## 1 Introduction  
The Huailai area is located on the southern margin of the Yan Mountain fold belt, the northern margin of the Taihang Mountain uplift, the Xuanhua Basin in the northwest, and the Fanshan Basin in the southeast (Figure 1). It is rich in geothermal resources, such as the Xijiabao, Houhaoyao, and Nuanquan hot springs, among others. Hot spring water in the area has medium to low temperature (<80 \({}^{\circ}\)C), but the temperature of deeper hot water can reach up to 120 \({}^{\circ}\)C [1; 2], which is closely related to the occurrence of earthquakes [3; 4]. The circulation depth of the geothermal water is generally greater than 2 km, which is categorized as the deep circulation type [5]. According to the viewpoint of plate tectonics, the heat source is deeper, and the primary task of studying this kind of geothermal system is to determine the origin of the heat source [6]. Based on the formation types of heat sources, geothermal systems can be roughly divided into magmatic and non-magmatic. The magmatic geothermal system is formed by magma, with a relatively simple structure: the upper layer is caprock, composed of high-conductivity clay and calcareous cement, and the lower layer is thermal storage, with relatively high resistance. Groundwater continuously circulates between the caprock and thermal reservoirs to form hot water. Non-magmatic geothermal systems are different in structure and composition [7]. According to hydrogeological surveys, most geothermal water in the Huailai area is mainly distributed in bedrock and the overlying strata, and there are clay and calcareous cements in the upper caprock [8], which conforms to the shallow characteristics of the magmatic geothermal type. However, there has been no Cenozoic volcanic activity or tectonic movement that may lead to magmatism in the Huailai area or the adjacent areas. No clear conclusion exists about the formation mechanism of the heat source. Based on the heat transfer mode, the geothermal system in the Huailai area was classified as convective [9]. One view was that this type of geothermal system has no additional heat source, and is instead supplied by normal or high regional geothermal flow, and that the heat storage temperature is proportional to the circulation depth [1]. Another view is that the heat source is radioactive, generated in the bedrock granite [10], but this lacks relevant evidence. However, helium isotope studies have shown that there is a considerable amount of mantle-derived helium in hot spring water, implying that the formation of deep heat sources is related to the upwelling of mantle-derived materials [2,11]. Seismic reflections have also provided evidence of magma intrusion in the Huailai area [12-14], but there is insufficient evidence to show that it is related to the formation of a geothermal system. Related hydrogeological research only involved the movement and storage of groundwater above the basement [8] and lacked considerations of deep heat sources and structural support. To explore the genetic model and establish a complete model of the geothermal system in the Huailai area, the types of deep heat sources and the structure of the crust and mantle must be identified. Electrical surveying is an important means of investigating geothermal systems. The conductivity of rocks is closely related to the abundance of water and temperature [15]. The rocks rich in geothermal water usually have lower resistivity than dry rocks and are generally connected to the structural fracture zone of the deep heat source [16]. Water-abundant and fractured zones usually have high conductivity. The magnetotelluric sounding method (MT) has a very sensitive response to high conductors, and its resolution can range from tens of meters to hundreds of kilometers. Combined with relevant regional geological conditions, the MT is effective in the exploration of geothermal systems [17-22]. In this study, we used the MT combined with seismic and hydrothermal constraints to further explore the geothermal characteristics of the Huailai area.