Collaborative Research: Relating Bulk Composition to Seismic Properties in Crustal Rocks

地球的大陆地壳是矿产和其他自然资源的储藏库。此外，地壳的化学成分控制其材料特性(例如，强度)，从而影响地壳在地质时间内变形和演化的方式。然而，虽然S地表的地壳组成可以通过直接采样来确定，但在深部的组成必须通过地球物理成像等间接方法来推断。最常见的地球物理成像方法之一是地震波，它在不同类型的岩石中以不同的速度传播。通过测量地震波速，可以使用一种类似于医学研究中的X射线计算机断层扫描(CT扫描)的方法来确定地壳的三维内部结构。在这个项目中，我们将采用地球-S大陆地壳的地震速度模型，并对它们进行反演，以推断地壳成分。这些成分模型将被用来确定地壳强度的变化，以及具有不同自然资源潜力的地区。在过去的几十年里，通过美国国家科学基金会的程序，如EarthScope、GeoPRISMS和Cascadia Initiative，可以获得丰富的大陆地壳地震数据，包括来自受控震源实验的共址P波数据，来自接收函数的VP/VS约束，以及来自环境噪声层析的S波数据。这些数据集为今后研究陆壳的性质和组成提供了基础。然而，尽管地震数据提供了丰富的信息，但缺乏岩石成分和地震速度之间的定量关系。在这个项目中，我们将开发一套新的工具，将地壳成分与地震速度联系起来。我们的方法分为四个步骤：(1)建立具有代表性的地壳成分空间数据库；(2)进行各种平衡P、T、H2O和fO2条件下各成分平衡矿物组合的热力学计算；(3)计算不同原位P、T条件下各相组合的Vp和VS；(4)利用所得到的数据集建立地壳成分与地震速度之间的正演关系。这些数据将汇编成一个公开可用的在线数据库，其他人可以使用该数据库推导出在特定地壳条件范围内主要元素化学与地震速度之间的关系。该项目将支持麻省理工学院/世界卫生组织联合项目的学生威廉·希尼瓦尔。为了最大限度地发挥成分反演对社区的效用，我们将开发一个基于Web的Java小程序，允许用户从他们自己的地震数据集中直接反演主要元素成分。

The Earth's continental crust is a reservoir for minerals and other natural resources. Moreover, the chemical composition of the crust controls its material properties (e.g., strength), and thus influences the way in which the crust deforms and evolves over geologic time. However, while crustal composition at the Earth?s surface can be determined by direct sampling, at depth composition must be inferred from indirect methods such as geophysical imaging. One of the most common geophysical imaging methods employs seismic waves, which travel at different speeds through different rock types. By measuring seismic wave speeds the 3-D internal structure of the crust can be determined using an approach similar to x-ray computed tomography (CT scan) in medical studies. In this project, we will take seismic velocity models for the Earth?s continental crust and invert them to infer crustal composition. These compositional models will then be used to determine variations in crustal strength, as well as regions with different natural resource potential.In the past decades a wealth of seismic data characterizing the continental crust have become available through NSF programs such as Earthscope, GeoPRISMS, and the Cascadia Initiative?including co-located P-wave data from controlled-source experiments, VP/VS constraints from receiver functions, and S-wave data from ambient noise tomography. These datasets provide the foundation for future research on the nature and composition of the continental crust. However, despite the abundance of information provided by the seismic data, a quantitative relationship between rock composition and seismic velocity is missing. In this project we will develop a new suite of tools to relate crustal composition to seismic velocity. Our approach is divided into four steps: (1) derive a representative database for crustal composition space, (2) perform thermodynamic calculations of the equilibrating mineral assemblages for each composition at various equilibrium P,T, H2O and fO2 conditions, (3) calculate VP and VS for each phase assemblage for different in situ P,T conditions, and (4) use the resulting dataset to establish forward relationships between crustal composition and seismic velocity. These data will be compiled into a publically available online database, which can be used by others to derive relationships between major element chemistry and seismic velocity under a specified range of crustal conditions. The project will support MIT/WHOI Joint Program student William Shinevar. To maximize the utility of the compositional inversion to the community we will develop a web-based Java applet to allow users to directly invert major element compositions from their own seismic datasets.