Orienting Ocean-Bottom Seismometers from P-wave and Rayleigh wave polarisations

We present two independent, automated methods for estimating the absolute horizontal misorientation of seismic sensors from their recorded data. We apply both methods to 44 free-fall ocean-bottom seismometers (OBS) of the RHUM-RUM experiment (http://www.rhum-rum.net/). The techniques measure the three-dimensional directions of particle motion of (1) P-waves and (2) Rayleigh waves of earthquake recordings. For P-waves, we used a principal component analysis to determine the directions of particle motions (polarisations) in multiple frequency passbands. We correct for polarisation deviations due to seismic anisotropy and dipping discontinuities using a simple fit equation, which yields significantly more accurate OBS orientations. For Rayleigh waves, we evaluated the degree of elliptical polarisation in the vertical plane in the time and frequency domain. The results obtained for the RUM-RHUM OBS stations differed, on average, by 3.1° and 3.7° between the methods, using circular mean and median statistics, which is within the methods’ estimate uncertainties. Using P-waves, we obtained orientation estimates for 31 ocean-bottom seismometers with an average uncertainty (95% confidence interval) of 11° per station. For 7 of these OBS, data coverage was sufficient to correct polarisation measurements for underlying seismic anisotropy and dipping discontinuities, improving their average orientation uncertainty from 11° to 6° per station. Using Rayleigh waves, we obtained misorientation estimates for 40 OBS, with an average uncertainty of 16° per station. The good agreement of results obtained using the two methods indicates that they should also be useful for detecting misorientations of terrestrial seismic stations.