Seismic tomography survey under the La Gileppe Dam

Abstract

AbstractA tomographic survey of 15 panels was performed under the La Gileppe dam (Belgium) to assess the degree of fracturing in the rock foundation which consists of steeply dipping beds of sandstone and shale. A particular aim of the study was to locate fracture zones likely to cause leakage under the dam. The panel dimensions range from 9 by 15 m (between 2 boreholes) to 60 by 40 m (between 4 boreholes). An explosive source and a 12 hydrophone string were used for the experiment. The data inversion was made with the Rai2D software of the L.C.P.C. (Laboratoire Central des Ponts et ChaussÉes), based on the SIRT algorithm with circular ray paths. The velocity values were influenced both by rock type and fracturing. The tomography method has been shown to image successfully low velocity fracture zones which could not have been determined by classical cross-hole tests. In some cases, tomograms showed cross shape images with a high velocity zone along the diagonals. These artefacts have been explained by numerical modelling as a result of the poor angular ray coverage of the section.