GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 23 200 m depth in correspondence to SPCA. In accordance with ambient seismic noise estimates performed at borehole stations installed in the Po Plain in sites characterized by similar geological settings (Franceschina et al. , 2014), we hypothesize a decreasing rate of 0.1dB/m for the mean noise level in the frequency band: 1-20 Hz. In order to establish the detection threshold of the network, we used point source simulations of earthquakes characterized by different values of magnitude and distance. For each installation site, the power spectrum of the simulated earthquake was compared with the observed (or hypothesized) power spectrum of ambient seismic noise. The amplitude Fourier spectrum of S-waves, recorded at hypocentral distance R , was modeled according to Brune (1970, 1971). The seismic wave attenuation was introduced by considering a R -1 dependence of geometrical spreading, a f 1 dependence of the quality factor, Q( f ), and a constant value of the k parameter of Anderson and Hough (1984). A seismic sequence with maximum M L magnitude equal to 3.0, was recorded by the SPCA station during the test installation period. We employed the corresponding data to calibrate the simulation parameters. After that, point source simulations were carried out for seismic sources placed in 121 equally-spaced points of 5 regular grids, located within DE at 1.0, 2.5, 4.0, 6.5 and 9.0 km depth. The levels at 1.0, 4.0 and 9.0 km coincide with the depth of the “Sant’Alberto” gas field, the bottom of DI and the bottom of DE, respectively. A microseismic network, with inter-station distances between 3 and 5 km, composed of 5 stations installed in DI was hypothesized. In addition, in all simulations we considered the station FIU of National Seismic Network, located within the surface projection of DE at 9.4 km distance from SPCA. By adopting the network configuration reported in Fig. 3, consisting of stations POV1, POV2, POV3, POV4, SPCA and FIU, we considered three possible simulation scenarios, obtained with different combinations of the ambient noise levels. Noise levels were chosen as follows: Case A: We adopted for all stations the noise levels really observed at the selected sites; Case B: The noise level observed at the most favorable site, POV1, was artificially extended to the remaining installations; Case C: As in case B, with the exception of the SPCA site, where a borehole installation at 200 m depth was hypothesized. In this case, a decreasing rate of 0.1dB/m was hypothesized for the mean level of noise in the frequency band: 1-20 Hz. For each source-station couple, the power spectrum of a seismic signal of 4 s duration, generated by earthquakes with locale magnitudes ranging between -1.0 and 3.0, has been computed, and then compared with the corresponding power spectrum of the observed (or hypothesized) noise. A seismic event is assessed as detected when the signal to noise ratio is equal to 5 (~ 14 dB) in correspondence to the corner frequency of the source. We defined the detection threshold as the minimum magnitude at which an earthquake can be recorded by at least one station and the localization threshold as the minimum magnitude at which an earthquake can be localized. In this work we classified a seismic event as localizable if it is detected by at least 3 stations of the network. The results obtained in cases A and B showed the need to install at least one borehole station in order to ensure the detection levels specified in MiSE-DGRME (2014) in the whole DI, also in the case of unfavorable noise conditions. The installation of a borehole station at 200 m depth ensures a general improvement of the detection threshold up to 4 km depth. In particular, the borehole station installed in correspondence of SPCA, is capable to detect M L -0.3 events, located at the depth of the reservoir. Also localization thresholds show a considerable improvement with M L values between 0.7 and 0.8 at the bottom level of DI. On the other side, Fig. 3 also shows that in these conditions, prescriptions indicated in MiSE-DGRME (2014) are not satisfied in the part of the extended domain of detection not included in DI. Localization thresholds greater than 1.0 are obtained in the deeper part of DE. Moreover, it is important to highlight that, under these conditions, the location of seismic events would be affected by remarkable errors due to the inadequate stations distribution. For a more accurate and homogeneous localization of seismic events occurring within the whole extended domain,