Global 3D Imaging Of Electrical Structure In Mantle Transition Zone

The conductivity of mantle transition zone?MTZ?is very sensitive to temperature,water content and melting state,so the electrical structure obtained by geomagnetic depth sounding?GDS?method can make good constraint on the properties of MTZ,supporting the study of geodynamics in deep Earth.However,a significant obstacle in the study of GDS is that the distribution of the available geomagnetic stations is sparse,and the data quality of some stations is poor,which is not suitable for the traditional L2-norm inversion.In order to reduce the influence of poor quality data on the reliability of inversion results,and involve more stations in three-dimensional inversion,improve the resolution of GDS inversion,and obtain more fine three-dimensional electrical structure characteristics of the Earth,this paper presents and implements a three-dimensional inversion technique of GDS based on the L₁-norm measurement.C-response for 129 observatories at middle and low latitudes are obtained by BIRRP method with self-reference.The data quality of 65 stations is high,while the rest 64 stations with poor data quality.The L₁-norm three-dimensional inversion is carried out on the C-response of 129 stations with periods ranging from 3.5 to 113.8days.In the inversion calculation,the L₁-norm is used to measure the data misfit term of objective function in order to reduce the influence of poor quality data on the inversion results.The forward simulation is based on the staggered-mesh finite difference method in spherical coordinate system,and the limited-memory quasi-Newton method is used to solve object function.The global average one-dimensional electrical structure is chosen as the starting model,and conductivity jump is allowed at the discontinuous interface of 410 km,520 km and 670 km.So the electrical structure of the global MTZ and the upper part of lower mantle is obtained.The three-dimensional conductivity model of the mantle obtained by GDS L₁-norm inversion shows that the electrical properties with strong lateral heterogeneities in depth of 410¹600 km,among which the lower part of MTZ is the most obvious,and the variation of conductivity value between conductivity and resistivity anomalies is more than two magnitude.The inverted model shares many common features with published results,such as the enhanced resistivity in southern Europe and North Africa,high conductivity anomalies in northeastern China,and increased conductivity in southern Africa.Because more stations are used in our L1-norm inversion,the results have higher resolution than before.And some new characteristics are found in MTZ,such as:1)an obvious conductivity anomaly are discovered beneath Bermuda;2)the eastern part of Asia in MTZ is characterized by a number of independent conductivity anomalies rather than a holistic one,especially,obvious high conductivity anomaly centers occure in the central and northeastern of China;3)the conductivity of MTZ increase significantly in south of South America.Combining the electrical structure with petrophysical model,it is found that the causes of these high conductivity anomalies mentioned above are different in origin:1)Some anomalies are related to water circulation:with the relationship between conductivity and water content in MTZ,it is found that the water distribution is inhomogeneity,and a large amount of water is stored in the upper part of MTZ,while the dehydration process of stagnant plate mainly occurs in the lower part of MTZ;2)Some anomalies are related to thermal abnormalities:the high conductivity anomaly beneath Bermuda is an important discovery in this paper.Based on the results of high temperature and high pressure rock conductivity measurement in laboratory,the electrical structure of Bermuda area is analyzed,and compared with seismic imaging results in the region,the author believes that there may be a hot and upwelling flow originated from the lower mantle in Bermuda.