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Research Progress

New Hybrid Correlation Stacking Method to Locate Weak Seismic Events

Jan 04, 2018

The seismic source location problem is encountered in different scales and applications in seismology, such as tremor and earthquake location, microseismic monitoring in geothermal, oil and gas reservoirs, as well as rock burst monitoring in mines and tunnels.

Waveform-based seismic location methods can reliably and automatically image weak seismic sources. Besides the classical diffraction stacking operator which is based on the one-way traveltime, correlation-based imaging method is another subcategory of waveform-based methods using differential traveltime.

After systematically analyzing the existing correlation-based methods, including single correlation stacking (SCS), double correlation stacking (DCS), and relative correlation stacking (RCS), researcher LI Lei from the Institute of Acoustics (IOA) of the Chinese Academy of Sciences, together with researchers from both China and Germany proposed a novel hybrid correlation stacking (HCS) method.  It was proved to be a reliable and efficient approach for seismic location, particularly in case of a less reliable velocity model.

In this new approach, the double differential traveltime from an event pair (i.e. a master event and a target event) to a receiver pair was used to stack the corresponding double correlation waveforms. Figure 1 shows the schematic diagrams of the four correlation-based seismic location methods.

 

Figure 1. Schematic diagrams of the correlation-based methods: (a) SCS (b) DCS (c) RCS (d) HCS. The reverse triangles are receivers, grey stars are target events and black stars are master events. The summing junction symbol denotes cross-correlation operation. (Image by LI Lei) 

Researchers conducted a thorough analysis of imaging operators above by using synthetic and field data examples, revealing characteristic differences in imaging resolution and level of redundancy of these methods. Figure 2 shows their basic 2D imaging patterns when only four receivers were considered.

 

Figure 2. Imaging results of the correlation-based methods: (a) SCS (b) DCS (c) RCS (d) HCS. The reverse triangles are receivers. The positions with highest imaging values denote the source locations. (Image by LI Lei) 

For SCS and HCS, their imaging patterns correspond to hyperbolae intersections in 2-D scenario. For RCS, circular arcs intersections are presented in the imaging profile. For DCS, there are hyperbolae intersections and more redundancy.

For general 3-D models with more receivers, deformed hyperboloids and spherical surfaces intersections with highly focused source energy will be presented in the final imaging profiles.

Researchers pointed out that a moderate level of redundancy could ensure both accuracy and stability of correlation-based imaging methods, while an extremely high or low level of redundancy would hinder their performance in locating weak seismic events.

Furthermore, they summarized these correlation-based methods using a unified formula and demonstrated their generalized relationship by approximation with the beamforming algorithm.

This research was supported by National Natural Science Foundation of China and China Scholarship Council.

Their work entitled "A systematic analysis of correlation-based seismic location methods" was published in Geophysical Journal International.

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