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

地震学家和勘探地球物理学家利用通常所说的地震波来成像地球？S内部。事实上，今天用于石油和天然气勘探的主要工具是地震法，它使用与声波相当的地震波来成像地球。地震波是如何传播的，对于理解地震是如何发生的也是至关重要的。地震学界和石油天然气行业用于描述地震波传播的标准模型提出了两个简化的假设，将在本项目中进行测试：(1)当地震波与地球相互作用时，地球？S表面就像一面镜子；(2)材料特性是各向同性的，这意味着控制地震波传播的物理特性不依赖于波的传播方向。众所周知，这两种假设经常是错误的，但事实证明，拆解时间和方式是很困难的。这是一项试验性提案，旨在解决之前数据中的一些缺陷。这项实验之所以可能，是因为过去几年中有一个独特的设施可供使用，该设施名为桑福德地下实验室，位于南达科他州的铅，位于霍姆斯塔克矿内。Homestake曾是北美最深的金矿，但现在已被改造成地下科学设施。该项目建立在明尼苏达大学物理学家与加州理工学院和印第安纳大学地球物理学家之间现有合作的基础上。该小组目前正在运营一种独特的三维被动地震阵列，在地下和地面都有传感器。该项目通过添加Active Source.组件来扩展该项目。数据采集。也就是说，当前的阵列很像被动声纳阵列，监听瞬变(地震和采矿爆炸)并测量背景噪声场。这个项目的核心是使用受控的、人为的资源来补充听力模式。有源源数据将对过程的物理提供强烈的附加约束，这在单独的监听模式下是不可能的。在这个项目中，研究人员将收集三种类型的有源源实验数据。(1)一种称为水平地震剖面(HSP)的新实验几何学。这是一种在石油和天然气勘探中常用的垂直钻孔方法的变体，但在这种情况下，钻孔是人工大小的，大致是水平的。(2)倒置反射测量。这很像石油和天然气行业中使用的反射剖面，但数据将在矿坑中收集，以研究波是如何被自由表面反射的。(3)是表面有源源实验，他们将使用加速重量下降源将受控脉冲放入地面，由运行中的无源阵列仪器记录。对这些数据的分析将集中于解决有关地震波传播性质的两个基本问题。(1)地震波是如何被地球散射的？S自由面和相关的复杂勘探地球物理学家通常称之为风化层？(2)现有的各向异性波传播理论模型和相关模型对于小于波长尺度的非均质性如何产生各向异性行为有多好？风化层问题将通过倒置反射测量和HSP实验直接解决，间接通过无源阵列粒子运动分析来解决。活跃的震源数据将使用高分辨率地形的基尔霍夫积分正演模拟方法和桑福德地下实验室正在开发的三维地质模型进行建模。组合的主动和被动数据将产生可能是有史以来最全面的单位体积数据集，以了解各向异性的尺度相关性。一个关键原因是矿场收集的观测数据提供了出色的控制，目前桑福德地下实验室正在为研究界收集这些数据。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和分裂的S模QS1和QS2的相速度测量来测量。粒子运动方法将为S波分裂提供一种独立的测量方法。面源数据将提供传播速度与方位角相关的密集体积采样。这些数据将被用来测试Homestake的整个岩石体积是否可以被描述为均匀的各向异性介质，或者是否需要被视为非均匀的各向异性介质。

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.