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Within the ICDP-funded project COSC (Collisional Orogeny in the Scandinavian Caledonides), mountain building processes are investigated with the help of two ~2.5 km deep fully cored boreholes in Central Sweden. Drilled in 2014, borehole COSC-1 near Åre studied the emplacement of the high-grade metamorphic allochthons and obtained a section through the Lower Seve Nappe as well as the underlying mylonite zone. The second borehole COSC-2, drilled in 2020 near Järpen/Mörsil, focuses on defining the character and age of deformation of the underlying greenschist facies thrust-sheets, the main Caledonian décollement and the Precambrian basement.An extended walkaway VSP survey at the COSC-2 drill site was performed in September-October 2021. This study aims to support the geological interpretation with a high-resolution 3D image of the subsurface in the direct vicinity of the borehole. This allows the determination of the origin of the basement reflections and reveals the nature of the main décollement as well as the degree of basement thrusting. Two 2D surface seismic lines approximately perpendicular to each other (North to South, West to East) and centered around the COSC-2 drill site were acquired using single (1C) and three-component (3C) geophones at 5-30m intervals. Furthermore, the West-East line was extended by 30 geophones at 100m intervals on each line end to allow the registration of wide-angle shots. A 32 t Vibroseis source operated along both lines with source point distances of 100 m within the central part of the line and 500 m at the wide-angle stations, respectively. Ocean bottom seismometers (OBS) were deployed on the bottom of a lake north of the borehole along a ~1.5 km portion of the North-South line. An airgun source was activated on this part of the profile. Along the entire borehole down to a depth of 2.26 km a 3C geophone chain recorded the seismic wavefield from all source points with a geophone spacing of 10 m, complemented by the recording from one single zero-offset source point with a geophone spacing of 2 m.The obtained surface seismic and VSP data set exhibits exceptionally good quality and shows many pronounced and clear reflections in the raw gathers. They are observed even at the largest source-receiver offsets (~11 km) and are visible at two-way-traveltimes up to 3-4 s, corresponding to structures at a depth of approximately 11 km. We present results of the ongoing surface seismic data processing and analysis, including a P-wave velocity model obtained from first arrival traveltime tomography, an analysis of seismic anisotropy related to the geological structures in the area and a first imaging result from the surface seismic data.

The ICDP funded project COSC (Collisional Orogeny in the Scandinavian Caledonides) is investigating mountain building processes with the help of two ~2.5 km deep fully cored boreholes in Central Sweden. While borehole COSC-1, drilled in 2014, studied the emplacement of the high-grade metamorphic allochthons, borehole COSC-2, drilled in 2020, focuses on defining the character and age of deformation of the underlying greenschist facies thrust-sheets, the main Caledonian décollement and the Precambrian basement.We have performed combined surface and borehole seismic investigations at both drill sites in order to characterize the Earth’s upper crust in the direct vicinity of the boreholes. Both surveys were designed as multi-azimuthal walkaway VSP surveys that have the potential to yield not only a 3D seismic image around the borehole both also to derive information about seismic anisotropy related to the drilled rock units.During the COSC-1 survey in 2014, three surface lines were acquired centered radially around the COSC-1 drillsite. In the central part up to 2.5 km away from the borehole a hydraulic hammer was used as the seismic source, while for larger offsets up to 5 km explosives were employed. The wavefield of both source types was recorded using an array of 15 three-component receivers with a spacing of 10 m deployed at 7 different depth levels in the borehole. Simultaneously, the wavefield was recorded at the surface by 180 standalone three-component receivers along each of the three up to 10 km long lines, as well as by a 3D array of single-component receivers in the central part of the survey area around the borehole.The COSC-2 survey in 2021 comprised two surface lines across the COSC-2 drillsite with densely spaced single- and three-component receivers and maximum source-receiver offsets of ~11 km. The location of the COSC-2 borehole right next to lake Liten made it necessary to design the survey as an amphibious seismic experiment using a 32 t Vibroseis truck and wireless geophones on land along the lake as well as an airgun and three-component OBS along the profile part across the lake. An array of 17 three-component receivers with a spacing of 10 m recorded the seismic wavefields of both sources along the entire borehole length.In both cases, a 3D velocity model including anisotropy information was obtained from the seismic data by first-arrival traveltime tomography. In the case of COSC-1, the anisotropic velocity model was used to perform an anisotropic prestack depth migration of the surface data, while for COSC-2 this part of the data processing and imaging is still ongoing. We show a comparison of the characteristics of both data sets, compare the obtained results and present lessons learnt for the planning of similar projects in the future.

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.

