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In artificial-source electromagnetic explorations, the shadow effects and source overprint effects are two of the key factors to affect the detection reliability. Compared with frequency-domain electromagnetic methods, the transient electromagnetic (TEM) shadow effect and source overprint effect are more complex, especially for the short-offset TEM (SOTEM) observed in the near-source region. However, there is less relevant research. Therefore, in this paper, we first realize 3D SOTEM simulations based on the vector time-domain finite element method, then further analyze the generation of SOTEM shadow effect and source overprint effect from view of the diffused electromagnetic fields, as well as their influence on ground horizontal electric field Ex and vertical induced voltage dBz/dt responses. Results show that the attraction or repulsion of the electrical inhomogeneity would affect the diffusion speed of the electromagnetic fields, which is the essential in the two source effects. Besides, from view of the ground Ex and dBz/dt responses, the relative anomaly of SOTEM shadow effect is almost twice those of the SOTEM source overprint effect, i.e. the SOTEM shadow effect is easier to record, especially for dBz/dt responses. In addition, owing to the shadow effect, SOTEM may fail to identify the real target anomaly, whereas the influence of the source overprint effect for recognizing the target anomaly mainly occurs on all-time dBz/dt responses and early-time Ex responses (before about 10 ms), that is, observing late-time SOTEM Ex responses can effectively avoid such influence. Hence, to fully ensure avoiding the influence of the source overprint effect, we suggest adopting high-power transmission to increase the late-time signal-to-noise ratio in field measurements.

Geologic and geochemical results from the Ni-Cu deposits of the Central Asian orogenic belt show that carbon from carbonaceous sediments was incorporated in the magma that formed the Ni-Cu deposit. Thus, the presence of an Ni-Cu deposit may be hypothesized at localities where carbonaceous sediments are in contact with mafic intrusions. However, the widely developed carbonaceous layer in the Central Asian orogenic belt has low-resistivity and high-chargeability characteristics similar to those of massive and semimassive orebodies, which makes it difficult to detect Ni-Cu deposits by conventional geophysical methods. To test whether the frequency-domain controlled-source electromagnetic (FDCSEM) method can circumvent this problem, a survey was conducted using horizontal electric fields to map the spatial distribution of rock types around gabbro intrusions containing an Ni-Cu sulfide deposit and barren gabbro intrusions of the Central Asian orogenic belt. The recovered resistivity structure resolved two low-resistivity layers: one was located at an elevation range of 850 to 950 m (the elevation of the surface is about 1,000 m) and widespread along the survey lines, whereas the other was present between 300 and 600 m and was limited to only part of the survey line. These structures are interpreted to represent carbonaceous layers. Gabbro intrusions are present as high-resistivity formations. Based on the drilling results, in the area where the deep carbonaceous layer exists, the Ni-Cu orebodies are associated with gabbro intrusions, whereas where the gabbro intrusions are not associated with a carbonaceous layer, no Ni-Cu orebodies are present. These associations can be detected using FDCSEM with a horizontal electric field and could be used to explore Ni-Cu deposits.

The concept of a smoke-ring in electromagnetic fields can be used to describe the spatial distribution and diffusion of electromagnetic fields with either frequency or time. This provides an intuitive basis for qualitative interpretation of artificial source electromagnetic measurement results. The existing discussions regarding this field of study have mainly focused on the smoke rings of a magnetic dipole or loop source. The smoke ring of the other commonly used grounded-wire sources in the time domain was investigated. The results show that the smoke ring of grounded-wire sources is more complex than that of magnetic sources. A grounded-wire source can generate induced current in both the horizontal and vertical directions. The horizontal induced current consists of two parts. At the early stage, when the source has just been turned off, there is a low-amplitude and fast-diffuse opposite return current. With time it becomes positive, and reaches a maximum, then decreases. The horizontal induced current maximum diffuses downward perpendicular to the surface, and the vertical induced current maximum diffuses at 45 degrees from the earth’s surface with a faster diffusion velocity, which is roughly 5 times that of the horizontal induced current. The smoke ring of the horizontal induced current more closely resembles the shape of a basin, while that of the vertical induced current is more like two separate jars. Finally, the respective imaging depths estimated based on the horizontal and vertical induced currents are 0.92 and 1.25 times the diffusion depth.

The diffusive electromagnetic field can be transformed into the wave domain by means of mathematical conversion. The transformed field can then be interpreted with the tools in seismic data processing so that the identification to the underground targets can be effectively improved. However, the conversion is typically an ill-posed problem that needs to be solved using regularization tools. Based on the conventional regularization with smooth constraints in the L2 norm, the inversion result is of low resolution, while that obtained using truncated singular value decomposition (TSVD) methods is typically accurate, but has poor stability. To obtain a stable and accurate transformed electromagnetic field value, this study proposed to combine conventional regularization tools and singular value decomposition algorithms by incorporating a set of weighting coefficients. The proposed method is validated on both synthetic and observed data. The results from the proposed method are more accurate at the early time, and at the late time are more stable compared with the other methods. Furthermore, the example of field data shows that the proposed method could potentially further improve the interpretation accuracy of future mining explorations.

Several magmatic Ni-Cu deposits associated with Permian-Triassic mafic-ultramafic intrusions have been discovered during the last three decades in the southern part of the Central Asian Orogenic Belt, the northeastern margin of Tarim, and the margin of the North China Craton. The accurate delineations of the large resistive rock masses are crucial for locating potential ore bodies when they are located beneath rock masse. The controlled-source electromagnetic method (CSEM) is an important tool for investigating resistivity contrast in earth subsurface. In this study, we present a comprehensive example of using frequency domain CSEM survey to delineate the resistive rock masses that are related to the forming of orebody. The horizontal electric field, which is more sensitive to resistive target than the other components, is used in our study. We first built a geophysical model of the survey area based on prior confirmed geophysical and geological information. Then, both forward and inverse modeling were performed in order to assess the detectability of the resistive ore-forming rock masses in the model. Finally, this study acquired field data in the Kalatongke Ni-Cu Deposit, which is located in the Kalatongke District of the southern section of the Central Asian Orogenic Belt, and is specifically part of China’s southern Altai Orogenic Belt. The resistive ore-forming rock masses were successfully identified during the survey process and further verified by the drilling results. It was observed that both the synthetic and field examples indicated that the adopted frequency domain CSEM method successfully measured the electrical fields, and could be utilized to explore deeply buried resistive targets in future prospecting activities.