Velocity calibration by incremental pseudo-master events for single-well microseismic monitoring

The velocity model plays a critical role in accurately determining microseismic event locations during hydraulic fracturing. Conventional velocity optimization methods, such as updating the velocity model using perforation shots, have the drawback of weak ray coverage during single-well monitoring. The microseismic events distributed in different areas can broaden the ray coverage and help improve the accuracy of the velocity model, whereas only a few perforation shots are available for a given stage. In this study, we propose an incremental pseudo-master (IPM) method to reduce the velocity errors. The master event location technique was used to determine the positions of representative events in each iteration of the IPM method. These events then act as new pseudo-masters to further optimize the velocity model using Bayesian inference. With progressive iterations, the number of pseudo-master events increases and their distribution range expands. The accuracy of the velocity model continuously improved, which meant that the final IPM based velocity model, generated based on the pseudo-masters distributed throughout the study area, more accurately located all microseismic events than the perforation shot alone method. One synthetic example and one field test were conducted, and the results demonstrated that the IPM method could overcome the drawbacks of weak ray coverage, and improved the accuracy of the inverted velocity model and event locations.