The Study Of Three Dimensional Lithospheric Conductivity Structure Of Lhasa-gangdese With MT Methods

MT is an important geophysical methods of natural electromagnetic field to study the internal electrical structure of the Earth.We use this method to obtain a three-dimensional electrical structure of the lithosphere.Lhasa-Gangdese is located in the south of Tibet-Pleatu,between the Bangonghu-Nujiang suture zone and Tsangpo suture zone.This paper is an application of MT method to study three-dimensional lithosphere electrical structure of Lhasa-Gangdese.We conclude that the main electrical axis of this area is near east-west direction,thus indicating that the underground electrical structure of this area is east-west direction.Six north-to-south profiles are set up to acquire data of electromagnetic field which then used to do inversion with two-dimensional algorithm.And the information of the three-dimensional electrical structure of the lithosphere is obtained with the location of faults in this research area.According to the six electrical models of magnetotelluric sounding profiles in Lhasa-Gangdese we suggest that the electrical structure of this region is complicated,it can be generally distributed as regions from south to north and blocks from east to west.The eastern part of this research area can be broadly divided into five electrical layers with the second layer and the forth layer of high conductivity.However the western part of this research area has different layers from south to north.The southern area can be broadly divided into five electrical layers with the second layer and the forth layer of high conductivity and the northern area can be broadly divided into three electrical layers with the middle layer of high conductivity.And we found the evidence of these conclusions from the results of the three-dimensional inversion using OCCAM algorithm and the results of two-dimensional inversion using NLCG algorithm of the long profile.And we speculate that Bangongcuo-Nujiang suture zone is a giant-super-deep fault zone,cutting the Earth's layers from south to north.This electrical fault zone's northern edge of is Bangongcuo-Cantor-Nujiang fault and the southern edge are Dingqing-Ritu-Gaize fracture and Gaer-Guchang-Wurucuo fault from west to east.The Yarlung Zangbo suture zone is also an steep fault zone,with the northern edge named Dajiweng-Langxian fault and the southern edge named Zhada-Lazi-Qiongduojinag fault.What's more,the three-dimension models of Lhasa-Gangdese area indicate that,the "double high conductivity layers" in the research area is ubiquitous,however it is not always obviously seen from the two-dimension inversion models,that is related to the distance of the two layers.The larger distance between the two layers,the more obvious they seems.But if the first conductive layer is too thick then these two high conductivity layers will be mixed together.Moreover,by appling the distance-weighting interpolation method in this research,we found that this method is effective in magnetotelluric field acquisition to eliminate the small local anomalous bodies.When the site is on the top of an anomalous body,we can still use other sites that around it to eliminate the effects of this local anomaly.