3D high-resolution seismic imaging of the iron oxide deposits in Ludvika (Sweden) using full-waveform inversion and reverse time migration

Abstract

Abstract. A sparse 3D seismic survey was acquired over the Blötberget iron oxide deposits of the Ludvika Mines in south-central Sweden. The main aim of the survey was to delineate the deeper extension of the mineralisation and to better understand its 3D nature and associated fault systems for mine planning purposes. To obtain a high-quality seismic image in depth, we applied time-domain 3D acoustic full-waveform inversion (FWI) to build a high-resolution P-wave velocity model. This model was subsequently used for pre-stack depth imaging with reverse time migration (RTM) to produce the complementary reflectivity section. We developed a data preprocessing workflow and inversion strategy for the successful implementation of FWI in the hardrock environment. We obtained a high-fidelity velocity model using FWI and assessed its robustness. We extensively tested and optimised the parameters associated with the RTM method for subsequent depth imaging using different velocity models: a constant velocity model, a model built using first-arrival travel-time tomography and a velocity model derived by FWI. We compare our RTM results with a priori data available in the area. We conclude that, from all tested velocity models, the FWI velocity model in combination with the subsequent RTM step provided the most focussed image of the mineralisation and we successfully mapped its 3D geometrical nature. In particular, a major reflector interpreted as a cross-cutting fault, which is restricting the deeper extension of the mineralisation with depth, and several other fault structures which were earlier not imaged were also delineated. We believe that a thorough analysis of the depth images derived with the combined FWI–RTM approach that we present here can provide more details which will help with better estimation of areas with high mineralisation, better mine planning and safety measures.

Three-dimensional reflection seismic imaging of the iron oxide deposits in the Ludvika mining area, Sweden, using Fresnel volume migration

“…In that sense, such imaging approaches are also interesting for the exploration of less well-known or explored potential mineral deposits. Potential avenues for future work include the incorporation of a more detailed P -wave velocity model derived from, e.g., full waveform inversion (Singh et al, 2021) for static corrections or even directly as part of a 3D migration velocity model. Furthermore, the acquired 3D dataset could be used for a 3D MVA using focusing pre-stack depth migration techniques to generate common offset images, which then can be sorted into common image gathers depending on not only the offset but also the angles of illumination.…”

Preface: State of the art in mineral exploration

“…The imaging results, verified by comparison to mapped mineralisation zones, will serve as the basis for further investigations, drillings and follow-up mine planning at the Blötberget mining site, and similar sites worldwide. The third contribution from the Ludvika mining area, by Singh et al (2022), presents the implementation of a timedomain 3D acoustic full-waveform inversion scheme to the same sparse 3D dataset. Within this approach, the authors used 216 shots to produce an accurate velocity model of the subsurface of the target area.…”

“…One of the most significant contributions of Earth scientists to the major climate challenges of the 21st century is to facilitate responsible access to minerals. Sustainable exploitation of minerals is needed to support the urgent transition to a low carbon economy (Valero et al, 2018) and meet the increasing need of high-tech metals for advanced computing and engineering (Wellmer et al, 2019;Cheng et al, 2021). The economics of mineral resources have some special features, such as that the resources are where they are -and thus cannot be delocalised -and that the conversion of an exploration target into a fully operating mine can take decades.…”

Pre‐stack depth imaging techniques for the delineation of the Carosue Dam gold deposit, Western Australia

“…The benefit of a high receiver count for imaging complex structures in a hard rock environment at all depths was quickly realized in the minerals industry and deployed even for 2D surveys (Naghizadeh et al., 2019). Similarly, modern vibroseis and electro‐kinetic vibrating sources provide a broad‐band seismic signal that is required for the utilization of novel inversion and imaging techniques (Naghizadeh et al., 2019; Pertuz et al., 2022; Singh et al., 2022). A combination of a large number of sensors (high data density) and broad‐band seismic sources enables the successful implementation of novel processing (signal enhancements, interpolations, etc.…”

“…), imaging techniques such as reverse time migration (RTM) and inversions such as full-waveform inversion (FWI). This led to early investigations into the potential of FWI in hard rock environments (Adamczyk et al, 2014(Adamczyk et al, , 2015, while investigations into imaging techniques were documented by several authors (Brodic et al, 2021;Broüning et al, 2020;Ding & Malehmir, 2021;Hloušek et al, 2015;Singh et al, 2022).…”