Estimating the orientation of borehole seismometers from ambient seismic noise

Abstract

The orientation of the horizontal components of borehole seismometers is unknown upon installation but is crucial information for any multicomponent seismic analysis. Here, we use ambient seismic noise (ASN) to estimate the orientation of a network of 12 seismic stations monitoring the Auckland volcanic field (AVF) in New Zealand. Eight of these stations are installed at varying depths in boreholes, whereas the remaining instruments are three short-period and one broadband surface seismic stations. Despite the variations in the station hardware and configuration, the ellipsoidal wave particle motion of Rayleigh waves extracted from ambient noise correlations between stations proves robust. When the maximum correlation between the estimates of the radial-vertical component and the (90° phase shifted) vertical-vertical component of the Rayleigh-wave signal exceeds 0.3, orientation estimates of the network stations have an average standard deviation of 11°. This value compares well to the 13° standard deviation in estimates of the horizontal orientation of the seismometers, based on the polarization of P waves from up to 40 earthquakes in eight years of regional earthquakes. This proves that the ambient noise surface-wave method for station-orientation estimation can be especially useful for borehole stations in regions of low seismicity or as an independent estimate to complement estimates from the P-wave recording of earthquakes.