Measuring of clock drift rates and static time offsets of ocean bottom stations by means of ambient noise
Measuring of clock drift rates and static time offsets of ocean bottom stations by means of ambient noise
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DOI：
10.1093/gji/ggt434
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发表时间：
2014-02-01
影响因子：
2.8
通讯作者：
Dahm, Torsten
中科院分区：
文献类型：
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作者：
Hannemann, Katrin;Krueger, Frank;Dahm, Torsten
海洋地震学通常依赖于独立的海底地震台站（OBS）的临时部署，这些台站在部署前在船上初始化和同步，并且在几个月后恢复后在船上重新同步和停止。在这两者之间，记录器时钟可能会以未知的速率漂移和浮动。如果时钟漂移很大或不是线性的，并且不能被校正，地震学的应用将限于不需要精确的公共定时的方法。因此，例如，阵列地震学方法，需要非常准确的定时之间的各个站，将不适用于这样的部署，我们使用的OBS测试阵列的12个站和75公里的孔径，部署了10个月的深海（4.5-5.5公里）的中东部大西洋。该实验旨在分析宽带阵列地震学在海底的潜力。恢复后，我们确定了一些站，要么显示不寻常的大时钟漂移和/或静态时间偏移的内部时钟和GPS信号之间的差异很大（skew）。我们测试的方法，环境噪声互相关同步时钟的深水OBS阵列与公里级站间距离。我们表明，小漂移率和静态时间偏移可以解决垂直分量与标准技术。较大的时钟漂移（每天几秒），只能准确地恢复，如果一个输入道的时间窗口，根据预期的互相关之前的站对之间的漂移移动。我们验证了从地震计数据中提取的漂移是线性的一阶。这对于大多数水听器都是有效的。此外，我们能够确定时钟漂移在一个站，没有偏斜可以测量。此外，我们发现，不稳定的视在某些水听器，这是不相关的地震检波器漂移记录在同一个数字化仪的漂移率，表明故障的水听器。
Marine seismology usually relies on temporary deployments of stand alone seismic ocean bottom stations (OBS), which are initialized and synchronized on ship before deployment and re-synchronized and stopped on ship after recovery several months later. 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, seismological applications will be limited to methods not requiring precise common timing. Therefore, for example, array seismological methods, which need very accurate timing between individual stations, would not be applicable for such deployments.We use an OBS test-array of 12 stations and 75 km aperture, deployed for 10 months in the deep sea (4.5-5.5 km) of the mid-eastern Atlantic. The experiment was designed to analyse the potential of broad-band array seismology at the seafloor. After recovery, we identified some stations which either show unusual large clock drifts and/or static time offsets by having a large difference between the internal clock and the GPS-signal (skew).We test the approach of ambient noise cross-correlation to synchronize clocks of a deep water OBS array with km-scale interstation distances. We show that small drift rates and static time offsets can be resolved on vertical components with a standard technique. Larger clock drifts (several seconds per day) can only be accurately recovered if time windows of one input trace are shifted according to the expected drift between a station pair before the cross-correlation. We validate that the drifts extracted from the seismometer data are linear to first order. The same is valid for most of the hydrophones. Moreover, we were able to determine the clock drift at a station where no skew could be measured. Furthermore, we find that instable apparent drift rates at some hydrophones, which are uncorrelated to the seismometer drift recorded at the same digitizer, indicate a malfunction of the hydrophone.