The exploration of podiform chromites in the Indus Yarlong Zangbo suture zone of southern Tibet has proved difficult because most known deposits pinch out and then reappear in the same direction. Several ground-based geophysical approaches such as gravity, magnetic, and controlled-source audio-frequency magnetotelluric (CSAMT) methods have been applied to explore for these chromite deposits but have mostly failed to delineate prospective areas. We have evaluated a successful podiform chromite exploration case history that is based on AMT. More than 8000 AMT stations were used in this study within a [ ] area of the ophiolite belt. Line separations were 80 or 40 m, and the station separation was 20 m. We implemented Bostick conversion and nonlinear conjugate gradient inversions for data interpretation, whereas 2D resistivity sections and 3D resistivity imaging were used to elucidate the inner structure and distribution of rock faces within the Luobusa ophiolite. Results from rock physics and drilling further indicate that resistivity-anomaly domains from these AMT results are correlated with rock faces in terms of fresh harzburgite, altered harzburgite and dunite, and they can thus be connected to concealed deposits. Therefore, we have developed three resistivity-anomaly models for chromite exploration, and we delineated several prospective regions containing exploitable deposits within the Luobusa ophiolite. Seven of the nine verified boreholes discussed in this paper intersected with chromite deposits; one comprises the largest and highest grade chromite deposit in China to date. Our AMT results provide the impetus for future chromite exploration in Tibet and enable a refined understanding of the structure and distribution of rock faces within the Luobusa ophiolite.

A successful case history of applying the high-frequency passive source electromagnetic (EM) method and controlled-source audiomagnetotellurics (CSAMT) to investigate the Qiyueshan (Q) Tunnel route is presented in this paper. The high-frequency EM system (EH-4, with frequency range from 90 KHz to 12.8 Hz) and the CSAMT system (V6-A Multipurpose Receiver, with frequency range from 8,192 Hz to 0.125 Hz) were used for the data acquisition. The orthogonal components of the electromagnetic field were measured in the high frequency EM method, while scalar measurements of the electrical and magnetic field components were used in the CSAMT method. The relevant electrical properties of the earth were extracted from the electromagnetic profiles. High frequency EM has high resolution in the shallow earth but a smaller depth of exploration, while the CSAMT method has a powerful signal but a lower resolution in the shallow earth. The integration of the two methods might be effective for the survey of the deep tunnel route. Q Tunnel, located in central south China, has a length of 10 km and a depth of up to 900 m. Half of the tunnel goes through karst terrain, where the geologic structures are very complex, due to cavities, underground rivers and faults. The EM mapping results distinguish the electrical resistivity of different rock formations. Five low-resistivity areas and four high-resistivity areas were found and nine faults were verified by the EM method. These findings were very useful for the later engineering design.

Southwestern Tibet plays a crucial role in the protection of the ecological environment and biodiversity of Southern Asia but lacks energy in terms of both power and fuel. The widely distributed geothermal resources in this region could be considered as potential alternative sources of power and heat. However, most of the known geothermal fields in Southwestern Tibet are poorly prospected and currently almost no geothermal energy is exploited. Here we present a case study mapping the Mapamyum (QP) geothermal field of Southwestern Tibet using audio magnetotellurics (AMT) and magnetotellurics (MT) methods. AMT in the frequency range 11.5–11,500 Hz was used to map the upper part of this geothermal reservoir to a depth of 1000 m, and MT in the frequency range 0.001–320 Hz was used to map the heat source, thermal fluid path, and lower part of the geothermal reservoir to a depth greater than 1000 m. Data from 1300 MT and 680 AMT stations were acquired around the geothermal field. Bostick conversion with electromagnetic array profiling (EMAP) filtering and nonlinear conjugate gradient inversion (NLCGI) was used for data inversion. The AMT and MT results presented here elucidate the geoelectric structure of the QP geothermal field, and provide a background for understanding the reservoir, the thermal fluid path, and the heat source of the geothermal system. We identified a low resistivity anomaly characterized by resistivity in the range of 1–8 Ω∙m at a depth greater than 7 km. This feature was interpreted as a potential reflection of the partially melted magma in the upper crust, which might correlate to mantle upwelling along the Karakorum fault. It is likely that the magma is the heat source of the QP geothermal system, and potentially provides new geophysical evidence to understand the occurrence of the partially melted magmas in the upper crust in Southwestern Tibet.

