College of Geo-exploration Science and Technology, Jilin University, Changchun 130026, China

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 10 Ω·m. Therefore, the resistivity iso-surface of 10 Ω·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.