I-Worms

One of the most difficult tasks of operating a seismic net- work is the verification of data quality. In particular, the accuracy the relative travel time measurements between seismic stations are crucial for tomographic experiments and event localization. With the recent development of larger and denser temporary networks in geophysics and seismology, array processing techniques imply the use of frequency-dependent phase matching algorithms for which a perfect array synchronization is also mandatory. Clock synchronization among the instruments constituting an array is, however, difficult without a direct communication between them. On land, data logger clocks can be ensured by frequent synchronization with GPS satellites acting as a highly accurate, external reference clock and time errors result mainly from changes to the electronic equip- ment at particular stations. Recent works also suggest that apparent clock jumps are caused by missing samples that correspond to cases when the data logger fail to write a short chunk of samples to disk. Marine seismology also relies on temporary deployments of stand- alone seismic ocean bottom seismometers (OBS). For typical OBS deployments, the problem is even worse because the clocks are only synchronized to GPS time before and after deployments, which can last up to several years. In between, the recorder clocks may drift and float at unknown rates. If the clock drifts are large or not linear and cannot be corrected for, array seismological methods cannot be applicable.