Evaluating and correcting short-term clock drift in data from temporary
seismic deployments
Abstract
Temporary seismic network deployments are quite common both in land and
offshore. The acquired data have significantly helped improve our
understanding of earthquake processes and internal structure of the
Earth. However, some temporary stations, especially these all-in-one
units without external GPS timing system, suffer from incorrect timing
record and thus pose a challenge to fully utilize the valuable data. To
inspect and fix such time problems, ambient noise cross-correlation
function (NCCF) is widely adopted by using daily waveforms. However, it
is difficult to identify short-term time drift after stacking the NCCF
output for several days to months. To detect such clock errors, travel
times of local and distant earthquakes are utilized along with NCCF. We
apply such a strategy on an Ocean Bottom Seismograph (OBS) dataset from
southern Mariana subduction zone and a dataset from a temporary dense
network from Weiyuan shale gas field, Sichuan, China. By inspecting
travel times from local and distant events, we identify a very
short-term clock drift (~25 sec) on the OBS data that
was not detectable using NCCF only. To overcome the problem, short
segments (3, 6, 12 hours) of daily wavefrom data is inspected as clock
errors become stable within the selected segments. In addition, the data
quality is carefully inspected with impact of different interstation
distance and period band on NCCF. In particular, we find that the 6-hour
segment with a period band of 2-5 sec is able to detect and correct
short term changes, including linear drift. For the dense array data, we
observe that NCCF symmetry is well-preserved for short interstation
distance (within 1 km) but becomes distorted for larger interstation
distances. Therefore, we split our dense array (79 stations) into 16
groups with a maximum interstation distance of 500 meters and 1-2 sec
period band was selected after testing. Short data segments improve the
time-drift detection efficiency in NCCF results, which is consistent for
both local and distant events. In a nutshell, the carefull selection of
data length and NCCF parameters can be helpful to identify and correct
the time drift errors of temporary seismic stations.