Borehole Antenna Considerations in the EMRE System: Frequency Band 312.5-2500 kHz

Korpisalo, Arto

doi:10.2174/1874262901307010063

Cited by 4 publications

(4 citation statements)

References 15 publications

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“…In the EMRE system, the electromagnetic fields are generated by insulated electric dipole antennas [19] that are aligned parallel to the borehole axis ( Figure 1). The electromagnetic field of the electric dipole has three effective components: a magnetic component ( ), an electric tangential component ( Θ ), and a radial component ( ).…”

Section: Theorymentioning

confidence: 99%

“…The effective performance of any radiating system depends on the characteristics of transmitter and receiver antennas, because they are the important intermediate devices that radiate and capture the propagating electromagnetic energy [19]. The core of the EMRE system Figure 4: The electrical properties of a granite sample (a) and a sulphide sample (b) from Vogt [7].…”

Section: Rim Instrumentmentioning

confidence: 99%

“…In situations where close proximity to a borehole wall changes from one transmitter position to another, resulting in large variations in antenna impedance, local source strengths can be used. Because antenna impedance is sensitive to the electrical properties of the material near the borehole, the radiated power can also differ considerably from one transmitter position to another [19]. If the electrical contrasts are moderate, the results from the above-mentioned methods are often very good and sufficient.…”

Section: International Journal Of Geophysicsmentioning

confidence: 99%

“…The highest measurement frequency (2500 kHz) was problematic, as no signals were received. The reason lay not only in the internal attenuation of the material but also in the unfavourable functioning of the transmitter antenna [19]. The theoretical scattering parameter s 11 , which defines the relationship between the input power and the reflected power in the transmitter, was ∼ −2 dB, thus meaning that ∼62% of the power was reflected back to the generator and only ∼38% was radiated.…”

Section: Field Case In Finlandmentioning

confidence: 99%

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Characterization of Geotomographic Studies with the EMRE System

Korpisalo

2014

International Journal of Geophysics

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Posiva Oy carries out research and development on spent nuclear fuel disposal in Finland. The repository will be constructed deep in the crystalline bedrock of Olkiluoto island in Eurajoki. Posiva Oy and ANDRA (France) have cooperated actively in examining methods for revealing properties of granitic bedrock. One of the considered methods was an electromagnetic cross-borehole survey, and RIM measurements were conducted in 2009. Olkiluoto migmatitic bedrock has undergone polyphasic ductile-brittle deformation and resistivity in the bedrock varies strongly in range of tens to tens of thousands of Ωm. Field work was successfully performed in one borehole pair. The results are presented in this paper. The tomographic reconstruction of the borehole section is based on the far-field approximation of the electric field. The results prove that the method can be used between boreholes to delineate and follow sulphide-bearing horizons. The detected low and high resistivity zones and their apparent shapes and orientations are in fair agreement with geological and other geophysical results. The obtained information can be used, for example, in assessing the integrity of the rock mass, as the increased electrical conductivity is often associated with rock mass deformation (clay and water bearing fractures, sulphide and graphite bearing zones).

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Section: Theorymentioning

confidence: 99%

Section: Rim Instrumentmentioning

confidence: 99%

Section: International Journal Of Geophysicsmentioning

confidence: 99%

Section: Field Case In Finlandmentioning

confidence: 99%

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Characterization of Geotomographic Studies with the EMRE System

Korpisalo

2014

International Journal of Geophysics

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Radiowave imaging research (RIM) for determining the electrical conductivity of the rock in borehole section OL-KR4−OL-KR10 at Olkiluoto, Finland

Korpisalo

Heikkinen

2015

Exploration Geophysics

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Full-waveform inversion of cross-hole radio frequency electromagnetic data

Zheglova,

Farquharson,

Malcolm

2024

Geophysical Journal International

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Summary We consider application of full waveform inversion (FWI) to radio-frequency electromagnetic (EM) data. Radio-frequency imaging (RIM) is a cross-borehole technique to image electromagnetic subsurface properties from measurements of transmitted radio-frequency waves. It is used in coal seam imaging, ore exploration and various engineering and civil engineering applications. RIM operates at frequencies from 50 kHz to several tens of MHz. It differs from other geophysical EM methods, because the frequency band includes the transition between the wave propagation and diffusion regimes. RIM data are acquired in two-dimensional cross-hole sections in a reciprocal manner. Traditionally, radio-frequency data are inverted by straight ray tomography because it is inexpensive and easy to implement. It is argued that due to attenuation, the sensitivity of the transmitted electric field is the strongest within the first Fresnel zone of the ray connecting the transmitter and receiver. While straight ray tomography is a simple to implement and fast method, the non-linearity in the relationship between model parameters and data is often strong enough to warrant non-linear inversion techniques. FWI is an iterative high resolution technique, in which the physical properties are updated to minimize the misfit between the measured and modelled wavefields. Full waveform techniques have been used and extensively studied for the inversion of seismic data, and more recently, they have been applied to the inversion of GPR data. Non-linear inversion methods for RIM data are less advanced. Their use has been hindered by the high cost of full wave modelling and the high conductivity contrasts of many RIM targets, and, to some extent, by the limitations of the measuring instruments. We present the first application of this methodology to simultaneous conductivity and permittivity inversion of RIM data. We implement the inversion in the frequency domain in two dimensions using L-BFGS optimization. We analyze the sensitivity of the data to the model parameters and the parameter trade-off and validate the proposed methodology on a synthetic example with moderate conductivity variations and localized highly conductive targets. We then apply the FWI methodology to a field data set from Sudbury, Canada. For the field data set, we determine the most appropriate preprocessing steps that take into account specific peculiarities of RIM: the insufficient prior information about the subsurface and the limitations of the measuring equipment. We show that FWI is applicable under the conditions of RIM and is robust to imperfect prior knowledge: we obtain satisfactory model recoveries starting from homogeneous initial models in all of our examples. Just as other methods, FWI underestimates large conductivity contrasts due to the loss of sensitivity of the transmitted electric field to the conductivity variations as the conductivity increases above a certain level. The permittivity inside high conductors can not be recovered, however, recovering permittivity variations in the resistive zones helps obtain better focused conductivity images with fewer artifacts. Overall, FWI produces cleaner, less noisy and higher resolution reconstructions than the methods currently used in practice.

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Borehole Antenna Considerations in the EMRE System: Frequency Band 312.5-2500 kHz

Cited by 4 publications

References 15 publications

Characterization of Geotomographic Studies with the EMRE System

Characterization of Geotomographic Studies with the EMRE System

Radiowave imaging research (RIM) for determining the electrical conductivity of the rock in borehole section OL-KR4−OL-KR10 at Olkiluoto, Finland

Full-waveform inversion of cross-hole radio frequency electromagnetic data

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