On continental margins, sediments cause significant and spatially variable delays in seismic phase arrival times. The strong impedance contrast of the sediment-bedrock interface causes P-wave splitting that is clearly seen on distributed acoustic sensing recordings of earthquakes, resulting in additional phase arrivals that must be picked separately. We introduce sediment corrections to correctly account for those additional phases in the hypocenter localization procedure. Conceptually, the sediment correction method differs from the commonly-used station corrections; instead of introducing a mathematically optimal constant time delay for each station and each phase, the corrections are derived from a physical, first-order modeling of the wave propagation in the sediments. To calibrate the sediment corrections, a two-step procedure is adopted: (a) the delay between the P-phase and the converted Ps-phase is taken as a proxy of the sediment thickness; (b) the P- and S-wave speeds are determined through inversion. We show that sediment corrections are able to account for most of the observed bias while considerably reducing the number of free parameters compared to classical station correction. Moreover, the observed local delays are almost fully explained by the presence of the sedimentary layer, rather than by the 3D velocity variations of the bedrock. We retrieve $v_{\mathrm{P}}$ and $v_{\mathrm{S}}$ values that are compatible with values commonly found for sediments. Given the simplicity and physical foundation of the proposed method, we recommend the use of sediment corrections over station corrections whenever significant P-wave splitting can be observed.

中文翻译：

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用于分布式声学传感的沉积物校正


在大陆边缘，沉积物会导致地震阶段到达时间的显著且空间可变的延迟。沉积物-基岩界面的强阻抗对比会导致 P 波分裂，这在地震的分布式声学传感记录中可以清楚地看到，从而导致必须单独选择额外的相位到达。我们引入了沉积物校正，以正确解释震源定位过程中的那些额外阶段。从概念上讲，沉积物校正方法不同于常用的测站校正;校正不是为每个测站和每个相位引入数学上最优的恒定时间延迟，而是从沉积物中波传播的物理一阶建模中得出的。为了校准沉积物校正，采用两步程序：（a） 将 P 期和转换后的 Ps 期之间的延迟作为沉积物厚度的代表;（b） P 波和 S 波速度是通过反演确定的。我们表明，与传统站点校正相比，沉积物校正能够解释大部分观察到的偏差，同时大大减少了自由参数的数量。此外，观测到的局部延迟几乎完全是由沉积层的存在来解释的，而不是由基岩的三维速度变化来解释的。我们检索了 $v_{\mathrm{P}}$ 和 $v_{\mathrm{S}}$ 值，它们与沉积物的常见值兼容。 鉴于所提出的方法的简单性和物理基础，我们建议在可以观察到明显的 P 波分裂时使用沉积物校正而不是站校正。