Array Science for the IRIS Community Wavefields Experiment

穿过地球的地震波提供了关于产生它们的震源以及它们所经过的地壳和地幔的物理性质的重要信息。地震波通常是由放置在地球表面一个地点的地震仪器记录下来的，这样记录下来的地震波运动可以随着时间的变化而变化。地震阵列是一组地震仪器的集合，按照一定的模式排列，这样就可以在空间和时间上对地震波进行成像。这些额外的信息对于解决油气勘探、震源物理、爆炸探测、地震危险性评估以及地壳和地幔结构的科学研究等许多实际问题非常重要。该项目的研究重点是研究新的阵列几何形状和数据分析方法，这些方法是由地震学联合研究机构（IRIS）联盟在俄克拉何马州北部部署的一个科学社区实验所实现的。为了验证新的阵列方法，在地震活动活跃的地区部署了几个实验阵列来收集地震数据。预期的结果对于未来阵列的设计和对俄克拉何马州相对较新的地震活动的理解将是重要的。此外，研究结果将被纳入夏季举办的IRIS数据分析和处理短期课程。IRIS联盟于2016年6月22日至2016年11月底在俄克拉何马州北部一个假定诱发地震活跃的地区进行了一项社区计划的实验。本实验的科学目标包括：密集记录小到中度诱发地震以了解其震源过程，部署实验阵列以检查某些阵列几何形状和方法在确定局部和区域地震活动的波属性（例如，水平相速度，传播方位角）方面的实用性，在两个5公里和一个12公里长的剖面上进行密集记录以探测地壳结构，并使用局部次声阵列来区分地震信号中看到的波源。目前的项目是关于PI-在测试两个实验阵列设计的实验中的贡献。其中包括一个由“Golay 3x6”空间几何结构定义的相控、频率波数阵列，3x6几何结构由18个6公里孔径（3x6几何结构）的宽带站组成，以及一个独立的7级、巢式梯度仪，该梯度仪由112个三分量工业节点地震仪组成，部署在一侧800米的正方形上。这两个阵列的部署提供了一个前所未有的机会，可以使用产生波场的相同震源来严格测试梯度仪与相控阵的性能。对于小得多的梯度仪来说，一个积极的结果将允许在难以进入或安全性差的野外条件下部署阵列。测试“Golay 3x6”阵列性能的目的是检查其稳定性作为频率的函数和由于现场选址要求而导致的阵列响应退化。波场采样的巨大冗余将允许测试计算空间波梯度的方法，并处理由于地点、安装和仪器响应因素而引起的振幅静力学问题。除了评估阵列外，地震数据和波属性将用于研究局部和区域波传播，并将用于培训年轻科学家的阵列处理技术。该项目将资助一名研究生。

Seismic waves that move through the Earth give critical information about the earthquake sources that create them and about the physical nature of the crust and mantle that they move through. Seismic waves are generally recorded by a seismic instrument situated at a single spot on Earth's surface so that wave motion is recorded can be seen as time changes. A seismic array is a collection of seismic instruments arranged in a pattern such that waves can be imaged over space as well as time. This additional information is important to the solution of many practical problems in hydrocarbon exploration, earthquake source physics, explosion detection, earthquake hazards assessments, and scientific studies of the structure of Earth's crust and mantle. Research in this project is centered on the study of new array geometries and data analysis methods that have been made possible by the deployment of a scientific-community experiment in Northern Oklahoma by the Incorporated Research Institutions in Seismology (IRIS) consortium. Several experimental arrays were deployed over an area of active earthquake activity to collect seismic data in order to validate the new array methods. Expected results will be important in the design of future arrays and to the understanding of the relatively new Oklahoma seismic activity. In addition, the results will be incorporated into the IRIS Data Analysis and Processing short course held during the summer.The IRIS consortium deployed a community-planned experiment in northern Oklahoma in an area of presumed active induced seismicity from June 22, 2016 until the end of November 2016. Scientific goals for this experiment included: densely recording small to moderate induced earthquakes to understand their source processes, deploying experimental arrays to examine the utility of certain array geometries and methods in determining wave attributes of local and regional seismicity (e.g., horizontal phase velocity, azimuth of propagation), dense recording over two 5- and one 12-km-long profiles to probe for crustal structure, and use of local infrasound arrays in discriminating wave sources seen in the seismic signals. The present project is concerned with the PI-'s contribution to the experiment in testing two experimental array designs. These include a phased, frequency-wavenumber array in the defined by the 'Golay 3x6' spatial geometry 3x6 geometry consisting of 18 broadband stations with a 6 km aperture (3x6 geometry) and a separate 7-level, nested gradiometer consisting of 112 three-component industry nodal seismometers deployed over a square 800m on a side. Deployment of these two arrays offers an unprecedented opportunity to rigorously test the performance of a gradiometer against a phased array using the same earthquake sources that generate the wavefield. A positive result for the much smaller gradiometer would allow array deployments in areas with difficult field conditions such as inaccessibility or having poor security. Objectives for testing the performance of the "Golay 3x6' array are to examine its stability as a function of frequency and degradation of the array response due to field siting requirements. The great redundancy in sampling the wavefield will allow testing of methods to compute the spatial wave gradients and dealing with amplitude statics problems due to site, installation, and instrument response factors. In addition to evaluating the arrays, the seismic data and wave attributes will be used to study local and regional wave propagation and will serve to train young scientists on array processing techniques. The project will support a graduate student.