Increasing the effectiveness of exploration for mineral resources is vital to meet future societal, economic and environmental challenges. Effective exploration drilling for mineral resources is an area where industrial innovation plays an important role. Measurements-while-drilling, data acquisition and next generation logging sondes represent three important areas that need development in the mineral exploration sector. Despite this need, there is a lack of test beds that allow to test novel drilling equipment. This limits the development and implementation of equipment with technology that has been proven, but does not yet fulfil the requirements of a product on the commercial market. Although a variety of test sites exist throughout Europe, they are constrained to existing infrastructure, which limits users to pre-existing conditions that may not fit their purpose or need. The I-EDDA-TC provides a unique environment for the development of drilling, and related, equipment used for exploration of mineral resources.The regional geology around the test center site is dominated by Svecokarelian age granitoid intrusive and acid volcanic rocks (rhyolites) that strike east-west and dip sub-vertical. During 2019 and 2020, two boreholes were drilled at the test center site, as part of an EIT Raw Materials upscaling project. The first borehole is a fully cored 970 m deep borehole drilled with diamond bit (HQ dimension). The second borehole was drilled in the late summer of 2020, and is a 200 m deep percussion-drilled borehole with ~220 mm diameter. Here we present a preliminary synthesis of results from a geophysical survey, borehole logging and geological logging of drill core.In summer 2019 a comprehensive geophysical surveying program was performed at the site, including 3D high resolution seismic, 2D deeper seismic with a large vibrator source, a series of high-resolution resistivity profiles and magnetic profiles. The 3D seismic data provided detailed velocity information in the near-surface at the site, allowing interpretation of depths to the groundwater table and bedrock in 3D. Data gained from two downhole logging campaigns provides a robust base for the detailed differentiation and characterization of the formations. A first look on the data shows well defined correlations amongst the various geophysical downhole parameters. Geological logging focused both on material properties (e.g. mineralogy, grain-size, texture, alteration and mineralization) and rock mass (joints and RQD). Magnetic susceptibility and ultrasonic pulse velocity were measured at regular intervals along the full core length, and 66 specimens were prepared and analysed with respect to porosity, density, abrasivity, major chemical elements, indirect tensile strength and uniaxial compressive strength. The integrated analysis of core data, surface and borehole seismic data, and the continuous logging profiles allows for the 3-dimensional characterization of the underground below the test center platform, as well as provides reference data for assessment of work conducted at the site (e.g. development of geophysical instruments, testing of drillabilaty and wear on drill bits). Our results will be open access published so that data can be compared to drilling and instruments test of commercial and academic parties utilizing this testing facility in future.

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.

In this paper, we present an in-situ study of the marginal area of a CO2- and brine-rich cavernous structure in an underground rock salt mine. To reveal sources and dynamics of fluid movement, the temporal and spatial distribution of fluids in the potentially weakened cavern rim in salt were investigated. Seismic imaging, followed by long-term gas monitoring were applied along a transect from the cavernous structure to the undisturbed rock formation. The underground seismic survey comprised travel time tomography and pre-stack depth migration and showed, in agreement with the results of gas monitoring, disturbed areas without correlation to the cavernous structure. During two years, the monthly gas inflow into isolated borehole sections revealed insights into short term changes of gas migration pattern in rock salt. The measurements showed gas influx of up to 4 kPa/day and CO2 concentrations of up to 1.2%, especially during the beginning of the campaign. The gas migration is coupled to discrete fracturing and was observed to be spatially and temporary confined. Overall, no correlation between gas occurrences and their distance to the cavern exists, suggesting no wide-ranging fluid-rock interaction in the rim of the investigated natural cavernous structure in rock salt. The occurrence of gas is coupled to spatially and temporary limited fracturing which evidently healed and regained rock salt typical tightness.

The Baltic Sea Underground Innovation Network BSUIN is a European research project funded by Interreg Baltic Sea Region. The BSUIN network consists of six underground laboratories in Finland, Sweden, Russia, Poland and Germany with associated business and research partners. Each of the underground laboratories is unique in its geology, underground space and use. The BSUIN aims to build up a platform for innovative research and business concepts for the use of underground infrastructures and also especially for applications after completion of mining activities.For an innovation management it is important to identify research and application fields in underground labs for the present but also research areas of interest in the future. Also it is significant to define the relevant research fields, which are more likely to result in innovations and business applications.Within BSUIN an innovation platform concept will be established as a guideline for innovation management and support for the innovation processes. For that purpose we questioned aspects of the use of underground labs for users from several kind of customers and users from research and business of small and medium-sized enterprises.Here we present an overview of the evaluation of the questionnaire. What are the main aspects which are important for the use of underground labs for research and innovation and especially for business activities? Within BSUIN a concept of an innovation platform concept will be integrated in the BSUIN web based tool. This will allow to apply innovation keywords to site-specific research activities in each BSUIN mine.

Since 20 years the GFZ (German Research Centre for Geosciences) operates in an underground lab in the research and education mine Reiche Zeche at Freiberg in eastern Germany. This underground lab is used as testing lab for newly developed geophysical equipment and methods for 3D seismics. Therefore, in the underground space several galleries and boreholes can be used for seismic exploration in 2D and 3D approaches and for testing and validation of seismic acquisition equipment.Now, a Helmholtz Innovation Lab will be established at the GFZ in Potsdam. The Helmholtz Innovation Lab 3D-Underground Seismic Lab (3D-US Lab) is a place where scientific expertise and the needs of industry and its customers will meet together. By involving partners from mining and tunnelling in joint development projects on a long-term basis and transferring approaches from research into commercially successful applications a sustainable 3D-US Lab will be established.The 3D-US Lab bundles the seismic methods developed at the GFZ in a single technology platform, standardizes and modularizes them. It combines the technological and methodological developments in tunnel and borehole seismics for 3D seismic exploration of underground structures. The technology platform and the GFZ underground laboratory in Freiberg are of great interest for various partners from mining and tunnelling. In the long term the Helmholtz Innovation Lab aims to establish 3D underground seismics as a key technology for the effective and safe construction and use of underground buildings.

