Using microseismic events to improve the accuracy of sensor orientation for downhole microseismic monitoring

Event locations are essential for microseismic monitoring endeavours to map hydraulic fractures. For downhole monitoring, the event back-azimuthal angle, which is commonly evaluated as the average of the P-wave polarization angle at each sensor, is necessary to project the 2D location in the vertical plane determined by the traveltime information to the 3D space. The accuracy of the P-wave polarization angle at each sensor depends on the signal-to-noise ratio of the waveforms, as well as relies on the accuracy of the sensor orientation. The conventional approaches of the sensor orientation require an event with known back-azimuth angle, such as surface orientation shots, perforation shots, ball-drop events or teleseismic events. In terms of availability, the signals of the perforation shots or ball-drop events are the most widely used information. But they are usually characterized by a low signal-to-noise ratio. We propose a new method to incorporate high-signal-to-noise ratio microseismic events with unknown back-azimuth angles into the sensor orientation process. It can significantly improve the accuracy of the relative orientation angles among all of the sensors, and also improve the accuracy of the absolute orientation angles by linking the relative orientation angles to the sensors with the high-signal-to-noise ratio waveforms of the perforation shots or ball-drop events. We use both synthetic and field data to test the feasibility and reliability of the new method. The results reveal that the new method can reduce the uncertainty of the orientation angle and improve the accuracy of the microseismic location, compared with the conventional approach when utilizing a perforation shot or ball-drop event alone.