A three-station broadband network was installed around the Bruce Nuclear site at the beginning of August 2007, to monitor microearthquakes within a 50-km radius of the plant. The seismic network was equipped with borehole stations installed at cased boreholes at depths of 25, 27 and 40 m, and temporary surface stations at the same sites. The aim of the doubled identical equipment (Geotech Instruments' KS2000) was to compare the records of local, regional and teleseismic events, and seismic noise and to obtain results about the noise reduction, attenuation and the site response at each station. During the design and installation of the seismic network different geophysical surveys were carried out: refraction seismic profiles, vertical seismic profiling, and noise level measurements at different depths along the borehole. The obtained velocity models were used for modeling of the site response and finally comparison with the real data from the parallel borehole-surface recordings, and measured predominant frequencies using the Nakamura's HVSR method. The real noise reduction estimated from the parallel recordings was compared with the predicted 10 dB noise reduction. A practical conclusion was drawn out about the optimum borehole depths for instrument installation based on the noise reduction / attenuation balance and signal-to-noise ratio with the depth. The seismic threshold magnitude for the monitored area estimated at the design stage was compared with the threshold magnitude obtained from the real data.
Upprättat; 2012; 20121019 (andbra)