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.

The Kalahari Manganese Field (KMF) in the Northern Cape Province of South Africa hosts some of the world's richest manganese deposits, largely concealed beneath thick Cretaceous to Cenozoic Kalahari Group and Karoo Supergroup sediments. To improve imaging of the concealed Transvaal Supergroup strata, a high-resolution 2D reflection seismic survey was conducted in November 2023 across the Severn farm area. The survey comprised five profiles totalling 18.9 km, acquired using 5 Hz 1C geophones connected to wireless nodes, enabling effective burial beneath loose aeolian sand for improved coupling. A compact 500 kg drop hammer, mounted on a Bobcat, served as the seismic source, offering excellent manoeuvrability across challenging sandy terrain. Shot spacing was 10 m, with four vertical stacks per shot to enhance signal-to-noise ratio. Refraction tomography using first-break travel times provided near-surface P-wave velocity models, revealing variable Kalahari sediment thicknesses ranging from 20 to 70 m and bedrock velocities of similar to 5500 ms-1 associated with Karoo Supergroup strata. Despite the challenges posed by the thick sand cover, lithified calcrete horizons within the Kalahari sediments significantly aided seismic energy propagation. The data were processed using a conventional pre-stack imaging workflow. We tested both Kirchhoff pre-stack time migration (KPreSTM) and Kirchhoff pre-stack depth migration (KPreSDM) and compared the results. Both migration approaches revealed a high degree of similarity in reflector geometries and structural patterns, suggesting minimal lateral velocity variation across the study area. KPreSTM results were then used in the final seismic interpretation. Pre-stack time migrated sections exhibit nine laterally continuous high-amplitude reflectors between 0.05 and 3.42 km depth, corresponding to major stratigraphic boundaries from the Kalahari Group down to the Ghaap Group. Of particular interest is the moderate-amplitude reflection pair at 1.05-1.35 km depth, interpreted as the Hotazel Formation, the primary host for manganese mineralization. This study demonstrates that, when appropriately designed, reflection seismic imaging can be a powerful tool for delineating deep mineralized strata beneath thick sedimentary cover in arid environments.

Characterizing near-surface fault architecture and structural framework is critical for geological carbon storage (GCS) sites, as it directly influences storage maturation, integrity, and associated risks. Building on earlier interpretations, this study revisits the Gassum domal structure, a potential GCS site in eastern Jutland, Denmark, using seismic data acquired in 2023. The main aim is to address primary risk zones and delineate structural features that were previously not delineated. We applied spectral decomposition and red-green-blue (RGB) co-rendering to migrated reflection seismic sections to improve the imaging of faults and weakness zones. Spectrally decomposed attributes were further combined with first-break traveltime tomography to derive tuning thickness, providing a first-order approximation of layer thicknesses. This approach revealed detailed and complex fault architecture, including internal faulting within the near-surface secondary seal, the Chalk Group. The highest fault densities occur at the dome crest and within the spill-point zone; two areas critical for CO2 leakage. Our results indicate that the Gassum structure poses structural risk and must be carefully studied for its fault network integrity to ensure long-term CO2 storage without significant de-risking.

In carbon capture and storage (CCS) projects, it is important to investigate the seal properties, which often include the shallower part of the investigated area. Reflection seismic data in most of the Danish Basin appear less strong and coherent in the top 500 m, and hence reflectivity alone cannot be used to study the integrity of these potential CCS reservoirs. This study seeks to bolster and connect near-surface with deeper reflection seismic data by leveraging the capabilities of seismic traveltime tomography and other elements of seismic data. By exploiting the advantages of high-resolution, large offset data acquisition, the traveltime tomography method was adopted for detailed structural characterization of the Quaternary sediments and the Chalk Group in the Central Jutland region, Denmark. Specifically, a focus on the potential for CO2 capture and storage in this area is pursued by constructing accurate velocity models and integrating them into reflection data processing. Our study aims to enhance reflection images and correlate them with the traveltime tomographic results for an updated geological interpretation of the area. The subsequent findings demonstrate the effectiveness of the tomography method with major improvements in the reflection seismic sections that provide additional features for improved large-scale geological mapping. The combined integration of the tomography and reflection seismic sections reveals that salt tectonics influence the region's topography. Adaptation of different velocity models shows significant alterations in the seismic imaging of the area that should be taken into consideration regarding the site's capture and storage potential.

