Application of geophysical techniques to support geological mapping projects

Better and more detailed information on the bedrock and near-surface geology is needed to assist geological risk assessments, efficient use of water resources or management of rapidly growing regions. Since 2007, the Cartographic and Geological Institute of Catalonia (ICGC) has developed a geological mapping programme, which includes information related to hydrogeology, urban geology or geological risks among others. Within this framework, geophysical techniques make an important contribution refining geological mapping by adding subsoil information to traditional geological data. In this work, we present two studies carried out by ICGC in order to map bedrock geometry or to characterize near-surface sediments in areas with scarce borehole information. The first example of this support focuses on the combination of passive and active geophysical techniques in order to image the Quaternary/Neogene boundary, the bedrock top and to locate faults or the geological map of the Girona urban area (NE of Spain). The second case study shows the potential of reprocessing vintage seismic oil datasets to increase knowledge of the near-surface geological structure in a Neogene Basin located north of Girona (Empordà Basin). The reprocessed seismic reflection image of the first km depth has been interpreted using the refraction velocity model and passive seismic information as constraints. The main objectives are to recover bedrock geometry and structure as well as fault imaging, which are critical for geological mapping projects. The common pattern of the methodologies applied to each case is the integration of different geophysical datasets and the use of both active and passive techniques.