An active source seismic experiment was carried out as part of the subsurface characterization studyof the first Spanish Underground Research Facility for Geological Storage of CO2in Hontomín (Burgos,Spain). The characterization experiment included a 36 km23D seismic reflection survey and two three-component seismic profiles. The target reservoir is a saline aquifer located at 1450 m depth within LowerJurassic carbonates (Lias). The main seal is formed by interlayered marlstones and marly limestonesof Early to Middle Jurassic age (Dogger and Lias). The seismic images obtained allow defining the 3Dunderground architecture of the reservoir site. The structure consists of an asymmetric dome crosscut bya relatively complex fault system. The detailed characterization of the fracture system is currently understudy to unravel the geometric distribution of the faults and their extent within the different formationsthat form the structure. The constrained model has guided the design of the injection and monitoringboreholes and provided the data for the baseline study. The resultant high resolution seismic model willbe used as a reference in future monitoring stages.