Search Results (9)

The transient electromagnetic method (TEM) is widely used in the exploration of mineral, petroleum, and geothermal resources due to its sensitivity to low-resistivity bodies, limited site constraints, and strong resistance to interference. In practical applications, the TEM often uses a long wire source instead of an idealized horizontal electric dipole (HED) source as the excitation source. This is due to the complex external conditions and the relatively large distance between the receiving zone and the transmitter source. Compared to the HED, the long wire source can provide a larger excitation current, generating stronger signals to meet the requirements of a higher signal-to-noise ratio or deeper exploration. It also produces longer-duration signals, thereby providing better resolution. Additionally, for the interpretation of TEM data, three-dimensional forward modeling plays a crucial role. However, the mature traditional TEM forward method is based on a simple, sometimes inappropriate model, as it is well established that the induced polarization (IP) effect is widely present in the deep earth, especially in oil and gas reservoirs. The presence of the IP effect results in negative responses in field data that do not conform to the traditional theoretical decay law of TEM, which can significantly impact data processing and inversion results. To address this issue, a TEM forward modeling method considering the IP effect based on the spectral element method (SEM) has been developed in this study. Firstly, starting from the Helmholtz equation satisfied by the time domain electric field, we introduce the Debye model with polarization information into the forward modeling by utilizing the differential form of Ohm’s law. As a result, we derive the boundary value problem for the time domain electric field that considers the induced polarization effect. Using Gauss–Lobatto–Legendre (GLL) polynomials as the basis functions, the SEM is employed to discretize the governing equations at each time step and obtain spectral element discretization equations. Then, temporal discretization equations are derived using the second-order backward Euler formula, and the linear system of equations is solved using the Pardiso direct solver. Finally, the electromagnetic responses at any time channel are obtained via SEM interpolation and numerical integration, thereby achieving three-dimensional TEM forward modeling considering the IP effect. The results indicate that this method can effectively reflect the spatial distribution of polarizable subsurface media. It provides valuable references for studying the polarization parameters of subsurface media and performing a three-dimensional inversion of TEM data considering the induced polarization effect. Full article

The technologies of undersea detection and communication, seabed sensor networks, and geophysical detection using electromagnetic waves have emerged as research focal points within the field of marine science and engineering. However, most studies have focused on the propagation of electromagnetic fields over long distances within the shallow “sea-seabed” environment. This paper introduces a quasi-static approximation method to address the Sommerfeld numerical integration challenge within the near-field region, employing the horizontal electric dipole (HED) as a model. It derives the Sommerfeld numerical integral expressions under conditions where the wave-number ratio at the “seawater-air” boundary does not adhere to the requirement of |k₀/k₁| << 1 (where subscripts 0 and 1 denote seawater and air media, respectively). Building upon this, the paper simplifies the Bessel-Fourier infinite integral term within the integral expression to obtain Sommerfeld numerical integral approximations for the propagation of electromagnetic fields in the near region of extremely low frequency (ELF) within seawater. The study further conducts simulations and calculations to determine amplitude variations in electromagnetic field intensity generated by an ELF HED at different frequencies, dipole heights, and observation point depths. It concludes with an analysis of electromagnetic field propagation characteristics at the seawater-air boundary. Experimental findings highlight the lateral wave as the primary mode of electromagnetic wave propagation at this interface. Full article

Abnormal electromagnetic radiation associated with seismic activity has been reported across a wide range of frequencies, but its primary energy is concentrated in the super-low-frequency (SLF) and extremely low-frequency (ELF) bands. To estimate the effect of the seismic radiation source, a radiator in a soil foundation was modeled as a horizontal electric dipole (HED), and the propagation characteristics of the electromagnetic fields were studied in the Earth–ionosphere cavity. The expressions of the electromagnetic fields could be obtained according to the reciprocity theorem. Therefore, a new algorithm named the numerical integral algorithm was proposed, which is suitable for both the SLF and ELF bands. The new algorithm was compared with the asymptotic approximation algorithm when the receiving point was not close to the field source and the antipode. The two algorithms were found to be in excellent agreement, confirming the validity of the new algorithm for SLF and ELF bands. Full article

Great attention has been paid to the propagation of electromagnetic (EM) waves across the sea surface due to its important applications. Most of the previous research, however, focuses on the half-space model illustrating the deep sea environment. In this paper, EM field distribution in the extremely low frequency (ELF) near-region under horizontal electric dipole (HED) excitation in homogeneous half-space seawater is analyzed based on the general expression of the Sommerfeld integral using the quasistatic approximation method. The focus is on deriving complete and effective solutions in air and seawater regions under the cylindrical coordinates for the EM near-field, which is generated by an HED in a shallow sea. The resulting formulas can be given by a few summands in closed form as the well-known Fourier–Bessel integrals. The analytical approximate expression of ELF Sommerfeld EM field integral excited by the HED in the homogeneous half-space seawater is deduced under the condition that the propagation distance ρ satisfies kρ << 1. To this end, the EM field distribution in the range close to the HED antenna in seawater is simulated, the results have shown that the minimum attenuation value of the vertical electric component E_z is about 15 dB, and that of the radical magnetic components H_φ is about 30 dB, and these values are found to be of greatest potential for the near-field region propagation among the electric and magnetic components. Finally, the correctness of the proposed method is verified by comparison with Pan’s approximation method and Margetis’s exact expression approximation method, which demonstrated the correctness of the proposed method. Full article

