大地电磁测深在火山区地热研究中的应用

大地电磁测深(MT)由于勘探深度范围较大,且对温度、流体、岩浆房和岩性等与热储相关的地质条件的敏感度较高,因而成为火山区地热勘探和岩石圈结构研究中常用的一种地球物理勘探手段。地壳和上地幔的电性结构与热结构之间存在着密切的联系,通过解读二者的关系,可以刻画更为精细的岩石圈结构模型,进而掌握火山区的构造特征和热演化过程,了解其岩石圈地球动力学机制。本文着重介绍了MT方法的原理以及从野外数据采集到后期数据处理的过程,综述了MT法的应用特点以及电导率与温度之间的关系,通过实例分析,介绍了国际上这一方法在火山区地热勘探和岩石圈热结构研究中的应用进展,展示了MT法在新西兰Taupo火山区Ngatamariki高温地热田0~3km地热资源勘探中的应用;以埃塞俄比亚Afar省的Tendaho地热田和Badi火山为例,分别讨论了0~20km和0~50km不同深度的电性结构特征及其与温度存在的内在联系,探讨了形成火山的驱动机制;以日本九州岛的Shinmoe-dake火山为例,介绍了大地电磁测深和温度监测在火山监测方面的应用。最后简述了国内MT法在火山区的应用进展以及存在的问题,并利用上地幔电导率与温度的关系以及岩石圈内硅酸盐熔体不同含水量引起的电导率随温度的变化关系,初步估算了长白山天池和阿尔山火山区的莫霍面以下的温度以及长白山天池火山区的高温岩浆房温度。

Application of magnetotellurics in geothermal exploration and research in volcano areas

Magnetotellurics (MT) sounding is one of the most widely used geophysical exploration methods in volcanic areas for its wide exploration range and high sensitivity to geothermal reservoir related information such as temperature of rocks, abundance of geothermal fluid, extent of magma chamber, and lithostratigraphic strata. The electrical structure of the lithosphere is related with its thermal state. By analyzing the correlation, the geological structural model of the crust and the upper mantle can be built to enhance our understanding of the geodynamic mechanism of volcanic processes. In this paper, we first provide a brief introduction to the theoretical foundation of MT technology and the practical procedure from field data acquisition to data processing. The introduction is followed by an overview of the relationship between the electrical conductivity and temperature of the rocks. We then present several case studies from overseas to illustrate the application of the MT method in the geothermal exploration and the study of the thermal structure of the lithosphere in volcano Areas. For examples, in the Taupo volcanic area, New Zealand, MT sounding was used in the exploration of the Ngatamariki high geothermal field for hydrothermal resource in the upper 0~3km depths for power plant development; while in the Tendaho geothermal field in the Badi volcano in Afar Province, Ethiopia, the correlation between temperature and electrical structure from 0~20km and 0~50km were respectively analyzed, and the mechanism of volcanic formation and is discussed; in the Shinmoe-dake volcano in Kyushu, Japan, routine MT and temperature measurements have been used jointly to monitor volcanic activities. In the later part of this paper, we summarize the key MT sounding results from four major Cenozoic volcanic areas in China, namely, the Tengchong volcanic area in Yunnan Province, the Changbaishan Tianchi volcanic area in Jilin Province, the Wudalianchi volcanic area in Heilongjiang Province, and the Alershan volcanic area in the Inner Mongolia. We further employ the empirical temperature-electrical conductivity to constrain the temperatures of the crustal-mantle boundary for the Changbaishan-Tianchi volcanic area and the Alershan volcanic area, and possible magma chambers for the Changbaishan-Tianchi Volcano.