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Reconnaissance geophysical surveying has mapped a previously unknown fault across the Down Ampney site, using a conventional resistivity profiling technique. The Schlumberger array, optimised for the local geological sequence, revealed a distinctive fault signature, characterised by a marked peak and a very steep gradient in each apparent resistivity profile. Subsequent consideration of the data acquisition and presentation method used in the apparent resistivity profiling techniques led to the conclusion that these methods were smoothing the electrical field response and thereby limiting the potential accuracy of fault location. To overcome this limitation a novel technique has been devised in which the potential field generated by an electric current flow is profiled directly in terms of resistance. With a measurement of resistance at each electrode position, no averaging or smoothing effect is introduced. This high resolution data has revealed a 5 m interval of intermediate resistance gradient or resistivity, which is located above the intersection of the main fault with the upthrown Kellaways Beds. It is highly probable that a borehole positioned at the intersection point on any profile would penetrate the fault zone at shallow to moderate depth. This method of resistivity mapping has provided significantly more data to a higher resolution than is possible by conventional methods. The rotation of the current flow allows the response from geological structure of unknown orientation to be optimised. Thus the strike of a fault could be ascertained from orthogonal current flow at one location.

Additional information

Authors: HALLAM J R ET AL., British Geological Survey, Fluid Processes Research Group, Nottingham (GB)
Bibliographic Reference: EUR 14686 EN (1993) 30 pp., FS, ECU 7
Availability: (2)
ISBN: ISBN 92-826-5958-5
Record Number: 199310830 / Last updated on: 1994-11-29
Original language: en
Available languages: en