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Inversion of magnetotelluric (MT) responses has been used to explore the electrical conductivity distribution of the Earth's interior. In three-dimensional (3-D) inversions, it is significant to use a good initial model, because final model obtained by most 3-D inversion methods is influenced by the initial model. Although uniform initial models are widely used in 3-D inversions, one-dimensional (1-D) initial models are alternatives, which could more appropriately represent the actual conductivity variations in the Earth's interior. This study presents a two-step 3-D inversion method, especially for marine cases. This inversion method first concentrates on obtaining a 1-D initial model and then inverts for 3-D conductivity structures with it, in both of which the 3-D topography is carefully taken into consideration. This method was tested by synthetic models of different topography variations (depression-shaped, smoothly varying, channel-shaped and square-shaped plateau topography) and of heterogeneous layers with different checkerboard-type anomalies (sharp or smooth lateral conductivity variations) embedded in 1-D model of depth-dependent conductivity. The comparisons were done about obtaining an initial model by the proposed method and that inverted from the corrected responses. The results of 3-D inversions by using the method of this study were also compared to that with different uniform initial models. Results of synthetic tests and comparisons were discussed by using directional information of newly introduced model-vector parameters. The performance and validity of this method was verified. It also revealed that some of the newly introduced model-vector parameters could be used to show the convergence of inversions and help to select inverse model. [ABSTRACT FROM AUTHOR]

Landslides are a type of natural disaster that can cause substantial harm to humanity. Monitoring and predicting the initiation of potential landslides is critical to avoiding losses due to disasters and economic activities. The impact of the controlled-source audio-frequency magnetotelluric method on investigating landslide surfaces is assessed through numerical simulations with a finite element approach. A Dirichlet boundary condition is selected to match the truncated boundary, resulting in a remarkable improvement in simulation efficiency. Rederivation of the formulas for a layered medium adept to the controlled-source audio-frequency magnetotelluric method is necessary to determine the electromagnetic field at any location along the truncated boundary. After the reliability evaluation of the new codes, a landslide model with a slide surface is designed, and the characteristics of its electromagnetic field and the apparent resistivity are studied. Instead of the total electromagnetic field, which is strongly influenced by topography variation, the apparent resistivity should be used for sliding surface detection. The normalized pure anomalous electromagnetic field may also be employed to quickly assess the detectability of the sliding surface. Overall, this study demonstrates that the controlled-source audio-frequency magnetotelluric method can be employed for investigating landslides, and recommends survey parameters, including configuration, frequency range, and length of survey line in landslide exploration. [ABSTRACT FROM AUTHOR]

Geophysics offers an important means to investigate the physical processes occurring inside the earth. In particular, since the 1960s, electromagnetic (EM) methods have played important roles in mineral exploration and engineering investigation. Such investigation requires extensive data acquisition and experimental analysis based on geophysical techniques. However, high-performance geophysical equipment, particularly EM exploration equipment, has been dominated by large geophysical companies from the United States, Canada, Germany, and other European countries for decades. This has limited the development of deep exploration technology in China. Recently, we have developed a high-resolution acquisition system with a wireless control unit and a high-power transmitting system for surface EM prospecting (SEP). The new system has been tested in the high-intensity, noisy environment in Jian-sanjiang area, Heilongjiang Province. We then conducted a field survey on the western edge of Qaidam Basin, Qinghai Province. A highly conductive anomaly was found in the upper mantle below the Qinmantage Mountains, which indicates a possible northward fluid channel that runs from below the Qinmantage Mountains to the bottom of the western crust of the Qaidam Basin. Identification of this significant feature was made possible by the new SEP for its better resolution than the previous systems. Also, geophysical analysis confirmed that the thick Cenozoic sediments of the Qaidam Basin are underlain by rigid Precambrian basement rocks and are characterized by a series of folds. The resistivity profile indicates that the Qaidam Basin was formed due to the folding structures in the northern part of the Qinghai-Tibet Plateau which provided an additional evidence for the uplifting of the Qinghai-Tibet Plateau. [ABSTRACT FROM AUTHOR]

The North China Craton is the oldest continental block, and has suffered from large-scale lithospheric thinning and destruction, which in turn led to gold deposits in northern China. The decratonic gold deposits in the North China Craton became the most important gold deposits in China, and geophysical methods are key means to detect and discover gold deposits there. In this paper, based on the geological and petrophysical characteristics of the North China Craton, the geological model of the decratonic gold deposits is transformed into a geophysical model. At present, two methods of geophysical exploration of decratonic gold deposits are in use: rapid and efficient exploration on the scale of the ore concentration area, and large depth exploration on the scale of the deposit area. In detail, the airborne electromagnetic, magnetic and gravity methods are used to detect the shallow (1,500 m) anomaly area on the scale of the ore concentration area. Through the ground-controlled source electromagnetic and ground magnetotelluric methods, explorations for targets at significant depth (5,000 m) are carried out in the mining area. Then, taking the Liaodong ore concentration area as an example, geophysical methods are used to discover two prospecting areas around the Jianshanzi Fault in the Qingchengzi ore concentration area, Baiyun-Xiaotongjiapuzi deep prospecting area, and Qingchengzi deep prospecting area. Next, three prospecting areas are delineated around the Jixingou Fault in the Wulong mining area, Wulong deep prospecting area, Weishagou deep prospecting area, and Chang'an deep prospecting area. The anomalies in the ore concentration area and mining area are revealed by means of three-dimensional exploration methods, thereby providing technical support for the exploration of metal minerals such as decratonic gold deposits. [ABSTRACT FROM AUTHOR]