August 2016, I was driving from Helsinki towards Polvijärvi with anticipation. I was being followed by groups from the University of Helsinki, the Institute of Geophysics Polish Academy of Sciences, Vibrometric Oy and Geopartner Ltd., with seismic equipment worth millions of Euros. We all had a common goal: to acquire multiple high-quality seismic data sets in the Kylylahti mine site owned and operated by Boliden. Like seismic waves, we chose the fastest route to the destination, even though it was not the shortest.
The field work was preceded by an almost as intensive period of project formation which involved the gathering together of project partners, applying for funding and after receiving a positive response from the ERA-MIN network, careful planning of how the field work would be conducted in practice. The COGITO-MIN project had multiple components; both in-mine and surface seismic measurements conducted in both active and passive mode. We had rented 1000 wireless seismic receivers from a Polish company Novaseis, and planned to deploy these in a ~10 km2 grid around the Kylylahti mine for a month to record all possible seismic vibrations. Vibrometric Oy brought their borehole seismic receivers to Kylylahti and a small size seismic Vibsist-source that could be used in the mine tunnels. We had subcontracted Silixa to operate the Distributed Acoustic Sensing (DAS) system, including the fibre optic cables that were used as receivers in the borehole, to see whether the results from the DAS system and the conventional borehole seismic measurements were comparable. Geopartner followed the project group with 600 wireless geophones and two Vibroseis source trucks in a transportation trailer. The Geopartner equipment was later used in the acquisition of two active seismic reflection profiles. All of these activities were scheduled to occur within a six week period in a synchronised manner without compromising either data quality or mine production rates.
For seismic data acquisition, teamwork is an integral part of a successful operation. We needed to have receiver location coordinates measured with GPS, someone to drill holes for explosives, a group of people to deploy geophones, capable person to operate the seismic source and someone with a license to control the explosions. Someone is of course also needed to ensure that the whole ‘circus’ is performing as planned and most of all, we needed a social license to operate. Even if the seismic reflection method leaves hardly any imprint on the terrain, the COGITO-MIN project nevertheless ensured that permission for deployment of the geophones was asked from each of the private landowners.
In the COGITO-MIN experiments, data processing and interpretation were based on the simple physical principle of seismic waves traveling along the fastest ray paths (subsurface ’highways’ instead of ‘gravel roads’). In seismic reflection profiling this principle has an important influence. First, with long source-receiver distances, the first vibrations recorded by receivers are not caused by the waves traveling directly from source to receiver along the overburden. Instead, the fastest wave first penetrates the overburden at a relatively slow velocity (1500 -2000 m/s) before reaching the surface of bedrock. Compared to the overburden, the bedrock is a ‘highway’ for seismic waves. The typical seismic velocity in the Kylylahti mica schist rocks is 5500 m/s though it can reach as high as 7000 m/s for the high-density sulphide bearing rocks. Clearly, waves that travel along the bedrock reach the receivers faster than the slow waves travelling through the loose overburden. In seismic surveys, we have several receivers and multiple source points and by measuring the wave travel times with different source-receiver combinations we can determine the overburden thickness and velocity in both the overburden and the bedrock.
Notwithstanding how fascinating the properties of the overburden and bedrock surface might be for some applications, within the COGITO-MIN project we were primarily interested in the deeper features of bedrock that could shed light on the crustal architecture of the Kylylahti massive sulphide deposit. As is typical for the mining and exploration environments in Finland, the geology of Kylylahti is characterised by steep folding and the complex geometries of geological structures and units. During the planning period of the COGITO-MIN project, it was already acknowledged that direct imaging of the geological interfaces would be a challenging task. Clearly however, if it was not difficult it would not be interesting or ambitious enough for a research project.
Seismic imaging is complicated by the fact that with one-component receivers we are only measuring the travel times of the seismic waves, not the direction. Basically, observed reflection can originate from any direction within the certain distance constrained by the travel time and velocity of the waves. In the preliminary processing results, all the reflections are assumed to originate directly under the observation point. In order to display the true subsurface position of these reflections, we need to utilise a process called migration. Seismic data can be migrated in different stages of data processing, depending on the available computing power and algorithms. In the COGITO-MIN project, we tested several different migration algorithms and studied their efficiency in imaging the subsurface geological interfaces. The resulting final seismic images show steep reflectors that can be correlated with the surface occurrences of black schist and rock units hosting the Kylylahti ore deposit and are clearly visible at several kilometres depth.
When driving back from the COGITO-MIN field experiment in September 2016, I missed a few important turns and instead of driving the quickest way back home ended up taking a rather complicated and slower route. This also happens to seismic waves: diffraction and scattering causes waves to propagate in rather unexpected paths. These later arrivals are often not the primary interest and have less energy, but nevertheless they make seismic images fascinating and full of details worthy of closer study that could inspire several more geoblogs.
Learn more about the COGITO-MIN project by watching the seminar “SEISMICS IN MINERAL EXPLORATION: WHY, HOW AND HOW MUCH? -LESSONS LEARNED FROM THE COGITO-MIN PROJECT” via Unitube: https://www.helsinki.fi/fi/unitube/video/21066
or read publications:
Chamarczuk, M., Malinowski, M., Draganov, D., Koivisto, E. A-L., Heinonen, S., Juurela, S. and COGITO-MIN Working Group, 2018. Seismic Interferometry for Mineral Exploration: Passive Seismic Experiment over Kylylahti Mine Area, Finland. EAGE Near Surface Geoscience Conference & Exhibition 2018: 2nd Conference on Geophysics for Mineral Exploration and Mining.
Heinonen, S., Malinowski, M., Hlousek, F., Gislason, G., Koivisto, E., Buske S. and the COGITO-MIN Working Group, 2018. Seismic Exploration in the Kylylahti Cu-Au-Zn Mining Area: Comparison of Time and Depth Imaging Approaches. EAGE Near Surface Geoscience Conference & Exhibition 2018: 2nd Conference on Geophysics for Mineral Exploration and Mining.
Riedel, M., Cosma, C., Enescu, N., Koivisto, E., Komminaho, K., Vaittinen K. and Malinowski, M., 2018. Underground Vertical Seismic Profiling with Conventional and Fibre-Optic Systems for Exploration in the Kylylahti Polymetallic Mine, Eastern Finland. Minerals 2018, 8 (11), 538.
Teksti: Suvi Heinonen
Dr Heinonen is a senior scientist at the Geological Survey of Finland (GTK). She completed her PhD on Seismic reflection profiling for massive sulphide exploration in Finland at the University of Helsinki in 2013. Her research focuses on hard rock seismic exploration and 3D common Earth modelling.