Studies Of Location Problem For Microseismic Data With Low Signal-To-Noise Ratio

Microseismic monitoring plays an important role in the process of hydraulic fracturing for shale gas/oil production. The accuracy of event location is an essential issue in microseismic monitoring. Although the quality of recording data is better in bole hole observe system, but the narrow aperture angle made the locating processing more sensitive to the noise on the arrival time data. This research focus on two aspects, one is the effect of arrival time noise on the location uncertainty, and another one is how to use the recorded weak data to locate the low-energy event.We can use different information to locate the microseismic event, such as arrival time curve shape of single phase, S-P time difference, and cross S-P time difference. Usually, people deal this issue with adding different physical terms together. But sometimes jointing different physical terms by adding method is not the best method. Here, we develop a new method which joint different physical terms by multiplying them together. And from the synthetic example and analysis, we can see that this method is better than the previous method. We apply this method to field data analysis, the results indicates this method is preferable. And it should be used in real data processing.For low-energy microseismic events, however, P waves recorded in a downhole array may be very weak, while S waves are generally dominant and strong. Numerical experiments suggest that inverting S-wave arrival times alone is not sufficient to constrain event locations. In this study, we perform extensive location tests with various noise effects using a grid search method that matches the travel time data across a recording array. We find that fitting S-wave travel time data along with at least one P-wave travel time of the same event can significantly improve location accuracy. Besides, adding more than one P-wave arrival have much smaller effect to improvement of the accuracy than adding just one P-wave arrival. In practice, picking all S-wave arrival time data and at least one P-wave pick is possible for many small events. We demonstrate that fitting the combination of the travel time data is a robust approach, which can help increase the number of microseismic events to be located accurately during hydraulic fracturing.