## 1 Introduction  
Electrical resistivity tomography (ERT) applied to geotechnical investigation has developed rapidly in recent years with the beginning of 1D exploration that is mainly used in groundwater and mining exploration. With the advancement of computer technology and forward and inverse computing skills, in the past ten years, 2D exploration has been widely used in the investigation and monitoring of geotechnical engineering, environmental engineering, and groundwater pollution. 3D ERT is gradually starting to be used, however, due to the limitations of the test environment, 3D is the trend of future development, so the application in current engineering is mainly based on 2D ERT [1; 2; 3; 4; 5; 6; 7]. Linearity 2D ERT assumes that the geological formation resistivity property is homogeneous in the vertical line direction. When the geological formation does not meet the condition where they homogenize in the vertical line direction, since the electric current is flowing in 3D, the 2D ERT result may be in error, so the distortion that this hypothesis may cause is often ignored in the interpretation of the test results. The 3D effect means that the geological structure outside the 2D resistivity profile will map to the 2D resistivity profile and cause such errors. The boundary effect means that when 2D ERT is measured on a straight line of finite length but the boundary of the survey line encounters terrain changes such as pools, valleys, or underground structures like concrete structures or metal pipelines, it may be mapped to the 2D electrical resistivity profile, causing exploration errors, thus affecting the test results and the accuracy of the interpretation. The application of ERT in geotechnical investigation has been developed quite completely, and there exist fairly complete procedures from the arrangement of the test to the inverse computation analysis of the data, but there has not been a detailed assessment of the 3D effect and the boundary effect. 2D ERT assumes that the geological formation is a 2D semi-infinite space. However, in the real state of geological formations, the electric current flows in the 3D (X, Y, Z) direction, therefore, the object in the non-2D section has a certain degree of disturbance to the ground resistivity electric field, which causes some irregular resistivity and noise on the 2D profile [8]. [PERSON] explored the application of 2D ERT in the environment and engineering in Sweden. In the in situ test, it was considered that there was a 3D effect, and they suggested avoiding this when setting the line [9], [PERSON] et al. explored the detection of dam leakage by 2D ERT and found that the 3D effect was generated by the structure around the line and projecting the feature of the structure onto the testing profile. Therefore, they suggested that the effects of the 3D effect should be noted when testing [10]. Many scholars often face the boundary effect caused by the boundary of the sandbox when conducting an indoor sandbox experiment [11; 12; 13; 14; 15; 16]. [PERSON], [PERSON] used a homogeneous thickness model made from dough to explore the indoor test method and criteria for 2D ERT. As the limited boundary on the three sides of the sand-box will affect the transfer of the electric current, this is inconsistent with the assumption that the theoretical electric current has boundless extended flows in the underground half-space, and will consequently cause the boundary effect. After discussion, it was found that the boundary of the sand-box will cause the resistivity to be in an unstable state until the boundary of the bilateral line is more than double spread, where the apparent resistivity will be stable [17]. It can be found from the above literature that the 3D effect and the boundary effect are the problems that most scholars may face when conducting electrical resistivity tomography. However, few scholars have explored the 3D effect and the boundary effect in depth.