Microseismic event location using an inverse method of joint P-S phase arrival difference and P-wave arrival difference in a borehole system

The accuracy of hypocenter location is the essential issue for microseismic monitoring, and is the basis for evaluating the effect of fracture. Although the signal obtained from a borehole monitoring system has a higher signal to noise ratio (SNR) than the surface system, a narrow monitoring aperture makes the location sensitive to noise and tends to be a misguided shape. In order to overcome this disadvantage and obtain a more accurate estimation of the source, we develop a 'jointing method', which combines the P-S phase arrival difference and P-wave arrival difference of each receiver pair (PSP) in the objective function. In the synthetic example, we compare the noise responses of three different location methods which are based on P-wave arrival time difference, P-S wave arrival time difference and the PSP method, respectively. This analysis shows that the P-wave arrival difference method is more sensitive to arrival time error than the others and the location results tend to be in a misleading line directed to the receivers. The P-S arrival difference method is more robust than the method using P-wave and its error distribution is perpendicular to the ray-path direction. The PSP method, as expected, is the most stable and accurate. When the P-S method and PSP method are applied to field data of a coal bed methane hydro-fracture process monitoring, the results indicate that the PSP method is preferable. The successful location with the PSP method proves that it is suitable for field data.