3D controlled-source electromagnetic inversion in the radio-frequency band

Smirnova, Maria; Shlykov, A.; Asghari, Shiva Fadavi; Tezkan, B.; Saraev, A.; Yogeshwar, Pritam; Smirnov, Maxim

doi:10.1190/geo2021-0626.1

Cited by 5 publications

(8 citation statements)

References 45 publications

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“…Nevertheless, this is not valid for near-field and transition zones (Smirnova et al, 2023). In an ideal condition where signals are being received orthogonal, E-and B-polarizations are completely independent and only E-polarization or TE mode contains tipper information.…”

Section: Transfer Function Calculationsmentioning

confidence: 99%

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Multidimensional interpretation of radiomagnetotellurics and controlled‐source radiomagnetotellurics data: A validation study

Asghari

Shlykov²,

Smirnova

et al. 2023

Near Surface Geophysics

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Radiomagnetotellurics (RMT) is an electromagnetic method that uses signals from radio transmitters broadcasting in the 10 kHz to 1 MHz frequency range. Due to its limited frequency range, RMT is commonly used as a shallow-depth investigation tool. However, in remote areas, there is a lack of radio transmitters and only signals from very low frequency (VLF) antennas (10-30 kHz frequency range) can be measured. This can give rise to low signal-to-noise ratio. To overcome this disadvantage of RMT, a controlled-source RMT (CSRMT) can be applied to measure signals of the low-frequency (LF) and mid-frequency ranges (30-1000 kHz). Moreover, the wider frequency range of the CSRMT method (down to 1 kHz) leads to a deeper sounding depth. We present the first RMT and CSRMT validation studies using two perpendicularly located horizontal electric dipoles to realize a 3D inversion of CSRMT data. The survey area in Alexandrova village in Kaluga region, Russia, a previously investigated area, was selected for a validation study. We acquired the data along 8 profiles with 175 stations. Transmitter lines for the CSRMT case were about 900 m long, and the minimum and maximum distances of the stations from transmitters were 450-1000 m, respectively. We applied 2D and 3D inversions over the far-field data and compared with the previous results. The available geophysical information as well as the borehole data indicate a high agreement between the obtained models and the geological structure. We can confirm that the CSRMT method is a reliable approach for near-surface explorations and that, the existing advanced and tested inversion tools for magnetotellurics, can be used to invert the RMT and far-field zone CSRMT data leading to comparable results.

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Section: Transfer Function Calculationsmentioning

confidence: 99%

“…The following study focuses only on far-field zone results. The results for transition zone are available in Smirnova et al (2023).…”

Section: Introductionmentioning

confidence: 99%

Multidimensional interpretation of radiomagnetotellurics and controlled‐source radiomagnetotellurics data: A validation study

Asghari

Shlykov²,

Smirnova

et al. 2023

Near Surface Geophysics

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“…Moreover, the convergence behaviour of normalized EM field component inversion can be severely impaired if a heterogeneous sensitivity distribution is generated by a single distant source setup (Bretaudeau & Coppo, 2016) that is a combination of two transmitter aerials employed to provide two independent source polarizations. When inverting transfer functions (TF) in the form of CSEM impedance tensors and vertical magnetic transfer functions (e.g., Bretaudeau & Coppo, 2016; Smirnova et al., 2023), also known as tensor controlled‐source audio‐magnetotellurics (CSAMT) (Zonge & Hughes, 1991), the source currents do not need to be known as they cancel out in this formulation of the CSEM data. This may reduce the footprint of the source in the resistivity models (Bretaudeau & Coppo, 2016).…”

Section: Introductionmentioning

confidence: 99%

“…This may reduce the footprint of the source in the resistivity models (Bretaudeau & Coppo, 2016). However, it is suspected that resolution properties of transfer function data are worse compared to using field components (Sasaki et al., 2015; Smirnova et al., 2023).…”

Section: Introductionmentioning

confidence: 99%

Research note: A comparison between normalized controlled‐source electromagnetic field components and transfer functions as input data for three‐dimensional non‐linear conjugate gradient inversion

Rulff,

Kalscheuer

2024

Geophysical Prospecting

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Controlled‐source electromagnetic methods are applied to survey the electrical resistivity distribution of the subsurface. This work compares normalized electromagnetic field components and transfer functions such as impedance tensors and vertical magnetic transfer functions generated by two independent source polarizations as input data for three‐dimensional inversion. As most other available inversion codes allow for inverting only one of the mentioned input data types, it is unclear which data type is preferable for controlled‐source electromagnetic inversion. Our three‐dimensional non‐linear conjugate gradient inversion code can handle both input data types, facilitating a comparison of normalized electromagnetic field components and transfer functions inversion. Examining inversion results for a three‐dimensional synthetic model with two anomalies, we infer that the transfer functions inversion is favourable for recovering the overall resistivity distribution below the receiver sites in fewer iterations. The inversion of normalized electromagnetic field components produces a sharper image of the anomalies and may be capable of detecting the resistivity distribution below the extended sources, which comes at the price of introducing a more heterogeneous background resistivity in the model.

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“…Along with the soundings in the far-field zone, the techniques and software tools were elaborated to use the HED source in the transition zone. Examples of the bimodal inversion of field data using the tensor variant of the CSRMT method in the transition zone have been reported in articles [19,24].…”

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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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3D controlled-source electromagnetic inversion in the radio-frequency band

Cited by 5 publications

References 45 publications

Multidimensional interpretation of radiomagnetotellurics and controlled‐source radiomagnetotellurics data: A validation study

Multidimensional interpretation of radiomagnetotellurics and controlled‐source radiomagnetotellurics data: A validation study

Research note: A comparison between normalized controlled‐source electromagnetic field components and transfer functions as input data for three‐dimensional non‐linear conjugate gradient inversion

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

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