The Krafla geothermal area in northern Iceland comprises one of the best studied examples of a high temperature geothermal field associated with an active volcanic rift zone. Of key importance to improved resource exploration and development in volcanic areas such as Krafla, is the interpretation and understanding of the subsurface geology. Within this study we present results from an integrated analysis of the downhole volcanic stratigraphy from the K-18 borehole within the Krafla caldera based on combined wireline, ditch cuttings, and zero-offset VSP (vertical seismic profile) analyses. This study presents the first published sonic log velocity data from a high temperature geothermal borehole in Iceland and clearly demonstrates the importance of borehole velocity data for improving volcanic facies interpretations. The shallow subsurface geology of the K-18 site from c. 0–380 m comprises an inter-layered sequence of sub-aerial basaltic lavas, hyaloclastites and tuffaceous lithologies of both felsic and basic composition, which are progressively replaced by basaltic sheet intrusions with increasing depth. An interval of variably basic to more evolved mixed tuffaceous facies is identified based on cuttings analysis, gamma and sonic velocities between c. 790–1120 m depth. Discrete high sonic Vp units cut the lower c. 100 m of this interval and are interpreted as either sheet intrusions or lava interiors. At c. 1800 m, a sharp increase in P-wave velocity from c. 4.5 to c. 5.5 km/s, represents the transition from a mixed lava and sheet intrusion dominated sequence, into a dense basic intrusion forming the local basement that persists to the bottom of the borehole at c. 2215 m. Reduced travel time analysis of processed zero-offset VSP results reveal good correspondence with the major stratigraphic boundaries in the borehole, supporting the potential for VSP studies to robustly characterize complex volcanic stratigraphy in high temperature geothermal fields. Finally, the application of percentage-based ditch cuttings analyses methods for volcanic facies analysis in geothermal boreholes is tested and assessed to have future potential.

Salt rocks serve as host rock for technical caverns due to their impermeability but their can also be influenced by fluid migration due to geological fracture zones. Seismic methods can be used to monitor cavernous structures in the transition zone between cavity and undisturbed salt rocks. Around an artificially created cavity (field-test cavern) in a salt pillar with a volume of approximately 100 litre, travel time tomography was utilized to image structures related to caverns and fluid-storage. Seismic surveys were performed at different stages of an experimental simulation of gas-water-rock interaction in the field-test cavern aiming for a better understanding of the multiphase system in the cavern-near area. The baseline survey (1) was carried out using 8 three-component piezo-electrical sensor rods and a seismic vibrator source at the surface of the salt pillar, first without an installed field-test cavern. After drilling and installing the field-test cavern, seismic cross-hole measurements were performed after producing partial vacuum in the test cavern (2) and infill of gas (3) and water (4). To finalize the field experiments the last seismic survey (5) was again conducted at the surface of the salt pillar as a repeat measurement to the baseline survey. The seismic monitoring of the salt pillar was carried out in a frequency range of 100 Hz to 14000 Hz allowing a spatial resolution in the cm-range. This was followed by pre-processing of the seismic data sets to apply the picked travel times in a tomography program. On the basis of the tomography results and reflection seismic data we want to assess the potential enlargement of the field-test cavern due to water-infill and to image the differences between unaffected salt rocks, cavernous structures and developing transition zones.

Among geothermal exploration methods, active surface seismic methods have played only a minor role to date. Especially in high-temperature volcanic systems, reflection seismic data often reveal poor delineation of volcanic features, due to the internal heterogeneity of volcanic sequences. To enhance the vertical resolution, one possibility is the application of downhole seismic methods like vertical seismic profiling (VSP). A test experiment was carried out in the Krafla high-temperature geothermal field, NE-Iceland, to assess the ability of VSP to image subsurface structures, such as fractures, zones of high permeability, magmatic bodies, and zones of supercritical fluids and steam. Logging in such hostile environments is technical challenging in many aspects, but mainly due to the high temperature impact on the downhole electronic components of the measuring equipment. This requires a thorough pre-examination and implementation of the measurement, especially to avoid delays and tool failures. This paper presents results of zero- and far-offset VSP data from the K-18 borehole from within the Krafla caldera, which reveal good correlation with the surrounding lithology. The raw three-component seismic data display a good signal-to-noise ratio and dominant signal frequencies between 20 and 40 Hz, down to c. 2200 m depth, for air gun and explosive sources, respectively. A zero-offset source comparison was also conducted to assess the use of different impulsive sources for future VSP surveys in similar settings. By applying a standard VSP processing, we identified stratigraphic boundaries between lavas, hyaloclastites, and intrusions, which are in good agreement with existing well data. For the zero-offset VSP, both P- and S-wave velocity models were calculated and a depth-converted corridor stack was determined. In addition, multicomponent Kirchhoff depth migration and Fresnel volume migration were tested around the borehole. The 3D results are promising, but the specific shape and lateral extent of the reflectors could not be determined due to the restriction to only two sources and the insufficient spatial coverage (aperture). Our study demonstrates that vertical seismic profiles can clearly detect variations in the subsurface volcanic stratigraphy in high-temperature geothermal fields. A more detailed reservoir characterization can be achieved by further data integration, enhanced survey design including more source positions, and improved processing and imaging techniques, such as full-waveform inversion.

