Technologies and Applications of Geophysical Exploration in Deep Geothermal Resources in China

According to the geophysical responses of deep geothermal resources, the geophysical techniques of geothermal exploration in China are categorized into thermal infrared remote sensing, gravity survey, magnetic survey, electrical methods, seismic survey, and well logging. On the basis of the previous study of geophysical exploration in high temperature geothermal fields and active geothermal areas in China, the paper summarizes a variety of geophysical methods in geothermal exploration and the relationship between these methods and the geothermal anomalies. The paper also compares the depths and precision of investigations, the range of applicability, and the advantages and disadvantages of these methods. To compensate the shortages of using a single method, the combined method has been proposed for geophysical exploration of deep geothermal resources which can not only reduce the blindness of deep geothermal explorations effectively, but also improve the accuracy of exploration results, as well as reduce the risk and cost in explorations at the same time. 1. INTRUDUCTION Geophysical exploration of geothermal resources is to use specialized equipment to detect the physical fields related to the geological structures and characteristics of a geothermal system. The temperature-sensitive physical parameters obtained by geophysical methods are used to find the geothermal prospect areas, evaluate the characteristics of the geothermal system, delineate the drilling area, locate the aquifers, and provide the basis for locating the borehole position. Compared to other fields, the geophysical exploration in geothermal resources is focused on the geology and the structure of geothermal fluids or hot dry rocks, and pays more attentions to some parameters which are particularly sensitive to the temperature. The geological background of geothermal resources in China is quite complex compared with that in Iceland, New Zealand, and other countries. Therefore, many things need to be determined before the exploration and the development, including the distribution and occurrence of regional faults especially those related to geothermal fluids, the distribution, size and nature of the igneous intrusive bodies, the location of geothermal reservoirs, and so on. Because there are multiple solutions existing in geophysical methods and each method has its own specific scope, a single method cannot meet the needs of the geothermal exploration. An optimal method or a combining method based on the different geological backgrounds in China is required in order to carry out the exploration of geothermal resources economically and effectively. 2. THE PRESENT SITUATION AND MAIN PROBLEMS IN DEEP GEOTHERMAL EXPLORATION IN CHINA China is so rich in geothermal resources throughout the country. The total reserves are 11×10 6 EJ/a accounting for 7.9% of the total global geothermal resource reserves. As seen in Figure 1, the distribution of geothermal resources in China has obvious regularity and zonality. The high-temperature geothermal fields are mainly located in the Himalayan geothermal belt and the Circum-Pacific geothermal belt covering south Tibet, west Yunnan, west Sichuan and Taiwan areas, while the low- and medium- temperature geothermal fields are distributed all around the country, mainly round the Ordos basin, the geothermal belt in the Southeast and Jiaoliao including the southeast coast, eastern and north-eastern China.