Environmental corrections to gamma-ray log data: Strategies for geophysical logging with geological and technical drilling

Correction concepts for the elimination of environmental influences on gamma-ray logs have been published in a number of articles and in guidelines by service companies. However, these widely-used processing rules for open- and cased-hole measurement conditions have been presented in different forms using different assumptions. For a detailed comparison, these concepts were reduced to simple analytical formula with the application of the same units and scaling to common, standardised conditions. For geological and technical drilling, environmental influences like borehole geometry, casing configuration and material, as well as mud density have turned out to represent the most important effects for a characterisation of environmental conditions. Typical case histories show that correction processing may lead to differing results, depending on the chosen presumptions. A thorough analysis of different types of influences and a careful choice of adequate correction procedures substantially improve the quality of gamma-ray log data. Further implications of these analytical correction rules permit also the quantitative interpretation of logs in dry cased-hole intervals and with the presence of tool joints or casing collars. The latter effect is widely underrated but is proven here to be a necessary step in precise gamma-ray log processing.