CSAMT EXPLORATION TO SHALLOW ELECTRIC STRUCTURE BENEATH THE DAIXIAN BASIN

doi:10.3969/j.issn.0253-4967.2011.04.016

Abstract

Abstract:

The Daixian Basin,a typical asymmetric half-graben basin,lies in the northern Shanxi Fault zone.It is bounded by the northeast trending Wutaishan northern piedmont fault zone and Hengshan southern piedmont fault zone.To investigate the shallow electric structures beneath this basin and the boundary faults,a CSAMT profile was applied which runs northwesterly through the basin.CSAMT data were recorded at 246 locations along the 12.55km-long profile.At each location data were recorded from 8533.333 to 1.333333Hz and processed to give estimates of apparent resistivity and phase.The profile is through the Wutaishan Fault in the southeast and arrives at the Hengshan piedmont loess hilly region in the northwest.Some data are of low signal-to-noise ratio due to ractive noises from power lines,pipelines,railways,highways and power substations.But useful data can be acquired 200meters away from the interference sources.The curves of apparent resistivity and phase have an obvious segmentation character.The shape of the curve of each section is consistent to the adjacent section and the segmentation shows good correlation with geomorphologic features.NLCG inversion was performed on the CSAMT data along this profile to reveal the electric structure within the depth of 1km from the surface in the basin.The result shows that the Wutaishan northern piedmont fault zone consists of a set of north-dipping normal faults which decline down stepwise,verging toward the Daixian Basin.Because the CSAMT profile does not run through the whole Hengshan southern piedmont fault zone,only partial features of it are revealed,i.e.the southeast-dipping high-resistivity foot wall bedrock and low-resistivity hanging wall alluvial deposits are underlain by thin-bed low-resistivity flood deposits.Besides,it is found that the Daixian Basin is laterally inhomogeneous,corresponding to its topography,where the tilting alluvial-flood plain is underlain by low-resistivity layer and the resistivity below the alluvial plain is slightly higher.The application of CSAMT to the Daixian Basin indicates that this method can be used to determine the location,dip and size of faults,and provide reliable basic data for survey of active faults.

Key words: controlled source audio-frequency magnetotellurics(CSAMT), electric structure, fault, Daixian Basin

摘要：

为研究代县盆地及其断裂的空间展布以及深部的延展情况,东南跨过五台山北麓断裂带,西北至恒山山前黄土丘陵区,布置了一条NW向穿过代县盆地的可控音频大地电磁(CSAMT)测深剖面,全长12.55km。共完成可控源音频大地电磁测点246个,观测频率为8 533.333～1.333 333Hz。视电阻率、相位曲线具有明显的分段特征,每段内测点的曲线形态和视电阻率值变化不大,其分段与地貌特征有很好的对应关系。采用NLCG二维反演方法对该剖面进行了反演,获得该测区1km以上范围内的电性剖面。沿剖面自西向东可以划分出6个电性分区,电性边界带与五台山北麓断裂和恒山南麓断裂对应得很好。结果显示,五台山北麓断裂带由一组北倾的正断裂组成,呈阶梯状逐渐下降,倾向代县盆地; 剖面没有完全通过恒山南麓断裂带,揭示了部分断裂带的特征,在薄层的低阻洪积物之下,断裂的下盘为高阻基岩,上盘为低阻冲洪积物,倾向SE; 此外还发现代县盆地横向上与地貌相呼应,具有非均匀性,洪积倾斜平原下电阻率较低,冲积平原下电阻率稍大。CSAMT在代县盆地应用的结果表明, CSAMT法能给出断裂的构造位置、倾向以及发育规模等信息,为活动断裂探测提供可靠的基础资料。

关键词: 可控音频大地电磁, 电性结构, 断裂, 代县盆地

CLC Number:

P631.3⁺25

WANG Li-feng, CHEN Xiao-bin, ZHAO Guo-ze, ZHAN Yan, TANG Ji. CSAMT EXPLORATION TO SHALLOW ELECTRIC STRUCTURE BENEATH THE DAIXIAN BASIN[J]. SEISMOLOGY AND GEOLOGY, 2011, 33(4): 928-937.

王立凤, 陈小斌, 赵国泽, 詹艳, 汤吉. 代县盆地可控音频大地电磁浅层电性结构探测[J]. 地震地质, 2011, 33(4): 928-937.

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