWNHOLE LOGGING AT DEEP SEA DRILLING PROJECT

Successful logs were taken in four holes in the area of the Costa Rica Rift during DSDP Legs 69 and 70. The logs clearly show the chert layers in limestone at the base of the sedimentary section of Holes 501 and 504A and the alternation of the massive flow units and pillow basalts in the basement section of Holes 501 and 504B. Nearly continuous neutron and resistivity logs of the 562 meters of basement section of Hole 504B show that the porosity of the basalt decreases gradually from about 13% near the sediment/basalt interface to about 6% 250 meters into the basement. Below that depth porosity is approximately constant. A comparison of the resistivity and neutron logs suggests that pores in regions of high porosity near the top of the basement are more connected than in similar regions deep in the hole. The caliper log for Hole 504B shows that the size of the hole is generally close to the size of the bit but has variations that are not clearly related to either porosity or permeability. The compensated density log, although it gives a good indication of the variation of porosity, is not a reliable indicator of the values of formation porosity in high porosity zones. The sonic log is noisy on a fine scale but gives an idea of large-scale structure that correlates well with the values of porosity derived from resistivity. INTRODUCTION During Legs 68 (501), 69, and 70 of the Deep Sea Drilling Project, logs were obtained in four holes drilled in two contrasting areas of the Costa Rica Rift to investigate both open and sealed circulation of heated ocean water within the ocean crust (Fig. 1). In the area where circulation is believed to be open to the ocean, where heat flow is low, drilling the brittle, fractured, little altered basalt proved to be very difficult. It was only with great effort that in one hole (Hole 5O5B) we penetrated 40 meters into basement. We managed to log most of that basement section. We also logged a section of the overlying sediment, but the cold, unaltered sediment had washed out beyond caliper range during drilling, and the logs were not very satisfactory. Drilling was much better in the high-heat-flow area, where hydrothermal circulation was believed to be sealed beneath a continuous blanket of sediments. Hole 501, the pilot hole for re-entry, penetrated 70 meters into basement beneath 250 meters of sediment that had been diagenetically altered to chert and limestone over its lowest 30 meters. Here the lower 120 meters of sediment and most of the basement was logged. We made a first attempt at re-entry only a few hundred meters from this site, at Hole 504A. The attempt failed when the casing bit broke up only a few meters into basement. We made one logging run, primarily to record temperature but also to acquire a gamma density record of the chert layers. The eventually successful re-entry hole, Hole 504B, was cased to basement and then drilled, first to 212 meters of basement penetration on Leg 69 and eventually Cann, J. R. , Langseth, M. G. , Honnorez, J., Von Herzen, R. P., White, S. M. , et al., Init. Repts. DSDP, 69: Washington (U.S. Govt. Printing Office). to 562 meters of basement penetration on Leg 70. Suites of logs were run in the basement section at the end of the drilling on both legs, with different rates of success on each leg. These logs provide the principal interest of the Costa Rica Rift drilling, if only because of their length, which allows a clear view of the extent of the fluctuations of the physical properties of the basement. The logging tools used for the standard logging runs were part of a logging package that was provided under contract by Gearhart-Owen, Incorporated. GearhartOwen logging engineers operated the tools: Larry Axline sailed on Legs 68 and 69 and David Collart on Leg 70. The technical specifications for the logging tools are given in Table 1. The logging results were interpreted by using the appropriate charts in the Gearhart-Owen manual, supplemented where necessary by charts in the Schlumberger logging manual. The results of all logs except the temperature log are discussed here. The temperature logs are discussed by Becker et al. (this volume) in conjunction with the other temperature measurements. Two special tools that were not part of the GearhartOwen package were also run during the logging program. They are both discussed in other chapters of this volume, the downhole magnetometer by Ponomarev and Nekhoroshkov and the borehole televiewer by Anderson and Zoback. Different suites of logs were run during five logging periods with varying degrees of success (Table 2). The sonic tool did not work at all effectively on Legs 68 and 69; it worked better, though still noisily, on Leg 70. Conversely, the gamma density tool worked well on the earlier two legs but failed during the logging of the deep Hole 504B on Leg 70. Despite occasionally good operation during Legs 68 and 69, the natural gamma tool was ineffective because of the scarcity of radioactive elements in the section, and it gave particularly noisy read-