Three‐dimensional topographic and bathymetric effects on magnetotelluric responses in Jeju Island, Korea
Three‐dimensional topographic and bathymetric effects on magnetotelluric responses in Jeju Island, Korea
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DOI：
10.1111/j.1365-246x.2008.03993.x
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发表时间：
2009-02
影响因子：
2.8
通讯作者：
M. Nam;H. Kim;Yoonho Song;T. Lee;J. Suh
中科院分区：
文献类型：
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作者：
M. Nam;H. Kim;Yoonho Song;T. Lee;J. Suh
摘要 大地电磁 (MT) 勘测旨在调查韩国南海济州岛的火山结构。岛呈椭圆形，宽31公里，长73公里，最高峰汉拿山位于岛中央，海拔1950米。 MT 数据是在中山地区以各种频率获得的，可能会受到地形和测深效应造成的电场畸变的影响。本文的目的是使用边缘有限元方法研究具有济州岛地形和测深起伏的四层模型的地形和测深对 MT 数据的影响。该模型有效地再现了岛上观测到的 MT 测深曲线的一般特征。地形效应仅出现在高频下，而海洋效应会导致低频下不同极化的视电阻率曲线分裂。由于除了汉拿山附近以外，该岛的坡度都小于10°，因此具有与顶层厚度变化类似的效果。相比之下，海洋效应可以沿岛的主轴线观察为准二维而不是三维，并在低频下产生对数视电阻率曲线的平行移动。
SUMMARY Magnetotelluric (MT) surveys have been conducted to investigate the volcanic structure in Jeju Island in the South Sea of Korea. The island is oval in shape, 31 km wide and 73 km long, with a peak elevation of 1950 m at Mt. Halla, located in the centre of the island. MT data were obtained in a wide range of frequencies in a mid-mountain region and may be affected by distortion of electric fields due to topographic and bathymetric effects. The purpose of this paper is to investigate the topographic and bathymetric effects on MT data for a four-layered model having the topographic and bathymetric undulation of Jeju Island, using an edge finite-element method. The model effectively reproduces a general feature of MT sounding curves observed on the island. The topographic effect appears only at high frequencies, whereas the sea effect causes a split of apparent resistivity curves in different polarizations at low frequencies. Since the island has a gentle slope of less than 10° except in the close vicinity of Mt. Halla, it has a similar effect of a change in thickness in the top layer. In contrast, the sea effect can be observed as quasi-2-D rather than 3-D along the major axis of the island and generates a parallel shift of the log-apparent resistivity curve at low frequencies.