Underground Vertical Seismic Profiling with Conventional and Fiber-Optic Systems for Exploration in the Kylylahti Polymetallic Mine, Eastern Finland

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.

Sparse 3D Seismic Imaging in the Kylylahti Mine Area, Eastern Finland: Comparison of Time Versus Depth Approach

“…Data were recorded quasi-continuously (20 h per day) with a 2 ms sampling interval for 30 days. During this period, other components of the COGITO-MIN survey took place, i.e., 2D seismic acquisition along two profiles (Line A and Line B) using both Vibroseis and dynamite sources [28] (Figure 1), and vertical seismic profiling (VSP) in underground boreholes, with dedicated Vibroseis and dynamite sources also on the surface [20]. In order to improve source density and create a sparse 3D active-source survey to benchmark results of seismic interferometry, additional Vibroseis and explosive sources were placed wherever permitted ( Figure 1).…”

“…Furthermore, as the sources form a major part of the costs of a 3D survey, the aim was to test if a sparse active-source 3D survey can provide a significantly cheaper alternative when the financial resources, terrain conditions or permissions do not allow for regular 3D source line geometries. The 3D array recorded Vibroseis and dynamite shots made for the 2D data acquisition [19], vertical seismic profiling (VSP) [20], as well as some additional "random" Vibroseis and dynamite shots made to complement the 3D source distribution. The resulting 3D survey is characterized by an irregular shooting geometry and relatively large receiver intervals (50 m).…”

Cost-Effective Seismic Exploration: 2D Reflection Imaging at the Kylylahti Massive Sulfide Deposit, Finland

“…Seismic methods can be used to image subsurface geological features at high resolution down to several kilometers depth but are often considered expensive by the mineral exploration industry. The COGITO-MIN project (COst-effective Geophysical Imaging Techniques for supporting Ongoing MINeral exploration in Europe) investigated and developed cost-effective seismic exploration techniques [1] including passive seismic interferometry [2] and distributed-acoustic sensing vertical seismic profiling [3]. A major part of the cost of seismic surveys is attributed to the seismic source and the large field crews operating and moving the seismic receivers.…”

Multi-task learning based P/S wave separation and reverse time migration for VSP

“…However, the whole process of wave separation is time-consuming. An Expert in data processing needs lots of preprocessing to master the main events of the data beforehand (Riedel et al, 2018), and the next step is to separate the data through wave decomposition methods. Furthermore, The difference between near and far offset makes it hard to use the same parameters to separate modes.…”