Analysis of Downhole Logs, Leg 57, Deep Sea Drilling Project

All sites drilled on Leg 57 were logged as weather conditions permitted. The logging was done by the Singapore office of Schlumberger, and because of short notice the special logging tools required to pass through the Challenger drill pipe were not available. The standard centralizer, excentralizer, and caliper tools were not appropriate, and those modified aboard ship were not completely satisfactory. In addition, we suspected that vertical motion from the ship's heave degraded the quality of logs. Both the sonic and neutron density tools were operated beyond their normal working ranges. Despite these operational difficulties, the logs from different runs in a hole, both repeat sections and runs with different tools, show generally the same features. Over intervals of 5 to 10 meters the field logs seem to be good indicators of the physical character of the section penetrated (Carson and Bruns, this volume). The recovery of continuous core as well as logs differs from normal industrial drilling practice, in which a major purpose of logging is to establish lithology. Since lithology is best established by direct examination, the log analysis focused less on identifying a specific lithology than on continuous measurement of in situ physical properties, especially at Sites 438, 439, and 440, where recovery was good. However, the logs were used to establish the lithology of missing intervals, especially at Site 441, where recovery was poor. Lithology and physical properties aspects of the log analysis are covered in the individual site chapters and in a comparative analysis of laboratory measurements of physical properties and logged physical parameters (Carson and Bruns, this volume). In the present chapter we concentrate on log precision, correlation of key horizons in seismic records and core lithology, and on fracturing as indicated by the logs. We do this by using the results of a study of corrected logs and the derivative products from computer programs developed by the Scientific Software Corporation (SSC) and Technology Research Center and of corrected logs from Schlumberger (available from DSDP)