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Abstract

Low permeability, high retention capacity and self-sealing ability are advantageous characteristics that are attributed to argillaceous rocks. In contrast, other properties of clay, such as internal heterogeneities, strong attenuation and anisotropic behavior, pose major challenges for underground exploration techniques. Although with regard to the underground storage of nuclear waste, the seismic exploration in the underground itself is of great importance to fill the gap between surface and borehole investigations. Furthermore, to prevent destruction of the host rock during exploration this demands low to non-invasive techniques. To approach these issues, a seismic survey was carried out in the Mont Terri Underground Rock Laboratory (Switzerland) using a gallery-based acquisition with an operating range up to several decameters. The seismic campaign included 3-component borehole sensors and two different seismic source types (pneumatic impact and magnetostrictive vibroseis source). An executed source comparison analyzed the characteristics of the different source types, e.g. frequency or amplitude behavior of the generated wavefield, to assess their performance under similar conditions at the meso scale and to reveal their strengths and weaknesses in clay. Based on these findings, we performed travel time and reflection analyses that demonstrate their potential to characterize clay formations and to map internal structures. The highest seismic velocities are found in the carbonate-rich sandy facies (vPmax = 4000 m/s, vSmax = 2050 m/s), slower velocities are found in the sandy facies (vPmax = 3720 m/s, vSmax = 1840 m/s) and the slowest velocities are found in the shaly facies (vPmax = 3220 m/s, vSmax = 1480 m/s). The seismic velocity anisotropy is larger within in the shaly facies (AvP = 23 per cent, AvS = 32 per cent) compared to the sandy facies (AvP = 9 per cent, AvS = 12 per cent) and it is more pronounced for S-waves than P-waves. Thus, non-invasive meso-scale seismic techniques are suited to characterize the Opalinus Clay in great detail.