非线性共轭梯度三维反演在山西河津铁矿音频大地电磁数据勘察中的应用

利用非线性共轭梯度反演方法,对采集到的近正交的两条音频大地电磁剖面数据构成的主阻抗数据集进行三维反演。结果显示,山西省河津市地下结构呈现较明显的三层构造特征,推测第一层高阻异常是侵入岩或变质片麻岩,第二层低阻异常是铁矿富存区,高阻的第三层异常反映研究区的花岗闪长岩基底。根据地球物理电性参数资料,铁矿体为低阻,电阻率<10Ω·m。因此,推测10Ω·m的电阻率等值面大致勾勒出大地电磁反演结果推测的铁矿的成矿有利区。在两条剖面正交位置下方,矿体规模较小;但在2号线60-100号点和3号线476-492号点的下方1 km深的位置附近发现了新矿体。结果表明,利用大地电磁法寻找类似热液型铁矿这类局部异常体,三维反演是获得可靠结果的必要技术手段;即使是剖面性观测数据,也可以进行三维反演,并取得良好效果。

Application of NLCG 3D inversion of AMT data exploration in Hejin iron deposit,Shanxi

Nonlinear conjugate gradient(NLCG)inversion method is used to conduct three-dimensional inversion of the impedance data set composed of two nearly orthogonal audio magnetotelluric profiles.The results show that the subsurface structure in Hejin of Shanxi presents an obvious three-layer structure.It is speculated that the first layer of high resistivity is intrusive rock or metamorphic gneiss,whereas the second layer of low resistivity is enriched with iron deposit,and the third layer of high resistivity represents the granodiorite basement of the study area.According to the geophysical electrical parameters,the ore body of iron deposit is of low resistivity less than 10Ω·m.Therefore,the resistivity iso-surface of 10Ω·m roughly outlines the favorable area of iron deposit predicted by magnetotelluric inversion.Under the intersecting position of two sections,the size of ore body is small,however,new ore bodies have been found at the depth of approximately 1 km below the points 60-100 of line 2 and points 476-492 of line 3.The results show that in searching for local anomalies related to hydrothermal iron deposit using magnetotelluric method,three-dimensional inversion is necessary to obtain reliable results.Three-dimensional inversion can applied even to sectional observation data to obtain good results.