1. n. [Formation Evaluation]
A wireline log of the yields of different elements in the formation, measured using induced gamma ray spectroscopy with a pulsed neutron generator. The elemental yields are derived from two intermediate results: the inelastic and the capture spectrum. The inelastic spectrum is the basis for the carbon-oxygen log, and can also give information on other elements. The capture spectrum depends on many elements, mainly hydrogen, silicon, calcium, iron, sulfur and chlorine. Since the elemental yields give information only on the relative concentration of elements, they are normally given as ratios, such as C/O, Cl/H, Si/(Si + Ca), H/(Si + Ca) and Fe/(Si + Ca). These ratios are indicators of oil, salinity, lithology, porosity and clay, respectively. To get absolute concentrations, it is necessary to calibrate to cores or, more often, use a model such as the oxide-closure model. The depth of investigation of the log is several inches into the formation. It can be run in open or cased hole. Pulsed neutron spectroscopy logs were introduced in the mid 1970s after a decade or more of investigation.
See: activation log, alpha processing, elemental capture spectroscopy, geochemical log, induced gamma ray spectroscopy, neutron interactions, oxide-closure model, pulsed neutron spectroscopy measurement, wireline log