The Schlumberger Geochemical Logging Tool (GLT) uses three types of nuclear measurement on a single string to estimate concentrations of ten elements: potassium, thorium, uranium (from the natural gamma ray spectrum); aluminum (by delayed neutron activation analysis); and silicon, calcium, iron, sulfur, titanium and gadolinium (from the prompt capture gamma ray spectrum measured after a 14 Mev neutron burst).
The geochemical logs have been used for a variety of purposes, but a major effort has been made to transform them to realistic mineral assemblages, using computer methods that are extensions of those described in the last section. The notion of using chemical analysis data to build hypothetical minerals is by no means new and dates back to Cross and others (1902). Such hypothetical minerals are often called "normative" to distinguish them from "modal" minerals that are actually observed in the rock. However, the aim of workers with geochemical logging data is to achieve as close a match as reasonably possible between the normative calculations and modal reality.
Harvey and others (1990) have described both the advantages and limitations of this approach as applied to sequences logged in the Ocean Drilling Program (ODP). Herron and Herron (1990) pointed out that the potentially overwhelming complexity of possible mineral associations in sedimentary sequences is offset to the degree that typically only ten minerals occur in the bulk of most rocks. Herron (1988) also discussed the possible need to introduce new methods of classification in order to integrate these new data with associations defined