A new method has recently been developed at NORSAR for providing improved stability for analyzing seismic phases using incoherent or partially coherent seismic arrays. Traditional beamforming and f –k analysis require waveform semblance over the full array aperture and cannot be applied in many situations where signals are incoherent between sensors. The NORSAR and MJAR arrays are two primary IMS stations where this is the case for high frequency regional phases. Large intersite distances and significant geological heterogeneity at these arrays result in waveform dissimilarity which precludes coherent array processing in the frequency bands with optimal signal-to-noise ratio.
Multitaper methods provide low variance spectral estimates over short time-windows and seismic arrivals can be detected on single channels using a non-linear spectrogram transformation which attains local maxima at times and frequencies characterized by an energy increase. This detection procedure requires very little a priori knowledge of the spectral content of the signal. The transformed spectrograms can be beamformed over large-aperture arrays or networks according to theoretical time delays resulting in an incoherent detection system which does not require waveform semblance at any frequencies.
A real-time automatic detection system has been developed for regional phase arrivals on the NORSAR array, providing stable and accurate slowness and azimuth estimates for quite marginal signals. In the case of partially coherent arrays, the procedure described may provide stable, if low resolution, estimates which can subsequently be refined using coherent processing over subsets of sensors.
As an example, the figure below illustrates that the spectrogram beamforming method facilitates a stable and accurate slowness estimate for the incoherent high-frequency Pn arrival at the MJAR array in Japan from the 2006 October 9 underground nuclear test in North Korea. Conventional analysis methods were unsuccessful for this high-frequency phase.
Spectrogram beamforming method for the underground
nuclear test in North Korea in October, 2006.
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