Hydrothermal system beneath Mt. Fuji volcano inferred from magnetotellurics and electric self-potential
Hydrothermal system beneath Mt. Fuji volcano inferred from magnetotellurics and electric self-potential
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DOI：
10.1016/j.epsl.2005.03.023
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发表时间：
2005-06-30
影响因子：
5.3
通讯作者：
Hurst, AW
中科院分区：
文献类型：
--
作者：
Aizawa, K;Yoshimura, R;Hurst, AW
在富士火山沿东北至西南方向进行了宽带大地电磁测深。利用最高质量的数据（频率范围为1-300 Hz）进行二维反演，发现峰顶下方有一个良好的导体（电阻率约为几欧姆），横向范围约为4公里。它开始于地表以下1英尺处；然而，它的深度无法解决。在我们之前的研究中，在一个直径约为3kin的山顶陨石坑周围发现了一个强烈的正自我电位（SP）异常（约2000 mV）。我们将良导体和正SP异常的存在解释为活跃热液系统的强烈指示。随后，我们寻找传导电流源，利用大地电磁法反演得到的电阻率结构解释地表SP分布。得到的结果是，在导体的顶部应该有一个1000 a左右的正传导电流源。从这些结果，我们推断导体代表一个热液系统，其中单相（液体）对流正在发生。由于离良导体一段距离处的电阻率可以用冷地下水的作用来解释，所以热液系统似乎并没有延伸到整个火山，而似乎仅限于山顶火山口下面的区域。最后，利用毛细管模型中流体体积通量与电流密度之间的关系，估计了地下热液流动的数量级。结果表明，富士山浅部渗透率较低。我们推测火山的低渗透率与热液系统的限制和火山活动的静止有关，如低地震活动，无气体排放，无天然温泉。(c) 2005 Elsevier B.V.版权所有
Wideband magnetotelluric (MT) soundings were carried out on Mt. Fuji volcano along a northeast to southwest axis. It was found by two-dimensional inversion using the highest quality data (in the frequency range 1-300 Hz) that a good conductor (resistivity of approximately a few ohm in) was located beneath the Summit with a lateral extent of approximately 4 km. It begins approximately I kin below the ground surface; however, its depth cannot be resolved. In our previous study, an intense positive self-potential (SP) anomaly (approximately 2000 mV), was found around a summit crater having a diameter of approximately 3 kin. We interpreted the presence of the good conductor and positive SP anomaly as a strong indication of an active hydrothermal system. Subsequently, we searched for conduction current sources to explain the SP distribution on the surface by using the resistivity structure determined by the MT inversion. The results obtained were that a positive conduction current source of the order of 1000 A should be located at the top of the conductor. From these results, we deduced that the conductor represents a hydrothermal system in which single-phase (liquid) convection is taking place. Since the resistivity at a distance from the good conductor can be explained by the effect of cold groundwater, the hydrothermal system does not seem to extend throughout the entire body of the volcano, but seems to be confined to the area beneath the summit crater. Finally, an estimate of the order of magnitude of the subsurface hydrothermal flow was performed using a relation between the fluid volume flux and electric current density in the capillary model. The result suggested that there exists fairly low permeability within the shallow part of Mt. Fuji. We speculate that the low permeability in the volcano has a correlation with the confinement of the hydrothermal system and quiescence of volcanic activities, such as low seismicity, no gas emanations, and no natural hot springs. (c) 2005 Elsevier B.V. All rights reserved.