Seismic Imaging of a Dam Using Pseudo-VSP Acquisition

The justification for the stability of many hydraulic structures as gravity dam is reviewed to adapt to new hydrological data or regulatory constraints. With respect to the stability of these structures, additional margins can be identified by the characterization of the macro-roughness of the concrete-rock interface. We present a field case. We show that a Pseudo VSP configuration can be implemented to detect and image the foundation dam contact. Measurements were done from the upstream and downstream facings of the dam with sensors (hydrophones and geophones) and impulse sources, positioned at the crest of the structure. The acquisition is done to generate and record P and S-waves. The upgoing reflected P and S waves are migrated by VSP-CDP transform. Hydrophone-geophone VSP- CDP stacked sections are merged after computation of a hydrophone-geophone transfer function by Wiener filter. Two composite VSP-CDP stacked sections are generated: the first one with VSP in P waves, the second with VSP in converted S-P waves on the upstream face and VSP in S waves on the downstream face (S-wave VSP). We show that the S-wave VSP has a better resolution to detect the dam-foundation interface. The results are consistent with nearest excavation line profiles.