Seafloor depth controls seismograph orientation uncertainty
Seafloor depth controls seismograph orientation uncertainty
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海底深度控制着地震仪方向的不确定性
DOI：
10.1093/gji/ggac397
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发表时间：
2022-10
影响因子：
2.8
通讯作者：
Y. Sawaki;Y. Yamashita;Shukei Ohyanagi;E. Garcia;A. Ito;H. Sugioka;Tsutomu Takahashi;M. Shinohara;Y. Ito
中科院分区：
文献类型：
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作者：
Y. Sawaki;Y. Yamashita;Shukei Ohyanagi;E. Garcia;A. Ito;H. Sugioka;Tsutomu Takahashi;M. Shinohara;Y. Ito
本研究基于海底地震仪水平传感器方位的校正分析，评估随水深变化的海底环境噪声环境。由于海底地震仪主要部署为“自由落体”装置，因此我们没有关于水平传感器在海底面向哪个方向的信息。准确的传感器定向对于基于地震波场的数据处理至关重要。在使用无源源校正水平传感器方位角的几种地震学方法中，远震瑞利波的质点运动被广泛用于宽带海底地震仪。我们在南海俯冲带西端的日向滩地区进行了海底地震观测，并部署了宽带和短周期地震仪。然而，尚未有研究探讨通过瑞利波偏振方法进行的定向校正对于短周期数据是否有效。根据我们的活动观测数据，瑞利波方法的结果表明，短周期传感器方向的估计不确定性随着水深的减小而增加；我们观察到不确定性的转变深度为 2200-2600 米。测量质量，即径向分量和希尔伯特变换垂直分量之间的互相关系数，在深度浅于 2000 米时也会下降。此外，对噪声功率谱密度的分析表明，长时间（> 10秒）的环境噪声水平随着深度的减小而增加。次重力波控制垂直长周期噪声水平，而洋流主导水平长周期噪声；这两者都减少了作为环境噪声函数的瑞利波信号。次重力波也可能扭曲瑞利波形。这两种机制都导致了浅层站（即 < 2000 m）方向不确定性的突然上升和低测量质量。我们证实，方向不确定度随水深的变化可以作为海底环境噪声环境的指标。
This study evaluates the seafloor ambient noise environment that varies with the water depth based on a correction analysis of the horizontal sensor orientation for ocean-bottom seismographs. As ocean-bottom seismographs are mainly deployed as ‘free-fall’ installations, we have no information on which direction a horizontal sensor faces at the seafloor. An accurate sensor orientation is crucial for data processing based on seismic wavefields. Among several seismological approaches that use passive sources to correct the horizontal sensor azimuth, the particle motion of teleseismic Rayleigh waves is widely used for broadband ocean-bottom seismographs. We performed seafloor seismic observations in the Hyuga-nada region at the western end of the Nankai subduction zone and deployed broadband and short-period seismographs. However, studies have yet to investigate whether orientation correction via the Rayleigh-wave polarization method is valid for short-period data. The results of the Rayleigh wave method from our campaign observation data showed that the estimation uncertainty of short-period sensor orientations increased with a decreasing water depth; we observed a transition depth for the uncertainty at 2200–2600 m. The measurement quality, or the cross-correlation coefficient between the radial and Hilbert-transformed vertical components, also decreased at depths shallower than 2000 m. Moreover, an analysis of the noise power spectral densities showed that ambient noise levels during long periods (> 10 s) increased with decreasing depth. Infragravity waves controlled vertical long-period noise levels, while ocean currents dominated horizontal long-period noise; both of these reduced the Rayleigh-wave signals as a function of environmental noise. Infragravity waves also likely distorted the Rayleigh waveforms. Both mechanisms contributed to the sudden rise in orientation uncertainty and low measurement quality at shallow stations (i.e. < 2000 m). We confirmed that the variation in orientation uncertainty with the water depth can be used as an index for the ambient noise environment of the seafloor.