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被称为地震波的东西来成像地球。室内。事实上，目前用于石油和天然气勘探的主要工具是地震方法，它利用地震波来成像地球。地震波的传播方式对于理解地震是如何发生的也至关重要。地震学学术界和油气行业用于描述地震波传播的标准模型做了两个简化的假设，将在本项目中进行测试：当地震波与S表面相互作用时，S表面就像一面镜子，并且(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传播的方向。众所周知，这两种假设都经常是错误的，但通过实验证明，要解开时间和方式是困难的。这是一个实验性的建议，以解决以前数据中的一些缺点。这个实验之所以成为可能，只是因为在过去的几年里，南达科他州铅市的桑福德地下实验室（Sanford Underground Laboratory）成为了一个独特的设施，它位于霍姆斯特克矿内。霍姆斯特克曾经是北美最深的金矿，但现在已被改造为地下科学设施。该项目建立在明尼苏达大学物理学家、加州理工学院和印第安纳大学地球物理学家现有合作的基础上。该小组目前正在操作一种独特的三维被动地震阵列，该阵列在地下和地面上都有传感器。这个项目通过添加一个组件来扩展那个项目。活跃的来源吗?数据采集。也就是说，当前阵列很像被动声纳阵列，监听瞬态（地震和采矿爆炸）并测量背景噪声场。这个项目的重点是使用受控的、人造的来源来补充听力模式。活动源数据将对过程的物理特性提供强大的附加约束，这是单独使用侦听模式无法实现的。在这个项目中，研究人员将收集三种类型的有源实验数据。(1)一种称为水平地震剖面（HSP）的新型实验几何。这是一种通常用于垂直井眼的油气勘探方法的变体，但在这种情况下，井眼是人类大小的，并且近似水平。(2)倒立反射测量。这很像石油和天然气工业中使用的反射剖面，但数据将在矿井漂移中收集，以研究波如何被自由表面反射。(3)是一个地面有源实验，他们将使用加速失重源将受控脉冲放入地面，由操作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。地震波是如何被地球散射的？地球物理学家通常称其为风化层。(2)现有的各向异性波传播理论模型和小于波长尺度的非均质性如何产生各向异性行为的相关模型有多好？风化层问题将通过倒立反射测量和热激反射实验直接解决，通过被动阵列粒子运动分析间接解决。活源数据将使用Kirchhoff积分正演建模方法进行建模，该方法使用高分辨率地形和Sanford地下实验室正在开发的3D地质模型。主动和被动数据的结合将产生可能是迄今为止最全面的单位体积数据集，以了解各向异性的尺度依赖性。一个关键的原因是，矿井收集的观测资料提供了出色的控制，目前桑福德地下实验室正在为研究界收集这些资料。HSP数据将提供不受自由表面效应影响的清晰信号。这些数据的各向异性参数将使用相速度测量来测量P和分裂S模式qS1和qS2。粒子运动方法将提供S波分裂的独立测量。表面源数据将提供传播速度的方位角依赖性的密集体积抽样。这些数据将用于测试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，还是需要被视为非均质各向异性介质。

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.