RUI: A laboratory study of ultrasonic scattering attenuation by possible microstructures in Earth's inner core

RUI：地球内核可能的微观结构对超声波散射衰减的实验室研究

• 批准号: 1619888
• 负责人: Michael Bergman
• 金额: $ 13.86万
• 依托单位: Simon's Rock of Bard College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-06-01 至 2019-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1619888&HistoricalAwards=false
• 关键词: RUI; laboratory; study; ultrasonic; scattering

Earth's inner core is elastically anisotropic, with seismic waves propagating parallel to the rotation axis about 3% faster than those parallel to the equatorial plane. The inner core also exhibits an attenuation anisotropy, with the faster waves having smaller amplitudes. There is now evidence that the pattern of both anisotropies is more complex, exhibiting depth dependence, hemispherical variations, and smaller scale regional variations. The isotropic seismic velocity may also exhibit regional variations. These puzzling seismic inferences are clues as to the structure and evolution of the solid iron alloy inner core. This grant will allow the PI to investigate the causes of inner core attenuation and its anisotropy, and thus help to better understand the most remote part of our planet. In particular, in laboratory studies the PI will use ultrasonic waves to probe metallic alloys with a variety of microstructures that have been suggested for the inner core, employing ratios of wavelengths to grain and sub-grain lengthscales that are relevant to the inner core. By comparison with seismic data, this will allow the PI to quantify the relative importance of scattering attenuation versus intrinsic attenuation (viscoelasticity). As an RUI grant, the project will involve diverse undergraduates in all aspects of the research, providing them with training and experience in doing science.Most explanations for the elastic anisotropy rely on a lattice preferred orientation (texturing) of hexagonal close-packed iron crystals, the most likely stable phase of iron under inner core conditions. Explanations for the texturing fall broadly into two classes, that due to solidification and that due to deformation. Directional solidification of metallic alloys typically results in columnar grains formed by primary and secondary dendrites of the primary compositional phase, with the secondary phase along grain boundaries and between dendrites, i.e., intragranular. Such microstructure has been proposed for the inner core, and scattering off grain boundaries of elongated crystals has been suggested as a cause for the attenuation anisotropy. Solidification microstructure is not thermodynamically stable, however, and annealing can result in coarsening of secondary dendrites, while maintaining the primary dendrites and columnar crystals, or possibly, in recrystallization and polygonal grain growth. The latter typically occurs in deformed materials exposed to high temperature. The PI will examine the ultrasonic scattering attenuation of three possible microstructures likely in Earth's inner core: directional solidified columnar grains composed of primary and secondary dendrites; directionally solidified and then annealed grains; and polygonal grains that result from recrystallization due to deformation and annealing. He will use Pb-Sn because of its simple eutectic phase diagram, ease of use, and relatively small single crystal elastic anisotropy; and microstructures with relative wavelength/scatterer dimensions thought to be similar to those in the inner core. The PI will use the shape and decay of the ultrasonic coda to determine the quality factor Q, and will compare the ultrasonic waveforms with seismic data to infer regional microstructure in the inner core, which will give insight into the evolution of the inner core.

地球内核具有弹性各向异性，平行于自转轴传播的地震波比平行于赤道面传播的地震波快约 3%。内核还表现出衰减各向异性，较快的波具有较小的振幅。现在有证据表明，两种各向异性的模式更加复杂，表现出深度依赖性、半球变化和较小规模的区域变化。各向同性地震速度也可能表现出区域变化。这些令人费解的地震推论是有关固体铁合金内核的结构和演化的线索。这笔赠款将使 PI 能够研究内核衰减及其各向异性的原因，从而有助于更好地了解地球最偏远的部分。特别是，在实验室研究中，PI 将使用超声波来探测具有各种已建议用于内核的微观结构的金属合金，采用与内核相关的波长与晶粒和亚晶粒长度尺度的比率。通过与地震数据进行比较，PI 可以量化散射衰减与固有衰减（粘弹性）的相对重要性。作为 RUI 的资助，该项目将让不同的本科生参与研究的各个方面，为他们提供科学方面的培训和经验。对弹性各向异性的大多数解释依赖于六方密堆积铁晶体的晶格择优取向（纹理） ，铁在内核条件下最可能的稳定相。对纹理的解释大致分为两类，一类是由于凝固引起的，一类是由于变形引起的。金属合金的定向凝固通常导致由主要成分相的初级和次级枝晶形成的柱状晶粒，其中次级相沿着晶界并且在枝晶之间，即晶内。这种微观结构已被提出用于内核，并且细长晶体的晶界散射被认为是衰减各向异性的原因。然而，凝固微观结构在热力学上不稳定，并且退火会导致二次枝晶粗化，同时保持初生枝晶和柱状晶体，或者可能导致再结晶和多边形晶粒生长。后者通常发生在暴露于高温的变形材料中。 PI将检查地球内核中可能存在的三种可能的微观结构的超声波散射衰减：由初级和次级枝晶组成的定向凝固柱状晶粒；定向凝固然后退火的晶粒；以及由变形和退火引起的再结晶产生的多边形晶粒。他会使用Pb-Sn，因为它的共晶相图简单，易于使用，而且单晶弹性各向异性相对较小；具有相对波长/散射体尺寸的微结构被认为与内核中的类似。 PI将利用超声波尾波的形状和衰减来确定品质因数Q，并将超声波波形与地震数据进行比较，以推断内核的区域微观结构，从而深入了解内核的演化。