Hydraulic fracturing is widely used for initiating and subsequently propagating fractures in reservoir strata by means of a pressurized fluid to release oil and gas or to store industry waste. Downhole or surface microseismic monitoring is commonly used to characterize the hydraulically induced fractures. However, in some cases, downhole microseismic monitoring can be difficult due to the limitation imposed by boreholes. Surface microseismic monitoring often faces difficulties acquiring high signal-to-noise ratio data because of the on-site noise from hydraulic fracturing process. Research and field observations indicate that injecting conductive slurry or water into a strata may generate distinct time-lapse electromagnetic anomalies between pre- and posthydraulic fracturing. These anomalies provide a means for characterizing the hydraulic fracturing using time-lapse electromagnetic methods. We examined the time-lapse variation over an hour, one day, one month, and two years of observed audio-magnetotellurics (AMT) resistivity and the 1D and 3D AMT modeling result of the variation pre- and posthydraulic fracturing. There is also a successful case history of applying the time-lapse AMT to map hydraulic fractures. Observed data indicate that the variation of AMT resistivity is normally less than 6% apart from the data of the dead band and some noisy data. Modeling results show the variation pre- and posthydraulic fracturing is larger than 30% at the frequency point lower than 100 Hz. The case history indicates that time-lapse magnetotelluric monitoring may form a new way to characterize the hydraulic fracture.

Karst cavity mapping is attracting great interest from engineering geologists because of its relation to the dangerous geohazards faced during engineering construction. Ground-based geophysical methods still face challenges in karst mapping, and concealed karst cavities potentially pose threats to tunnel construction in southwest China. Given the significant contrast in electrical resistivity between karst cavities and their host rocks, geoelectrical methods are widely used for mapping these cavities. We have developed a successful case history of mapping karst cavities on a planned railway route using controlled-source audio magnetotellurics (CSAMT). Scalar CSAMT, with frequencies ranging from 0.5 to 8192 Hz, was used for field data acquisition. A full-frequency domain apparent resistivity correction method was used for near-field corrections. Electromagnetic array profiling (EMAP) filtering was used for topographic and static shift corrections, and the Bostick conversion was used for data interpretation. Our study indicated that the results of the Bostick conversion with EMAP filtering were more acceptable than the results of rapid relaxation inversion and nonlinear conjugate gradient inversion in this case. The G Tunnel is a key tunnel along the Gui-Guang high-speed railway in southwest China. Initial geophysical and engineering geologic results suggest that the bedrock of the survey section of the G Tunnel route is sandstone. A CSAMT survey with three inline sections and three crossline sections over the tunnel route was conducted in two phases to verify the rock conditions of the tunnel route. A concealed karst cavity with a low-resistivity anomaly was found on the tunnel route and was verified by the borehole. Data from the CSAMT survey significantly refined our understanding of the subsurface engineering geologic conditions along the tunnel route.

The Bayan Obo deposit in Inner Mongolia is a crucial source of rare earth elements-niobium-iron, with significant economic importance for China. The ore-bearing dolomite is believed to have originated from mantle-derived magma, indicating high prospectivity in the deeper sections. However, the deep distribution of the ore-bearing dolomite remains poorly understood. Here, we develop a 3D S-wave velocity (VS) structure down to 2.5 km depth beneath the Bayan Obo using ambient noise tomography techniques, which used 25 days of continuous waveform data recorded by 312 short-period seismometers. Combined with previous geological, geochemical, and geophysical studies, our results reveal the possible 3D spatial distribution of the ore-bearing dolomite beneath the Bayan Obo. The results indicate that the VS structures beneath the main, east, and west pits change from relatively high-velocity (high-V) zones (with VS of approximately 2.8–3.2 km/s) at shallow depths (depth to approximately 1.5–1.8 km) to significant low-velocity (low-V) zones (with VS < 2.6 km/s) at deeper depths (depth below approximately 1.8 km). We suggest that the content of ore-bearing dolomite may decrease significantly at deeper depths (depth below approximately 1.8 km) beneath these pits, which possibly indicates a diminished prospecting potential there. Our results also indicate a significant high-V zone (with VS > 3.4 km/s) at depths of approximately 1–2.5 km between the main and west pits. Its lithology remains unclear and is proposed to be a key scientific issue in the future to improve our understanding of the ore-controlling structure associated with the tectonic events in the Bayan Obo deposit.