Reverse modelling for seismic event characterization

The localization of seismic events is of utmost importance in seismology and exploration. Current techniques rely on the fact that the recorded event is detectable at most of the stations of a seismic network. Weak events, not visible in the individual seismogram of the network, are missed out. We present an approach, where no picking of events in the seismograms of the recording network is required. The observed wavefield of the network is reversed in time and then considered as the boundary value for the reverse modelling. Assuming the correct velocity model, the reversely modelled wavefield focuses on the hypocentre of the seismic event. The origin time of the event is given by the time where maximum focussing is observed. The spatial extent of the focus resembles the resolution power of the recorded wavefield and the acquisition. This automatically provides the uncertainty in the localization with respect to the bandwidth of the recorded data. The method is particularly useful for the upcoming large passive networks since no picking is required. It has great potential for localizing very weak events, not detectable in the individual seismogram, since the reverse modelling sums the energy of all recorded traces and, therefore, enhances the signal-to-noise ratio similar to stacking in seismic exploration. The method is demonstrated by 2-D and 3-D numerical case studies, which show the potential of the technique. Events with a S/N ratio smaller than 1 where the events cannot be identified in the individual seismogram of the network are localized very well by the method.