A microseism-based method to determine borehole seismic sensor orientation and its practical application

Borehole seismometers are widely installed to avoid ambient noises originating from the surface. However, the borehole seismometers suffer from ambiguity in the azimuthal orientation. It is important to determine the orientation of borehole seismometer instantly for prompt use of the borehole seismometer records. In this study, we propose a novel method for the instant determination of borehole seismometer orientation using microseisms that are ubiquitously present in the seismic records. The microseisms are dominantly composed of Rayleigh waves that construct plane wavefronts over local stations. We determine the azimuthal orientation of borehole seismometer from the difference in the apparent radial directions of Rayleigh waves in microseisms between the surface and borehole seismometers. The radial directions of Rayleigh waves are determined using a polarization analysis based on the correlation between the radial and phase-shifted vertical components of Rayleigh waves. Both primary and secondary microseisms can be used for the analysis. We apply the proposed method to determine the orientations of borehole seismometers in the Yonsei Earth Observatory in Seoul, South Korea. The borehole seismometer orientations are determined stably using 18-hour-long microseism records. Also, the orientations are consistently estimated while the microseism properties change with time. The determined borehole-seismometer orientations are verified by comparing regional earthquake waveforms at the collocated borehole and surface seismometers. The observation suggests that the proposed method allows stable and prompt determination of borehole seismometer orientation using short-time records in any time.