1. n. [Formation Evaluation]

A model for converting relative elemental yields from a pulsed neutron spectroscopy log to absolute weight concentrations using the assumption that the sum of all oxides in the rock matrix is 1. The model is based on the observation that, with few exceptions, sedimentary minerals are oxides, so that the sum of the dry weight percent of all oxides must be 100%. The weight percent of an oxide can be calculated from the dry weight percent of the cation by knowing the chemical formula (for example, SiO₂ from Si). The absolute dry weight percent, W, of element i is given by W_i = F * Y_i / S_i where F is the unknown normalization factor, Y_i is the measured spectral gamma ray yield and S_i is the tool sensitivity to that element, measured in the laboratory. The dry weight percent of the oxide is then O_i = F* X_i * Y_i / S_i where X_i is the oxide association factor, given by the chemical formula. Since the sum of all O_i equals 1, it is possible to calculate F and determine each W_i .

See related terms: elemental capture spectroscopy,  geochemical log,  natural gamma ray spectroscopy