## 1 Introduction  
S salinity is the total amount of ions per unit volume of fluid. It is an important indicator of reservoir fluid. Salinity is used to evaluate the salt content in fluids, not only for pore fluid identification but also for the evaluation of formation fluid pollution [1, 2]. The salinity of the pore fluid directly affects the electrical properties of the formation. Since the converted electric field of the acoustic field in the porous media is sensitive to the electrical properties, it becomes one of the potential methods for detecting the salinity level of the pore fluid [3, 4, 5]. The phenomenon of acoustic waves propagating in the pore fluid to generate the coupling electromagnetic field is called the seismoelectric effect [6, 7, 8, 9, 10, 11, 12, 13]. The electric double-layer structure and the seepage of the pore fluid are important factors for this phenomenon. The prospecting method based on seismoelectric signals receives both elastic wave and electromagnetic wave information, which is convenient for obtaining the formation's elastic information as well as electrical information. Studies have shown that the seismoelectric signal is not only sensitive to the rock modulus, porosity, permeability, saturation, and other parameters that affect the acoustic properties but is also closely related to the electrical properties of the rock such as electrical conductivity [14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. This also makes researchers pay close attention to the phenomenon of acoustic-electric coupling. Research on the theory and experiment of acoustic-electric coupling has been extensively carried out, and has obtained many meaningful results [24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. Some researchers also proposed a seismoelectric logging method based on the experience of sonic logging [37]. In addition to obtaining formation elasticity and electrical information at the same time, this method also has advantages such as deep exploration. [PERSON] et al. [38; 39] were the first to simulate the seismoelectric waves in the borehole. For inducing electric fields, in addition to the accompanying electric fields that propagate with the velocity of the acoustic waves, there also exists the interface responses propagating with the speed of the electromagnetic waves in the formation. Later, they proposed a quasi-stable method for independently calculating the acoustic fields and the electric fields [40], and verified the feasibility and applicability of the method [41]. Some investigators also used the finite difference [42] and the finite element [43; 44] methods to simulate the borehole seismoelectric wave fields. The results show that the wavefield characteristics obtained by multiple methods are consistent. In actual exploration, factors such as geological structures or fluid pollution will make the elastic or electrical properties of the formation inhomogeneous. The interface-converted electromagnetic wave generated by the elastic wave impinging the interface of the medium is one of the important characteristics [45; 46; 47]. This characteristic can be used to detect the position and nature of the formation interface or the structural characteristics of the geological body next to the borehole. [PERSON] et al. [48] simulated the acoustoelectric coupling fields for the while-drilling condition when the formation has an interface separating two materials having different elastic properties and found that the elastic properties on both sides of the interface affect the intensity of the interface response. [PERSON] et al. [49] simulated the borehole seismoelectric waves for the case of a formation having an interface separating fluids with two different levels of salinity. However, this article did not make an in-depth investigation of the interface response law for this formation situation. [PERSON] et al.