Determination of Borehole Seismic Sensor Orientation Using Microseisms

ABSTRACT We introduce a novel method to determine the azimuthal orientation of borehole seismometer using microseisms that are ubiquitously present in seismic records. The microseisms are dominantly composed of Rayleigh waves that construct plane wavefronts over local stations. We determine the borehole seismometer orientation by comparing the polarization directions of Rayleigh waves in microseisms between borehole and surface stations. The Rayleigh wave polarization directions are determined using the nature of Rayleigh wave ground motions that present a 90° phase difference between the radial and vertical components. The azimuthal differences in apparent Rayleigh wave polarization directions between borehole seismometer and local surface seismometer provide information on the misorientation angle of borehole seismometer. Both the primary and secondary microseisms can be used. The method is applied to determine the misorientation angles of two deep borehole seismometers in the Yonsei Earth Observatory in Seoul, South Korea. The misorientation angles of borehole seismometers are determined stably using 18-hour-long ambient noise records, suggesting instant determination of borehole seismometer orientation based on short-time ambient noise records. Regional-event seismic waveforms at borehole seismometers match well with those at collocated surface station, supporting the correct determination of borehole seismometer orientation. The proposed method does not require information on seismic source locations and ray paths unlike conventional methods, allowing prompt determination of borehole seismometer orientation with high precision.