Downhole Logging at Deep Sea Drilling Project Sites 501, 504, and 505, near the Costa Rica Rift

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.