Characterization of an earth dam in Brazil from seismic refraction tomography and multichannel analysis of surface waves

Effective methodologies to obtain information regarding the internal features in an earth dam are fundamental for rapid and technically correct decision making. Although several geophysical methods have potential applicability for investigations on dams, the contribution of seismic methods is well known, since the P and S-waves velocities can be associated with elastic modules of geotechnical interest. The study area was the Paranoá dam, the largest dam in Brazil's federal capital. We analyzed the internal structure of the dam and identified anomalies that could be associated with water saturation. We used traveltimes tomography to obtain Vp models and the MASW method to obtain Vs models. Four profiles of seismic data were acquired downstream of the dam. Borehole's information was used to corroborate with the geophysical interpretation of the profiles. The dam's abutments and the foundation ground could be identified within most models, and the velocity values obtained were mainly related to the clay material that forms the dam's massif, with higher values associated with the quartzite rocks. The rockfill material was well marked as a low velocity zone. Saturated zones were interpreted as local anomalies of high Vp/Vs ratio. An analysis of the depth range obtained within velocity models suggests that the depth of investigation from both methodologies are likely to be site specific, rather than exclusive matter of source and receiver instrumentation.