SUMMARYA remarkably well preserved representation of a deeply eroded Palaeozoic orogen is found in the Scandinavian Caledonides, formed by the collision of the two palaeocontinents Baltica and Laurentia. Today, after 400 Ma of erosion along with uplift and extension during the opening of the North Atlantic Ocean, the geological structures in central western Sweden comprise far transported allochthonous units, the underlying Precambrian crystalline basement, and a shallow west-dipping décollement that separates the two and is associated with a thin layer of Cambrian black shales. These structures, in particular the Seve Nappes (upper part of the Middle Allochthons), the Lower Allochthons and the highly reflective basement are the target of the two approximately 2.5 km deep fully cored scientific boreholes in central Sweden that are part of the project COSC (Collisional Orogeny in the Scandinavian Caledonides). Thus, a continuous 5 km tectonostratigraphic profile through the Caledonian nappes into Baltica’s basement will be recovered. The first borehole, COSC-1, was successfully drilled in 2014 and revealed a thick section of the seismically highly reflective Lower Seve Nappe. The Seve Nappe Complex, mainly consisting of felsic gneisses and mafic amphibolites, appears to be highly anisotropic. To allow for extrapolation of findings from core analysis and downhole logging to the structures around the borehole, several surface and borehole seismic experiments were conducted. Here, we use three long offset surface seismic profiles that are centred on the borehole COSC-1 to image the structures in the vicinity of the borehole and below it. We applied Kirchhoff pre-stack depth migration, taking into account the seismic anisotropy in the Seve Nappe Complex. We calculated Green’s functions using an anisotropic eikonal solver for a VTI (transversely isotropic with vertical axis of symmetry) velocity model, which was previously derived by the analysis of VSP (Vertical Seismic Profile) and surface seismic data. We show, that the anisotropic results are superior to the corresponding isotropic depth migration. The reflections appear significantly more continuous and better focused. The depth imaging of the long offset profiles provides a link between a high-resolution 3-D data set and the regional scale 2-D COSC Seismic Profile and complements these data sets, especially in the deeper parts below the borehole. However, many of the reflective structures can be observed in the different data sets. Most of the dominant reflections imaged originate below the bottom of the borehole and are situated within the Precambrian basement or at the transition zones between Middle and Lower Allochthons and the basement. The origin of the deeper reflections remains enigmatic, possibly representing dolerite intrusions or deformation zones of Caledonian or pre-Caledonian age.

The 2.5 km deep scientific COSC-1 borehole (ICDP 5054-1-A) was successfully drilled with nearly complete core recovery during spring and summer of 2014. Downhole and on-core measurements through the targeted Lower Seve Nappe provide a comprehensive data set. An observed gradual increase in strain below 1700 m, with mica schists and intermittent mylonites increasing in frequency and thickness, is here interpreted as the basal thrust zone of the Lower Seve Nappe. This high strain zone was not fully penetrated at the total drilled depth and is thus greater than 800 m in thickness. To allow extrapolation of the results from downhole logging, core analysis and other experiments into the surrounding rock and to link these with the regional tectonic setting and evolution, three post-drilling high-resolution seismic experiments were conducted in and around the borehole. One of these, the first 3D seismic reflection land survey to target the nappe structures of the Scandinavian Caledonides, is presented here. It provides new information on the 3D geometry of structures both within the drilled Lower Seve Nappe and underlying rocks down to at least 9 km. The observed reflectivity correlates well with results from the core analysis and downhole logging, despite challenges in processing. Reflections from the uppermost part of the Lower Seve Nappe have limited lateral extent and varying dips, possibly related to mafic lenses or boudins of variable character within felsic rock. Reflections occurring within the high strain zone, however, are laterally continuous over distances of a kilometer or more and dip 10–15° towards the southeast. Reflections from structures beneath the high strain unit and the COSC-1 borehole can be followed through most of the seismic volume down to at least 9 km and have dips of varying degree, mainly in the east–west thrust direction of the orogen. Collisional Orogeny in the Scandinavian Caledonides (COSC)

Geophysical exploration and in particular active-source seismic imaging of geothermal fields is important to assess and optimize the exploitation of natural heat sources for energy production and direct use. The first multi-offset (moving-source) vertical seismic profiling (VSP) experiment over the high-temperature geothermal field in Krafla (Iceland) was carried out in spring 2014 with the aim to test whether VSP is a suitable method to map volcanic stratigraphy, fractures, dykes, steam zones and magmatic bodies at this site and for volcanic environments in general. In this study, we present a workflow for processing the sparse Krafla VSP dataset recorded with receivers in either of two boreholes. The analysis involved first-arrival traveltime inversion and seismic reflection processing. The seismic velocity model obtained by traveltime tomography reveals structural information between the two boreholes and can be linked to an existing geological model, showing that the seismic velocities are mainly controlled by lithology. The zero-offset seismic reflection data were processed into two corridor stacks. Walk-away VSP reflection data were migrated with a novel multicomponent Kirchhoff migration algorithm that includes P- and S-wave isolation to obtain separate PP, PS and SS migrated images. The reflections imaged in the corridor stacks can be linked to the main lithological units known from borehole logging information. Migrated images from the walk-away data reveal reflectors below and to the sides of the two boreholes. Considering a priori information, such as hypocenter locations from earthquake seismology studies, the reflectors can be related to changes in lithology, fault zones, dykes and possibly the top of the Krafla magma chamber. We found that VSP is potentially a useful method to image the key lithological boundaries and volcanic stratigraphy in the complex magmatic environment at Krafla, but a priori information proved to be essential to constrain the processing and interpretation of the sparse array dataset.

