Seismic reflection methods have been used for the exploration of mineral resources for several decades. However, despite their unmatched spatial resolution and depth penetration, they only have played a minor role in mineral discoveries so far. Instead, mining and exploration companies have traditionally focused more on the use of potential field, electric and electromagnetic methods. In this context, we present a case study of an underground Vertical Seismic Profiling (VSP) experiment, which was designed to image a (semi-)massive sulfide deposit located in the Kylylahti polymetallic mine in eastern Finland. For the measurement, we used a conventional VSP with three-component geophones and a novel fiber-optic Distributed Acoustic Sensing (DAS) system. Both systems were deployed in boreholes located nearby the target sulfide deposit, and used in combination with an active seismic source that was fired from within the underground tunnels. With this setup, we successfully recorded seismic reflections from the deposit and its nearby geological contrasts. The recording systems provided data with a good signal-to-noise ratio and high spatial resolution. In addition to the measurements, we generated a realistic synthetic dataset based on a detailed geological model derived from extensive drilling data and petrophysical laboratory analysis. Specific processing and imaging of the acquired and synthetic datasets yielded high-resolution reflectivity images. Joint analysis of these images and cross-validation with lithological logging data from 135 nearby boreholes led to successful interpretation of key geological contacts including the target sulfide mineralization. In conclusion, our experiment demonstrates the value of in-mine VSP measurements for detailed resource delineation in a complex geological setting. In particular, we emphasize the potential benefit of using fiber-optic DAS systems, which provide reflection data at sufficient quality with less logistical effort and a higher acquisition rate. This amounts to a lower total acquisition cost, which makes DAS a valuable tool for future mineral exploration activities.

A precise velocity model is necessary to obtain reliable locations of microseismic events, which provide information about the effectiveness of the hydraulic stimulation. Seismic anisotropy plays an important role in microseismic event location by imposing the dependency between wave velocities and its propagation direction. Building an anisotropic velocity model which accounts for that effect allows for more accurate location of microseismic events. We utilize downhole microseismic records from a pilot hydraulic fracturing experiment in Lower-Paleozoic shale gas play in Baltic basin, Northern Poland to obtain accurate microseismic events locations. In this paper we develop a workflow for a VTI (i.e. vertical transverse isotropy) velocity model construction when facing a challenging absence of horizontally polarized S-waves in perforation shots data, which carry information about Thomsen’s γ parameter and provide valuable constraints for location of microseismic events. We extract effective ε, δ and V_P0, V_S0 for each layer from P- and SV-waves arrivals of perforation shots, while the unresolved γ is retrieved afterwards from SH-SV-waves delay time of selected microseismic events. Inverted velocity model provides more reliable location of microseismic events which then becomes an essential input for evaluating the hydraulic stimulation job effectiveness in the geomechanical context. We discuss the influence of the pre-existing fracture sets and obliquity between the borehole trajectory and principal horizontal stress direction on the hydraulic treatment performance. The fracturing fluid migrates to previously fractured zones, while the growth of the Microseismic Volume in consecutive stages is caused by increased penetration of the above-lying lithological formations.