Collaborative Research: Understanding free-surface scattering in an anisotropic medium with active and passive seismic methods at the Homestake Mine, South Dakota

合作研究：在南达科他州 Homestake 矿使用主动和被动地震方法了解各向异性介质中的自由表面散射

• 批准号: 1526894
• 负责人: Gary Pavlis
• 金额: $ 18.1万
• 依托单位: Indiana University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-08-15 至 2018-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1526894&HistoricalAwards=false
• 关键词: Collaborative; Research; Understanding; free; surface

Seismologists and exploration geophysicists utilize what are commonly called seismic waves to image the Earth?s interior. In fact, the major tool used for oil and gas exploration today is the seismic method, which use seismic waves comparable to sound waves to image the earth. How seismic waves propagate is also of fundamental importance to understanding of how earthquakes work. The standard models used to describe seismic wave propagation in both the academic world of seismology and the oil and gas industry make two simplifying assumptions that will be tested in this project: (1) the Earth?s surface acts like a mirror when seismic waves interact with it, and (2) material properties are isotropic meaning physical properties that control seismic wave propagation are not dependent upon the direction a wave is propagating. Both assumptions are known to be frequently wrong, but unraveling the when and how has proven experimentally difficult. This is an experimental proposal to address some of the shortcomings in previous data. The experiment is possible only because of the availability of a unique facility that has become accessible in the past few years called the Sanford Underground Laboratory in Lead, South Dakota, that is located within the Homestake Mine. Homestake was once the deepest gold mine in North America, but has now been converted to a facility for underground science. The project builds on an existing collaboration between physicists at the University of Minnesota and geophysicists at the California Institute of Technology and Indiana University. That group is currently operating a unique three-dimensional passive seismic array with sensors in the underground and on the surface. This project extends that project by adding a component of ?active source? data acquisition. That is, the current array is much like a passive sonar array listening for transients (earthquakes and mining explosions) and measuring background noise fields. This project centers on the use of controlled, manmade sources to supplement the listening mode. The active source data will provide strong added constraints on the physics of the process not possible with the listening mode alone. In this project the reseachers will collect three types of active source experimental data. (1) A novel experimental geometry called Horizontal Seismic Profile (HSP). This is a variant of a method commonly used in oil and gas exploration with vertical boreholes, but in this case the borehole is human sized and approximately horizontal. (2) an upside down reflection survey. This is much like reflection profiling using in the oil and gas industry, but the data will be collected in a mine drift to study how waves are reflected by the free surface. (3) Is a surface active source experiment where they will use an accelerated weight drop source to put a controlled pulse into the ground to be recorded by the operational passive array instruments. Analysis of these data will center on addressing two fundamental questions about the nature of seismic wave propagation. (1) How are seismic waves scattered by Earth?s free surface and the related complexity exploration geophysicists commonly call the weathered layer? (2) How good are existing theoretical models of anisotropic wave propagation and related models for how heterogeneity at scales smaller than a wavelength yield anisotropic behavior? The weathered layer problem will be addressed directly by the upside down reflection survey and HSP experiments and indirectly by analysis of passive array particle motions. The active source data will be modeling using a Kirchhoff integration forward modeling method using high resolution topography and a 3D geologic model under development by Sanford Underground Laboratory. The combined active and passive data will yield what is likely the most comprehensive data set per unit volume ever assembled to understand the scale dependence of anisotropy. A critical reason is the outstanding control provided by the observations collected by the mine and now being assembled for the research community by the Sanford Underground Laboratory. The HSP data will provide clean signals isolated from the free surface effect. Anisotropic parameters from these data will be measured using phase velocity measurements for P and the split S modes qS1, and qS2. Particle motion methods will provide an independent measure of S wave splitting. The surface source data will provide a dense volume sampling of azimuth dependence of propagation speeds. These data will be used to test if the entire rock volume of Homestake can be described as a uniform anisotropic medium or needs to be treated as a heterogeneous, anisotropic medium.

地震学家和探索地球物理学家利用通常称为地震波的东西来形象地球的内部。实际上，今天用于石油和天然气勘探的主要工具是地震方法，它使用与声波相当的地震波来形象地球。地震波如何传播对于理解地震的工作方式也至关重要。用于描述地震学学术界和石油和天然气行业的地震波传播的标准模型做出了两个简化的假设，这些假设将在该项目中进行测试：（1）当地震波相互作用时，地球表面就像镜子一样借助它，（2）材料特性是各向同性含义的物理特性，即控制地震波传播的物理特性并不取决于波动传播的方向。已知这两个假设经常是错误的，但是可以揭示何时何时和如何证明实验上的困难。这是一个实验建议，可以解决以前数据中的某些缺点。 该实验之所以可能仅是因为在过去几年中可以使用独特的设施，该设施已被称为南达科他州铅的桑福德地下实验室，该实验室位于霍姆斯特矿山内。宅基地曾经是北美最深的金矿，但现在已转变为地下科学的设施。 该项目建立在明尼苏达大学的物理学家与加利福尼亚理工学院和印第安纳大学的地球物理学家之间的现有合作。该组目前正在操作一个独特的三维被动地震阵列，地下和表面上有传感器。该项目通过添加Active源的组件来扩展该项目？数据获取。 也就是说，当前的阵列就像一个被动声纳阵列聆听瞬态（地震和采矿爆炸）和测量背景噪声场。该项目集中在使用受控的人造资源来补充听力模式的基础上。仅在听力模式下，主动源数据将对该过程的物理物理物理学提供强大的添加约束。在这个项目中，研究人员将收集三种类型的主动源实验数据。 （1）一种新型的实验几何形状，称为水平地震谱（HSP）。 这是用垂直钻孔用于石油和天然气探索的方法的一种变体，但在这种情况下，钻孔是人尺寸且大致水平的。 （2）颠倒的反思调查。 这就像在石油和天然气行业中使用的反射分析，但是数据将在矿山漂移中收集，以研究自由表面如何反射波。 （3）是一个表面主动源实验，他们将使用加速的重量降源将受控的脉冲放入地面，以通过操作的被动阵列仪器记录。 对这些数据的分析将集中在解决有关地震波传播性质的两个基本问题上。 （1）地震波如何散布着地球的自由表面和相关的复杂性探索地球物理学家通常称为风化层？ （2）各向异性波传播的现有理论模型和相关模型的现有理论模型如何小于波长的量表如何产生各向异性行为？ 风化层问题将直接通过颠倒的反射调查和HSP实验来解决，并通过分析被动阵列粒子运动间接解决。 主动源数据将使用Kirchhoff集成向前建模方法进行建模，并使用高分辨率地形图和Sanford Underground Laboratory开发的3D地质模型进行建模。 合并的主动数据和被动数据将产生有史以来最全面的数据集，以了解各向异性的规模依赖性。一个关键的理由是矿山收集的观察结果提供的出色控制，现在由Sanford Underground Laboratory为研究社区组装。 HSP数据将提供与自由表面效应隔离的干净信号。这些数据的各向异性参数将使用P和Split S Modes QS1和QS2的相位速度测量值进行测量。粒子运动方法将提供对S波分裂的独立度量。 地表源数据将提供繁殖速度方位角依赖性的致密体积采样。这些数据将用于测试整个宅基地的岩石体积是否可以描述为均匀的各向异性培养基，或者需要将其视为一种异质，各向异性培养基。