## 1 Introduction  
In recent years, urban disasters have happened constantly. People's lives and properties have been increasingly severely threatened. If the geological conditions of the base could provide accurate and detailed information from the prevention perspective of urban disasters, disaster prevention would achieve better results. The incidence of disasters should decline spontaneously. The investigation of geotechnics aims at the geological conditions of the site. The accurate and detailed information of stratum distribution and material parameters are much required for reference in planning analysis, design, and construction. The more accurate and detailed the geotechnical investigation is, the more suitable countermeasures and construction methods could be proposed in the planning and design stages. It could let the construction period be shortened and expenditures saved. The main traditional method of geotechnical investigation is drilling. However, drilling is expensive and only obtains the information about that point. In addition, the drilling of large sites sometimes fails to meet the planning needs of planners. So, geophysical detection is often used as an assistance method [1, 2]. In the past, geophysical exploration was used in geotechnical investigation mostly for seismic detection (refraction, reflection, and surface wave techniques). The elastic wave velocity of strata is closely bound up with the material engineering characteristics [1; 2; 3], but the seismic detection is likely to be influenced by ambient vibration noise. Moreover, the refraction seismic detection cannot positively detect the low-velocity layer under the high-velocity layer, and the shallow layer (less than 50 m) reflection seismic detection is difficult to be implemented. The surface wave seismic detection consists of mainly 1D and 2D probing techniques; the 3D detection method is still in the preliminary research stage. Ground penetrating radar is a kind of electromagnetic method. The ground penetrating radar is similar to reflection seismic detection, detecting the distribution of the bed boundary, and the localization of depth and the depth of investigation are limited. If a bistatic antenna is used, it can increase the detection depth capability [4; 5; 6; 7]. Electrical resistivity tomography is a kind of electrical method; the present electrical resistivity tomography technique has been able to explore 2D, even 3D earth-resistivity distributions, and the depth of investigation can be adjusted flexibly by the length of the measuring line, and the earth-resistivity is highly correlated with the geomaterial and groundwater characteristics, so it has gradually become one of the main geophysical methods for geotechnical investigation. This study discusses the spatial resolution of 2D ERT in geotechnical investigation. In the past decade, 2D ERT exploration has been extensively used in geotechnical engineering and environment engineering investigations and monitoring, such as geologic surveys [8; 9; 10], fault surveys [11; 12; 13; 14; 15; 16; 17], slipping plane surveys and monitoring [18; 19; 20; 21; 22; 23], groundwater investigations and pollution monitoring [24; 25], seawater intrusion investigations [26; 27; 28; 29], dam leakage investigations and monitoring [30; 31; 32; 33], landfill leakage investigations and monitoring [34; 35; 36; 37; 38], and archaeological surveys [39; 40]. The 3D DC resistance method has been used by scholars [41; 42; 43], but it is difficult to be implemented, and the site is often limited to the measurement space, so the 2D DC resistance method is still the main method in engineering. 2D ERT has been extensively used in geotechnical engineering investigations, but there are many uncertainties hidden behind the vivid color earth-resistivity profiles.