Abstract: The paper presents the trial method and results with Long-offset & window Transient Electromagnetic Method (LoTEM) to investigate detailed geological structure in the area covered with carbonatite. We chose a well known local structure in Sangzhi Shimen synclinorium, Siwang mountain structure whose area is about 200 km²,to test the LoTEM. The rugged topography area is covered with carbonatite, and the surface geological condition gives priority to the complicated structure. The resistivity of the surface carbonatite is greater than 500Ω·m, and the target layer is Silurian marlite with resistivity value about 10Ω·m, and its buried depth is one to two kilometers. These give the better geophysical preconditions to the working area. Two sources were setup in the field. We finished eight lines with total 212 geophysical sites. The grid was arranged with equality (2km×0.5km).Althought there is strong cultural noise in the area, LoTEM features good anti disturbance. The rate of higher quality data is more than 80%. The EMF curve is credible before 1s and it guaranteed the data quality for interpretation. Through data processing and inversion, resistivity versus depth profiles of eight lines were obtained, and they have good coherence with the seismic profile. The results show that LoTEM can be used in prospecting for detailed structure in complicated areas.

Key words: Transient electromagnetic method, Resistivity, Structure

摘要： 详述了长偏移距瞬变电磁测深法(LoTEM)在碳酸盐岩地区进行局部构造落实的试验与效果。选择了面积约200km²的湖南桑植-石门复向斜中的四望山构造区作为试验区。工区地形复杂,地表地质条件以高陡构造为主;表层为灰岩覆盖,电阻率一般大于500Ω·m,目标层为志留系泥岩,电阻率在10Ω·m左右,埋深一般为1～2km。试验布设场源2个,测线8条,共完成物理点212个。测网基本均匀,线距2km,点距500m。尽管测区内文化干扰十分严重,由于LoTEM法具有较强的抗干扰能力,资料质量的优良率达到了80%以上,单点电动势曲线1s以前可靠,确保了资料解释质量。通过资料处理与反演,获得了该区各测线的电阻率深度剖面图,与工区内地震剖面对比的一致性较好,表明该方法能在如此复杂的地区进行局部构造落实。

关键词: 瞬变电磁测深, 电阻率, 构造