The development of cost‐effective and environmentally acceptable geophysical methods for the exploration of mineral resources is a challenging task. Seismic methods have the potential to delineate the mineral deposits at greater depths with sufficiently high resolution. In hardrock environments, which typically host the majority of metallic mineral deposits, seismic depth‐imaging workflows are challenged by steeply dipping structures, strong heterogeneity and the related wavefield scattering in the overburden as well as the often limited signal‐to‐noise ratio of the acquired data. In this study, we have developed a workflow for imaging a major iron‐oxide deposit at its accurate position in depth domain while simultaneously characterizing the near‐surface glacial overburden including surrounding structures like crossing faults at high resolution. Our workflow has successfully been showcased on a 2D surface seismic legacy data set from the Ludvika mining area in central Sweden acquired in 2016. We applied focusing prestack depth‐imaging techniques to obtain a clear and well‐resolved image of the mineralization down to over 1000 m depth. In order to account for the shallow low‐velocity layer within the depth‐imaging algorithm, we carefully derived a migration velocity model through an integrative approach. This comprised the incorporation of the tomographic near‐surface model, the extension of the velocities down to the main reflectors based on borehole information and conventional semblance analysis. In the final step, the evaluation and update of the velocities by investigation of common image gathers for the main target reflectors were used. Although for our data set the reflections from the mineralization show a strong coherency and continuity in the seismic section, reflective structures in a hardrock environment are typically less continuous. In order to image the internal structure of the mineralization and decipher the surrounding structures, we applied the concept of reflection image spectroscopy to the data, which allows the imaging of wavelength‐specific characteristics within the reflective body. As a result, conjugate crossing faults around the mineralization can directly be imaged in a low‐frequency band while the internal structure was obtained within the high‐frequency bands.

Abstract. We present pre-stack depth-imaging results for a case study of 3D reflection seismic exploration at the Blötberget iron oxide mining site belonging to the Bergslagen mineral district in central Sweden. The goal of the study is to directly image the ore-bearing horizons and to delineate their possible depth extension below depths known from existing boreholes. For this purpose, we applied a tailored pre-processing workflow and two different seismic imaging approaches, Kirchhoff pre-stack depth migration (KPSDM) and Fresnel volume migration (FVM). Both imaging techniques deliver a well-resolved 3D image of the deposit and its host rock, where the FVM image yields a significantly better image quality compared to the KPSDM image. We were able to unravel distinct horizons, which are linked to known mineralization and provide insights on their possible lateral and depth extent. Comparison of the known mineralization with the final FVM reflection volume suggests a good agreement of the position and the shape of the imaged reflectors caused by the mineralization. Furthermore, the images show additional reflectors below the mineralization and reflectors with opposite dips. One of these reflectors is interpreted to be a fault intersecting the mineralization, which can be traced to the surface and linked to a fault trace in the geological map. The depth-imaging results can serve as the basis for further investigations, drilling, and follow-up mine planning at the Blötberget mining site..

Abstract. We present the pre-stack depth imaging results for a case study of 3D reflection seismic exploration at the Blötberget iron-oxide mining site belonging to the Bergslagen mineral district in central Sweden. The goal of this case study is to directly image the ore-bearing units and to map its possible extension down to greater depths than known from existing boreholes. Therefore, we applied a tailored pre-processing workflow as well as two different seismic imaging approaches, Kirchhoff pre-stack depth migration and Fresnel Volume Migration (FVM). Both imaging techniques deliver a well resolved 3D image of the deposit and its host rock, where the FVM image yields a significantly better image quality compared to the KPSDM image. We were able to unravel distinct reflection horizons, which are linked to known mineralisation and provide insights on lateral and depth extent of the deposits beyond their known extension from borehole data. A comparison of the known mineralization and the image show a good agreement of the position and the shape of the imaged reflectors caused by the mineralization. Furthermore, the images show a reflector, which is interpreted to be a fault intersecting the mineralisation and which can be linked to the surface geology. The depth imaging results can serve as the basis for further investigations, drillings and follow-up mine planning at the Blötberget mining site.

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<p>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.</p> <p>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.</p> <p>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.</p>