Automated seismic event location for hydrocarbon reservoirs
An automatic monitoring system has been developed to process continuously recorded microseismic data and locate the associated events. To this end we use P- and S-wave travel times and the direction of the incoming wave field. The processing is organised in four modules: (i) a multi-channel detection algorithm based on signal-to-noise ratios, (ii) a P-wave onset determination based on error prediction filtering with an auto-regressive model, (iii) a P-wave polarisation analysis providing the direction of the incoming wave field, and (iv) a rotation of seismic traces into the ray coordinate system with subsequent S-wave onset determination. For a homogeneous velocity model the event hypocenter is determined by a linearised inversion technique, and for a three-dimensional (3D) velocity model a directed grid search method or the neighbourhood algorithm is applied. We applied the monitoring system to a microseismic data set from the Ekofisk oil field in the North Sea and located about 2000 microseismic events close to the borehole receiver string. Most of the microseismic events occurred in clusters and in a depth range from 2800 to about 3050m depth. Location errors were estimated by bootstrapping, and a comparison between results from directed grid search and neighbourhood algorithm revealed a high level of consistency.
event location microseismicity inversion event detection ray tracing