Geophysical exploration is indispensable for planning deep drilling. Usually 2Dor 3D-seismics investigations are applied and, depending on the resulting geologic model for the underground, the drill site and drilling path are determined. In recent years the focus of exploration has shifted towards small-scale geological structures such as local layers and faults. Depending on the source frequencies and the target depth, 2Dor 3D-seismics from surface cannot always resolve such structures in particular at larger depths. In general, signal frequencies of about 30–70 Hz are typical for surface seismic methods. The deeper and smaller the sought-after structures are, the worse will be the resolution. Therefore, borehole seismic measurements like Vertical Seismic Profile (VSP) or Seismic While Drilling (SWD) have been developed (Fig. 1). For the VSP method geophones are normally integrated in the borehole, while the seismic source generates seismic waves at the surface. The SWD method uses the drill bit as the seismic source. Hence, the quality of the seismic signals is highly dependent on the drilled rock and the type of drill bit, but even well-suited rock conditions and adequate drilling may not provide sufficient data quality. Compared to 2Dand 3D-seismics, the distances between source and receiver to the target are shorter for VSP, and for SWD in particular. Signal frequencies up to about 100 Hz are observed yielding a slightly better resolution compared to surface seismics. However, subtle yet important features (e.g., fault zone) often cannot be identified with certainty. Accordingly, a method or tool that would improve the resolution is of high importance since this would allow adjusting the drilling path to minimize risks and costs of drilling.

The choice of an appropriate seismic source for a specific application is crucial for the success of seismic surveys. In this study, the utilization of two novel seismic sources is discussed for geoengineering applications in the bandwidth of 80–3000 Hz. These are a pneumatic impact and a magnetostrictive vibrator source mainly designed for seismic transmission and reflection applications. Seismic test surveys were carried out on Paleozoic sediment deposits to study a near surface fault zone at the southern rim of the Taunus mountain range in Germany and in crystalline rocks to investigate fracture zones along the Piora Adit of the Gotthard Base Tunnel in Switzerland, respectively. Both sources are evaluated with respect to their seismic wave energy, the sum of the squared amplitudes of first P- and S-arrivals and frequency content, signal to random noise ratio, as well as their suitability with regard to the specific site conditions and applications. Common Midpoint-stacked data of the Taunus survey and pre-stack depth migrated data of the Piora survey were analysed to assess the ability of the seismic sources to image geological structures and to give support for geotechnical aspects. For both test sites, the vibrator source proved to be more suitable to generate body waves than the impact source, which generates strong ground roll noise signals due to its non-controlled signal amplitude spectrum. In fact the control system of the vibrator source enabled improved adaption to varying ground conditions due to a controlled adjustment of the signal amplitude spectrum to optimize resolution and penetration for a given exploration depth.

Summary Vertical seismic profiling (VSP) was tested for mapping volcanic stratigraphy, fractures, dykes, fluid and steam in the geothermal area of Krafla in Iceland. Seismic imaging in magmatic environments is very challenging, largely due to the intense scattering of seismic waves traveling through the highly heterogeneous volcanic rocks. VSP offers means to image structures beneath and away from the well in complex volcanic environments. The VSP survey at Krafla was carried out in two wells, for each of which a zero offset, a far offset and a walk-away experiment were recorded. The zero offset data is of good quality, with the observed reflections corresponding to stratigraphic boundaries that can be explained by a simple 1D velocity model. The corridor stacks of the synthetic and field data look similar to each other, apart from a constant time shift and amplitude differences. High scattering in the subsurface leads to low amplitude reflections from deeper horizons. The walk-away data shows little coherent reflectivity. Furthermore, a complex 2D velocity model involving heterogeneities in the horizontal as well as vertical directions will be required to explain the observed seismograms.

The GFZ Underground Laboratory is operated by the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences. It is located in the research and education mine “Reiche Zeche” in Freiberg, Germany allows testing of geophysical and geotechnical tools and methods in boreholes and galleries. The lab is ideally suited for seismic system components such as receivers and sources for three-dimensional high resolution seismic imaging and tomography surveying. The lab layout of a basement rock block surrounded by galleries around a vertical as well as two horizontal boreholes enables the realization of various underground survey geometries e.g. well-to-well and well-to-gallery. The galleries are equipped with thirty 3-component geophone anchors installed in 1 m and 2 m depths for tomographic measurements or the recording of radiation pattern of seismic borehole sources.

To monitor the migration of the injected CO2 in the Ketzin project (Germany) a permanently buried multi-component seismic array has been installed in August 2009. Since then the array has been continuously recording passive seismic data. Additionally an active seismic survey resulting in a 230 m long 2D seismic line roughly passing over the monitoring well Ktzi 202/2007 has been acquired in November 2009. This relatively small set-up is meant as a pre-screening study to demonstrate the added value of the buried sensors before installing a larger 3D array. A first analysis of the active seismic data shows, that the use of an array of buried receivers results in (time-lapse) seismic data that offer a superior S/N ratio suppressing both ambient noise and surface related coherent noise. This setup is expected to lead to an increased repeatability. This paper describes the experimental set-up and first results of the acquired baseline data using the buried sensors. © 2011 Published by Elsevier Ltd.

A 3D 25-fold seismic survey with a bin size of 12 by [Formula: see text] and about [Formula: see text] of subsurface coverage was acquired in 2005 near a former natural gas storage site west of Berlin, as part of the five-year EU funded [Formula: see text] project. Main objectives of the seismic survey were to verify earlier geologic interpretations of structure based on vintage 2D seismic and borehole data and to map, if possible, the reservoir pathways in which the [Formula: see text] will be injected at [Formula: see text] depth, as well as providing a baseline for future seismic surveys and planning of drilling operations. The uppermost [Formula: see text] are well imaged and show an anticlinal structure with an east-west striking central graben on its top that extendsdown to the target horizon. About [Formula: see text] of throwis seen on the bounding faults. No faults are imaged near the planned drill sites. Remnant gas, cushion and residual gas, is present near the top of the anticline in the depth interval of about [Formula: see text] and has a clear seismic signature; both higher amplitudes in the reservoir horizons and velocity pulldown are observed. Amplitude mapping of these remnant gas horizons shows that they do not extend as far south as the injection site, which is located on the southern flank of the anticline. Amplitude anomalies, gas chimneys along an east-west striking fault, show that the stored or remnant gas either has been or is presently migrating out of the reservoir formations. Summed amplitude mapping of the planned injection horizon indicates that this lithologically heterogeneous formation may be more porous at the injection site.

