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We develop methodological improvements on the back-projection imaging approach. The standard processing of back projection is beamforming, which suffers from low resolution especially when simultaneously emitting sources are present. We successfully introduced a sophisticated MUlitiple SIgnal Classification (MUSIC) (Schmidt, 1986; Meng et al., GRL, 2011) algorithm into BP and improved the practical applicability of BP to non-stationary seismic signals by incorporating multi-taper cross spectrum estimates (Thomson, 2000). The MUSIC-enhanced BP provides superior resolution over the standard back-projection approach based on beamforming, yielding a sharper image of the rupture process (Meng et al., JGR, 2012).
Figure 1 shows the comparison of resolution between MUSIC (left panel) and beamforming (right panel). Two plane waves, A and B, impinge on a linear array as shown by the inset cartoon of the left panel. The azimuth of A is fixed at 0 degrees while the azimuth of B is varied from 10 to 0 degrees. The color maps indicates the normalized stack of beamforming or MUSIC pseudospectra as a funtion of the relative azimuth with respect to A for a given azimuth separation between A and B. The white dots mark the half width of the two largest maxima at a given azimuth of B.
The resolution of a method is defined as its ability to separate closely spaced sources. The comparison in Figure 1 shows that MUSIC can resolve waves with azimuth separation as small as 3 degrees, achieving at least twice higher resolution (minimum resolvable azimuthal separation) than the beamforming method.
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