3D joint inversion of controlled-source audio-frequency magnetotelluric and magnetotelluric data

Different geophysical exploration methods have significant differences in terms of exploration depth,especially in frequency domain electromagnetic(EM)exploration.According to the definition of skin depth,this difference will increase with the effective detection frequency of the method.As a result,when performing three-dimensional inversion on single type of EM data,it is not possible to effectively distinguish the subsurface geoelectric structure at the full scale.Therefore,it is necessary to perform joint inversion on different type of EM data.In this paper we combine the magnetotelluric method(MT)with the controlled-source audio-magnetotelluric method(CSAMT)to study the frequency-domain three-dimensional(3D)joint inversions,and we use the unstructured finite-element method to do the forward modeling for them,so that the numerical simulation accuracies of different electromagnetic methods can be satisfied.By combining the two sets of data,we can obtain the sensitivity of the electrical structure at different depths,and depict the full-scale subsurface geoelectric structures.In actual mineral exploration,the 3D joint inversion is more useful for identifying subsurface veins in the shallow part and blind mines in the deep part.It can delineate the morphological distribution of ore bodies more completely and provide reliable EM interpretations to guide the mining of minerals.

Thre-dimensional topographic responses in MT using finite diference method

In this paper, the magnetotelluric response to 3-D earth topography was studied using the finite difference method for rectangular cells. Three numerical modeling calculations were carried out; the results indicate that the finite difference method is applicable to the magnetotelluric problem in estimating the 3-D topographic responses.

Magnetotelluric data reveals subduction polarity and reactivation of the Mudanjiang suture zone,Northeast China

The Jiamusi and Songnen blocks converged in the easternmost segment of the Central Asian Orogenic Belt as a result of the subduction and subsequent closure of the Mudanjiang oceanic plate during the Permian-Jurassic.The Mudanjiang suture zone was later directly affected by subductions of the Paleo-Pacific plate and Pacific plate and is therefore an ideal place to study the subduction polarity and later transformation of a paleo-suture zone.Using three-dimensional inversion of magnetotelluric data collected along a 160-km-long profile across the Mudanjiang suture zone,we established a resistivity model of the suture zone and adjacent area.Our results reveal the subduction polarity and subduction trace of the Mudanjiang oceanic plate and provide geoelectrical evidence for reactivation of the Mudanjiang suture zone induced by the(Paleo-)Pacific plate subduction.The suture zone shows a complex conductive structure.The west-dipping crustal-scale conductor beneath the Songnen-Jiamusi collision zone represents the fossil subduction zone and indicates the westward subduction polarity of the Mudanjiang oceanic plate.Furthermore,the Mudanjiang fault identified by surface geology does not fully represent the deep structure of the Mudanjiang suture zone.The definition of the suture zone should be extended to the whole conductive region with a lateral extent of~70 km.Solid conductive minerals beneath the arc in front of the subduction zone were exhumated up from deep to the upper crust.The“chimney”-shaped conductor connected with the mantle represents the intrusive pathways of mantle-derived materials,suggesting that the Mudanjiang suture zone was reactivated by subductions of the Paleo-Pacific plate and Pacific plate,leading to remelting of the cooled and crystallized materials in the pathways.Therefore,subduction of the(Paleo-)Pacific plate destroyed the lithospheric structure of the paleo collision zone in the eastern segment of the Central Asian orogenic belt,and the large-scale crustal conductor beneath the suture zone reflects reactivation of the paleo-suture zone.

Three-dimensional electrical resistivity structure beneath the Cuonadong dome in the Northern Himalayas revealed by magnetotelluric data and its implication

The North Himalayan gneiss domes(NHGD),as one of the extensional structures widely distributed across the southern Tibetan Plateau,are an important window for studying post-collisional diastrophism and magmation as well as polymetallic mineralization.However,the deep mechanism for the formation of NHGD remains controversial.The magnetotelluric(MT)method was adopted to study the deep structure of the Cuonadong dome in the Northern Himalayas.The characteristics of the dome were explored by using the MT sounding curves and phase tensors.Three-dimensional(3D)MT inversion was performed to determine the electrical resistivity structure beneath the Cuonadong dome.The preferred 3D electrical resistivity model shows that an obvious low-resistivity anomaly develops beneath the Cuonadong dome which is overlaid by a high-resistivity body and surrounded by an apparent subcircular zone of low-resistivity anomalies.The integrated conductivity(longitudinal conductance)from depths of 1-20 km indicates that the average longitudinal conductance at the core of the Cuonadong dome is about 10,000 S.The high-conductivity anomaly at the core is found to be analogous to that of lava,mainly resulting from the crustal partial melting,and the estimated melt content is 11.0-17.3%.The high conductance surrounding the dome reaches 20,000 S on average,which is mainly attributed to saline fluids.MT results in this study support that the Cuonadong dome experienced magmatic diapirism.Taken together with previous geological and geochemical studies,we suggest that under the east-west(E-W)extensional tectonic setting in southern Tibet,deep crustal partial melting constantly accumulated beneath the dome,and therefore the magmatic diapirism resulted in the formation of the Cuonadong dome.In addition,the MT results also indicate that the development of the Cuonadong dome provides abundant mineralizing fluids and the space for migration of metallogenic fluids for(rare-metal)polymetallic mineralization.