A statistical evaluation of the comparability of resistivity logging tools

Both the shallow and deeper parts of the underground are surveyed by geophysical borehole resistivity logs for several decades. For this, a number of logging tools in different electrode configurations are used. Each electrode configuration has a volume of investigation given by the specific electrode configuration. Before data from different resistivity log types are combined in e.g. geostatistical modelling, an evaluation of the comparability of the resistivity log data is suggested. The aim of the evaluation is to justify that resistivities measured with different equipment using different logging approaches during borehole logging campaigns carried out over decades are consistent and can be compiled.

An example of a resistivity log comparability study is presented here using resistivity logging data and borehole lithological data from Danish national databases. A dataset consisting of normal logs, focused resistivity logs and Ellogs associated with borehole lithological description is compiled. The evaluation is based on pairs of closely spaced resistivity logs both acquired using the same type of logging tool as well as different types of tools and involves visual comparison and statistical hypothesis test of the observed relative resistivity differences. Furthermore, the geological variability (represented by resistivity) on a regional scale is evaluated to ensure that it is larger than the variability related to the evaluation of the resistivity logging tools.