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As the mining industry shifts toward more responsible and lower-impact practices, mine tailings are increasingly regarded as a secondary resource that can be sourced into the raw-material supply chain. However, limited knowledge of subsurface geometry at many historical tailings sites leads to uncertainties in volume and resource estimates. High-resolution seismic imaging can provide improved characterization of such sites. Here, we present the results of a seismic survey conducted in 2024 on tailings at Bl & ouml;tberget in central Sweden. The data were acquired using a combined, partly collocated setup consisting of a sparse 3D nodal array, a landstreamer and distributed acoustic sensing (DAS) systems. The objective was to characterize the tailings and to optimize the acquisition and processing strategies for our application. Through 2D reflection seismic imaging, layer-based refraction travel-time modelling and travel-time tomography, we obtain well-constrained P-wave velocities for the tailings on the order of 300-350 m/s. Additional 3D travel-time modelling yielded volumetric estimates, from which we estimate a total tailings tonnage of approximately 4 Mt. Finally, we demonstrate that surface DAS can serve as a complementary recording system, capable of delineating the tailings when fibre cable-ground coupling conditions are favourable. The survey serves as a pilot for future mine tailings investigations and related near-surface characterization efforts, and we highlight the benefits of using multi-element seismic recording arrays for such purposes.

The Petra morphological depression on Lesvos Island hosts one of Greece's most promising low- to intermediateenthalpy geothermal prospects, yet its internal structure and fluid pathways remain poorly constrained. To improve the characterization of the geothermal system, an integrated geophysical survey combining seismic, gravity, and magnetic methods was conducted in 2024 to delineate basin geometry and identify fault and fracture zones with enhanced permeability. Reflection seismic data were processed to image subsurface structure, while point-diffraction modelling was applied to constrain migration velocities and distinguish structural discontinuities from faults and small-scale intrusive bodies. First-break traveltime tomography provided complementary constraints on fault geometry and on the thickness of fresh volcanic units (Upper Lava), and residual gravity data were used to resolve lateral density variations. The integrated interpretation reveals a fault-bounded depression filled with low-density, low-velocity Upper Lava overlying higher-velocity, higher-density Lower Lava. Two major sets of steeply dipping faults define the Petra depression; minor and intersected faults are considered as conduits that enhance permeability and promote hydrothermal circulation. The Petra depression is interpreted as a small transtensional pull-apart basin that localizes geothermal fluid flow along its major fault zones. From a geothermal perspective, these results reduce structural uncertainty and highlight two domains along the northern and southern bounding faults as the most promising targets for future geothermal drilling.

Accurate near-surface characterization is essential to ensure public safety and operational integrity in geological carbon storage (GCS) projects. This study focuses on the imaging of near-surface features within the Havnsø domal structure, a potential GCS site in northern-central Denmark, using seismic data acquired in 2022. We combined high-resolution seismic reflection imaging with first-break travel-time tomography to characterize features and key horizons within Quaternary sediments and the Chalk Group. We achieved more accurate and detailed near-surface sections down to 500 m by migrating and time-to-depth-converting seismic sections using the tomographic velocities. This approach, along with superimposed tomography-based models, revealed important features such as buried valleys, Chalk Group horizons and fault zones. Our results demonstrate that the potential GCS prospect area is suitable for long-term CO₂ storage, supported by the lack of major faults in situ and in the vicinity of buried valleys.

Kefalonia island, in front of the Greek west coast, is located in a peculiar tectonic setting characterized by a transition from an oceanic subduction contact to a continental collision. This tectonic setting makes the island a test bed for geological, geophysical, and archaeological studies. To improve the subsurface knowledge and shed light in the top hundreds of metres, we acquired three seismic profiles in the isthmus connecting the main part of the island to the Paliki peninsula, in the Thinia valley, where the presence of a possible channel has been disputed. A total of approximately 3.5 km of seismic data were acquired using a 5 m receiver and shot spacing and a 25 kg accelerated weight drop as the main source. The sharp topographic changes and morphological features of the valley made the survey challenging, limiting the spread, precluding uniform shot points, and resulting in crooked profiles. The acquired data, however, show visible reflections with variable quality down to 0.5 s and occasionally to 1 s. First-break travel time tomography and 3D reflection travel time modelling were performed to complement the seismic reflection processing work together with lithological columns from three boreholes located along the profiles. Results show a low-velocity zone with no reflectivity from the surface to approximately 100 m depth probably related to the presence of loose material, under which two main east-dipping reflections are imaged. Considering the surface geology and tectonic history of the valley, we interpret these reflections as the same lithological boundary displaced by three highly east-dipping thrust/reverse faults probably part of the Hellenide thrusts. These findings further constrain the contentious presence of a historic water channel in the valley.