The Kheis Tectonic Province of southern Africa represents a key, yet under-constrained, component in the tectonic history of the Kaapvaal Craton and its surrounding terranes. The complex geological framework is masked by extensive sedimentary cover and limited outcrop, making geophysical investigations essential. In this study, we present an integrated seismic analysis using the combination of a legacy deep reflection data (GS-02), a shallower reflection profile (KBF-01), teleseismic receiver functions, and refraction tomography to refine the crustal architecture and major tectonic boundaries across the region. Prestack time migration of the GS-02 profile reveals significant improvement in reflector clarity, enabling the identification of thrust faults, fold structures, and previously unresolved reflective packages. Refraction tomography constrains the thickness of the Kalahari Group cover, averaging similar to 250 m, while receiver function analysis at three broadband seismic stations yields new Moho depth estimates ranging from 32 to 46 km and delineates crustal stratification. Our interpretation supports a model in which the Kaapvaal Craton is underthrusting westward beneath the Kheis Province, with partial crustal imbrication. We find no strong seismic evidence for the Dabep Thrust as a major tectonic boundary, aligning with recent challenges to its significance. In contrast, the Blackridge Thrust and the Kalahari Line show coherent seismic and geophysical expression, supporting their role as first-order structures. Additionally, we image a deeply buried, high-reflectivity zone in the west, suggestive of a possible plutonic body or relict basin structure. This multi-method seismic investigation advances the understanding of the crustal-scale structure and tectonic evolution of the Kheis Province, providing new constraints for regional tectonic models and highlighting the value of reinterpreting legacy seismic data with modern techniques.

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 CO 2 storage, supported by the lack of major faults in situ and in the vicinity of buried valleys.

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 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;.

Improving the exploration of deep-seated mineral deposits and assessing the stability of the mine pillars require that geophysical techniques are deployed in a fast and cost-effective manner with minimal environmental impact. This research presents results from in-mine reflection seismic experiments and a ground penetrating radar (GPR) survey conducted at the Maseve platinum mine, South Africa. The research aims to develop and implement methods to image platinum group metal (PGM) deposits and geological structures near mine tunnels and assess the stability of pillars. The seismic experiments were conducted using a sledgehammer source (10 lb), conventional cabled geophones (14 Hz), and a landstreamer with 4.5 Hz vertical component geophones. The GPR survey was conducted using a Noggin 500 GPR system with 500 MHz centre frequency. An image of the underlying orebody and geological structures down to 100 m from the mine tunnel floor (& SIM;500 m below ground surface) was produced. We correlated the coherent reflections beneath the tunnel floor with a known Upper Group (UG2) PGM orebody. The final seismic section shows that the UG2 mineralisation is dissected by near-vertical dykes, faults and fractures. These structures, faults in particular, are interpreted to have been active post-mineralisation, implying that they may have contributed to the current complex geometry of the deposit. Four GPR profiles were collected around a stability pillar adjacent to the seismic lines. The radargram sections were processed to improve the signal-to-noise ratio (S/N). The results show different patterns of fracturing and stress-induced structures. These fractures were shown to be sub-vertical and, possibly, constitute complex micro-structures within the pillar, which could compromise the pillar stability and integrity. The study demonstrates that in-mine seismic and GPR surveys can be cost-effective and valuable for mineral exploration.

Summary This study focuses on the characterization and effect of near-surface materials at a potential geological carbon storage (GCS) site in Denmark. The seismic data, acquired using a dual seismic acquisition setup comprising 10-m-spaced wireless units and 2-m-spaced landstreamer microelectromechanical units (MEMs), provided high-quality reflections that can be associated with lithological units that are important for GCS such as the Gassum Formation that is the prime target reservoir. However, near-surface materials and their heterogeneity pose challenges for obtaining an accurate near-surface velocity model and thereby affect the quality of the seismic image. Given the high-resolution aspects of the data and dense receiver spacing, it was possible to overcome near-surface challenges and correct for their effects to obtain a high-resolution image of the deeper reflections of interest, which is a key aspect for GCS applications. The estimated velocity model agrees with expected near-surface sediment velocities, but details in the layers are not fully resolved. Further research will include seismic refraction tomography and the integration of the landstreamer data to obtain an accurate velocity model that will improve the GCS site characterization.

Summary The method of carbon capture and storage is among the top attempts to reduce carbon emissions into the atmosphere. This has necessitated more accurate ways of investigating and characterising potential geological storage sites, such as high-resolution reflection-seismic surveys. In this study, we present a multi-profile 2D seismic survey that was conducted in the Central Jutland region of Denmark. We incorporated the use of two seismic vibrator trucks in combination with a parallel dual-recording system comprising both a landstreamer and wireless nodal units. The imaging target of the survey was a ∼233 km2 dome structure within the series of sandstones that form the Gassum Formation. Dense spacing (2 m) of the streamer units in combination with the large spread (up to 10 km) of the nodal units yielded seismic sections with high resolution up to depths of 4 km, well below the target.