A VSP test experiment at the high temperature geothermal field Krafla in Iceland has been carried out. In two boreholes a zero-, far- and moving-source VSP has been applied to delineate subsurface fracture sets, zones of supercritical fluids, superheated steam and zones of magma with the background of a magmatic, high temperature and high attenuating basement. A careful preparation and evaluation of the survey is needed in order to provide a high resolution image of the surrounding subsurface. Under such special conditions high-quality three component data have been recorded and will grant a good basis for further processing and imaging techniques. It can be assumed that VSP techniques will provide a good alternative and addition to hitherto only applied surface seismic, teleseismic or microseismic procedures.

In this paper the method for estimating the statistical parameters of the medium from traveltime measurements of refracted waves is applied to study the statistical characteristics of crystalline rocks at the Multifunctional Station Faido (Gotthard Base Tunnel, Switzerland). The method is based on the geometrical optics (GO) approximation. A covariance function for traveltime fluctuations has been obtained by considering quasihomogeneous fluctuations of sound velocity in a plain-stratified medium. Strongly anisometric (having unequal dimensions in different directions) random inhomogeneities were embedded in this medium. To estimate the statistical parameters around the tunnel, the traveltime fluctuations are calculated. It is assumed that each observation of traveltime-distance relation for a given shot-receiver group corresponds to a particular realization of a medium statistical ensemble. By calculating the variance and the zero cross intervals of the first derivative of traveltime fluctuations, the standard deviation of the velocity fluctuations and the characteristic horizontal scale of the inhomogeneities are estimated. Although the method allows to obtain the characteristic lengths of the inhomogeneities in vertical as well as in horizontal direction, the limited offset of the field data made it only possible to measure the latter. The estimated horizontal characteristic scale is about 13 m, which is reasonably close to the direct geological measurements in the studied region, where quartz lenses are dominant among the inhomogeneities. The standard deviation of the velocity is estimated as 4.5%, which might be caused by the fractured structure around the tunnel and also by the fault zone near the study area.

The EUROBRIDGE'96 seismic refraction and wide-angle reflection profile in Belarus crosses the 2.0–1.9 Ga Palaeoproterozoic Central Belarus belt (CB) subdivided by NE-trending Minsk Fault into the northwestern (NWCB) and southeastern (SECB) sub-terranes differing in their styles of tectonothermal evolution. The first interpretation of the EUROBRIDGE'96 profile, however, did not indicate any difference between the NWCB and SECB and revealed a high-velocity zone beneath the SECB, with a P-wave velocity of 6.6 km/s at a depth of 6 km, which does not correlate with any pronounced positive Bouguer anomaly. We present a detailed analysis of the seismic and gravity data obtained along the EUROBRIDGE'96 profile, complemented with petrological and petrophysical observations on the main rock types of the CB. Our integrated interpretation shows that the dip orientation of the main reflectors and the P-wave velocity and density are different in the NWCB and SECB. The Minsk Fault zone is distinguished as an area of relatively high velocities (>6.2 km/s) in the upper crust. Increased reflectivity in the middle crust SE of the Minsk Fault may be explained by a laminated structure composed of alternating tectonically deformed amphibolite-facies and granulite-facies rocks that produce a certain seismic anisotropy. This seismic anisotropy is proposed as an alternative explanation for the high P-wave velocity in the middle crust beneath the SECB that does not result in any corresponding positive gravity anomaly. The laminated structure beneath the SECB might be formed during the uplift of the NWCB and juxtaposition of the NWCB and SECB.

The construction of large tunnels and underground infrastructures faces increasingly large dimensions and complex geological conditions. Under these conditions, exploration techniques are needed which enable for a detection of potentially hazardous structures during construction. Seismic sensors, integrated into rock anchors, and small seismic signal sources using defined pneumatic impulses or sweep signals generated by magnetostrictive actuators are the components of an exploration system which can be easily integrated into different types of underground excavation work and which can also be deployed for the long-term monitoring of already existing tunnels or caverns. However, for a continuous acquisition of seismic signals during tunnel excavation, the strong and broadband signal generated by a tunnel boring machine (TBM) may be used as a continuously operating source. Within the collaborative project SOUND, the seismic equipment at the Underground Lab of the Reiche Zeche Research Mine in Freiberg (Germany) has been used for a tomographic monitoring study during the excavation of an inclined gallery. A synthetic, but realistic seismic data set was simulated using a randomly heterogeneous velocity model which can be regarded as a realistic prototype of the velocity distribution in the real Gneiss block. The simulated acquisition geometry has been derived from the actual source and receiver point distribution in the Underground Laboratory. It can be shown that the analysis of the modelled seismic data by full waveform inversion (FWI) was able to reveal the lateral heterogeneity of the velocity model with significantly higher resolution compared to traveltime tomography of the direct P-wave arrivals. The analysis of field data from the Underground Laboratory has shown that there are complex interactions in close vicinity to the receiver location, and before FWI can be applied to this real data set, source and receiver dependant signatures need to be removed by inversion and deconvolution. A further field experiment, performed during gallery excavation in the Underground Laboratory, has shown that the setup of seismic receivers in rock anchors and a sparse array of adaptive vibro-sources is able to detect subtle changes in seismic wave propagation related to stress changes due to the excavation of an inclined gallery. After the deployment in the Underground Laboratory, a field survey was carried out on a tunnel construction site. A broadband seismic data set, using the tunnel boring machine could be acquired providing a basis for high resolution imaging of structures ahead of the construction site and geotechnical characterization of the imaged volume.

