## 1 Introduction  
Underground voids are sometimes attributable to natural phenomena, such as karst processes and the dissolution of permafrost. Karstification results from the gradual dissolution of carbonate rocks, while the dissolution of permafrost is driven by climate change, which accelerates the melting of frozen ground [1; 2; 3; 4]. In addition, human activities can create underground voids, such as old mine shafts or abandoned tunnels [5; 6; 7]. The area of underground voids left behind after mining operations are completed is commonly referred to as an abandoned mining tunnel area [8]. By the end of 2025, the total length of expressways in China had reached 183,600 km, making it the longest expressway network in the world. With the rapid development of expressways, numerous cases have arisen where they must traverse mining areas. The residual mining tunnels in these mining areas have a significant impact on the stability of the expressway infrastructure [9]. When an expressway crosses an abandoned mining tunnel area, the voids disrupt the initial stress balance of the overlying rock layers. The weight of the overlying rock mass may cause uneven surface deformation, which can damage both the roadbed and the pavement [10; 11; 12; 13; 14]. Furthermore, most abandoned mining tunnels are filled with water, and, under the influence of water disturbance, surface subsidence in these areas may further develop, leading to catastrophic consequences [15; 16]. Therefore, conducting thorough surveys of abandoned mining tunnel areas in mining regions before the construction of expressways becomes crucial [17]. Various non-destructive geophysical methods are available for detecting underground abandoned mining tunnels [18; 19]. The fundamental principles of these methods are based on the differences in physical properties between the voids and the surrounding rock [20; 21]. Depending on factors such as backfilling status and groundwater table fluctuations, abandoned mining tunnels may be empty, partially filled, fully filled, or completely water-filled [22; 23]. While geophysical methods have been shown to be effective in locating buried objects, their spatial resolution decreases with increasing depth. Therefore, applying these methods for detecting underground abandoned mining tunnels still presents significant challenges [24]. Currently, the primary geophysical methods for detecting underground abandoned mining tunnels include Ground-Penetrating Radar (GPR), seismic methods, gravity surveys, and Electrical Resistivity Tomography (ERT) [25; 26]. Ground-Penetrating Radar is particularly effective for detecting shallow voids at depths of just a few meters [27; 28]. Due to its wide high-frequency range, GPR provides high-resolution images and can detect even small voids within concrete structures. As a result, GPR is widely used for identifying voids in infrastructure [29; 30]. Other methods, such as seismic surveys [31; 32; 33], gravity surveys [34; 35], and ERT [36; 37; 38; 39; 40], are also commonly applied. Among these geophysical techniques, ERT has been confirmed as a highly effective method for detecting underground voids [41; 42]. In recent years, due to its low cost, simple automated data collection, and efficient data acquisition, ERT has gained increasing popularity in civil engineering, mining operations, and underground void detection, as it can reliably reflect the underground characteristics [43; 44; 45]. This study focuses on the abandoned refractory clay mine located on the eastern side of Chenxi County. By utilizing ERT in combination with borehole data, the study analyzes the resistivity parameters of the strata to determine the distribution of the underground abandoned mining tunnels in the mining area. The findings provide valuable insight into the next steps in handling the abandoned mining tunnels and ensure the smooth progress of the Chenxi-Yuanling Expressway project.