Radio‐magnetotelluric and controlled‐source magnetotelluric surveys on a frozen lake: Opportunities for urban applications in Nordic countries

Bastani, Mehrdad; Wang, Shunguo; Malehmir, Alireza; Mehta, Suman

doi:10.1002/nsg.12180

Cited by 4 publications

(1 citation statement)

References 39 publications

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“…By expanding the frequency range of the Enviro-MT equipment down to 1 kHz, the investigation depth of the method was increased to 100-150 m. Experience has been gained in the application of the CSRMT method with the HMD source for solving various problems such as geological mapping [5], searching for ore [6] and groundwater resources [7,8], and studying landslides [9]. Offshore test measurements in summer and winter conditions have been performed [10][11][12].…”

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confidence: 99%

Use of Subharmonics of Base Frequencies in the CSRMT Method with Loop Sources

Saraev

Bobrov

Shlykov

2023

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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.

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confidence: 99%

Use of Subharmonics of Base Frequencies in the CSRMT Method with Loop Sources

Saraev

Bobrov

Shlykov

2023

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Imaging mofette structures in the Ohře Rift System, Czech Republic, using radio-magnetotelluric data

Rulff,

Weckmann,

Kalscheuer

et al. 2024

Preprint

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The pathways of fluids and mantle originated carbon dioxide in the seismically active Ohře (Eger) Rift system appearing as mofettes at the surface are currently subject to investigation, especially by the International Continental Scientific Drilling Program “Drilling the Eger Rift”. If the aquifers show significant contrast in electrical resistivity to the host rocks, they can be investigated with geo-electromagnetic methods. However, imaging complex fluid and CO2 pathways in detail in near-surface structures is challenging, because, in contrast to the background stratigraphy, they are often oriented in near-vertical directions. Therefore, we aim to investigate how the shallow aquifer structures can be examined best with an inductive electromagnetic method. For this purpose, we collected radio-magnetotelluric data in the Hartoušov mofette field and evaluated them by two- and three-dimensional inversions. Data of a nearby magnetotelluric station, drill hole data, gas flux measurements and electrical resistivity tomography models were used to assess reliability and robustness of our inversion results. We concluded that the near-surface fluid reservoirs are adequately depictable, while the migration paths of gaseous CO2 cannot be traced properly due to a lack of resistivity contrast. Our model analyses suggest that imaging the given geological setting with fluids and gases ascending in anastomosing pathways, benefits from a fine-scale three-dimensional inversion approach, because the fluids mostly appear as local conductive reservoir-like anomalies, which can be falsely projected onto the profiles during inversion in two dimensions. The resistivity models contribute with detailed images of the near-surface aquifers to the geodynamic model of the Ohře Rift.

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PDE-Based Two-Dimensional Radiomagnetotelluric forward Modelling Using Vertex-Centered Finite-Volume Scheme

Xie,

Zhu,

Tong

et al. 2024

Mathematics

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An efficient finite-volume algorithm, based on the vertex-centered technique, is proposed for solving two-dimensional radiomagnetotelluric forward modeling. Firstly, we derive the discrete expressions of the radiomagnetotelluric Helmholtz-type equation and the corresponding mixed boundary conditions using the vertex-centered finite-volume technique. Then, the corresponding approximate solutions of the radiomagnetotelluric forward problem can be calculated by applying the finite-volume scheme to treat the boundary conditions. Secondly, we apply the finite-volume algorithm to solve two-dimensional Helmholtz equations and the resistivity half-space model. Numerical experiments demonstrate the high accuracy of the proposed approach. Finally, we summarize the radiomagnetotelluric responses through a numerical simulation of a two-dimensional model, which enables qualitative interpretation of field data. Furthermore, our numerical method can be extended and implemented for three-dimensional radiomagnetotelluric forward modeling to achieve more accurate computation.

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Radio‐magnetotelluric and controlled‐source magnetotelluric surveys on a frozen lake: Opportunities for urban applications in Nordic countries

Cited by 4 publications

References 39 publications

Use of Subharmonics of Base Frequencies in the CSRMT Method with Loop Sources

Use of Subharmonics of Base Frequencies in the CSRMT Method with Loop Sources

Imaging mofette structures in the Ohře Rift System, Czech Republic, using radio-magnetotelluric data

PDE-Based Two-Dimensional Radiomagnetotelluric forward Modelling Using Vertex-Centered Finite-Volume Scheme

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