Seismic travel-time and attenuation tomography were applied to characterize the excavation damaged zone and the adjacent rock mass in the GFZ-Underground-Lab within the research and education mine Reiche Zeche of the Technical University Bergakademie Freiberg (Germany). The lab is situated in gneiss rocks at 150 m depth and comprises three galleries which enclose an area of approximately 50 m×100 m. Along these galleries two seismic surveys were performed before and after the excavation of a new ramp and chamber. For both measurements, travel-time and attenuation tomographies for P-waves were performed with the ray-based inversion algorithm SIMULPS14. The seismic velocities were calculated from first-arrival travel times whereas a logarithmic-spectral-ratio approach was used to calculate the corresponding quality factors (Q) for attenuation tomography. A comparison of the tomograms reveals a decrease of average P-wave velocity values from 5.64 km/s to 5.54 km/s and of average Q-values from 29.8 to 26.5 in the whole area after the excavation of the new cavities. The maximum changes are located at already weakened zones either at the conjunction of two galleries or of a major fracture zone with a gallery. The attenuation tomography shows a higher sensitivity to rock mass changes than the travel-time tomography. However, the calculation of the Q-values demands a higher signal quality than the determination of the seismic travel times.

TOAST (Toolbox for Applied Seismic Tomography) makes methods of full-waveform inversion of elastic waves available for the practitioner. The inversion of complete seismograms is an utmost ambitious and powerful technology. One of its strengths is the enormously increased imaging-resolution since it is able to resolve structures smaller than the seismic wave length. Further it is sensitive to material properties like density and dissipation which are hardly accessible through conventional techniques. Within the TOAST project algorithms available in academia were collected, improved, and prepared for application to field recordings. Different inversion strategies were implemented (global search, conjugate gradient, waveform sensitivity kernels) and computer programs for imaging the subsurface in 1D, 2D, and 3D were developed. The underlying algorithms for the correct numerical simulation of physical wave propagation have thoroughly been tested for artefacts. In parallel these techniques were tested in application to waveform data. They proved their potential in application to synthetic data, shallow-seismic surface waves from field recordings, and microseismic and ultrasonic data from material testing. This provided valuable insight to the demands on seismic observation equipment (repeatability, waveform reproduction, survey layout) and inversion strategies (initial models, regularization, alternative misfit definitions, etc.). The developed software programs, results of benchmark tests, and field-cases are published online by the OpenTOAST.de initiative.

The EUROBRIDGE deep seismic sounding (DSS) profile is a key component of a EUROPROBE project to examine Palaeoproterozoic processes of continental collision and crustal accretion. Its purpose is to establish the deep lithospheric structure of the East European Craton between the exposed Proterozoic and Archaean complexes of the Baltic and Ukrainian Shields. In 1994 a DSS experiment was recorded across the Baltic Sea from Vastervik (Sweden) to Shventoji (Lithuania). We report on EUROBRIDGE'95, the first onshore stage of the seismic profile. It is 280 km long, recorded from NW to SE on the Lithuanian part of the East European Platform, traversing the Proterozoic West Lithuanian Granulite Domain (WLG) and East Lithuanian Belt (EL) terranes. Explosive shots of up to 1000 kg TNT were detonated at 10 shotpoints (SP01–SP10) at intervals of about 30 km. Arrivals were recorded at 76 3-component seismograph stations with an average station spacing of 3.5 km, providing high quality records. A 11th shot of 3000 kg (SP00) was fired in the Baltic Sea close to Gotland. Raytracing analysis of refracted and reflected P-waves has been used to determine a 2-dimensional seismic velocity model for the crust and uppermost mantle below EUROBRIDGE'95. The thickness of the Phanerozoic sedimentary cover decreases from 2.2 km in the north–west near the Baltic Sea coast to 0.4 km at the south–east end of the profile near the Lithuania/Belarus border. Crust in the north–west and central part of profile consists of two major layers with a thickness of about 44 km, increasing to 50 km and three layers in the south–east. Crystalline upper crust is about 20 km thick, thinning in the south–east, with P-wave velocities of 6.0–6.3 km/s. A very weak low velocity zone, with a velocity contrast of 0.1–0.2 km/s, occurs at 8–13 km depth below the north–west and central part of the profile only. Lower crust exhibits velocities of commonly 6.5–6.9 km/s, and thickens to the south–east with P-wave velocities up to 7.0 km/s in the deepest parts. Crystalline crust is characterised by low velocity gradients and small velocity contrasts at most seismic boundaries. Major lateral changes in crustal velocity structure at all depths can be spatially correlated with the WLG–EL boundary determined from near surface geological information. Very strong reflections from the Moho boundary are observed. The mantle P-wave velocity immediately below the crust is 8.2–8.35 km/s. A reflector in the lower lithosphere at a depth of almost 70 km was found below Lithuania. Reflectivity modelling of the Gotland shot data suggests that this interface is absent offshore, where mantle velocities are lower. The DSS model supports the interpretation of the WLG and EL as terranes of Proterozoic age forming part of Fennoscandia. Later modification of crustal structure may have occurred, possibly by a mantle-heating episode centred on the Baltic Sea area.

