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Because of the relatively weak useful signal and noises with complex characteristics, the data processing of airborne electromagnetic observation is very difficult. As the downstream of data processing, inversion generally cannot further distinguish whether there are residual noises in the processed data. That is, in the current processing flow, the signal‐noise distinguishing is isolated from the signal‐model mapping. The reliability of the estimated earth resistivity model will be seriously affected due to the probable disadvantages in the denoising process. To this end, we propose a manifold assumption, and so establish one "feedback" mechanism between signal‐noise distinguishing and signal‐model mapping. On this basis, we propose a deep learning method: through simultaneous optimal training the network parts for signal‐noise distinguishing and earth resistivity model estimation respectively, the entire neural network can perform the denoising and inversion in the traditional sense at the same time, so as to obtain more objective and reliable estimation results of earth resistivity model. We use the Stacked Auto‐encoder neural network structure to implement the proposed method, and test the network performance with simulation and measured data. The results show that the proposed method can obtain a more reliable earth resistivity model directly from the noisy data. Plain Language Summary: The Transient Electromagnetic (TEM) method is a geophysical method based on the law of electromagnetic induction. The traditional TEM method has low efficiency because of the ground observation. The airborne TEM (ATEM) was introduced to overcome this limitation. However, due to the flight‐movement observation, the useful signal in ATEM data is relatively weak, while the characteristics of noises are complex. So far the denoising process generally consists of many steps, and each step deals with one kind of noise, so the requirements placed on the processing personnel will be very high. After the denoising is completed, the data will be brought into the inversion to obtain an estimate of the earth resistivity model. The inversion generally cannot further distinguish whether there are residual noises in the processed data. Therefore, the reliability of the estimated earth resistivity model will be seriously affected due to the probable disadvantages in the denoising process. So we proposed a Manifold assumption and an alternative approach based on this assumption. The concept of the new approach is similar to adding one more "force" to achieve a more effective separation of the signal and noises, so as to obtain more objective and reliable detection results. Key Points: In the traditional processing flow of the Airborne Transient Electromagnetic data, denoising and inversion are independent of each otherA manifold assumption is proposed, and a "feedback" mechanism is established between signal‐noise distinguishing and signal‐model mappingAn extensible stacked auto‐encoder neural network is used to realize the direct estimation of the earth resistivity model from noisy data [ABSTRACT FROM AUTHOR]

Subsurface geoelectric structures and resources can be detected based on the propagation of the electromagnetic field. Near‐source electromagnetic methods have recently become the most popular survey mode. However, calculation of the electromagnetic field in the near‐source region with artificial sources has always been performed based on far‐source dipole approximation. Owing to the introduction of artificial sources in the near‐source region, it is challenging to obtain accurate solution of the electromagnetic field based on the relationship between the observation point and the source, thereby affecting the detection accuracy. Specifically, the main problem lies in the position vector of the transmitting source, which is generally neglected in the response calculation based on the plane‐wave theory. Moreover, only far‐source data can be observed in the plane‐wave theory. However, near‐source data have been collected in a near‐source survey, with the dipole approximation resulting in a large error. We introduce a point charge element to decrease or eliminate the error caused by the dipole approximation. The non‐homogeneous Helmholtz equation is first solved using the Green's function. Then, the response expression of the real source, including the source position vector, is obtained by integrating the response of the harmonic point charge element. The substitution of the dipole elements with the point‐charge elements not only reduces the dipole approximation errors but also displays the electromagnetic characteristics of the entire zone, which are verified by the forward modeling and field survey results. The near‐source (offset is 0.8 of the target depth) survey can be realized based on the point charge element. Key Points: A point charge element is introduced to decrease or eliminate the error caused by dipole approximationThe response expression of the real source, including the source position vector, is derivedThe electromagnetic characteristics of the entire zone are displayed [ABSTRACT FROM AUTHOR]

The Fengtai Pb–Zn metallogenic deposits located in China's western region have good minerogenetic conditions and exploration potential. However, the Fengtai mining area passed through complex orogenic process zones, such as subduction, collision, and integration between the Yangtze Plate and the North China Plate. Existing geological research and drilling results showed that the ore bodies of large‐scale Pb–Zn deposits in the ore‐gathering area were steeply inclined, and the detection depths of the majority of the metal deposits were generally less than 300 m. Deep ore‐controlling structures at approximately 500–1,000 m are not well understood. Therefore, high‐precision and large‐depth geophysical detection techniques are necessary to achieve the required accurate detection results. This study revealed that the resistivity values of the ore and host rocks were quite different, which is essential for effective and efficient geophysical prospecting. Based on the petrophysical result, the grounded‐wire transient electromagnetic method system has been found to be one of the better choices for the deep prospecting of the metal deposits and was applied to detect Pb–Zn deposits in the Bafangshan‐Erlihe area. First, the grounded‐wire transient electromagnetic method's effectiveness was verified using results from survey line 1 at a known ore deposit verified by the drilling result. Then, the technique was used to identify deposits in an unexplored area. A deep anticline and ore body were successfully imaged. Most lead‐zinc ore bodies are controlled by the hinge and two wings of the tight anticline, and distributed in the contact zone between limestone and phyllite. The results provide a critical basis for deep prospecting and drilling design. [ABSTRACT FROM AUTHOR]

Xiaoshan gold–polymetallic concentrated area is located in Henan Province, central China. The area is principally underlain by typical metamorphic core complexes (MCCs), along with essential components of the basins and ranges of western Henan. Mineralization occurs in the regions where the geochemical anomalies of Au and related elements are widely distributed and consist predominantly of mineralized altered rocks and auriferous quartz veins, both of which are hosted within shear zones or fractures of varying sizes and orientations. Several small‐ to medium‐sized gold–polymetallic ore deposits, along with numerous clustered occurrences, have been found in the area in recent years. However, despite the fact that the Xiaoshan District has traditionally been deemed as an excellent geological endowment area with enormous potential for large‐sized deposits, no significant discoveries have been made to date. It is suggested that the main reason responsible for the present exploration situation could be the scarcity of applications of deep‐penetration geophysical prospecting in the area. It is anticipated that integrating geological and geophysical data would be crucial to enhance the successful mineral explorations. In this contribution, a conceptual geological model for the study area was initially reviewed so that the corresponding electromagnetic responses could then be modelled, which would simulate a geophysical type of survey, with appropriate uncertainties added into the analyses. Then, based on such an integrated approach, the required precision of detecting deep‐buried deposits could then be evaluated. In addition, by employing supplementary geophysical data acquired over known deposits, it is possible to gain more instructive insights into the practical geological model. The viabilities of this approach were demonstrated through a case study in which a hidden gold deposit in the studied area was successfully detected. Geophysical surveys were carried out by employing a grounded‐wire short‐offset TEM (SOTEM) technique and revealed two new anomalies (one of which was subsequently tested by drilling). The drilling then confirmed that this anomaly was caused by an ore body 30 m in thickness occurring at about 550 m below surface. [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]

China is facing many challenges in the area of high‐precision exploration for deep mineral resources, and also in environmental and engineering fields. It has become an urgent task to realize geophysical explorations with high precision and high efficiency in complex environments. It has been found that the traditional transient electromagnetic method (TEM) had a series of problems related to theory, method, and instrument, as well as data interpretation. Based on the reviews of present TEM achievements, three new improved measurements are introduced. The central‐loop system was improved by a large fixed‐loop system; the long‐offset system was changed to the short‐offset one; the resistivity profile was transformed into a virtual wave‐field profile. Finally, this paper points out that the future research direction of the TEM method is in terms of equipment, data processing, and big data information. [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]