One-dimensional Velocity Model Estimation And Migration-based Earthquake Location & Fracture Imaging For Microseismic Monitoring

For microseismic monitoring, in order to obtain accurate microseismic locations a good estimation of the velocity model for the region covering the monitoring stations and microseismic sources is crucial. Perforation shots are generally used to estimate a velocity model suitable for microseismic location. However, the origin times of the perforation shots are generally not accurate. To mitigate the effect of inaccurate origin times of perforation shots on calibrating the velocity model, we propose to search for a velocity model fitting for station-pair differential arrival times instead of absolute arrival times from perforation shots. Another advantage of using station-pair differential arrival times is that waveform cross-correlation can be used to estimate more accurate differential times because of waveform similarity among stations for perforation shots. Due to high nonlinearity of the objective function for estimating one-dimensional velocity model, the differential evolution (DE) method for solving high-dimensional global optimization problems is utilized in the optimization. Compared to the grid-search method, the DE method is much more efficient. Synthetic test based on a downhole microseismic monitoring system shows the effectiveness of the proposed method to recover the velocity model. We also test our proposed DE-based method using perforation shots for a real microseismic monitoring project. Compared to the well sonic velocity model, with the calibrated velocity model station-pair differential arrival times are better fitted and perforation shots are also more accurately relocated.Generally, the accurate 1D velocity model will play a crucial rule in locating microseismic events during the tracking processes. Traditional arrival time based earthquake location methods require accurate arrival time pickings, which is challenging in the case of low signal to noise ratios for microseismic signals. In addition, it is difficult to realize arrival based location in real time because arrival pickings are usually performed by analysts and automatic pickings have large uncertainties. Here we introduce a migration-based microseismic location method to deal with this problem. Compared with microseismic location results from manual pickings, our proposed method shows similar location accuracy, and is capable of characterizing fracture distribution and development induced during hydro-fracking.