Determining temporal change of seismic properties inside the Earth using repeated earthquakes

我们建议建立理论框架，将差异波的传播概念化，定义为重复源之间的波场差异，并使用差异波场来有效地成像介质中特性的时间变化。我们还建议利用密集排列和地震对研究全球地震性质的可能的时间变化，并探索地震性质的时间变化与许多有趣的地质现象的关系，如地震过程、断裂带行为、冰川融化、火山喷发和地热活动。我们提出的概念化将使实验设计和分辨率分析变得简单，并克服当前确定介质中时间特性变化的方法中的许多具有挑战性的问题。这种概念化也使得精确定位和量化媒介内部的时间变化成为可能。在研究过程中，我们还将制定经验程序，以处理实际数据处理中震源重复性不理想的情况。利用地震或超声波探测介质中的时间变化是超声实验、土木工程、医学成像和地质科学中的一个重要课题。在地质科学中，对介质时间变化的探测不仅为地震成核、断裂带破坏与修复过程、火山活动、地壳应力聚集和地核顶部的热化学过程等地球物理现象提供了基本的物理基础，而且也成为监测油田演化和地下碳汇的实用有效的工具。

We propose to establish theoretical frameworks that conceptualize propagation of difference waves, defined as the wave field difference between the repeated sources, and use difference wave fields to effectively image temporal change of properties in the media. We also propose to study possible temporal changes of seismic properties globally using dense arrays and earthquake doublets, and to explore the relationship of temporal changes of seismic properties with many interesting geological phenomena, such as earthquake process, fault zone behavior, glacier melting, volcano eruption and geothermal activities. The conceptualization we propose would make experiment design and resolution analysis straightforward and overcome many challenging issues in the current methods for determining temporal property changes in the medium. The conceptualization also makes it possible to pinpoint and quantify temporal changes inside the medium. During the course of research, we will also formulate empirical procedures to handle the situation of imperfect source repeatability in the real data processing. Using seismic or ultrasonic waves to probe temporal change in the medium is an important topic in ultrasound experiments, civil engineering, medical imaging, and geological sciences. In geological sciences, probing temporal medium change has not only started to provide fundamental insights into physics of many geophysical phenomena, such as earthquake nucleation, fault zone damage and healing process, volcano activities, stress build-up in the crust and the thermo-chemical processes in the top of the Earth?s inner core, they have also become a practical and effective tool for monitoring evolution of oil fields and underground carbon sequestration.