In borehole seismics several methods are in use while drilling. In the case of small layer structures, for instance thin layers or faults, the resolution of surface seismic methods is often not sufficient to determine and locate these structures for exploration. Combining a seismic source and receiver in a common device in the drill string would increase the seismic resolution. In the project SPWD such a prototype of a device has been designed, manufactured and tested. The used source signal frequencies are up to 5000 Hz, which allows a resolution of up to one metre. The resolution is therefore significantly higher than for VSP and SWD. With a penetration depth of about 100 metres an exploration of the borehole surrounding is possible. By applying this method while drilling the risks and costs of drilling can be minimized because of the possibility of adjusting the drill path and locating small sought-after structures.

In this paper, we report first acoustic communications experiments along deep drill strings enabling data rates up to 20 kbit/s. To reduce costs for geothermal energy, search for water-bearing fault zones in the rocks hundreds of meters below the ground will be assisted by seismic prediction while drilling (SPWD) enabling control of the drilling direction. The SPWD tool needs higher data rates to the ground. In contrast to common mud-pulse telemetry offering few bit/s, our approach is based on acoustic wave propagation along the steel wall of the drill string. Based on waveguide theory, we develop a tractable channel model for drill-string communication predicting the pass and stopbands created by regular screw joints between the pipes. For data transmission, we use closed-loop adaptive orthogonal frequency division multiplexing (OFDM) following the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). First results indicate that, in this way, much higher data rates can be realized in future deep drilling applications.

CLEAN was a scientific programme in support of a pilot Enhanced Gas Recovery (EGR) project and it was planned to inject nearly 100,000 t of carbon dioxide (CO2) into the Altmark natural gas field. Due to delays in the permitting process, injection did not occur within the time frame of the project. Therefore a test case was studied of the theoretical injection of CO2. Modelling CO2 injection in the Altensalzwedel segment of the Altmark depleted gas field shows that effective injection can be carried out at a tubing head pressure as low as 3.5 MPa and at tubing head temperature of 10 °C. The history matching of the simulation model and the injection prediction were successfully conducted with a volume ratio of injected CO2 to the gas in place of approximately 0.06 which did not show any enhancement in the gas recovery (no EGR effect). Within the pilot project period of 2 years there would be no CO2 arrival at the production well located at about 1,600 m from the injector. Penetration of the CO2 into the reservoir was maximum approximately 800 m and minimum around 250 m depending on the permeability of the formation layer. The gas mixing zone reaches the observation well which is located at about 660 m from the injector already before 1 year of injection. Under the simulated conditions direct monitoring of the CO2 front with active seismic measurements will not be feasible as the reservoir layers are too thin and the effects of CO2 gas replacing reservoir gas are too small. Seismic monitoring has to concentrate on the detection of possible leakages in shallower aquifers where larger velocity and density changes would occur.

One aim of the CLEAN project was to develop and test monitoring methods for the reservoir cap rock and the reservoir itself. It is shown here that advanced injection and production profile evaluation can be achieved using a combination of pressure, temperature and spinner flow meter data. Using distributed temperature sensing, temperature profiles in gas-filled wells can be acquired, as the sensor cable can be stationary during the measurement allowing for simultaneous thermal equilibration along the entire logged profile. A first field test of the developed hybrid wireline logging system was successfully performed under static conditions and the feasibility of warm-back monitoring was shown based on the results of numerical simulations for a possible CO2 injection scenario. A combined approach using the developed hybrid system for enhanced production logging during injection followed by warm-back monitoring within a subsequent shut-in period would allow for accurate determination of the spatial extent and injectivity of individual CO2 injection intervals.

The demand towards a high resolution image of seismic measurements rises in every sphere of interest. We want to test an elastic full waveform inversion approach for inverting for crystalline host rock for future application below ground. This will decrease the safety and investment risks for underground developments. We performed seismic measurements with multi-component receivers using transmission geometry in the GFZ Underground Lab in the research and education mine ”Reiche Zeche” of the TU Bergakademie Freiberg. A 2-D elastic full waveform tomography is applied to gain a high-resolution velocity model by using only the waveform of the first arrival P-wave. For this approach, based on the field measurement geometry, a synthetic test investigated the applicability of the inversion. A preprocessing flow was applied on the synthetic data. As the true model, a random scattered velocity model which represent the crystalline rock was used to estimate the possible resolution of the inversion. The starting model was obtained from a first arrival travel time tomography. After 250 iterations, the waveforms match almost perfectly. Also, the velocity model was reconstructed very well. The unresolved structures are in the range of or even smaller than the smallest wavelength used for the inversion. Therefore, our preprocessing approach for a crystalline rock medium by only considering the P-wave arrivals is suitably for a high resolution image compared to other inversion schemes.

Ketzin is an European pilot site for onshore CO2 storage in a saline aquifer. Injection started in summer 2008 and to date, more than 37.000 tons of CO2 have been injected into the Stuttgart Formation at approximately 650 m depth. A wide range of monitoring methods are being applied at Ketzin, among which are active seismic observations at various scales. 3D reflection seismic, combined surface-downhole measurements and crosshole tomographic surveys were performed before injection and after the start of injection in order to image the reservoir and to track the CO2. Time-lapse signatures of the injected CO2 were observed by all active seismic methods. The CO2 could be detected by increased reflectivity at the top of the injection reservoir, by a change in the attenuation behaviour and also by reduced propagation velocity within the reservoir. The ongoing injection of CO2 during the next years will be followed by further repeat surveys. Current investigations focus on the still difficult problem of the quantification of the CO2 imaged by the geophysical measurements.