## 4 Results
### 2D Inversion Models  
Prior to performing a 2D inversion, a mean geoelectric strike of N45E direction was considered for the inversion models. Then, different inversions models were performed separately. Thus, inversion models of (TM): transverse magnetic field parallel to the structure or B polarisation, (TE): transverse electric field parallel to the structure or E polarisation, invariant model and (TE-TM): joint inversion of TE and TM modes, were run and data are rotated to a mean strike of N45E. The choice of geoelectric strike is not relevant for the derived apparent resistivity from the invariant impedance tensor as the quantity is invariant under any rotation angle and remain the same regardless of the geoelectric strike/rotation angle [26; 41]. Inversion code performing Occam philosophy with finite difference mesh was used. This code is Occam2D and is an open-source tool which is available on MTNet. The number of MT stations inverted is 14 as showed by the 2D profile across the rift in Figure 1. The width of mesh cell is fixed at 100 m where the number of total layers is 100. The initial model was set to a homogeneous half-space with a resistivity of 100 \(\Omega\)m. Another inversion model with an initial homogeneous half-space of resistivity of 10 \(\Omega\)m was tested. The best final 2D inversion that fitted the data with the lowest RMS misfit was selected, which corresponds to the initial model of homogeneous half-space of 100 \(\Omega\)m.  
The results of the two models (TM and invariant) exhibit similar patterns of subsurface resistivity (Figure 9). While the TM mode is considered stable and robust, the TE data are very sensitive to the choice of geoelectrical strike and to the 3D effects of galvanic and inductive origin [42]. However, inversion model of TE data is presented in Figure 9 and the result is in agreement with the TM and invariant models. And higher conductivity is observed at the center of the TE model at a depth of 2 to 5 km. At the same time, the TE model displays lower resistivity at the shallow depth than the TM and invariant models (Figure 9). The mechanism that could generate this difference will be discussed in the discussion (Section 5). Due to the restrictions or limit of available 2D anisotropic inversion codes, a 2D inversion for isotropic models was used and followed the practical approach developed by [40]. The latter authors proposed that a 2D isotropic inversion model could resolve well macro-anisotropy or structural anisotropy when join inversion of TE and TM data are rotated to the anisotropy strike that coincide with the geoelectric strike direction and when the anisotropy ratio is higher than five. Joint inversion of TE and TM data rotated to N45\({}^{\circ}\)E is presented in Figure 9 while another joint inversion of TE and TM data rotated to N120\({}^{\circ}\)E is presented in Appendix A. As evidenced, the joint inversion model of TE-TM recovers the anisotropy signature as vertical conductive zone that can be interpreted as conductive dyke as proposed by [40]. The resistivity pattern displayed in Figure 9 can be described as a succession of different subsurface resistivity from surface up to 10 km of the bottom model. The amplitude and phase of the observed impedance tensor data fit well with the model response (Figure 10). The dashed triangle indicates the position of the MT stations along the 2D profile (Figure 1). Figure 9: _Cont._  
Figure 9: Inversion models. (**A**) TM resistivity model, (**B**) Invariant resistivity model, (**C**) TE resistivity model and (**D**) Joint inversion model of TE-TM. Figure 10: Observed vs. modeled apparent resistivity and phase data. (**A**) for the TM data and (**B**) for TE\(-\)TM data. Near the surface, the high resistivity **R1** layer can be related to the dry volcanic deposit close to the surface or to volcanoclastic rock with low water content.