Comparison of hypocenters and their uncertainties determined by three location methods: A case study on hydro-fracturing microseismic events recorded at a dense sub-surface array

Abstract:

The hypocenters of microseismic events induced by hydro-fracturing for developing shale gas provide important information on the characteristics of the fracture networks. This information is crucial to evaluate the production rate and also make a plan for efficient hydro-fracturing design. Therefore, the accurate hypocenter determination of microseismic events is very important from the economic point of view and the tolerable location error should be only dozens of meters for this purpose. In this study, we test three methods, which are widely used for locating earthquakes in seismology, to determine the hypocenters of microseismic events generated during the commercial shale gas development and recorded at a dense sub-surface array. We selected 1-hour time window and applied automatic picking algorithm to identify events and measure P and S arrival times of them. Our automatic picking algorithm identified 84 events in given time window and 73 of them are reliably located by all three methods. The first method is based on linear and iterative inversion and locates individual events separately. The second method simultaneously locates clustered events and reduces location errors due to incorrect velocity model by introducing the station terms. The third method locates only relative rather than absolute locations of clustered events using double difference algorithm. 1D P and S-wave velocity models are constructed based on well-logging data. Automatically measured P and S arrival times are used for determining absolute locations and relative arrival times measured by waveform cross-correlation are utilized for relative locations. Our preliminary result indicates that double difference method combined with waveform cross-correlations is more applicable than other two methods in that it satisfies the required accuracy for the microseismic monitoring.