## 1 Introduction  
Various infrastructure projects, such as railway, subway, and coal mining operations, require extensive underground roadway construction. During excavation, unforeseen geological hazards, like water innyh and mudflows, frequently arise. To ensure the uninterrupted progress of these projects, it is critical to anticipate such hazards in advance. The transient electromagnetic (TEM) method is widely employed for the prediction and exploration of underground disasters, owing to its high resolution in detecting low-resistance anomalies, ease of operation, and cost-effectiveness [1]. The TEM method is a time-domain method that utilizes a grounded wire or loop to inject pulsed currents into the subsurface, subsequently measuring the spatiotemporal decay patterns of secondary electromagnetic fields induced by eddy currents. The TEM method is widely employed to investigate a variety of geological issues, providing valuable insights into subsurface structures [2]. Common TEM configurations include surface TEM (STEM), mining TEM (MTEM), and surface-to-roadway TEM (SRTEM) methods [3]. However, when the TEM transmitter and receiver are positioned far from the underground roadway, detection accuracy for deeper targets is often inadequate [4]. Additionally, low-resistance overburden may shield underground conductors, hindering the precise localization of anomalies [5]. In response to the needs of underground engineering exploration, the MTEM method has been proposed [6]. The MTEM method typically uses a multi-turn, overlapping rectangular loop as the transmitter to suit the limited space within roadways, with side lengths ranging from 2 to 5 m. During the exploration process, EM field parameters in various directions are acquired by rotating the coil [7]. The MTEM method offers a longer exploration range and improved directivity. However, it is influenced by detection blind zones and significant interference from metallic structures, which degrade data quality [8]. The SRTEM method is a novel approach proposed in recent years. It involves a transmitter on the ground while receivers are within the roadway, effectively minimizing interference from the complex underground environment [9]. This configuration necessitates real-time communication between the surface and the roadway, yet it faces the challenge of inadequate shallow-depth resolution. When integrated with the STEM method, it allows for the detection of targets at greater depths [10]. It is clear that conventional EM exploration techniques have limitations when applied in underground environments. In contrast, drilling directly provides geological information during excavation. Consequently, the advanced exploration of underground hazards currently mainly relies on boreholes, supplemented by geophysical methods [11]. Due to cost constraints, existing boreholes may not offer comprehensive coverage of the target area. Therefore, neither EM exploration nor borehole coring alone can achieve optimal outcomes. Given that boreholes are spatially closer to the target and are less affected by electromagnetic interference, while EM methods provide multi-directional depth detection, deploying EM configurations within boreholes represents a promising approach. This has led to the development of in-borehole EM methods [12]. Borehole EM methods typically employ either frequency-domain or time-domain methods [13; 14]. The frequency-domain borehole controlled-source electromagnetic (CSEM) method is commonly used to detect large-scale geological targets, such as geothermal resources, metal deposits, and hydrocarbons, by utilizing surface-based borehole stations [15; 16; 17; 18; 19; 20]. In contrast, EM exploration in underground engineering focuses on small-scale and low-resistivity targets, such as water-bearing zones, making the borehole TEM method more widely used [21]. Currently, the TEM configurations used in boreholes include surface-to-borehole TEM (SBTEM), roadway-to-borehole TEM (RBTEM), and borehole TEM (BTEM) methods.