Background
The Pyhäsalmi ore mine in Central Finland is the oldest operating metal mine in Finland and the deepest in Europe extending to a depth of 1441 meters and is owned by Inmet Mining Co. It is quarried by sublevel and bench stoping, achieving an annual production of about 1.3 Mt of ore. Massive sulfide ores form a potato-shaped body, which seems to be located in the hinge of a large synform fold. The ore body is medium to coarse grained and consists of sphalerite-pyrite with some zinc in the outer rim and in an inner rim of chalcopyrite-pyrite with a copper content of 1-6%, whereas the innermost part of the ore body is uneconomical pyrite. The ore body is surrounded mainly by quartz-rich felsic volcanics (80-90%) with amphibolite bands that vary in width from millimeters to meters. There are no distinct faults in the active mining area between 1100 and 1450 m depth that could have been activated by mining. However pegmatite veins, varying in size and orientation and being common near ore contacts, have been identified as possible fault planes. Furthermore, severe shearing has occurred in the geological past in Pyhäsalmi, and active faults exist outside the current mining area and near the surface. The complete mining infrastructure including tunnels, stopes and passes has been surveyed by laser scanning, and a model has been built using a mine design software.
Model of Pyhäsalmi ore body, the surrounding
infrastructure, and the seismic stations. Most
seismic activity is observed close to KN1,
which is a pass for quarried ore.
The seismic network
The monitoring network within the Pyhäsalmi mine consists of 18 sensors including 6 three-component stations leading to source-receiver distances of 60 to 400 m. The geophones are cemented in 10.5-m-long boreholes that were drilled vertically in the roof of the tunnels. The installation was performed by ISS (Integrated Seismic System International Ltd, South Africa) and the monitoring is conducted routinely by Inmet Mining Co.
NORSAR obtains data for scientific purposes from the seismic monitoring network by courtesy of Inmet Mining Co and a good part of the microseismic monitoring software MIMO was developed using these data sets. The network has been operational in a continuous mode since January 2003 and, until March 2004, about 18,000 events were detected. About two thirds of those events were identified as mining blasts, the remaining events are microseismic activity. The range of moment magnitudes is -1.8 to 1.2, but most events have moment magnitudes from -1.5 to -0.5. In general, sampling rates of the geophones are 3000 Hz, with occasional down-sampling to 1000 or 500 Hz.
Beside the automatic location of events and analysis of source parameters, we currently work on analysis methods related to both travel paths and source mechanism. We use the 3D finite difference code E3D from Larsen and Schultz (1995) to calculate full waveforms for waves traveling through the mining infrastructure including the host rock, the ore body, tunnels and cavities.
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Full waveforms calculated for Pyhäsalmi mining infrastructure using E3D, wave field shown by red and blue colors, green outline depicts arrival time computed by Eikonal solver. Left: 2D computation, outline of ore body and cavities marked by pink lines (partly hidden by wave field); right: 3D computation, a slice of the wave field is shown on the gray plane, ore body visible as 3D structure.
Until now, basic source parameters (e.g. seismic moment, moment magnitude, corner frequency) can be estimated by MIMO, using integrals of S-wave spectra. We currently develop an automated routine to compute fault plane solutions by analyzing P-wave first motion polarities. The figure below displays waveforms of an event recorded on the 21st of December 2005 along with plausible fault plane solutions.
Computation of fault plane solutions from P-wave first motion polarities. Top: waveforms for an event recorded on the 21st of December 2005, orange lines: P-wave onset time, red lines: positive first motion, blue lines: negative first motion; bottom: fault plane solutions, red circles mark positive first motions, blue circles mean negative first motion, filled and empty circles stand for impulsive and emergent onsets; left: solutions reproducing all polarities, middle: solutions reproducing all impulsive polarities, right: mean solution.
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