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Electromagnetic (EM) fields excited by a vertical electric dipole (VED) at the super‐low frequency and the extremely low frequency have been widely used in different studies such as space weather, earthquake predictions, geophysical investigations, and communications. Accurate and efficient algorithms are necessary for quick estimations of worldwide EM fields. This study proposes a recursive algorithm to calculate EM fields excited by a VED embedded in a layered spherical model. A set of local/general reflection and transmission coefficients is defined to describe propagations of EM fields in each spherical layer and the outgoing‐type waves and the standing‐type waves of EM fields are expressed by normalized spherical Bessel/Hankel functions. The normalized spherical Bessel/Hankel functions are not only useful to overcome the overflow problem in calculations of spherical Bessel/Hankel functions but also helpful to speed up the convergence of summation expressions of EM fields, because the normalized spherical Bessel/Hankel functions mitigate small oscillations terms. Calculating the primary fields and the secondary fields of the total EM fields, respectively, is also helpful to reach convergent summations, because their convergence radiuses are different. Key Points: We propose a new convergent expression of electromagnetic fields with normalized spherical Bessel and Hankel functions at super‐low frequency/extremely low frequencyThe normalized spherical Bessel and Hankel functions are helpful to overcome the overflow problem and then to accelerate its convergenceWe estimate the primary electromagnetic fields and secondary fields separately and it is helpful to accelerate its convergence [ABSTRACT FROM AUTHOR]

Full‐waveform inversion of surface waves can provide high‐resolution S‐wave velocity (Vs) of the shallow subsurface and is becoming a popular shallow‐seismic method. We propose a misfit function based on instantaneous‐phase coherency, which can measure the amplitude‐unbiased coherency between measured and synthetic data. The instantaneous‐phase coherency was once the key component that was used in the phase‐weight stacking technology to enhance the weak but coherent signals. Using synthetic data, we show that our full‐waveform inversion approach based on the proposed misfit function is robust in reconstructing subsurface anomalies from data contaminated by random noise. We also show that our misfit function is robust against the errors in the estimated source wavelets. We then choose to use published field data acquired at an archaeological site as a benchmark dataset to test the performance of our full‐waveform inversion in a real environment. Subsurface structures identified in our inversion results are verified by an independent archaeological excavation, while the results from conventional full‐waveform inversion are dominated by artefacts. The results of synthetic tests and field data experiments demonstrate the robustness of our full‐waveform inversion approach in reconstructing the shallow subsurface structure from field data, where amplitude information of recorded wavefield may not be correctly recorded. [ABSTRACT FROM AUTHOR]

The article discusses research which assessed the behavior of downhole electromagnetic telemetry (EMT) in deep subsurface prospecting and the characteristics of uphole EMT with repeater system. Topics covered include the performance of downhole EMT during sudden water influx into adjacent formation during directional drilling, the characteristics of downhole EMT in deep horizontal well, and the behaviors of the EMT system with oil based mud (OBM) and water based mud (WBM).

