Microseismic Elastic Time-reversal Imaging For Single Monitoring Well

Seismic source location is very important in both earthquake and exploration seismology.Traditional techniques are based on the fact that the arrival time can be picked on the recorded seismic data.Some events with low signal-noise ratios are very difficult to be identified,so we could pick different wave modes' arrival times manually.The Time-Reversal Imaging(TRI)method utilizes the wave equation to back-propagate recorded seismic data to locate the passive seismic source.Compared to diffraction stack value method based on ray theory,it can locate seismic events in complex models.Time-reversal imaging method based on acoustic scalar wave does not utilize time difference between P wave and S wave,thus the uncertainty of location results could be very large.So we carry our research on time-reversal imaging based on elastic wave.The procedure of elastic TRI method is consisted of these two follow steps:the first step is to take the seismograms as the boundary condition to calculate the elastic wave equation,the reconstructed wavefield can focus on the seismic source location automatically at onset time during time-reversal modelling and separate wavefield during time-reversal modeling during time-reversal modeling;the second step is to apply a proper imaging condition to highlight the seismic source.In this paper,our research could be divided into the following two parts:In part one,we first compare the effect of different wave modes decomposition methods and apply decouple method which is able to remain the vector and amplitude information in our research.Then we combine P and S wave modes decomposition method and interferometry technology to propose PS interferometric cross correlation imaging condition to solve pseudo radiation pattern in traditional methodology.In this paper we compare our method with traditional interferometric imaging condition without separating P and S wave modes,the diffraction stack value imagingmethod based on ray theory and acoustic TRI method to illustrate our method's advantage.This paper discuss the effect of sparse and non-uniform acquisition geometry,velocity errors to location result and analyze the ability to distinguish overlapping events.In part two,we apply elastic TRI method to microseismic events location under single well acquisition geometry.For the case that the single well acquisition geometry only record seismograms through one,the application of elastic TRI method could not solve seismic source azimuth problem.Our paper analyze the the character of different wave modes.Our paper analyze the characters of different wave modes to propose a novel procedure,which is to determine the depth and radius distance through the cross correlation information of different wave modes,then find the azimuth using decomposed P wave modes in imaging process.Compared to traditional grid search and calculate the polarization of P wave mode,our methodology is able to avoid the uncertainty when we need to pick the arrival times manually and 180 degree uncertainty in azimuth information.We discuss the sensitivity to data noises and the application under various deployment of monitoring well.