The hydrology and geology of the Xinzhou geothermal field in South China and its surrounding area have been well studied. Previous studies have indicated that mantle heat, granite radioactive heat, and partial melting are the likely key thermal sources. However, neither the geometry of the geothermal reservoir nor the groundwater circulation pattern has been fully characterized. Toward that aim, a grid of magnetotelluric (MT) data has been acquired to reveal the subsurface distribution of electrical conductivity and to detect the geothermal reservoir. An analysis of the phase tensors suggests that 3D MT inversion is essential for accurate interpretation of the data. We perform a 3D full-impedance MT inversion and observe pronounced low electrical conductivity zones in the inverse model. We interpret these materials as widely developed granites that form the bedrock, which is cut by a crisscross of faults. The cracks in the bedrock provide pathways for the vertical and lateral movement of the hydrothermal fluid. The geothermal reservoir appears as a prominent high electrical conductivity anomaly beneath the hot spring. A northwest-dipping zone of enhanced electrical conductivity reaching approximately 6–7 km in depth is found in the southeast part of the study area, and it is interpreted as the geothermal reservoir recharge channel. We develop an inferred underground fluid circulation model, informed by the electrical conductivity of the area, in combination with hydrologic data. We suggest that the water supply of the Xinzhou geothermal field mainly comes from meteoric water, which is mixed with the invading seawater and shallow cold groundwater as it rises along the fault. The results of our MT study confirm earlier inferences that the Xinzhou geothermal field belongs to an intraplate deep-circulation type originated from the mantle heat flow.

中文翻译：

---

基于3D大地电磁成像的东南沿海地热场电导率模型

华南及周边地区忻州地热田的水文地质已得到很好的研究。先前的研究表明，地幔热、花岗岩放射性热和部分熔化可能是关键的热源。然而，地热库的几何形状和地下水循环模式都没有得到充分表征。为实现这一目标，已获得大地电磁 (MT) 数据网格，以揭示电导率的地下分布并检测地热储层。对相位张量的分析表明，3D MT 反演对于准确解释数据至关重要。我们执行 3D 全阻抗 MT 反演并在反演模型中观察到明显的低电导率区域。我们将这些材料解释为形成基岩的广泛发育的花岗岩，基岩被纵横交错的断层切割。基岩中的裂缝为热液的垂直和横向运动提供了通道。地热储层表现为温泉下方突出的高电导率异常。在研究区东南部发现了约6-7km深度的电导率增强的西北倾斜带，解释为地热储层补给通道。我们根据该地区的电导率以及水文数据开发了一个推断的地下流体循环模型。我们建议忻州地热田的供水主要来自大气水，当它沿着断层上升时，它与入侵的海水和浅层冷地下水混合。我们的MT研究结果证实了早先的推论，即忻州地热田属于源自地幔热流的板内深环流类型。