| Abstract |
A high-temperature geothermal system significantly influences the resistivity of its environment, directly by the physical and chemical properties of the geothermal fluid and indirectly by the chemical changes experienced by the host rocks. Now that deeply magnetotelluric (MT) surveys are routinely available,this resistivity anomaly should be readily detectable and used to target the geothermal resource. However, interpretation of the resistivity measurements is not always straightforward, especially in steep terrain, and can often mislead investigators and result in failure. This paper presents MT surveys four different drilled geothermal fields, and presents an anomaly, namely the shape of the base of the conductive layer overlying the geothermal system, that is directly related to reservoir temperature and often to well productivity. In contrast, conventional resistivity anomalies are shown to be often located in less prospective parts of the field. While the proposed interpretation method is simple and does not require sophisticated analysis techniques or computing power, it does require widespread and reasonably close-spaced MT measurements over the prospect area. As long as the initial survey is well designed and executed, the geothermal system can be revealed - as plain as the nose on your face. |