Reference window Strategy
The back-projection (BP) imaging is an effective tool to image the ruptures of large earthquakes. However, one of the well-known problem in the back projection community is the "swimming artifact", which appears as systematic transients migrating across the BP image towards the direction of the receiver array. These artifacts degrades the quality of the images and makes it difficult to build confidence on the details of the source imaging. A common practice is to smooth the images to suppress the artifacts, which comes at the expense of the resolution. The origin of the "swimming artifact" is related to the non-stationary signature of seismic signals. Unlike stationary signals such as ambient noise and radio communications, earthquake waveforms are impulsive and transient. Specifically, the amplitude and coherence of the signal decays quickly after the initial arrival. Conventional BP algorithms are is designed for stationary signals. When they are applied to non-stationary signals, The trade-off between space and time results in artificial migration even for a fixed point source. We have introduced modifications in the choice of sliding windows to adapt to the non-stationary signal. By adopt the "reference window" as oppose to the "absolute window" in conventional BP, we are able to mitigate the "swimming artifact" (Meng et al., EPS, 2013). An example of imaging an kinematic rupture scenario is shown below. The synthetic seismograms of a unilateral rupture composed of six moving sources is processed by BP with the "absolute window" and the "reference window" strategies. The "reference window" BP clearly reproduces the linear rupture well. The "absolute window BP" suffers significant "swimming artifacts".