Mid-lower mantle scatterers: Detection methods, research progress and prospect

Thanks to the rapid development of seismic wave propagation theory, emerging of data analyzing methods, and the increasing coverage of the seismic exploration, our ability to probe deep into the Earth has been increased from the scale of 100 kilometers to the scale of kilometers at present. The existence and wide-distributed heterogeneities at a scale of thousands of kilometers in the massive lower mantle have been revealed by seismic tomography for a long time, while the knowledge of smaller scales (~10～100 km) has come from high-frequency seismic scattered wave detection techniques that are mainly based on array analysis. A growing body of evidence shows that multiple-scale heterogeneities distribute through the Earth's entire lower mantle; the heterogeneities have been attributed to basaltic oceanic crust and its underlying lithospheric mantle. Therefore, detecting and characterizing of their distribution and formation mechanism will help understand the material composition, mineral phase transformation, and thermochemical structure of the Earth's interior. Furthermore, they shed light on the thermochemical and dynamic processes of the Earth's interior, such as mantle rheology, convection, and mixing efficiency. This review focuses on the small-scale heterogeneities/scatterers distributed in the mid-lower mantle at a depth of ~700～2000 km. Firstly, from the perspective of the definition of the scatterers and statistical description of small-scale heterogeneities, we introduce a variety of seismic waves, acting as the "probes" for the small-scale heterogeneities in the lower mantle, and then summarize the characteristics and limitations of available detection methods. Moreover, some representative studies are briefly reviewed. Based on the data of more than 200 lower mantle scatterers collected from previous publications, we characterize the distribution of scatterer's depths. Finally, we point out some unsolved problems in the detection method of lower mantle scatterers and propose the prospect for future research directions.