Abstract
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.
Highlights
Seismic reflection methods have been used for mineral exploration and mine planning for several decades (e.g., [1,2,3,4] and the references therein)
The Kylylahti underground Vertical Seismic Profiling (VSP) experiment presented in this study provides several important conclusions
Measurements for mineral exploration and resource delineation, which is due to various reasons
Summary
Seismic reflection methods have been used for mineral exploration and mine planning for several decades (e.g., [1,2,3,4] and the references therein). Hardrock settings are usually dominated by igneous and metamorphic rocks that have undergone severe deformation due to tectonic processes [1] This causes various challenges for seismic methods: (1) The predominant rock types usually exhibit high seismic velocities (≈4–8 km/s [24]), causing the spatial resolution to be lower compared to sedimentary environments. One way is to integrate seismic reflection methods with other geophysical and geological techniques that are conventionally used in mineral exploration (e.g., [22,27]) Another promising approach is to combine strategically different forms of seismic acquisition. The COGITO-MIN project involved acquisition and integration of multi-scale seismic methods including surface-based 2D and 3D active-source and passive seismic measurements and an underground in-mine VSP survey using conventional and fiber-optic technologies. We present reflectivity images of the acquired and synthetic VSP datasets and discuss their geological interpretation in relation to borehole data
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