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After 70 years of development, magnetotelluric (MT), a remote sensing technique for subsurface electrical resistivity imaging, has been widely applied in resource exploration and the deep tectonic evolution of the Earth. The electrical resistivity anomalies and their quantitative interpretation are closely related to or even controlled by the interconnected high-conductivity phases, which are frequently associated with tectonic activity. Based on representative electrical resistivity studies mainly of the deep crust and mantle, we reviewed principal electrical conduction mechanisms, generally used conductivity mixing models, and potential causes of high-conductivity including the saline fluid, partial melting, graphite, sulfide, and hydrogen in nominally anhydrous minerals, and the general methods to infer the water content of the upper mantle through electrical anomaly revealed by MT.

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In China, gas hydrates in onshore permafrost areas have so far only been found in the Juhugeng Mine of the Qilian Mountains. However, their subsurface distribution remains unclear. Electrical resistivity logs have revealed that zones containing gas hydrates have higher resistivity than surrounding zones, which makes electromagnetic methods viable for detecting gas-hydrate deposits. We have deployed a natural-source audio-magnetotelluric (AMT) survey at the Juhugeng Mine. AMT data were collected at 176 sites along five profiles, and resistivity models were derived from 2D inversions after detailed data analysis. After the available geologic and geophysical observations were combined, the inversion results from profile 1 suggested that permafrost near the surface with high resistivity and thickness is essential for underlying gas hydrates to be present. The decrease in resistivity and/or thickness of permafrost due to climate change may lead to gas-hydrate dissociation. The other four AMT transects suggested three prospective gas-hydrate sites. Our results indicate that the AMT survey technique is suitable for exploring gas hydrates in permafrost areas and analyzing the impact of permafrost characteristics on gas-hydrate occurrence.