ABSTRACT
Traditional ray-based methods for microseismic event localization require picking of P- and S-wave first arrivals, which is often time consuming. Polarization analysis for each event is often also needed to determine its absolute location. Location methods based on reverse time extrapolation avoid the need for first-arrival time picking. Traditional reverse time extrapolation only incorporates particle velocity or displacement wavefields. This is an incomplete approximation of the acoustic representation theorem, which leads to artifacts in the back-propagation process. For instance, if the incomplete approximation is used for microseismic event locations using three-component (3C) borehole recordings, it produces a ghost event on the opposite side of the well, which leads to ambiguous interpretations. We have developed representation-theorem-based reverse time extrapolation for microseismic event localization, combining the 3C particle velocities (displacements) and the pressure wavefield. The unwanted ghost location is removed by explicitly incorporating a wavefield and its spatial derivative. Moreover, polarization analysis is not needed, because wavefields will focus at its absolute location during back propagation. Determination of microseismic event locations using wavefield extrapolation also necessitates a robust focusing criterion. The Hough transform allows for accurate determination of source timing and location by summing wavefront energy in the time-space domain. Synthetic examples demonstrated the good performance of the wavefield extrapolation scheme and focusing criterion in complex velocity fields for borehole acquisition geometries.
