Hydrothermal system beneath Aso volcano as inferred from self-potential mapping and resistivity structure
Hydrothermal system beneath Aso volcano as inferred from self-potential mapping and resistivity structure
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DOI：
10.1016/j.jvolgeores.2004.12.005
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发表时间：
2005-05-30
影响因子：
2.9
通讯作者：
Tanaka, Y
中科院分区：
文献类型：
--
作者：
Hase, H;Hashimoto, T;Tanaka, Y
自1998年8月至2001年12月，我们在阿索火山的中央锥体上逐段进行了自然电位（SP）测量。编制的SP图显示，大SP异常的中央锥。SP图的主要特征是沿中心锥上方NS样带呈沿着“W”形剖面。这种特征性的自然电位剖面可能是中部热液上升流和地形作用共同作用的结果。为了评价这种SP异常，我们实现了一个数值代码，计算由任意位置的电流源和电流汇在任何三维电阻率结构产生的电位。从时域电磁（TDEM）场实验获得的层状结构的电阻率模型。估计电流源为300 A，位于海平面周围的导电层中。同时，汇估计坐在一个圆形区域对应的导电层的边缘部分。水和热收支的研究给出了水的质量传输从深度到底部的火山口湖的Nakadake的下限。该值用于估计在动电（EK）的任一情况下的等效电流[Mizutani，H.，Ishido，T.，1976.对与松代地震有关的磁场变化的新解释。地电，28，179-188.]或快速流体破裂（RFD）方法[约翰斯顿，M.J.S.，Byerlee，J.D.，Lockner，D.，2001.快速流体破裂：火山自电位异常的来源，J. Geophys。Res. 106（133），4327-4335.]。这种比较表明，前者的过程是更好地解释所观察到的SP异常。从这些结果中，我们推断一个大规模的热液系统下的中央锥的阿索火山，其中的流体流从周围地区发起，汇聚到中央喷口的热量和物质输送到火山口湖的Nakadake通过一个充满蒸汽的管道。(c)2005 Elsevier B. V.保留所有权利。
We conducted self-potential (SP) surveys sequentially from part to part over the central cones of Aso volcano since August 1998 by December 2001. The compiled SP map revealed large SP anomalies on the central cones. The main feature of the SP map is a `W-shaped' profile along the NS-transect over the central cones. It is probable that this characteristic SP profile is produced by the combination of hydrothermal upwelling in the middle and topographic effect. A positive anomaly showing a large concentric pattern has appeared after correcting the topographic effect.To evaluate this SP anomaly, we implemented a numerical code that calculates electric potential produced by arbitrarily positioned current sources and sinks in any three-dimensional resistivity structure. A layered structure obtained from a time-domain electromagnetic (TDEM) field experiment was used for the resistivity model. The estimated current source is 300 A, being located in a conductive layer around the sea level. Meanwhile, sinks were estimated to sit on a circular area corresponding to the marginal part of the conductive layer.Water and heat budget study gives a lower limit of water mass transfer from depth to the bottom of the crater lake of Nakadake. This value was used to estimate the equivalent current in either case of electro-kinetic (EK) [Mizutani, H., Ishido, T., 1976. A new interpretation of magnetic field variation associated with the Matsushiro earthquakes, J. Geomag. Geoelectr., 28, 179-188.] or rapid fluid disruption (RFD) process [Johnston, M.J.S., Byerlee, J.D., Lockner, D., 2001. Rapid fluid disruption: A source for self-potential anomalies on volcanoes, J. Geophys. Res. 106(133), 4327-4335.]. This comparison suggests that the former process is preferable to explain the observed SP anomaly. From these results we infer a large-scale hydrothermal system beneath the central cones of Aso volcano, in which the fluid flow initiates from the surrounding area, converging to the central vent to transport the heat and materials up to the crater lake of Nakadake through a vapor-filled conduit. (c) 2005 Elsevier B.V. All rights reserved.