Resolving the Degree of Chemical Heterogeneities in the Mantle by Dual Bootstrap Stack

在地球上，表层板块通过俯冲不断地循环进入深部地幔，但人们仍然很少了解这些俯冲板块被地幔对流混合和均化的速度有多快。尽管对地幔的化学状态有无数的猜测，但观测的限制是有限的。以前试图确定地幔小尺度非均质性的尝试依赖于相对较高幅度的散射波，这可以在叠加前从原始地震数据中识别出来。这种对信号强度的限制导致了我们探测能力的偏差，限制了我们在空间上广泛的特征，如横向不连续和倾斜反射层。最近，一种被称为双自举堆栈(DBS)的强大的信号检测技术被开发出来。DBS通过对信号重要性和一致性进行严格的统计测试，可以实现信号和噪声的清晰分离。与传统的非线性叠加不同，它即使在信噪比(S/N)低于1的情况下也能检测到信号，并且与简单的线性叠加相比，明确信号检测所需的迹线数量最多减少了两个数量级。拟议的项目旨在通过重新查阅公布的数据对这一新的信号探测技术进行系统测试，并更好地了解其对地幔化学状态产生令人信服的观测约束的潜力。更具体地说，我们将回顾Kaneshima和Helffrich[2010]的研究，他们报告了一项关于环太平洋地区中下地幔小规模不均一性的广泛研究。为此，将修改现有的基于DBS的迁移代码，以处理S到P的散射。他们使用的项目数量是：伊豆-博宁地区23个，千岛地区11个，斐济地区28个，南美洲12个。除了这项研究利用的S到P散射之外，我们还将寻找P到P散射的证据，如果时间允许，我们还将调查其他事件，看看基于DBS的远震迁移，探测到的散射体的重复性有多大。

On Earth, surface plates are continuously recycled into the deep mantle by subduction, but it is still very poorly understood how quickly these subducted plates are mixed and homogenized by mantle convection. Observational constraints are limited in spite of innumerable speculations on the chemical state of the mantle. Previous attempts to deterministically detect small-scale heterogeneities in the mantle have relied on scattered waves with relatively high amplitudes, which can be identified in original seismic data before stacking. This limitation on signal strength leads to a bias in our detection capability, restricting us to spatially extensive features such as lateral discontinuities and dipping reflectors. Recently, a powerful signal detection technique called dual bootstrap stack (DBS) has been developed. Built with rigorous statistical tests for signal significance and coherency, DBS can achieve a clean separation of signal and noise. Unlike conventional nonlinear stacks, it can detect signals even when the signal-to-noise ratio (S/N) is lower than unity, and compared to simple linear stack, the number of traces required for unambiguous signal detection is reduced by up to two orders of magnitude. The proposed project aims to conduct a systematic test of this new signal detection technique by revisiting published data and to better understand its potential for producing convincing observational constraints on the chemical state of the mantle. More specifically, we will revisit the study of Kaneshima and Helffrich [2010], who reported an extensive study on small-scale heterogeneities in the mid-lower mantle beneath the circum-Pacific area. The existing DBS-based migration code will be modified to handle S-to-P scattering for this purpose. The number of events they used is: 23 for the Izu-Bonin region, 11 for the Kuril region, 28 for the Fiji region, and 12 for the South America. In addition to S-to-P scattering, which was exploited by this previous study, we will also seek evidence for P-to-P scattering, and if time permits, we will investigate other events to see how reproducible detected scatterers are, in light of DBS-based teleseismic migration.