Geochemical Logging

The Ocean Drilling Program is a major international collaborative venture aimed at sampling the ocean crust through continuous coring. As part of the program a suite of open-hole (Schlumberger) nuclear logs are run which enable the insitu determination of most major rock-forming chemical elements. Using spectral natural gamma radiation we can determine K. U. and Th concentrations, whilst capture gamma ray spectroscopy provides concentrations of Si, Fe, Ca, S, Ti, Gd, K, H, and CI. The determination of AI is made through activation, whilst an estimate can also be made of the combined proportion of Na and Mg from the residual of the low energy gamma ray photoelectric effect. The relative concentrations are converted to absolute weight percent oxides on the assumption that these sum to unity. These data represent a unique view of both the physical and chemical nature of the subsurface formations and include responses from the fluid components. Their acquisition and interpretation represents a unique challenge for geophysicists and geochemists and relies on the mutual support and expertise of both.