## 1 Introduction  
Mantle plumes and subducting slabs can transport material upwards and downwards in the earth, and they play essential roles in Earth's convection and evolution [1, 2]. Although the plumes and slabs can be disclosed by geophysical observations [3, 4, 5, 6], their small scale compared with the mantle has hindered their direct observation. Plumes and slabs will react with the mantle transition zone (MTZ) when passing through it, particularly with the 410 km and 670 km phase transition interfaces. The 670 km discontinuity hinders the subduction of slabs, resulting in a stagnant slab along the interface, such as the stagnant Pacific Slab beneath Northeast Asia [6, 7, 8, 9], and it blocks the upwelling mantle plumes, resulting in the accumulation of plume materials and forming a broad head thousands of kilometers in diameter, such as the plume models established by [PERSON] [4, 5]. These reactions make the detection of MTZ significant in the search for the secrets of Earth. Geophysical methods, particularly seismic imaging, have been widely used to detect the structure of Earth [10, 11, 12]. Seismic imaging has shown strong evidence for mantle plumes and subducting slabs [6, 7, 11]. The observed heterogeneities of Earth based on seismic velocity variations may be caused by the thermochemical fluctuations associated with the underlying processes in the mantle, but a defined description has not yet been given. Obtaining additional information on the structure of Earth is an effective way to solve this challenge. The electrical conductivity of the mantle is strongly sensitive to variations of temperature, mineral composition, oxidation-deoxidation environment, extent of hydration, and melt state [13; 14; 15; 16], even more obviously than elasticity, which governs seismic velocity. Therefore, measuring the electrical conductivity of the mantle has great potential for mapping the chemical and physical structure of Earth and offers a complementary approach to tomography investigation. The deep electrical structure of Earth can be probed with geomagnetic depth sounding (GDS) [17; 18]. GDS has been used to detect variations in the MTZ beneath Australia [15] and East Asia [19; 20], demonstrating its advantages in the research of deep Earth. The first global-scale three-dimensional (3D) image of electrical conductivity variations in Earth's mantle was derived from long-term geomagnetic data through electromagnetic (EM) induction [3; 17; 21]. It demonstrates a heterogeneous MTZ and provides evidence for the influence of subducting slabs and mantle plumes [3; 17; 21]. In particular, beneath North Africa and Southern Europe, a large-scale resistive structure is found in the MTZ, which should be caused by less water, lower temperature, or changed mineral composition in the region compared with the normal MTZ [22]. However, little attention has been paid to conducting a detailed analysis of the origin of this structure. This could be attributed to the rough model obtained by global-scale inversion, which used a large grid interval of approximately \(10^{\circ}\) to parameterize Earth's electrical field and limits the cognition of structures on regional or local scales. Besides the African and European plates that meet in this area, the Calabria (CAL) and Hellenic (HEL) slabs have subducted into the deep mantle, as imaged by seismic tomography [23]. These subducting slabs cause a series of impacts on the mantle, such as reduced temperature and increased water content near the pathway of the slab, which can introduce a significant reduction in electrical conductivity [14; 24]. Therefore, it is necessary to undertake further analysis of the electrical structure of the area. In this paper, we will reprocess the geomagnetic data in North Africa and Southern Europe, especially regions in Southern Europe with dense observatories, and perform 3D EM inversion with a finer grid in order to obtain a more precise electrical structure of the mantle by applying the method we presented previously [3].