## 1 Introduction
The surface wave seismic detection is mainly 1D and 2D probing technique, and the 3D detection method is still in the preliminary research stage. Ground-penetrating radar is an electromagnetic method similar to seismic reflection detection. Here detecting the distribution of bed boundary and the localisation of depth and the depth of investigation are limited. If a Bistatic antenna is used, it can increase the detection depth capability [9; 10; 11; 12]. The ground-penetrating radar has been used in underground structure and archaeological investigations to a great extent, for instance, in underground tunnel mining [13; 14], underground pit mining [15; 16], and historical building mining [17; 18]. However, the ground-penetrating radar only judges the location of possible tunnel structures derived from the discontinuity of radar wave velocity. The shape, size, depth, and distribution range of underground blockhouses are unknown. Electrical Resistivity Tomography (ERT) is a geophysical method of mapping underground structures using electrodes placed in boreholes or electrical resistivity measurements from the surface. The present electrical resistivity tomography technique can explore 2D and even 3D resistivity distribution, and the depth of investigation can be adjusted easily by the length of the measuring line. The resistivity is highly correlated with the geomaterial and groundwater characteristics, so it has gradually become one of the primary geophysical methods of geotechnical investigation. In the past decade, the ERT has been extensively used in a variety of geotechnical and environmental engineering investigation and monitoring, for instance, in geologic surveys [19; 20], fault line surveys [21; 22; 23; 24; 25], slipping plane survey and monitoring [26; 27; 28], groundwater investigation and pollution monitor ing [29; 30], reservoir/dam leakage investigation and monitoring [31; 32], refuse landfill leakage investigation and monitoring [33; 34; 35], and underground tunnel mining [36; 37; 38]. Kinmen Island is located between Taiwan and China. The test site is located in an abandoned barracks area in the central region of Kinmen Island, shown in Figure 1. A large-scale military defensive infrastructure was built in this area, including multiple underground tunnels. The Shuangru Mountain Tunnel was built in 1949. It is 1142 m long, extending in all directions, and had a significant strategic position and value. After the troops' withdrawal, the Kinmen County Government declared this tunnel an important cultural heritage in 2019, as it had significant historical value. However, the site has been in disrepair; multiple tunnel intersections have collapsed, and it is difficult to determine the tunnel path and true underground location. This study attempts to evaluate the feasibility of using ERT to detect the basic data on the location, size, and depth of the underground tunnel. Figure 1: Test site location and measurement wiring diagram. The measurement results of ERT in the geotechnical investigation is influenced by soil water content, geological structure, groundwater level, and ambient noise, among other things. These factors affect the sensitivity of ERT measurement and spatial analysis, so this technology still faces problems in spatial resolution capability. Therefore, this study intends to construct a numerical model with the collected data to discuss the reliability of ERT results for detecting underground tunnels. As all geologic configurations are essentially 3D, theoretically, the 3D resistivity survey is supposed to obtain the most accurate result [39]. At present, the 3D ERT survey has been studied by most scholars actively, but it is not yet as extensively used as 2D ERT. The main reason is that the Conduct 3D ERT test needs a larger area site, 3D ERT has a relatively higher cost, and field tests take a long time. Therefore, the use of the 3D ERT survey is not yet widespread. In recent years, artificial intelligence has been extensively used in various domains [40].