Summary CCS is an efficient and an inevitable way to tackle climate crisis. Denmark aims at carbon neutrality by exploring and utilizing potential storage sites for CO² sequestration. A series of upscaled high-resolution reflection seismic surveys have been planned and are currently being carried out across the country to investigate potential reservoir sites; the largest one being around the town of Gassum and targeting the reservoir sandstones of the Gassum Formation. A dual element recording system has been adapted and spaced MEMs-based recorder on a moving landstreamer system. Here, we present the longest profile of the Gassum survey, P9, from the nodal array. Given the long offset range of the nodal arrays and high-quality data acquired, we aim at employing various velocity model building approaches to characterize the Gassum structure and its four-way closure or domed-shaped structure. Preliminary processing work reveals the domed Gassum structure with quality. Future works in the velocity model employing traveltime tomography and full-waveform inversion would help to further improve the quality of the sections and better characterize the target reservoir.

Summary Due to the increasing interest towards geological CO2 storage as an important contribution to the reduction of atmospheric CO2 levels, large-scale land seismic acquisitions have been prioritized in Denmark, to characterize subsurface conditions suitable for long-term CO2 storage. We present an up-scaled seismic survey performed in Havnsø in the Zealand of Denmark in order to image a major anticline structure and interpret the entire stratigraphy relevant for the storage. The survey covered different profiles having a total length of approximately 130 km and, following previous, smaller-scale successful examples, it benefited from wireless recorders spaced at 10 m and a moving landstreamer system consisting of 2 m spaced MEMs units for the data recording. As confirmed by the data quality, this fast and efficient acquisition work can achieve detailed imaging of the deeper sedimentary structures, while maintaining high resolution at shallower depths, making it valuable for large-scale CO2 storage projects onshore.

Two legacy reflection seismic profiles were acquired in 1988, north of the Kloof–Driefontein Complex East Mine in the West Rand goldfield (South Africa), for the purpose of gold exploration and mine planning. These legacy 2D seismic data have been reprocessed using the latest processing tools to improve imaging. Special interest is given to the Black Reef Formation, which hosts a known gold orebody. The original legacy data are of poor quality, especially in areas that are dominated by dolomitic outcrops. To improve the quality of the data, special attention was given to the refraction static correction to enhance the continuity of the reflections below dolomitic rocks. Refraction seismic tomograms from both profiles exhibit three‐layer P‐wave velocity models: (1) topsoil (1000–2000 m/s), (2) a weathered layer ranging from ca. 100 to 300 m in thickness (2000–5000 m/s) and (3) bedrock (> 5000 m/s). Seismic profile OK‐212 shows poor imaging of the Black Reef Formation because of the scattering of seismic energy in the near‐surface due to dolomites from the Transvaal Supergroup, while seismic profile OK‐213 exhibits south‐dipping reflections that are associated with the Black Reef Formation. To improve the structural imaging resolution, we tried pre‐stack time migration, pre‐stack depth migration and post‐stack time migration using the Kirchhoff algorithm. PreSDM most improved the imaging of deeper reflections due to its ability to honour complex lateral variations in the velocity field. Both pre‐stack time migration and post‐stack time migration enhanced the continuity of the near‐surface reflections below the dolomitic rocks.

Summary Two reflection seismic profiles are reprocessed to investigate the structural and lateral constraints of the 1.9 Ga Trompsburg Igneous Complex in South Africa. Incorporation of the seismic data with magnetic and gravimetric data, as well as surface and borehole geological data, allow for an integrated interpretation with a fair degree of confidence. The reprocessed seismic data yield images of substantially improved quality compared to the original data. Results reveal that the Trompsburg Complex intruded into Paleoproterozoic supracrustals after significant metamorphism took place. The complex is buried beneath 1.5 km thick sediments and has a lateral expanse of ∼60 km.