Understanding the structural intricacies of subsurface halokinetic formations is crucial for various geological applications, including geological capture and storage (geological carbon storage (GCS)). This study focuses on the seismic imaging of the Gassum structure in eastern Jutland, Denmark, employing high-resolution, dual-element acquisition, and processing techniques. The investigation aims to unravel details in the evolution of the salt dome and its implications for GCS potential. High-resolution seismic data processing and interpretation reveals a skewed dome structure with steeper flanks on the western and northern sides, characterized by faults and stratigraphic thinning. The asymmetric growth of the dome suggests uneven salt loading during its genesis, influencing local stress fields and structural development, with evidence of syn-tectonic subsidence that produced salt welds. This is supported by the presence of stratigraphic wedges and an increased depth of imaged horizons within steeper flanks of the dome. A mild piercement of the salt into overlying sediments, onlapping features, and the presence of normal faults that originate from the dome apex and extend radially, all indicate a reactive piercement process in the salt pillow's development stage. This produced an extensional regime in overlying strata, inducing sequence thinning and graben structures. Analysis of reservoir and seal properties unveils adequate conditions for GCS, with a continuous reservoir and thick primary and secondary seals. However, the presence of faults intersecting these formations raises concerns regarding long-term storage stability. Further investigations into reservoir porosity, migration paths, and volumetric analysis are warranted for conclusive GCS assessments.

In June 2021, a novel Danish national carbon capture and storage strategy was ratified by the Danish Parliament, and this was followed by the initiation of the project ‘CCS2022–2024’, led by the Geological Survey of Denmark and Greenland. In collaboration with other institutions, we acquired and interpreted new 2D seismic data between 2022 to 2024 to investigate and mature eight sites for potential subsurface storage of CO₂ in Danish onshore and offshore areas. This Bulletin contains a series of papers that present important results of the work. In this introduction paper, we provide an overview of seismic acquisitions and the interpretation of seismic data together with existing deep wells. The study sites selected are large subsurface structures located in onshore Jylland, Sjælland and Lolland and offshore Denmark in the eastern North Sea. The onshore targets are the Gassum, Havnsø, Rødby, Stenlille and Thorning structures, while the offshore sites comprise the Inez, Jammerbugt and Lisa structures. The project work comprises a series of reports regarding extensive seismic acquisition, processing and interpretation of the new and pre-existing seismic data as well as other publications emanating from the project. This Bulletin and the technical reports present an improved understanding of the formation, composition and geometry of the investigated structures. The studies include the mapping of the reservoir and seal formations, identification of principal faults, interpretation of the stratigraphic and structural development, reservoir and seal characterisation and estimates of the static storage capacity. Hence, this research provides a significant step forward concerning characterisation of the geology and maturation of the potential storage sites. In addition, it has inspired new ideas, including an updated regional stratigraphic interpretation of the Triassic succession of the Danish Basin and correlation with adjacent basins.

Advancements in technologies and acquisition techniques in reflection seismology have increased the quantity of data that can be recorded in a single survey. The spreading of three-component (3C) recorders has resulted in multicomponent data acquisitions to be more cost-effective than single-component (1C) acquisitions, making 3C datasets more common. Therefore, the need for processing workflows aiming to enhance S-wave from P-wave-designed acquisitions is increasing. As part of a larger survey for geological carbon storage site investigations, a 1-km-long seismic profile was acquired near the city of Randers in Denmark and is considered in this study. The acquisition setup consisted of two vertical vibrating sources, operating every 10 m, recorded simultaneously from a 1C nodal array, 10 m spaced and a landstreamer system, equipped with 60 3C micro-electro-mechanical sensors in 2 m intervals. Analysis of the 3C data recorded from the landstreamer system shows PP-, PS- and SS-wavefield reflections from a shallow horizon nearly 50 m below the ground surface. Although the PP-wavefield data processing was straightforward, the SS-wavefield imaging was challenged by strong surface waves recorded in the data, thus requiring a tailored workflow for its processing. The workflow employs a reflection-picked moveout correction specifically designed on the observed reflections with respect to the data acquisition geometry. Results demonstrate that this tailored method is effective, yielding improved continuity of the SS-wavefield reflectivity with respect to the traditional normal moveout corrections. P- and S-wave interval velocity models are estimated in depth, and their ratio is computed to complement the geological interpretations of the shallow subsurface. Similar imaging scenarios can benefit from the proposed method to sharpen SS-wavefield reflections in stacked section.

