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SUMMARY We provide new results from a controlled-source seismic experiment on the deepest part of the Val Sesia crust–mantle section of the Ivrea–Verbano zone (IVZ) in the Italian Alps. The IVZ is a tilted, almost complete section through the continental crust and exposes gabbros and peridotites in the structurally deepest level, coinciding with high-resolution gravity anomalies imaging the Ivrea geophysical body. The seismic experiment SEIZE (SEismic imaging of the Ivrea ZonE) was conducted along two crossing profiles: an NNE-SSW profile of ∼11 km length and an E-W profile of ∼16 km length. 432 vibration points were recorded with 110 receivers resulting in 24 392 traveltime picks. Inversion methods using Markov chain Monte Carlo techniques have been used to derive an isotropic 3-D P-wave velocity model based on first break traveltimes (refracted phases) from controlled source seismic data. Resulting seismic P-wave velocities (Vp ) range from 4.5 to 7.5 km s−1, with an expected general trend of increasing velocities with depth. A sharp velocity change from low Vp in the West to high Vp in the East marks the Insubric Zone (ISZ), the Europe–Adria plate boundary. The most prominent feature of the 3-D tomography model is a high-velocity body (Vp increases from 6 to 7.5 km s−1) that broadens downwards. Its pointy shape peaks the surface East of Balmuccia at a location coincident with the exposed Balmuccia peridotite. Considering rock physics, high-resolution gravity and other geophysical data, we interpret this high-velocity body as dominantly composed of peridotite. The dimension of this seismically imaged peridotite material is far bigger than interpreted from geological cross-sections and requires a revision of previous models. The interpretation of ultramafic bodies in the IVZ as fragments of mantle peridotites interfingered in the crust during pre-Permian accretion is not supported by the new data. Instead, we revive a model that the contact between the Balmuccia peridotite and the Permian mafic magmas might represent a fossil continental crust–mantle transition zone.

The Ivrea-Verbano Zone (IVZ) located in the Italian Alps is known as one of most complete archetypes of continental crust–upper mantle section on Earth (e.g. Pistone et al., 2017). Because of its accessibility at the surface it can be used as natural laboratory to improve the understanding of the crust–mantle transition zone. Several geophysical observables indicate the presence of mantle rocks (high density, high seismic velocity) in the shallow sub-surface (~ 1 km), commonly known as the “Bird’s Head” or Ivrea body (Berckhemer, 1968; Diehl et al., 2009; Scarponi et al., 2021). The project SEIZE images and characterizes the shallow upper crust at the Balmuccia site (Italy) providing depth, extent and shape of the outcropping Ivrea body as well as its rock properties. Our tomographic study covers the crust down to about 3 km depth, while seismic reflection imaging is possible down to 6 km depth or deeper. With SEIZE we contribute to the comprehensive ICDP Drilling program in the Ivrea-Verbano ZonE (DIVE, www.dive2ivrea.org).To tackle this task, a controlled source (vibroseis) seismic experiment was carried out in the region around Balmuccia in October 2020. The seismic survey comprised two crossing profiles with a total length of 28 km which ran along (NNE-SSW) and across (W-E) the Balmuccia peridotite. In total, 432 vibro points were acquired with a nominal distance of ~60 m which were recorded using a fix-spread (110 receivers, ~250 m spacing) and a roll-along setup (330 receivers, ~20 m spacing).To obtain a structural image of the shallow upper crust various seismic techniques are applied: The fix-spread data set is used to recover the velocity structure down to 3 km depth. By using a 3D Markov chain Monte Carlo travel time tomography a shallow, distinct high velocity body is imaged in 3D near Balmuccia, at the proposed drill site. Reflection seismic processing is applied to the roll-along data set. However, the difficult terrain setting (deep mountain valleys) results in complex wave propagation that is challenging for conventional processing methods (e.g. static and dynamic corrections, CDP stacking). Therefore, pre-stack migration techniques are applied enabling the imaging of steeply dipping structures.

SUMMARY 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 behaviour, 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 decametres. The seismic campaign included three-component borehole sensors and two different seismic source types (pneumatic impact and magnetostrictive vibroseis source). An executed source comparison analysed the characteristics of the different source types, for example frequency or amplitude behaviour 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 traveltime 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−1, vSmax = 2050 m s−1), slower velocities are found in the sandy facies (vPmax = 3720 m s−1, vSmax = 1840 m s−1) and the slowest velocities are found in the shaly facies (vPmax = 3220 m s−1, vSmax = 1480 m s−1). The seismic velocity anisotropy is larger within 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.

Since clay formations are heterogeneous and anisotropic, their seismic characterization at the meso scale is challenging. To tackle this problem, experiments using different seismic sources were undertaken in the Mont Terri Underground Rock Laboratory (URL). The first experiment was carried out using impact and vibroseis sources which were particularly designed for seismic exploration in the underground. The second experiment was conducted using an ELVIS vibration source (Polom et al. 2011) which was mainly designed for near-surface investigations on roads or in open terrain. The first experiment focused on the applicability and performance of the modular underground system (Borm & Giese 2003) in clay. It demonstrates the successful application of impact and vibroseis source in Opalinus clay. The impact source generates signals with high signal-to-noise ratios and strong lower frequencies (above 100 Hz). Due to that, the impact source is preferred for applications at large offsets. In contrast the vibroseis source has more control of the frequency generation and is able to excite higher frequencies (up to 12 kHz) than the impact source. Therefore, the vibroseis source is preferred for high-resolution applications at near offsets. Both sources are also suitable for clay characterization and reflection imaging. Travel time analyses resulted in average P- and S-wave velocities that show a clear azimuthal dependence. The carbonate-rich sandy and the sandy facies are characterized by faster velocities than the shaly facies which is stronger anisotropic than the sandy facies. Our findings are in good agreement with seismic velocities and anisotropy determined by Schuster et al. (2017), Popp & Salzer (2007) and Siegesmund et al. (2014). Although the sparse acquisition geometry was not optimal for reflection imaging of the geological conditions around the URL, later arriving shear wave reflections could be extracted from the impact data. A 3D migration focuses these reflections at a distance of ~50 m at the transition from the lower sandy facies to the upper shaly facies.The second experiment of our pilot survey focused on seismic reflection measurements using near-surface equipment to evaluate its applicability in URLs. Since the ELVIS source was combined with the 3-C geophones of the main experiment, the acquisition geometry was not optimal to image settings beneath the URL. The acquired ELVIS data were dominated by strong surface waves. After their removal, surface wave reflections appeared which mainly map the structural elements of the URL. The test measurements confirmed a general applicability of ELVIS in the tunnel, however it also indicates the need to improve the acquisition geometry.

