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Hybrid method: high-resolution imaging in lower mantle region

In the deep interior of Earth, the core-mantle boundary (CMB) at the depth of ∼ 2890 km, separates turbulent flow of liquid metals in the outer core from slowly convecting, highly viscous mantle silicates. The lowermost mantle, extending several hundred kilometres above the CMB, is of considerable interest because it includes the boundary layer of thermochemical mantle convection. Core specific, the D” region in the lowermost mantle, whose top is detected as 150 - 300 km above the CMB, exhibits the heterogeneity. The complexity of D” has been mostly attributed to the phase transition from perovskite (Pv) to postperovskite (pPv) in the dominant mantle silicate, and this association offers new opportunities for estimation of (local) temperatures above and heat flux across the CMB. However, lowermost mantle structure remains enigmatic, and in recent years many promising methods are developed for better imaging of lowermost mantle. With constraints provided by seismic imaging, it paves the path to better understand the evolution of Earth in terms of geochemistry and geodynamics. Nevertheless, advanced techniques are still needed to obtain better result with less limitation. Full-waveform Inversion (FWI) is such a technique, which can be used for scatter/coda wavefield imaging, and then illuminate small heterogeneous structure in dramatic high resolution. Spectral Element Method (SEM) is one of the most popular solvers for wavefield simulation which is involved in FWI. However, the bottleneck of this implementation to image lowermost mantle is that prominent since computation cost will be considerably expensive for 3D global model. A possibility is to treat this problem into two domains, i.e., ‘simple’ global domain and complicated local domain as our imaging target. This so-called hybrid method has been proposed and tested for subsurface imaging in synthetic model. But application to real teleseismic data and extension to deep mantle region remains to be explored.