The microseismic monitoring system employed in the Yufutsu oil/gas field was assessed in terms of the geophone-to-ground coupling and the smallest detectable magnitude of microseismic events. The surface shooting was conducted to calibrate the orientation of the geophones and the shooting signal was also utilized to evaluate the monitoring system. The energy of the surface shot was small. However, the signal was successfully obtained even at the deepest station (at the depth of 3,196 m BSL) with reasonable signal-to-noise ratio at an appropriate number of stacks.
The shooting signals measured at all the stations have similar level of amplitude after the correction for geometrical spreading and, therefore, we can conclude that all the stations are reasonably coupled to the formation through the casing.
According to the scaling law in seismology, we proposed a method to estimate the displacement spectrum of microseismicity for various magnitudes observed at a certain distance from the hypocenter in the field where some specific parameters (i.e. the density, velocity of S-wave and the quality factor) are available. The modeled displacement spectrum with respect to the actual background noise allows one to estimate the lower bound of the moment magnitude of microseismicity detectable by the monitoring system adopted in the field. We applied this method to assess the monitoring network employed in the Yufutsu oil/gas field. The quality factor, which is one of the key parameters required to model the displacement spectrum, was estimated by using the shooting signals. The moment magnitude of actually observed microseismicities in the Yufutsu field showed consistency with the expected lower bound of moment magnitude of detectable microseismicity, implying the validity of the proposed method in this study.