In 2007, a 2D reflection seismic survey was conducted at the Kevitsa Ni-Cu-PGE (platinum group elements) deposit in northern Finland. The aims of the survey were to delineate the overall extent of the ore-bearing Kevitsa ultramafic intrusive complex, to study the seismic response of the disseminated ore deposit, to potentially find indications for new ore deposits, and to extract structural information at depth that may be associated with mineralization. In the processing sequence, specific focus was given to finding optimal CDP-line geometries for the crooked-line survey profiles and, due to highly variable bedrock velocities, to detailed velocity analysis. Our conventional processing sequence, involving prestack DMO corrections followed by poststack migration, resulted in high-quality images of the subsurface. First, the data were used to establish the shape and extent of the Kevitsa intrusion, thus providing an overall framework for future exploration in the area. In particular, the data suggest deeper, up to about 1.5 km depth, continuation of the intrusion than previously thought. Furthermore, the images reveal variable reflectivity characteristics within the intrusion from nonreflective to internally reflective. The Kevitsa deposit is located within a part of the intrusion which is associated with distinct, gently dipping reflectivity fabric down to a depth of about 1 km, spatially constrained within a restricted zone internal to the intrusion. This zone can be used as a guideline for the near-mine exploration efforts, and the reflectivity is dominantly associated with magmatic layering controlling the extent of the bulk of economic mineralization. The seismic data also reveal a complex pattern of faults, in particular a series of major fault and shear zones bracketing and crosscutting the Kevitsa intrusion as a whole. Additionally, our interpretation of the data indicates a possible shared origin of the Kevitsa intrusion and the nearby Satovaara intrusion.

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.

We tested the applicability of a newly developed broadband (0–800 Hz) digital-based seismic landstreamer for open-pit mine planning in the apatite-bearing Siilinjärvi mine in central Finland. Four seismic profiles, in total approximately 2.5 km long (2–4 m source and landstreamer receiver spacing), two inside the pit and two on its margins, were acquired in combination with wireless recorders connected to 10 Hz geophones and fixed at every 10 m spacing along the seismic profiles while the streamer data were being acquired. Downhole logging and laboratory physical property measurements on core and rock samples were carried out to not only support the seismic interpretations but also to provide information about the possible geophysical signature of these unique types of deposits. In spite of a highly noisy mining environment, seismic data of high quality were acquired; however, reflection processing and interpretations were challenged by the geologic complexities of several generations of basic and carbonatite dikes. To complement the reflection data imaging and to account for the steep elevation changes and crookedness of some of the seismic profiles, 3D first-arrival traveltime tomography and 3D swath reflection imaging were also carried out. Clear refracted arrivals from the open-pit profiles suggest the possibility of low-velocity zones associated with either blasting or several shear zones intersecting the seismic profiles. In terms of reflectivity, reflections have a different appearance from short and flat to longer and steep ones. The downhole- and borehole logging data suggest that some of these reflections are associated with diabase dikes and some are likely from zones of weaknesses in the alkaline-carbonatite complex. We determine the potential of using seismic streamers for cost- and time-effective open-pit mine planning and encourage further testing in simpler geologic settings to be established.

The main issues related to passive-source reflection imaging with seismic interferometry (SI) are inadequate acquisition parameters for sufficient spatial wavefield sampling and vulnerability of surface arrays to the dominant influence of the omnipresent surface-wave sources. Additionally, long recordings provide large data volumes that require robust and efficient processing methods. We address these problems by developing a two-step wavefield evaluation and event detection (TWEED) method of body waves in recorded ambient noise. TWEED evaluates the spatiotemporal characteristics of noise recordings by simultaneous analysis of adjacent receiver lines. We test our method on synthetic data representing transient ambient-noise sources at the surface and in the deeper subsurface. We discriminate between basic types of seismic events by using three adjacent receiver lines. Subsequently, we apply TWEED to 600 h of ambient noise acquired with an approximately 1000-receiver array deployed over an active underground mine in Eastern Finland. We develop the detection of body-wave events related to mine blasts and other routine mining activities using a representative 1 h noise panel. Using TWEED, we successfully detect 1093 body-wave events in the full data set. To increase the computational efficiency, we use slowness parameters derived from the first step of TWEED as input to a support vector machine (SVM) algorithm. Using this approach, we detect 94% of the TWEED-evaluated body-wave events indicating the possibility to limit the illumination analysis to only one step, and therefore increase the time efficiency at the price of lower detection rate. However, TWEED on a small volume of the recorded data followed by SVM on the rest of the data could be efficiently used for a quick and robust (real-time) scanning for body-wave energy in large data volumes for subsequent application of SI for retrieval of reflections.

In the Pyhäsalmi case study, the seismic data is used in direct targeting of shallowly dipping mineralized zones in a massive sulfide ore system that was deformed in complex fold interference structures under high-grade metamorphic conditions. The Pyhäsalmi volcanic-hosted massive sulfide (VHMS) deposit (>50 Mt) is located in a Proterozoic volcanic belt in central Finland. Acoustic impedance of Pyhäsalmi ore (32×106 kg/m2 s) is distinct from the host rocks (16–19 ×106 kg/m2s), enabling its detection with seismic reflection methods. Drill-hole logging further indicates that the seismic imaging of a contact zone between mafic and felsic volcanic rocks possibly hosting additional mineralizations is plausible. Six seismic profiles showed discontinuous reflectors and complicated reflectivity patterns due to the complex geology. The most prominent reflective package at 1–2 km depth was produced by shallowly dipping contacts between interlayered felsic and mafic volcanic rocks. The topmost of these bright reflections coincides with high-grade zinc mineralization. Large acoustic impedances associated with the sulfide minerals locally enhanced the reflectivity of this topmost contact zone which could be mapped over a wide area using the seismic data. Seismic data enables extrapolation of the geologic model to where no drill-hole data exists; thus, seismic reflection profiling is an important method for defining new areas of interest for deep exploration.