We show that by using an advanced pre-stack depth imaging algorithm it is possible to retrieve meaningful and robust seismic images with sparse shot points, using only 3–4 source points per kilometer along a seismic profile. Our results encourage the use of 2D seismic reflection profiling as a reconnaissance tool for mineral exploration in areas with limited access for active seismic surveys. We used the seismic data acquired within the COGITO-MIN project comprising two approximately 6 km long seismic reflection profiles at the polymetallic Kylylahti massive sulfide mine site in eastern Finland. The 2D seismic data acquisition utilized both Vibroseis and dynamite sources with 20 m spacing and wireless receivers spaced every 10 m. For both source types, the recorded data show clear first breaks over all offsets and reflectors in the raw shot gathers. The Kylylahti area is characterized by folded and faulted, steeply dipping geological contacts and structures. We discuss post-stack and pre-stack data processing and compare time and depth imaging techniques in this geologically complex Precambrian hardrock area. The seismic reflection profiles show prominent reflectors at 4.5–8 km depth utilizing different migration routines. In the shallow subsurface, steep reflectors are imaged, and within and underneath the known Kylylahti ultramafic body reflectivity is prominent but discontinuous.

A 10.5 km² 3D seismic survey was acquired over the Kylylahti mine area (Outokumpu mineral district, eastern Finland) as a part of the COGITO-MIN (COst-effective Geophysical Imaging Techniques for supporting Ongoing MINeral exploration in Europe) project, which aimed at the development of cost-effective geophysical imaging methods for mineral exploration. The cost-effectiveness in our case was related to the fact that an active-source 3D seismic survey was accomplished by using the receiver spread originally designed for a 3D passive survey. The 3D array recorded Vibroseis and dynamite shots from an active-source 2D seismic survey, from a vertical seismic profiling experiment survey, as well as some additional “random” Vibroseis and dynamite shots made to complement the 3D source distribution. The resulting 3D survey was characterized by irregular shooting geometry and relatively large receiver intervals (50 m). Using this dataset, we evaluate the effectiveness of the standard time-imaging approach (post-stack and pre-stack time migration) compared to depth imaging (standard and specialized Kirchhoff pre-stack depth migration, KPreSDM). Standard time-domain processing and imaging failed to convincingly portray the first ~1500 m of the subsurface, which was the primary interest of the survey. With a standard KPreSDM, we managed to obtain a good image of the base of the Kylylahti formation bordering the extent of the mineralization-hosting Outokumpu assemblage rocks, but otherwise the image was very noisy in the shallower section. The specialized KPreSDM approach (i.e., coherency-based Fresnel volume migration) resulted in a much cleaner image of the shallow, steeply dipping events, as well as some additional deeper reflectors, possibly representing repetition of the contact between the Outokumpu assemblage and the surrounding Kalevian metasediments at depth.

This paper describes the principles of three novel seismic imaging techniques and their application to two deep seismic reflection data sets from the vicinity of the German Continental Deep Drilling Site (KTB). These imaging techniques are based on Kirchhoff prestack depth migration and use an inherent restriction of the migration operator to focus the wavefield to its actual reflection point. For Fresnel volume migration, the emergent angle at the receivers is estimated and then used to propagate the wavefield back into the subsurface along which the Fresnel volume is determined. The migration operator is restricted to this volume, thereby focusing the image to the part of the isochrone which physically contributes to the reflection. For coherency migration, the coherency of the wavefield at neighboring traces is calculated and used as a weighting factor within the migration integral, leading to a comparable focusing to the reflection point. For coherency‐based Fresnel volume migration, both approaches are combined, resulting in an even more focused seismic image with significantly increased image quality. We applied these methods to two seismic data sets from the area around the KTB: a survey with standard split‐spread geometry (KTB8502) and a sparse data set with a small number of source points in combination with short receiver lines (INSTRUCT93). The focusing approaches yield major improvements in the final images for both data sets. Incoherent noise and migration artifacts are reduced and the visibility of crustal structures is strongly enhanced, allowing for an improved geologic and tectonic characterization.

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..

This paper demonstrates the capabilities of Fresnel Volume Migration (FVM) for 3-D single-component seismic data in a crystalline environment. We show its application to the ISO89-3D data set, which was acquired in 1989 at the German continental deep drilling site (KTB) near Windischeschenbach (Southeast Germany). A key point in FVM is the derivation of the emergent angle for the recorded wavefield. This angle is used as the initial condition of the ray-tracing-algorithm within FVM. In order to limit the migration operator to the physically relevant part of a reflector, it is restricted to the Fresnel-volume around the backpropagated ray. We discuss different possibilities for an adequate choice of the used aperture for a local slant-stack algorithm using the semblance as a measure of the coherency for different emergent angles. Furthermore, we reduce the number of used receivers for this procedure using the Voronoi diagram, thereby leading to a more equal distribution of the receivers within the selected aperture. We demonstrate the performance of these methods for a simple 3-D synthetic example and show the results for the ISO89-3D data set. For the latter, our approach yields images of significantly better quality compared to previous investigations and allows for a detailed characterization of the subsurface. Even in migrated single shot gathers, structures are clearly visible due to the focusing achieved by FVM.