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.
Highlights
We present the pre-stack depth imaging results for a case study of 3D reflection seismic exploration at the Blötberget iron15 oxide mining site belonging to the Bergslagen mineral district in central Sweden
We applied a tailored pre-processing workflow as well as two different seismic imaging approaches, Kirchhoff prestack 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 Kirchhoff pre-stack depth migration (KPSDM) image
Bräunig et al (2019) showed that the application of focusing pre-stack depth migration techniques, such as Fresnel Volume Migration (FVM) (Lüth et al, 2005; Buske et al, 2009), Coherency Migration (Hlousek et al, 2015a) or Coherency based Fresnel Volume Migration (Hlousek et al, 2015b) can improve the resulting image of the mineralization for the 2D data set and allows for a more detailed interpretation 75 compared to a simple KPSDM approach
Summary
In the last decades the need for raw materials has increased worldwide. This increasing demand accounts for the European Union. 2D reflection seismic profiles had been acquired already in 2015/2016 crossing the known mineralization perpendicular to its main strike direction This data set was successfully processed using a standard time-domain processing and post-stack imaging workflow (Markovic et al, 2020) and Kirchhoff 60 pre-stack depth migration (KPSDM) as well as focusing imaging approaches (Bräunig et al, 2019). Bräunig et al (2019) showed that the application of focusing pre-stack depth migration techniques, such as Fresnel Volume Migration (FVM) (Lüth et al, 2005; Buske et al, 2009), Coherency Migration (Hlousek et al, 2015a) or Coherency based Fresnel Volume Migration (Hlousek et al, 2015b) can improve the resulting image of the mineralization for the 2D data set and allows for a more detailed interpretation 75 compared to a simple KPSDM approach. Yes 3 sweeps per source location 3D, sparse Surface-consistent for shots and receivers Based on matching filter using theoretical sweep Apply static correction based on refraction traveltimes, shift to final datum of 250 m using 5000 m/s replacement velocity 200 ms window length 80 ms operator length, single trace 15-35-145-165 Hz Wavelet-transform based (v ≤ 2700 m/s) Yes Whole-trace RMS amplitude balancing 30 ms below the picked first-arrivals
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