In Denmark, Geological Carbon Storage (GCS) has been prioritized as an immediate solution for climate action. The Havns & Oslash; domal structure has been identified as one of the most promising locations for GCS because its size and properties are believed to be suitable for GCS. However, the preliminary assessments, based mainly on old, sparse, and low-quality seismic data, are uncertain regarding the prospective storage resource and the integrity of the structure. To enable informed decisions and planning of the storage operations and as part of a large-scale acquisition campaign targeting several similar onshore structures throughout Denmark, a seismic data acquisition work was conducted in 2022 in the area. The purpose of the survey was to delineate the structural closure and map possible geologic features, such as faults, that could jeopardize GCS operations. In total, 132 km of highfold and high-resolution 2D profiles were acquired using an innovative dual-element recording system for both deep and shallow subsurface imaging purposes. The recording comprises two vibrating sources and a combination of nodal recorders spaced at 10 m, and 2-m-spaced microelectromechanical systems (MEMS)-based recorders attached to a moving landstreamer. The seismic data contain information on all horizons of interest for GCS. The structure is estimated as a well-defined four-way closure, where the reservoir is continuous. A thick, mostly uniform sealing rock is interpreted and no large-scale faults are found in the near surface. The results, supported from existing background information, provide crucial information to assist further decisions and actions related to future storage operations in Havns & Oslash;.

Strong global actions for climate change include carbon capture and storage (CCS) as a feasible solution to reach carbon neutrality and raise opportunities for detailed subsurface investigations. An acquisition set-up designed for onshore-offshore zones was maximized for wide-scale high-resolution structural imaging and implemented to cover a domal structure of interest for CCS utilization close to the town of Havnsø (Denmark). The challenges of the combined acquisition and processing of land and marine multisensor data along a 42 km seismic profile are analyzed, the suggested solutions are applied, and the limitations are discussed. On the onshore side, a nodal array and a seismic landstreamer system were simultaneously used, whereas along the transition zone, a marine seismic streamer and ocean-bottom seismometers were added to record the seismic response generated by two seismic vibrator sources. The adopted sensing domains (velocity, acceleration, and pressure) were studied, and different processing steps were evaluated to enable their processing and subsequent data set merging. Results suggest, as the best approach, a separate prestack processing of the different data sets and the computation of new geometries and new surface-consistent residual static correction after their merging. The data acquired in the transition zone illuminate, for the first time, the subsurface geology of the region, delineating an expected domal closure. The final seismic section shows high continuity of the reflections with good resolution along the entire profile, identifying the main reservoir structure and the surrounding areas, which are important to ensure reservoir integrity and safe exploitation over longer time scales. Shallow and deep reflections are consistent with the stratigraphic column from a well log near the profile. The presented study shows a comprehensive workflow for processing such a multisensor data set in onshore and transition zone settings.

During 2022 and 2023 five different four-way-closure domal structures have been investigated with several 2D reflection seismic profiles in onshore Denmark as part of the CCS2022-2024 project. The main objective of the surveys was to evaluate the potential storage capacity of these structures and their integrity for CO2 sequestration purposes. The surveys were conducted with a dual element acquisition system that guarantee the simultaneous recording of high spatial resolution data for shallow subsurface, as well as large offset to image the deeper structures. Furthermore, the rapidly adaptive setup helped to address the different acquisition scenarios encountered during the surveys, as country sides and urban areas. Despite the numerous challenges the acquisitions were all successful, results allowed a characterization of the domal structures and the detection of eventual related fault systems. These results were fundamental for a first potential storage capacity evaluation and an integrity characterization of each site, that will lead the future steps in their exploitation. An overview of all sites is shown, integrating the most updated results and commenting their future implications.