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1 British Geological Survey, Nottingham NG12 5GG, UK pdj{at}bgs.ac.uk
2 Department of Geology, University of Leicester LE1 7RH, UK
3 Geotek Limited, 3 Faraday Close, Daventry, Northampton NN11 5RD, UK
4 Department of Aeronautical & Automotive Engineering, Loughborough University LE11 3TU, UK
5 Adrian Wood Associates, Danehill, Brookhill Road, Copthorne, Crawley RH10 3PS, UK
6 British Geological Survey, Edinburgh EH9 3LA, UK
7 Department of Geosciences, University of Bremen, P.O. Box 330 440, D-28334 Bremen, Germany
We demonstrate a non-contact approach to whole-core and split-core resistivity measurements, imaging a 15 mm-thick, dipping, conductive layer, producing a continuous log of the whole core and enabling the development of a framework to allow representative plugs to be taken, for example. Applications include mapping subtle changes in grain fabric (e.g. grain shape) caused by variable sedimentation rates, for example, as well as the well-known dependencies on porosity and water saturation.
The method operates at relatively low frequencies (i.e. low induction numbers), needing highly sensitive coil pairs to provide resistivity measurements at the desired resolution. A four-coil arrangement of two pairs of transmitter and receiver coils is used to stabilize the measurement. One ‘coil pair’ acts as a control, enabling the effects of local environmental variations, which can be considerable, to be removed from the measurement at source.
Comparing our non-contact approach and independent traditional ‘galvanic’ resistivity measurements indicates that the non-contact measurements are directly proportional to the reciprocal of the sample resistivity (i.e. conductivity). The depth of investigation is discussed in terms of both theory and practical measurements, and the response of the technique to a variety of synthetic ‘structures’ is presented.
We demonstrate the potential of the technique for rapid electrical imaging of core and present a whole-core image of a dipping layer with azimuthal discrimination at a resolution of the order of 10 mm. Consequently, the technique could be used to investigate different depths within the core, in agreement with theoretical predictions.
