Locating passive-source positions by using microseismic events is essential for monitoring hydraulic fractures. Among all microseismic source locating approaches, time-reversal imaging (TRI) is a promising one that is based on the principle that all of the back-propagated receiver wavefields should coincide at the source position when the velocity is accurate. It can image microseismic sources by applying a focusing imaging condition to the reconstructed receiver wavefields. However, the TRI method is highly sensitive to velocity errors and it is time-consuming or even challenging to refine the velocity model when the subsurface structure is complex. Instead of updating the velocity model, we have adopted a new method to locate microseismic events on a seismic reflection image under the condition that the seismic data are colocated with surface or near-surface microseismic observations. This method does not correctly place the microseismic events in depth; rather, it makes them consistent with the seismic reflection image; thus, it is still capable of providing correct local structure information around the passive source without the need of building an accurate velocity model. We have theoretically analyzed the variation of the imaged source location when the velocity model is inaccurate. Our result shows that faster velocities cause shallower depths whereas slower velocities cause deeper depths of source locations on the image. This is similar to the behavior of the focusing depth, but it is opposite to that of the migration depth in active-source seismic reflection imaging. Then, we match the locations of microseismic events with the well-focused seismic reflection image, which is extracted by slicing the time-shift common-image gathers at the time lag where the image has the maximal energy. Finally, synthetic tests validate the effectiveness of the proposed approach.

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