Correction of nonlinear clock drift: the BRAVOSEIS Ocean-Bottom Seismometer Network in the Bransfield Strait, Antarctica

SUMMARY Ocean-bottom seismometers (OBSs) are reliable instruments to record ground motions and acoustic signals on the sea floor. Precise timing of the data is essential for most seismological analyses. The internal clocks of the OBSs are not global navigation satellite system (GNSS)-controlled, so the clock drift mainly caused by ageing of the quartz crystal and temperature effects must be corrected. As part of the BRAVOSEIS experiment, eight OBSs were deployed in the Antarctic Bransfield Strait for 13 months. All OBSs suffered from a large (−15.3 to 5.4 s) and nonlinear (−1.2 to −0.6 s residual to linear drift) clock drift. We used noise cross-correlations to determine the clock drift. The parameters for data pre- and post-processing such as filtering and normalization had to be selected carefully. Overlapping correlation windows were stacked to derive daily Green’s functions. The time-shift between consecutive days was calculated and distributed linearly over the data to obtain a continuous data set without gaps or overlaps. Airgun shots were used to constrain the cumulative clock drift of one station without initial synchronization. Two onshore stations served as GNSS-controlled reference for four OBSs in the northern part of the Central Bransfield Basin. A different noise regime prevailed in the southern part of the basin; therefore, two already corrected OBSs from the northern part were used as reference stations for the southern OBSs. In this way, the clock drift of all OBSs could be corrected accurately.