Improvements to Seismic Monitoring of the European Arctic Using Three-Component Array Processing at SPITS

Abstract

The detectability of low magnitude seismic events in the European Arctic is determined primarily by the small-aperture IMS arrays ARCES and SPITS. In August 2004, the SPITS array was upgraded to a broadband array with an increase in the sampling rate from 40 to 80 Hz. Most importantly however for the detection and location of small magnitude seismic events was the deployment of three-component instruments at 6 of the 9 sites. Detection and correct classification of secondary phases are of paramount importance for events observed by only a small number of stations at regional distances and, in the absence of the strong Lg phases typically observed for continental propagation paths, multiple three-component stations were deemed necessary to exploit the higher S-phase amplitudes anticipated on the horizontal sensors. We demonstrate improved SNR for S phases on horizontal beams for several events close to Novaya Zemlya. Horizontal component fk analysis improves direction estimates and phase classification for low SNR signals. We demonstrate secondary phases which are misidentified by verticalonly f-k analysis but which are correctly classified by 3-C array processing.A significant problem with array processing at SPITS is the overlap in slowness space of regional P and S phases. Phase identification is improved greatly by comparing the coherence between vertical traces with the coherence between horizontal traces. Considerations in the routine array-processing of SPITS data are reviewed, including the need for elevation-corrections in slowness

Cross-correlation beamforming

“…It is known that wavefronts arriving at SPITS from the NNW reach larger apparent velocities than may be expected based on a 1D velocity model (Fig. 6 in Gibbons et al (2011)). This is likely due to dipping interfaces in the crust below SPITS.…”

“…We use the station configuration of the SPITS array (Gibbons et al 2011), which is a concentric distribution of nine stations. Figure 1 shows the location of the array.…”

“…This is likely due to dipping interfaces in the crust below SPITS. Moreover, to further improve the directivity estimate, for both methods also corrections would need to be made for elevation differences over the SPITS array (Gibbons et al 2011). …”

“…Frankel et al (1991) employed a method akin to co-phase that they termed ZLCC (zero-lag cross-correlation) to scan the slowness parameter space. This method and related procedures have been applied to near-field (e.g., Almendros et al 1999; Wassermann and Ohrnberger 2001; Cros et al 2011; Hillers et al 2014), regional (e.g., Gibbons et al 2011), and global-scale (e.g., Landès et al 2010) parameter estimation scenarios. A probabilistic expansion of ZLCC has been derived in Saccorotti and Del Pezzo (2000).…”

Could the IMS Infrasound Stations Support a Global Network of Small Aperture Seismic Arrays?

“…Here, we have bandpass filtered in a suitable frequency band (2-5 Hz), taken a small segment of this signal (3.5 seconds long), and delayed across the different sensors to simulate the predicted speed and direction of the incoming signal. We follow the principle described by Gibbons et al (2011), that parameter measurement of S-phases is likely to be more reliable on the rotated horizontal components than on the vertical components. The classification is unambiguous for these regional phases.…”