Application and processing of geophysical images for mapping faults

This paper describes the application of geophysical prospecting techniques to locate the active faults in superficial Quaternary sediments. Both elect rical tomography and seismic refraction tomography presented here were part of a geophysical campaign performed in the Roer Graben area as a reconnais sance tool before trenching for paleoseismological observations. The aim of this investigation was, in the first place, to determine the exact positio n of the active fault in order to determine the best emplacement of a later trench. An additional objective was to image the fault zone at shallow dep th, allowing both a comparison with trench data, and a downward extrapolati on of these direct observations. Both methods proved to be successful but g enerated relatively smooth images with a poor spatial resolution that were prone to a subjective interpretation by the operators. In order to restrict the interpretation of tomographies with quantitative measures of gradients , image analysis techniques were tested. This paper presents some prelimina ry results obtained on the use of crest line location methods issued from m athematical morphology. Synthetic models of simple resistivity anomalies al ong a faulted section were used to check the reliability of the image proce ssing method. (C) 2001 Elsevier Science Ltd. All rights reserved.