Correction of OBS clock errors using Scholte waves retrieved from cross-correlating hydrophone recordings

Abstract

One of long-standing problems in underwater seismic studies is the inaccurate timing due to the fact the internal clock of Ocean Bottom Seismograph (OBS) is unable to synchronize with GPS. Here we present correcting large OBS clock errors and temporal drifts in a passivesource OBS array experiment in South China Sea by cross-correlating hydrophone recordings of OBS pairs. We show that, in this experiment, the noise cross-correlation function (NCCF) from hydrophone signals can retrieve higher signal-to-noise ratio (SNR) Scholte waves than the NCCF from seismometer. Because the hydrophone is positioned in the water above the seafloor, their NCCFs are thus less contaminated by more complicated solid-mode phases at the sediment-covered seafloor, leading to higher SNR for hydrophone's NCCFs. The relatively high SNR of Scholte waves enables us to use the daily NCCF (or stacked NCCFs of a few days) to constrain the temporal variations of the clock errors. A two-step approach is employed in this study to tackle large OBS clock errors: using predicted traveltimes of P phases from earthquakes to roughly correct the time, and then using the time asymmetry of two Scholte waves on NCCF to fine-tune the clock drifts. The uncertainty analysis indicates that the average error of our corrections is less than 0.2 s, suggesting the clock-corrected OBS data are valuable for seismic studies using surface waves and S waves.