Taohemu deposit is a superlarge silver‐polymetallic deposit discovered in the Great Xing'an Range metallogenic belt of China in recent years. The deposit occurs in the terrestrial volcanic rock sequences of Late Jurassic age. The ore bodies consist mainly of rhyolitic tuffs, brecciated tuffs, and volcanic breccia bearing Ag‐Pb‐Zn mineralization, and are largely conformable with the surrounding layered volcanic and volcaniclastic rocks with a gradual transitional relationship manifesting as strata‐bounded features in general. The mineralization system appears to be generally controlled by a paleo‐volcanic chamber. It is suggested that the late volcanic hydrothermal fluids flowed within the permeable layers of rhyolitic pyroclastics consisting mainly of volcanic breccias and breccia‐bearing tuffs. This resulted in the formation of superlarge Ag‐Pb‐Zn polymetallic ore deposit metasomatism and fracture filling. The majority of the deposit occurs as shallowly dipping concealed ore bodies, as defined by sparsely distributed drill holes. Because of its short discovery history, new mineralization style and low degree of comprehensive research, the boundaries of most ore bodies have not been effectively delineated along vertical and lateral extensions. At present, it is crucial to strengthen the comprehensive studies on the deposit, conduct more detailed geological exploration, and establish the metallogenic and prospecting models. For this reason, we have designed and implemented long sections of magnetotelluric (MT) and controlled‐source audio magnetotelluric (CSAMT) sounding across the Taohemu ore field and obtained a large number of geological structures and valuable underground exploration results that provided reliable geophysical support for promoting deep resource exploration. Plain Language Summary: The geological structures associated with metallogenic system is usually covered by surface vegetation and Quaternary layer. Furthermore, surface mineralization is not relevant to deep ore bodies. To reveal the shape of ore bodies and geoelectrical structures in this study area, we applied geophysical electromagnetic methods to achieve this purpose. The results showed that the ore bodies are largely conformable with the surrounding rocks as strata‐bounded features in general tested by borehole information. The mineralization system appears to be generally controlled by a paleo‐volcanic chamber. Key Points: Underground geoelectric structures are imaged from MT and CSAMT exploration over Taohemu superlarge silver‐polymetallic deposit, Inner MongoliaOre bodies are conformable with surrounding volcanic and volcaniclastic rock with a gradual transitional relationship manifesting as strata‐bounded featuresThe mineralization system appears to be generally controlled by a paleo‐volcanic chamber [ABSTRACT FROM AUTHOR]

Previously, the multi‐channel transient electromagnetic (MTEM) system has been successfully developed and applied to hydrocarbon reservoirs exploration on land and offshore environments in France. In order to explore the mineral resources buried at the depths up to 2,000 m in China, we developed a new MTEM system. In this paper, we present the components of the system, methods, principles, the parameters of the system, and the data interpretation method. To validate the effectiveness of the methodology and the developed system, we carried out an MTEM survey to locate the controlling faults of a gold deposit in the east of China. The survey contains 30 receivers distributed over 12‐km survey. By comparing the MTEM interpretation results with the known geological background, we find that MTEM methods can provide a good estimation of the ore‐controlling faults and this method could be a potential tool for the future deep mineral exploration. [ABSTRACT FROM AUTHOR]

Three emerging techniques for surface electromagnetic (EM) exploration with high detection accuracy and large depth of investigation, namely, WEM (wireless electromagnetic method), MTEM (multi‐channel transient electromagnetic method) and SOTEM (short‐offset grounded‐wire TEM), are introduced. In detail, we proposed a newly developed sky‐wave theory in which EM wave can propagate in full space of ionosphere‐atmosphere‐lithosphere. Correspondingly, we developed the WEM recording system to measure the sky‐wave to obtain the structures down to ∼10 km depth. We then introduced MTEM as a highly effective method for mineral exploration with target depth of 3km. Using self‐developed MTEM system, we presented the results obtained from mineral exploration study carried out in China. Lastly, we proposed the SOTEM method for a greater detection depth (1.5km) than traditional loop source TEM (500m) with a small offset and high resolution. [ABSTRACT FROM AUTHOR]

Electromagnetic telemetry is bi‐directional transmission of borehole data for effective drilling, especially in unconventional wells. In this study, numerical simulations was used to investigate the possibility of applying EM telemetry for measurement while drilling (MWD) in an ultra‐deep borehole in the Tahe region of the Tarim basin, using the drill string as an antenna excited as a gaped dipole source to quantify spatial variations of the electromagnetic field from subsurface to the surface. Results confirm that the distribution of the electric field along the drill string is sensitive to vertical variations of electric conductivity in the host rock. The simulated results show a high attenuation within the conductive formations, thereby limiting the effectiveness of EM telemetry in data transfer to the surface. In addition, the changes in thickness and resistivity of the target region affects the amplitude of the desired response. [ABSTRACT FROM AUTHOR]