P-S Travel-Time Detection and Hypocenter Location of Low-SNR Events Using Polarization in the Time-Frequency - Frequency Domain

The detection of microseismic events with low signal-to-noise ratios (SNRs) can expand the seismic catalog and provide opportunities for a deeper understanding of subsurface reservoir features. We propose a novel polarization analysis method for comprehensively detecting S-wave arrival and P-S travel time of low-SNR events from the particle motion of P and S waves in the time and frequency domain. In most circumstances, the direct Swave particle motion shows a flat plane, and that is perpendicular to the direct P-wave motion direction. We combine these two properties to detect the S-wave arrival of lowSNR events. Our previous study applied spectral matrix (SPM) analysis to characterize the 3D particle motion of P waves. However, SPM analysis had limitations in detecting S-wave arrivals. We then introduce the time-delay components of the SPM (complex spectral matrix [cSPM]) to characterize the S-wave particle motion, separate the S-wave from the noise, and detect S-wave arrivals. Using the cSPM analysis method, we assess the planarity and perpendicularity of the S-wave polarization in the time and frequency domains. We then define a characteristic function that detects S-wave arrivals by combining two properties, planarity and perpendicularity, to detect more low-SNR events. The P-S travel time is obtained by setting the threshold values for the P- and S-wave characteristic functions. We apply our method to 4 hr and 2 months of field data recorded at the Groningen field in the Netherlands. Our method successfully detects the P-S travel time of all catalog events and several additional undetected events. We locate the hypocenter of all events using the detected P-S travel times with a grid-based search method.