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Steep geological structures are critical for improved understanding of tectonic processes and fluid circulation, particularly in crystalline settings. However, accurately determining their geometry at depth remains a challenge for conventional 2D surveys. In this study, we present an automated three-dimensional (3D) reflection traveltime modelling approach to estimate 3D subsurface geometries of dipping reflectors. The method utilises the traveltime hyperboloid equation adapted for dipping reflected events. The required inputs include the acquisition geometry, the known location where the dipping layer intersects or projects to the surface, pre-stack gathers, first-break picks and reflection hyperbola picks. The traveltime equation is modified and adapted to incorporate common available information from seismic acquisitions, and the resulting function is analysed to optimise the input parameters. The study also highlights the sinusoidal behaviour of the objective function and demonstrates the importance of multi-azimuth acquisitions in constraining the possible reflector geometries. The output consists of a root mean square (RMS) error map for all modelled dip-strike pairs relative to the picked reflection traveltime, the best-fitting reflector geometry and its corresponding modelled reflection traveltime. If desired, the best-fitting reflector can also be modelled in the migrated stacked section for comparison purposes. Compared to traditional manual techniques, the proposed method improves accuracy, reduces the operator dependency and provides quantitative reliability metrics. Owing to its simplicity and application efficiency, this method can be largely applied in onshore reflection seismic data interpretations, where 2D crooked profiles commonly provide multi-azimuth coverage, enabling improved delineation of steep structures such as faults and dykes.

This thesis presents the development of an innovative 2D reflection seismic acquisition system and its processing. The dual-element system integrates a nodal geophone array employed for deep imaging, and a MEMS-based landstreamer system employed for near-surface imaging, enabling high-resolution seismic data acquisition across multiple depth ranges. In the Korean Peninsula, where seismic activity has increased following the 2011 Tohoku-Oki earthquake, this system was applied to image crustal-scale fault systems. Three major systems were partially imaged, and the integration of the two datasets helped constrain fault locations in the densely populated, hard-rock environment of metropolitan Seoul, improving the understanding of seismic hazards and earthquake preparedness in the region. In Denmark, the system was employed for large-scale geological surveys to assess potential CO₂ storage structures, contributing to climate change mitigation efforts. A novel data merging technique was developed to integrate the two datasets, enhancing the imaging of reservoirs, seals, and fault structures. In addition, offshore sensors were considered and analysed to cover an onshore transition to offshore zone. The applied acquisition setup and developed merging technique were crucial to reach the desired resolution at all pertinent depths. A reflection-picked moveout correction processing step was developed for implementing high-resolution near-surface imaging through S-wave reflections as a by-product of large-scale acquisitions. The application of this method increased the reflection continuity in the stacked section that, complemented with velocity analyses, permitted the identification of key geological markers such as the water table depth and the top of the pre-Quaternary layers. Throughout the thesis, application of complementary analyses highlights the importance of leveraging different seismic data characteristics to improve subsurface imaging and geological reconstruction. The adaptability of this system demonstrates its effectiveness in complex environments, supporting both urban seismic risk mitigation and carbon capture and storage (CCS) applications. By addressing seismic hazards and climate challenges, this research underscores the crucial role of reflection seismology in tackling global environmental and societal issues.

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.

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.

In arid regions like the Eastern Bahira Basin, communities mainly rely on groundwater for drinking and irrigation. However, efficiently managing these vital resources requires a deep understanding of the underlying aquifers' structure and identifying the most suitable areas for exploitation. This presents a significant challenge for the success of water supply and irrigation programs in the Eastern Bahira Basin. This study is based on an integrative approach, combining Electrical Resistivity Tomography data, with the compilation and reinterpretation of pre-existing seismic, gravimetric and vertical electrical sounding data. This approach is based on compiling old gravimetric data and applying advanced processing techniques to determine the horizontal gradient maxima, which helps highlight the major structural alignments in the basin. Furthermore, the approach utilizes seismic data in order to enhance understanding of the deep structure of the basin, reinterpreting it in light of recent drilling data. The interpretation of the gravimetric and seismic data has also been validated by the results of vertical electrical soundings and electrical tomography that we recently acquired in the Eastern Bahira basin. The outcomes of this research provide new insights into the deep structure of the Eastern Bahira Basin and suggest the most promising hydrogeological prospects, thereby contributing to the success of the ongoing drinking water supply and irrigation program in the Eastern Bahira Basin.

The increasing global interest in geologic carbon storage as a feasible way of reducing CO2 atmospheric levels requires extensive onshore high-resolution seismic investigations to characterize suitable storage sites, for example, close to major CO2 emitters. To partly address this challenge and to acquire quality data at shallower and greater depths in a cost- and time-effective approach, a tailored acquisition scheme is adopted and tested at a candidate site in Stenlille, Denmark. The survey aims to understand whether an anticline reservoir structure known to exist at a 1000 m depth can serve for long-term CO2 storage and presents the structural integrity for this purpose. The data are recorded using a combination of nodal arrays, spaced at 10 m in a fixed geometry, and a set of more closely (2 m) spaced digital recorders, mounted on a landstreamer, which was moved at each shot location. Two 12 t mini vibrators are used as seismic sources. The nodal and landstreamer data sets are compared and combined into a unique data set for reflection imaging purposes along five profiles with a total length of approximately 12 km. The seismic sections obtained using this tailored combination of different recorders provide images of the entire shallow and deeper structures with an unprecedented resolution at the different depth levels necessary to assess the full potential of the suggested CO2 storage reservoir. The results significantly increase the existing knowledge of the extent and structural closure of the reservoir as well as a possible fault, all of which are critical for future risk analysis and planning of the storage.