Microseismic event location using the first and reflected arrivals

Abstract

Locating microseismic events accurately is essential to characterizing hydrofracked reservoirs. Several location approaches have been developed to solve this problem. We have developed a global grid search method based on the Bayesian probabilistic approach. In addition to retrieving the global minimum of the cost function, it enabled the computation of the uncertainty that was essential to quantify the quality of the results. The location uncertainty was tightly related to the amount and the quality of observed data used to perform the location. Using multiple arrivals increased the quantity of information collected in a cost-effective manner (no additional receivers were required). In this location algorithm, we have used the first and reflected arrivals to constrain the locations. To locate the events, it was essential to model the first and reflected arrivals given a velocity model and acquisition geometry. We have developed a ray-tracing algorithm capable of computing traveltimes and polarization of direct, refracted, and reflected waves in layered velocity models. We have applied this methodology for several examples. The synthetic data set was generated using full-waveform finite-difference modeling and recorded in a single vertical well. The first and reflected traveltimes were picked using the 3C seismograms. In the last example, we used the Cotton Valley real data set in which reflections could be seen for strong events due to the high velocity contrasts. The applications revealed that the location uncertainty was significantly reduced when using both arrivals compared with the case when only first arrivals were used. Combining the first and reflected arrivals even allowed us to improve the location accuracy when the velocity model was inaccurate.