Petrophysical properties of cylindrical core specimens from three boreholes from the International Continental Scientific Drilling Program, the DSeis project, measured at ambient pressure and room temperature conditions in various laboratories are presented and compared with downhole petrophysical data (sonic and density). The measured properties are from sixty-six rock specimens constituting metasediments, metabasalts and intrusives. Seismic velocities were measured using 0.5 MHz P- and S-wave transducers. To investigate the source of seismic reflectivity observed on the 2D legacy seismic data, we computed synthetic seismograms for adjacent rock units using downhole petrophysical data and compared them with seismic reflections from the reflection seismic profile. The experimental measurements show that the metasediments exhibit lower bulk densities and seismic velocities than the metabasalts and intrusive specimens. The porosity was found to be less than 2% for all the samples. No clear trends emerge when the Poisson's ratio is plotted against the P-wave velocities and porosities of the samples. A positive relationship is observed between the bulk modulus and P-wave velocities of the rock samples. The highest calculated reflection coefficients (RC) are associated with the metasediment-intrusive interfaces in all three boreholes. The intrusive-metabasalt and the metasediment-metabasalt interfaces exhibit low RC. Synthetic seismograms reveal strong reflections that coincide with high RC calculated using the bulk density and velocity data. The synthetic seismograms also revealed additional strong reflections that were not identified using the reflection coefficients calculated from the rock specimens, due to core loss in some lithological units. Successful correlations are carried out between the synthetic seismic data and the real seismic data, enabling us to correlate the stratigraphic sequence drilled in the boreholes to the seismic reflections observed on the legacy 2D reflection seismic data.

We demonstrate the application of seismic methods using in-mine infrastructure such as exploration tunnels to image platinum deposits and geologic structures using different acquisition configurations. In 2020, seismic experiments were conducted underground at the Maseve platinum mine in the Bushveld Complex of South Africa. These seismic experiments were part of the Advanced Orebody Knowledge project titled “Developing technologies that will be used to obtain information ahead of the mine face.” In these experiments, we recorded active and passive seismic data using surface nodal arrays and an in-mine seismic land streamer. We focus on analyzing only the in-mine active seismic portion of the survey. The tunnel seismic survey consisted of seven 2D profiles in exploration tunnels, located approximately 550 m below ground surface and a few meters above known platinum deposits. A careful data-processing approach was adopted to enhance high-quality reflections and suppress infrastructure-generated noise. Despite challenges presented by the in-mine noisy environment, we successfully imaged the platinum deposits with the aid of borehole data and geologic models. The results open opportunities to adapt surface-based geophysical instruments to address challenging in-mine environments for mineral exploration.

The discovery and characterization of layered intrusions around the globe have been predicated to a large degree on the imaging capabilities of the reflection seismic method. The ability of this tool to detect mineralization zones and structural controls such as faults and folds has been critical in unlocking the economic potential of igneous complexes, most notably the Bushveld Complex in South Africa. In this study, we present novel seismic constraints on the lesser-known Trompsburg Complex in South Africa. Two yet-unpublished seismic profiles were conducted end-to-end in the early 1990s, with a southwest-to-northeast trend through the centre of the ∼2,400 km2 Trompsberg potential field anomaly in South Africa, attributed to a 1915 ± 6 Ma buried layered intrusion complex. The complex was first detected by magnetic and gravity measurements near the town of Trompsburg in 1939 and was subsequently confirmed as a layered intrusion by borehole cores drilled thereafter. The combined length of the two profiles is 108 km. Both profiles have been reprocessed and interpreted to further constrain the subsurface expanse of the Trompsberg Complex along the seismic traverse. Processing and interpretation of the seismic profiles were aided by a handful of studies found in the literature: stratigraphy and physical property measurements of borehole cores that were drilled into the complex in the 1940s; pre-Karoo (∼317 Ma) lithological maps that were constructed based on boreholes in and around the investigation area; and potential field maps of the intrusion area near the town of Trompsburg. Most of the seismic reflection energy is concentrated within the top 1 km in both profiles, where localized reflectors with strong amplitudes are observed, due likely to the dolerite sills that permeate the Karoo cover. These sills obstruct seismic illumination of underlying structures due to their high acoustic impedance contrast with the surrounding soft rock sediments, rendering underlying reflections challenging to identify and enhance. The base of the Karoo is confidently identified to be at an average depth of 1.5 km and several reflection packages have been identified thereunder. These are linked to Proterozoic supracrustals associated with the Witwatersrand, Ventersdorp, Transvaal/Griqualand West, and Kheis Supergroups, as well as the Trompsburg Complex that intruded into them. The geometry of the Trompsburg Complex along the seismic traverse has been constrained with a moderate degree of confidence. It comprises a series of 30° northeasterly dipping reflectors near its southwestern boundary, flat reflectors near its centre at the town of Trompsburg, and 45° southwesterly dipping reflectors near its northeastern boundary. The lateral sub-Karoo extent of the complex is 60 km and its total thickness is difficult to constrain due to lack of deep reflections, but is likely between 6.6 and 7.5 km. The complex subcrops against the Karoo cover except near the southwestern region, where it is overlaid by Waterberg Group sediments.