Monitoring changes of seismic properties at depth can provide a first-order insight into Earth’s dynamic evolution. Coda wave interferometry is the primary tool for this purpose. This technique exploits small changes of waveforms in the seismic coda and relates them to temporal variations of attenuation or velocity at depth. While most existing studies assume statistically homogeneous scattering strength in the lithosphere, geological observations suggest that this hypothesis may not be fulfilled in active tectonic or volcanic areas. In a numerical study we explore the impact of a non-uniform distribution of scattering strength on the spatio-temporal sensitivity of coda waves. Based on Monte Carlo simulation of the radiative transfer process, we calculate sensitivity kernels for three different observables, namely travel-time, decorrelation, and intensity. Our results demonstrate that laterally varying scattering properties can have a profound impact on the sensitivities of coda waves. Furthermore, we demonstrate that the knowledge of the mean intensity, specific intensity, and energy flux, governed by spatial variation of scattering strength, is key to understanding the decorrelation, travel-time, and scattering kernels, respectively. A number of previous works on coda wave sensitivity kernels neglect the directivity of energy fluxes by employing formulas extrapolated from the diffusion approximation. In this work, we demonstrate and visually illustrate the importance of the use of specific intensity for the travel-time and scattering kernels, in the context of volcanic and fault zone setting models. Our results let us foresee new applications of coda wave monitoring in environments of high scattering contrast.

中文翻译：

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横向变化散射特性对使用尾波敏感性内核进行地下监测的意义：在火山和断层带设置中的应用

监测深度地震特性的变化可以提供对地球动态演化的一级洞察。尾波干涉测量法是用于此目的的主要工具。该技术利用地震尾波中波形的微小变化，并将它们与深度衰减或速度的时间变化联系起来。虽然大多数现有研究假设岩石圈的散射强度在统计上是均匀的，但地质观测表明，这一假设在活动构造或火山区可能无法实现。在数值研究中，我们探讨了散射强度的非均匀分布对尾波时空敏感性的影响。基于辐射传输过程的蒙特卡罗模拟，我们计算了三种不同可观测值的灵敏度内核，即旅行时间、去相关、和强度。我们的结果表明，横向变化的散射特性会对尾波的灵敏度产生深远的影响。此外，我们证明了由散射强度的空间变化控制的平均强度、比强度和能量通量的知识，分别是理解去相关、旅行时间和散射内核的关键。许多关于尾波敏感性内核的​​先前工作通过使用从扩散近似外推的公式忽略了能量通量的方向性。在这项工作中，我们展示并直观地说明了在火山和断层带设置模型的背景下使用特定强度对走时和散射内核的重要性。