NORSAR has developed and applied processing and interpretation software for microearthquakes in hydrocarbon reservoirs and mines. In this paper we will discuss the challenges and limitations of borehole installations compared to subsurface installations in mines with respect to data analyses and interpretation. Since 2003 we have been working with microseismic data from the 1.4 km deep Pyhäsalmi ore mine (Oye et al., 2006). The data are recorded using an ISS International (Integrated Seismic System) network consisting of 12 vertical and 6 three-component geophones deployed at depths between 1 and 1.4 km around the active part of the mine. The Pyhäsalmi mine and ISS have kindly made the raw seismic data available to us, which we are processing independently with in-house software (Oye and Roth, 2003). In hydrocarbon reservoirs, the most commonly applied geometry is an array of geophones that is lowered into a single borehole. We have primarily been working with data from an 18-day monitoring of the Ekofisk field in the North Sea and with various data from the San Andreas Fault Observatory at Depth (SAFOD) based on different receiver array configurations within Pilot Hole and Main Hole installation (Oye et al., 2004). In principal, event localization can be based on phase arrival times and polarization information, the latter being essential for single borehole installations only. However, the polarization is strongly affected by local heterogeneities, especially at the receiver site, and the reliability of polarization determination depends to a high degree on the geophone quality and on proper installation/orientation. Uncertainties of 10-20 deg are typical. An automatic estimation of basic source parameters such as e.g. seismic moment, seismically radiated energy and corner frequency, can generally be determined under the assumption of a theoretical source model (e.g., Oye et al., 2006). The signal spectra are corrected for propagation path effects such as geometrical spreading and attenuation and source spectra are fit to the corrected signal spectra. Amplitude effects from the source radiation pattern can only be compensated for if the fault plane solution is assumed known or by averaging, provided that the receivers have sufficient spatial coverage of the source. The spatial coverage of the source is even more indispensable to achieve reliable fault plane solutions, since the solution space is non-linear and ambiguous. The assignment of fault plane solutions already implies the assumption of a pure shear failure, which might not necessarily be the general case. To resolve for such non-shear or volumetric components in the source, a moment tensor inversion is required, which in turn relies on even better spatial coverage. Due to a generally superior 3D geophone configuration in mining environments compared to hydrocarbon reservoirs, the quantity and quality of results obtained from passive seismic monitoring are significantly higher. This becomes also evident when considering the amount of microseismic installations, which have been proven valuable, if not indispensable.


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