There are many successful geothermal projects that exploit the Upper Jurassic aquifer at 2–3 km depth in the German Molasse Basin. However, up to now, only P-wave seismic exploration has been carried out. In an experiment in the Greater Munich area, we recorded S-waves that were generated by the conventional P-wave seismic survey, using 3C receivers. From this, we built a 3D volume of P- to S-converted (PS) waves using the asymptotic conversion point approach. By combining the P-volume and the resulting PS-seismic volume, we were able to derive the spatial distribution of the vp/vsratio of both the Molasse overburden and the Upper Jurassic reservoir. We found that the vp/vsratios for the Molasse units range from 2.0 to 2.3 with a median of 2.15, which is much higher than previously assumed. This raises the depth of hypocenters of induced earthquakes in surrounding geothermal wells. The vp/vsratios found in the Upper Jurassic vary laterally between 1.5 and 2.2. Since no boreholes are available for verification, we test our results against an independently derived facies classification of the conventional 3D seismic volume and found it correlates well. Furthermore, we see that low vp/vsratios correlate with high vpand vsvelocities. We interpret the latter as dolomitized rocks, which are connected with enhanced permeability in the reservoir. We conclude that 3C registration of conventional P-wave surveys is worthwhile.

Summary The modular seismic acquisition system at the GFZ is basically suitable for underground exploration in crystalline, salt and clay rocks. Special features of the application result from the different properties of the media such as degree of heterogeneity, damping, anisotropy and their interaction with the excavation zone (EDZ) induced by the cavity (drifts, tunnels). This manifests itself in different frequency damping behaviour for compressional, shear and surface waves, which require a specific use of impulse and vibration sources at the joint depending on the task. Seismic borehole measurements can contribute to a significant improvement of the resolution.

Seismic anisotropy and attenuation make claystone formations difficult to characterize. On the other hand, in many geotechnical environments, precise knowledge of structure and elastic properties of clay formations is needed. In crystalline and rock salt underground structures, high-resolution seismic tomography and reflection imaging have proven a useful tool for structural and mechanical characterization at the scale of underground infrastructure (several deca- to hundreds of meters). This study investigates the applicability of seismic tomography for the characterization of claystone formations from an underground rock laboratory under challenging on-site conditions including anisotropy, strong attenuation and restricted acquisition geometry. The seismic tomographic survey was part of a pilot experiment in the Opalinus Clay of the Mont Terri Rock Laboratory, using 3-component geophones and rock anchors, which are installed 2 m within the rock on two levels, thus suppressing effects caused by the excavation damage zone. As a source, a pneumatic impact source was used. The survey covers two different facies types (shaly and carbonate-rich sandy), for which the elliptical anisotropy is calculated for direct ray paths by fitting an ellipse to the separated data for each facies. The tomographic inversion was done with a code providing a good grid control and enabling to take the seismic anisotropy into account. A-priori anisotropy can be attributed to the grid points, taking various facies types or other heterogeneities into account. Tomographic results, compared to computations using an isotropic velocity model, show that results are significantly enhanced by considering the anisotropy and demonstrate the ability of the approach to characterize heterogeneities of geological structures between the galleries of the rock laboratory.

To study the salt-related tectonic evolution of the Leinetal Graben, located in the southernmost part of the Central European Basin (CEB) in Germany, we acquired two (1.8- and 3.2-km-long) P-wave reflection seismic profiles across the eastern border faults of the graben. The profiles were acquired with a minivibro along a 1.8-km active spread, densely sampled by geophones spaced at 5 m. The resulting sections showed stratigraphic and fault geometries to a depth of approximately 1200 m. Using two deep boreholes for calibration, we interpreted Mesozoic strata down to the Triassic Zechstein salt and the faults that affected these strata. We recognized two sets of faults: (1) steep, planar faults that are closely clustered and terminated in the Zechstein salt (type 1) and (2) shallow faults that connected between the first set of faults and have very variable dip, depending on the lithology they intersect (type 2). We discovered that the faults do not cross cut the Zechstein salt, but instead they decoupled at this layer. The present-day structure can be interpreted using a two-stage tectonic model. Either there was “downbuilding” during the Triassic, “rafting” of lower Buntsandstein blocks on the Zechstein salt, or both. This resulted in a proto-Leinetal Graben Zechstein diapir surrounded by depocenters. During the Late Cretaceous/Early Tertiary inversion phase, the diapir collapsed, first along type 1 steep faults that detached in the salt layer, and later along type 2 faults; the latter formed as the result of continued extension. Recognition of such early halokinesis was important for the understanding of the behavior of the salt in the CEB and salt tectonics in general.