In the controlled source radiomagnetotelluric (CSRMT) sounding method, a horizontal magnetic dipole, HMD (vertical loop) or a horizontal electric dipole, and HED (grounded line) are used as sources. When working with HMD, the source is usually tuned to resonance to increase the current in the loop. However, the disadvantage of this approach is the narrow frequency range realized in the CSRMT method (1–12 kHz) and the short operating distance from the source (600–800 m). The need to tune the source to resonance at each selected frequency reduces the efficiency of the survey. In the case of using HED for sounding, measurements are performed in a wider frequency range of 1 to 1000 kHz, and along with the signal of the base frequency, its subharmonics are measured. In this case, emitted signal measurements are possible at a distance of up to 3–4 km from the source. At the same time, the disadvantage of using HED is that it requires grounding, the arrangement of which requires additional time when working on frozen ground or dry stony soil. We consider the possibilities of generation and registration of signals of subharmonics of base frequencies when applying the CSRMT method with loop sources—HMD and VMD (horizontal loop). A matching unit (MU) based on a step-up transformer was developed, which increases the output voltage of the CSRMT transmitter. In a field test with base frequencies of 20, 40, and 80 kHz, the signal amplitudes increased by a factor of two to four for subharmonics at frequencies of 60–200 kHz and by up to 10–13 times for subharmonics at frequencies of 200–500 kHz due to transformation of signal spectrum provided by MU. The possibility of using odd subharmonics of base frequencies for inversion has been demonstrated in the results of field experiments with different sources (HED, HMD, and VMD). This expands the frequency range of the method when working with loop sources and increases the survey’s effectiveness. The use of loop sources in the CSRMT method is especially advantageous for winter work in Arctic regions. Full article

The controlled source extremely low frequency (CSELF) method bears the potential for deep resource exploitation utilizing the skywave. The “Skywave” denotes the electromagnetic wave propagating through the waveguide formed by the Earth and ionosphere. It has a considerable penetration depth into the lithosphere due to its low-frequency band. Previous research on extremely low-frequency electromagnetic fields with the coupled lithosphere, atmosphere, and planar ionosphere models ignored the effect of the Earth’s curvature. Thus, we aimed to present the exact formulas for horizontal electric dipoles (HED) in a spherical “Earth-ionosphere” model. These new formulas consider the Earth’s curvature as a multilayer medium rather than a homogeneous underground. We introduce three techniques: function combination pairs, addition and subtraction terms, and Padé approximants, to handle slow convergence in numerical calculation. In the spherical waveguide, electromagnetic fields are mutually interfered with and produce oscillations, which is different from the planar model. The influence of Earth’s curvature cannot be neglected with the increase in source–receiver distance, though it is negligible within 3000 km. Furthermore, it is worth noting that apparent resistivity ρ_θφ enters the waveguide area earlier than ρ_φθ. This method can be used as Green’s function to simulate the electromagnetic field of actual antennas and 3-D models. Full article

With the rapid exploitation of deep mines by digging new tunnels, the advance forecast of water inrush has become increasingly important. The land-based controlled source electromagnetic method (CSEM) is commonly used to detect water-bearing structures. To increase its sensitivity, we propose a new measuring configuration for CSEM by placing EM sensors in an underground steel-cased well. The numerical modeling is conducted by COMSOL to overcome the difficulties of investigating the feasibility of the measuring configuration. The current distribution and electromagnetic field along an in-seam horizontal casing are investigated based on a synthesis three-layered model. The results illustrate that the casing can be treated as antennas that enhance the electric fields at large depths. The water-bearing structures can be observed by a magnetic field (with a perpendicularly horizontal electric dipole (HED) source) rather than an electric field (with a parallelly HED source). Numerical simulations demonstrate that the proposed method is a feasible and effective technique for the detection of water-bearing structures during deep mineral exploration. Full article

A numerical analysis on field distribution along the Earth’s surface of a line current source submerged in the ground is conducted in this paper to investigate the potential of the extremely low frequency (ELF) technology in the envisioned long-distance communication techniques. The problem is modeled as a submerged horizontal electric dipole (HED) in a two-layered homogeneous half space and solved by the combined numerical methods of the Romberg-Euler method and Gauss-Laguerre method. The model is validated by experimental results with only a maximum 10% error at 9 Hz around 490 m. Meanwhile, the study shows that the ELF signals emitted by a submerged line current source can transmit at least 1 km with a current sensor sensitivity of 0.1 pT. These results indicate the possibility of applying of ELF technology to long-distance communication or the long-distance transmedia detection. Full article

The semi-airborne electromagnetic (EM) method has the potential to reach deeper exploration depths than purely airborne EM approaches. The concept of the method is to deploy high-power transmitters on the ground, which excite subsurface currents and induce strong magnetic fields, and to measure the corresponding EM fields with a passive airborne receiver instrument. Following recent conceptual developments of the semi-airborne EM technique deployed on helicopters, we performed a 10 km${}^2$ semi-airborne EM survey near Münster (Germany) based on a multicopter aircraft system. For this purpose, horizontal electric dipole (HED) transmitters were installed in the survey area and were surveyed individually. Magnetic transfer functions were determined and a model of the conductivity of the study area was derived. Despite restrictions such as low payload capacity and multicopter-related EM noise, we were able to estimate spatially and spectrally consistent transfer functions of high quality up to a distance of 2 km from the respective transmitter. Our results could be validated with independent results from a magnetotelluric and a direct current sounding. The study demonstrates that an unmanned aircraft system (UAS) is suitable for semi-airborne EM application and that such a system can be beneficial where ground-based methods and manned techniques become impractical. Full article