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With fast development of Chinese economy, a large amount of garbage have been produced. Especially, high level radioactive waste (HLRW) bring dangers to the security of human being. Finding a suitable site to dispose HLRW is a key management to government. A potential HLRW disposal site has been chosen in west north of China at first, however, it is necessary to understand whether there exist electrical structures inside the rock mass, whether the rock mass is intact or not. We applied the controlled source audio-frequency magnetotelluric method (CSAMT) to confirm that. Three-dimensional numerical modeling of finite difference method was performed for typical structure in the study area, and response characteristics were analyzed to confirm which actual frequency range can be chosen and what structure dimension can be detected. Then we laid out 14 parallel survey lines to carry out data acquisition. By analyzing typical sounding curves, combined with drilling information and the inversion results, we suggest that the rock is intact and suitable for building a repository, which is a good recommendation for government department. [ABSTRACT FROM AUTHOR]

We conducted broadband Magnetotelluric survey to obtain 2D lithospheric electrical structures of Alxa block through two NE-SE trending profiles, which revealed the electrical evidence of bidirectional subduction of Paleo-Asian Ocean with spatial difference beneath Enger Us fault belt. [Display omitted] • Lithospheric electrical structures of the Alxa block were imaged through two NW-SE BMT profiles. • There was bidirectional subduction of the Paleo Asian Ocean beneath the Enger Us fault belt. • The low resistivity bodies of the middle and upper crust in the northwest are related to large-scale horizontal nappe tectonics. • The high conductivity bodies that extend deep in the southeast are attributed to the partial melting. The Alxa Block, situated between the Tarim and North China cratons, is essential to understand the tectonics of northwest China. It has undergone several tectonic cycles, including rifting from northern Gondwana in Neoproterozoic, subduction-accretion of the Paleo-Asian Ocean during Paleozoic, and intracontinental deformation in Mesozoic. Broadband magnetotelluric data covering a frequency range of 320 Hz to ∼ 10000 s was collected along two NW-SE trending lines across major tectonic units of the Alxa region to investigate the deep structures of the Alxa Block. By applying the nonlinear conjugate gradient (NLCG) inversion approach, two-dimensional (2D) electrical resistivity models were obtained at a 100 km depth. These findings indicate that the Zhusileng-Hangwula and Zongnaishan-Shalazhashan tectonic zones have high resistivity in their lower crust and upper mantle, with an overall layered high-low–high resistivity pattern. The upper crust of the Nuru-Langshan tectonic zone is primarily characterized by high resistivity values, whereas the lower crust and upper mantle contain a large-scale low-resistivity (or conductive) zone that is likely the result of partial melting. The large-scale Early–Middle Jurassic nappe structures may be connected to sub-horizontal low-resistivity anomalies in the shallow strata, which are located up to 20 km below the Yagan and Zhusileng–Hangwula tectonic zones. Below the Engger Us fault, two low-resistivity bands are interpreted as the product of bidirectional subduction and final closure of the Paleo-Asian Ocean. The Alxa Block may have subducted southwards beneath the Ordos Block, as suggested by a clear low-resistivity anomaly between the two blocks. Furthermore, partial melting may have happened for high-conductivity bodies in the upper mantle based on the modified Archie formula's evaluation of their melting degree. [ABSTRACT FROM AUTHOR]

Equivalence causes non-uniqueness in interpretations of the grounded-wire transient electromagnetic method (TEM). Many different layered models may produce the same response curves practically. However, the conventional theory of equivalence is based on the three-layer geo-electric models while the earth may not be reducible to a simple three-layer structure. Therefore, it is important to perform research on equivalence based on more complex models to reduce the uncertainty the grounded-wire TEM interpretations. Here, we extend the theory of equivalence based on a multi-layer target in the grounded-wire TEM, consisting of SS-, HH-, and TT-type. The analysis of relative error is applied to investigate the effect of measured components and receiver locations on equivalence. The results show that equivalence of multi-model occurs when any of three conditions is satisfied: the longitudinal conductance is constant in the far area, the thickness of targets do not change, or the transverse conductance of a multilayer target remains unchanged. In order to largely resolve the effect of equivalence on data interpretation, a strategy combined with short-offset survey and joint inversion of the electric component and magnetic component is presented and verified by the field survey in Shanxi Province, China. • Equivalence causes non-uniqueness in interpretations of the grounded-wire TEM method. • The conventional theory of equivalence is based on the three-layer geo-electric models. • It is important to perform research on equivalence based on more complex models. [ABSTRACT FROM AUTHOR]

• The gold deposits of the Qingchengzi orefield are controlled by faults. • The Jianshanzi fault zone is the major ore-controlling structure. • A magnetotelluric study was undertaken along the The Jianshanzi fault suggesting excellent potential for new, deep-seated gold discoveries in the Qingchengzi orefield. The Liaodong Peninsula in northern China has undergone multi-stage magmatic and tectonic modifications, similar to those recorded by the Jiaodong gold province that is the largest gold-producing district in China, indicating that Liaodong Peninsula has great potential for new discoveries. However, no significant gold deposit has been discovered thus far at depths below 1.5 km, the typical depth of the gold-bearing system in the comparable Jiaodong gold province. This study focused on the Qingchengzi orefield on the northern Liaodong Peninsula. Qingchengzi recorded multiple post-magmatic hydrothermal gold-mineralizing events along the Jianshanzi fault zone, the main ore-controlling structure in the orefield. Given the potential for undiscovered, deep-seated gold mineralization along the Jianshanzi fault zone, we performed a magnetotelluric (MT) study designed to identify any deeper exploration targets. The surface electromagnetic prospecting (SEP) system, a portable instrument suitable for use in electromagnetically mountain environments, was employed for the purpose of acquiring the MT data. Shallow conductive anomalies identified in the vicinity of the Jianshanzi fault zone were tested by known drilling recorder. The survey not only helped to generate a detailed map of the deep geological architecture, but also allowed us to estimate the vertical extent of the known gold orebody to a depth of more than 2 km. [ABSTRACT FROM AUTHOR]

The Tianshan-Xingmeng Orogenic Belt (TXOB) is located in China's northern region, and is one of the country's most important Au-Pb-Zn-Ag-Mo-Cu metallogenic belts. This metallogenic belt is known to have good minerogenetic conditions and exploration potential. However, the engineering detection depths of the majority of the metal deposits are generally less than 300 m. Therefore, high precision geophysical detection techniques are necessary to achieve the required accurate detection results. Among the commonly used techniques, the Stratagem EH4 system has been found to be the better choice for the deep prospecting of the metal deposits in the TXOB. Furthermore, a good understanding of the physical characteristics of the different rock formations in the region is essential for effective and efficient geophysical prospecting. This study revealed that the resistivity values of the ore and rock formation walls were different. In previous related research, the EH4 Conductivity Image System, which is a system combining MT and CSAMT, has been successfully used for ore explorations in China. The results of the geophysical prospecting processes indicated that there was good exploration potential of the aforementioned system in the TXOB. In the current study, the hidden gold ore bodies of disseminated (altered rock-type) gold deposits within the study area were defined by resistivity values of less than 150–1000 Ωm. The concealed gold orebodies in the quartz-type gold deposits were confirmed by resistivity values of 300 Ωm. The mineralization anomalies of the epigenetic lead-zinc deposits were defined by the smaller resistivity values, which were lower than the 100–1000 Ωm range. [ABSTRACT FROM AUTHOR]