Application of Multi-channel Wiener Filters to the Suppression of Ambient Seismic Noise in Passive Seismic Arrays

We are concerned with the detection and location of small seismic events, such as can be encountered in monitoring hydro-fracturing with surface sensors. Ambient seismic noise is the main problem in detection of weak seismic phases from these events, particularly as the sites of interest are often within or near producing fields. Bandpass filtering and stacking are the most widely used technique for enhancing the signal-to-noise ratio (SNR) in passive seismic experiments, but they are of limited value when both noise and signal share the same frequency band. Seismic arrays can be used to reduce the unwanted noise (e.g. traffic noise, pumping noise, scattering ground roll) by delay-and-sum techniques (also called beamforming) or by frequency-wavenumber filtering. Beamforming maxi-mizes the array response for the assumed direction and slowness of the signal. Whereas in some situations it can be highly effective, and the azimuth and slowness of the sig-nal can be determined by a grid search approach, it is vulnerable to contamination by side-lobe energy, particularly for broadband signals and noise (Rost and Thomas, 2002). Frequency-wavenumber filtering can be very effective but requires regularly spaced arrays and implicitly assumes plane wave propagation. Both methods perform poorly when th