We relocate microseismic events from the Ekofisk reservoir in the North Sea that were recorded over an 18-day period in April 1997. A array of six 6-component sensors deployed in a vertical borehole with a 20 m spacing detected nearly 4500 events. Events are located using P- and S-wave travel times and azimuthal particle directions, which are calculated using a recently developed noise-weighted analytical-signal singular-value-decomposition method. A inherent 180 degree ambiguity in event locations exists with data recorded by sensors in a signal borehole and 1D velocity model. We remove this ambiguity using dip information estimated as part of the particle motion analysis. The relocation yields 1333 events, which are primarily concentrated in the producing part of the reservoir. In general, our location accuracy is 15m, but most events locate to within 10m. In comparison to the original event locations, the new locations show less scatter and cluster into more linear patterns, we interpret as fault structure.


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