Clock errors in land and ocean bottom seismograms: high-accuracy estimates from multiple-component noise cross-correlations
Clock errors in land and ocean bottom seismograms: high-accuracy estimates from multiple-component noise cross-correlations
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DOI：
10.1093/gji/ggy236
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发表时间：
2018-09-01
影响因子：
2.8
通讯作者：
Hadziioannou, Celine
中科院分区：
文献类型：
--
作者：
Hable, Sarah;Sigloch, Karin;Hadziioannou, Celine
地震学中的许多应用依赖于地震图的精确绝对计时。然而，地震地面台站和海底地震仪（OBS）都可能受到时钟误差的影响，这会导致地震图的绝对定时偏离通常由GPS卫星提供的高度准确的参考时间信号。当与全球定位系统信号暂时或永久失去同步时，地面站可能会出现定时问题。由于环境条件的变化，这可能导致复杂的、与时间相关的时钟相对于GPS时间的漂移。海底的地震仪不能接收GPS卫星信号，但比陆地台站在更稳定的环境条件下工作。标准协议是在部署之前和恢复之后立即将OBS与GPS信号同步。被称为“偏斜”的所测量的定时偏差被假设为在部署间隔上线性地累积，这是一个看似合理但通常无法验证的假设。近年来，环境微震噪声的互相关已被用于校正定时误差，但限于站间距离至多几十公里，而不会降低时间分辨率。我们应用噪声互相关的时钟误差在4个宽带陆地站和53个宽带和宽带OBSs，这是安装在和周围的留尼汪岛在西印度洋的RHUM-RUM（留尼汪热点和上地幔留尼汪Unterer曼特尔）实验的评估。我们将所有三个地震分量，加上OBS台站中的水听器通道相关联。每日互相关函数推导出中间距离（类似于20公里）的陆地对陆地站对;稳定，10 d堆栈获得非常大的站间距离> 300公里的陆地到OBS和OBS到OBS的配置。平均多个站对，每个站多达16个组件对，提高了四个因素相比，以前的研究的单通道方法的准确性。我们的方法的定时精度估计是类似于20 ms的标准偏差或一个样本在50 Hz的采样率。在地面站，非线性时钟漂移和时钟跳到6分钟的检测和成功地纠正。对于53个OBS中的52个，我们成功地获得了随时间的漂移函数，这验证了线性时钟漂移的常见假设。这些OBS中有29个的偏斜值与我们在其观测误差条内的独立估计值一致。对于缺少偏斜测量的23个OBS，线性OBS时钟漂移范围在0.2和8.8 ms d（-1）之间。除了线性漂移外，三个OBS还受到类似于1 s的时钟跳变的影响，这可能表明存在丢失样本的问题，否则无法检测到。因此，我们证明了常规的可行性，高精度的时钟校正在陆地和OBS在广泛的站间距离。
Many applications in seismology rely on the accurate absolute timing of seismograms. However, both seismological land stations and ocean bottom seismometers (OBSs) can be affected by clock errors, which cause the absolute timing of seismograms to deviate from a highly accurate reference time signal, usually provided by GPS satellites. Timing problems can occur in land stations when synchronization with a GPS signal is temporarily or permanently lost. This can give rise to complicated, time-dependent clock drifts relative to GPS time, due to varying environmental conditions. Seismometers at the ocean bottom cannot receive GPS satellite signals, but operate in more stable ambient conditions than land stations. The standard protocol is to synchronize an OBS with a GPS signal immediately before deployment and after recovery. The measured timing deviation, called 'skew', is assumed to have accumulated linearly over the deployment interval, an assumption that is plausible but usually not verifiable. In recent years, cross-correlations of ambient microseismic noise have been put to use for correcting timing errors, but have been limited to interstation distances of at most a few tens of kilometres without reducing the temporal resolution. We apply noise cross-correlations to the evaluation of clock errors in four broad-band land stations and 53 wideband and broad-band OBSs, which were installed on and around the island of La Reunion in the western Indian Ocean during the RHUM-RUM (Reunion Hotspot and Upper Mantle-Reunions Unterer Mantel) experiment. We correlate all three seismic components, plus a hydrophone channel in OBS stations. Daily cross-correlation functions are derived for intermediate distances (similar to 20 km) for land-to-land station pairs; stable, 10 d stacks are obtained for very large interstation distances up to > 300 km for land-to-OBS and OBS-to-OBS configurations. Averaging over multiple station pairs, and up to 16 component pairs per station, improves the accuracy of the method by a factor of four compared to the single-channel approaches of prior studies. The timing accuracy of our method is estimated to be similar to 20 ms standard deviation or one sample at a sampling rate of 50 Hz. In land stations, nonlinear clock drifts and clock jumps of up to 6 min are detected and successfully corrected. For 52 out of 53 OBSs, we successfully obtain drift functions over time, which validate the common assumption of linear clock drift. Skew values that were available for 29 of these OBSs are consistent with our independent estimates within their observational error bars. For 23 OBSs that lacked skew measurements, linear OBS clock drifts range between 0.2 and 8.8 ms d(-1). In addition to linear drift, three OBSs are affected by clock jumps of similar to 1 s, probably indicating a missing sample problem that would otherwise have gone undetected. Thus we demonstrate the routine feasibility of high-accuracy clock corrections in land and OBSs over a wide range of interstation distances.