## 3 Results and Discussion
### Influence of Fluid Salinity
#### Influence of Pore Fluid Salinity
As fluid salinity \(C_{f}\) increases, the value of \(L(\omega)\) also decreases monotonically, which indicates that the greater the pore fluid salinity, the weaker is the electrokinetic coupling effect. Next, we compare the electric fields in the borehole for different \(C_{f}\) to illustrate the influence of salinity on the electric field. This paper considers the typical sandstone formation for which the media parameters are given in Table 1. This set of parameters can  
Figure 2: Dependence of electrokinetic coupling on frequency and fluid salinity. well reflect the properties of the typical sandstone formation [39; 64]. For the parameters given in Table 1, the velocity of SH wave in the formation is 2492.2 m/s, thus it is a fast formation. The acoustic source frequency is 10 kHz, and the density of the borehole fluid is 1000 kg/m\({}^{3}\). The fluid salinity in this part is fixed at \(C_{b}=0.001\) Mol/L. From the above discussion and Equation (5), since \(C_{f}\) has no effect on \(\eta_{e}\), the influence of \(C_{f}\) on the borehole electric field is reflected on its influence on the electromagnetic response coefficient \(A_{H}\). Figure 3 shows the borehole electric fields for different \(C_{f}\), and the source distance corresponding to the waveforms in Figure 3 is z = 3 m. It should be noted that, according to the assumption of [PERSON] et al. [9], the salinity for pore fluid should not be greater than 1 Mol/L. Here we combine the work of [PERSON] et al. [5] to limit the fluid salinity range of 10\({}^{-4}\)-10\({}^{-2}\) Mol/L, which is a common groundwater salinity grade range. According to Figure 3, the general shape or composition of the electric field for different \(C_{f}\) does not change. The complete waveform of \(E_{\theta}^{b}\) consists of two parts--the interface response and the accompanying electric field [68]. When the salinity of the pore fluid increases, the arrival time of each component does not change but the amplitude decreases monotonically. Since the interface responses are generated at the borehole wall and propagate with the speed of the formation's electromagnetic waves, the salinity of the pore fluid hardly influences their arrival times. Besides, the pore fluid salinity mainly \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Symbols** & **Units** & **Values** \\ \hline Porosity & \(\phi\) & - & 0.20 \\ Permeability & \(\kappa_{0}\) & \(\upmu\)m\({}^{2}\) & 0.01 \\ Frame Shear modulus & \(G^{\circ}\) & Gpa & 14.41 \\ Frame density & \(\rho_{s}\) & kg/m\({}^{3}\) & 2650 \\ Pore fluid density & \(\rho_{f}\) & kg/m\({}^{3}\) & 1000 \\ Frame permittivity & \(\varepsilon_{s}\) & \(\varepsilon_{0}\) & 4 \\ Fluid permittivity & \(\varepsilon_{f}\) & \(\varepsilon_{0}\) & 80 \\ Fluid viscosity & \(\eta_{w}\) & Pa-s & 0.001 \\ Tortuosity & \(\alpha_{\infty}\) & - & 2.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Formation parameters. Figure 3: Electric fields for different levels of \(C_{f}\). The solid line, dashed line and dot-dashed line correspond to the salinity level \(C_{f}=0.01,0.001,0.0001\) Mol/L respectively. influences the formation's electrical properties but hardly influences the elastic properties. Therefore, when the pore fluid salinity changes, the SH wave velocity remains almost unchanged, thus the accompanying electric field's starting time also remains unchanged. The variation of the amplitude is mainly caused by the influence of pore fluid salinity on the formation conductivity and the electrokinetic coupling. Between the two, the influence of fluid salinity on the electrokinetic coupling plays the major role.