## 3 Results and Discussion
### Effect of Salinity Discontinuity Next to the borehole
#### 3.2.1 Wave Fields Simulation  
Figure 7 shows the normalized wavefields when the formation has a salinity difference across an interface. The interface is located at d = 1.5 m away from the borehole axis. The parameters used in this part are consistent with Table 1. The salinity of the inner porous medium is 0.01 Mol/L and the outer porous medium is 0.001 Mol/L. The placement of the source and the receivers is the same as Figure 1. \(u_{\theta}\) and \(E_{\theta}\) represent the displacement field and the electric field in the borehole respectively. Figure 7: Borehole SH-TE acousto-electric fields when the formation has a salinity difference across an interface. The solid line is \(E_{\theta}\), and the dashed line is \(u_{\theta}\). The components a-a and b-b are the electromagnetic wave interface responses generated at borehole wall and salinity difference interface respectively, and the component c-c is accompanying electric field. As shown in Figure 7, when there is a salinity difference interface near the borehole, the acoustic field is almost unaffected. Under this circumstance, the acoustic field still has only one set of components, namely the formation's SH wave. Unlike the previous simulation results, the electric field component becomes complicated. In addition to the accompanying electric field (component c-c in Figure 7) of the SH wave, there are two sets of components that arrive earlier (components a-a and b-b in Figure 7). These two components reach the receivers at different distances almost simultaneously, which indicates that they are the electromagnetic wave interface responses generated by the formation's sound wave impinging the interface of the medium. As discussed earlier, component a-a is the interface response produced by SH waves impinging the borehole wall. In order to further explain the generation cause of component b-b, we calculate the secant integral of the formation's electromagnetic wave branch point, and the interface converted electromagnetic waves are obtained. For a detailed discussion of this specific method readers are referred to [PERSON] et al. [54]. Figure 8 shows the simulation results. In order to distinguish them from the electric field full waves obtained by the real axis integral, we denote the electromagnetic wave interface response obtained by the secant integral as \(E_{\theta-cut}\). Figure 8a is the normalized waveforms of the interface electromagnetic wave, and Figure 8b is the waveform comparison between \(E_{\theta-cut}\) and \(E_{\theta}\), taking z = 3 m as an example. In addition, we also compare the waveforms of \(E_{\theta-cut}\) for different interface positions in Figure 8c. As shown in Figure 8a, the interface electromagnetic wave obtained by secant integral has only two groups of components, denoted as a-a and b-b. Unlike the case of the elastic  
Figure 8: Interface converted electromagnetic waves. (a) The normalized waveforms of \(E_{\theta-cut}\). (b) Comparison of \(E_{\theta}\) and \(E_{\theta-cut}\). (c) \(E_{\theta-cut}\) for different interface positions. The components a-a and b-b are same as in Figure 7.  
difference interface next to the borehole [53], the interface response for the electrical difference interface is relatively simple. This is because the elastic properties of the formation are uniform, and the SH wave will not be refracted and reflected multiple times in the inner porous medium. As shown in Figure 8b, the interface electromagnetic wave response calculated by secant integral is completely consistent with the first two components of the time-domain waveform of the electric field full wave, which further illustrates that the components a-a and b-b shown in the electric field full wave (Figure 7) are the electromagnetic wave interface responses. In addition, it also shows that it is feasible to calculate interface waves by secant integral. Under the parameters given in Table 1, the theoretical SH wave velocity of the formation is 2492.2 m/s.