Some comments on seabed propagation of ULF/ELF electromagnetic fields

Chave, Alan D.; Flosadóttir, Á. H.; Cox, Charles S.

doi:10.1029/rs025i005p00825

Cited by 39 publications

(22 citation statements)

References 44 publications

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“…The real part of the complex Poynting vector was used by Chave et al (1990) to visualize the time-averaged energy flux for an earth model containing a low conductivity lithospheric zone bounded above and below by higher conductivity regions. Weidelt (2007) and Chave (2009) presented analogous results for a hydrocarbon reservoir model.…”

Section: Theorymentioning

confidence: 99%

On the physics of frequency-domain controlled source electromagnetics in shallow water. 1: isotropic conductivity

Chave¹,

Everett

Mattsson³

et al. 2016

Geophys. J. Int.

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S U M M A R YIn recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. It is the purpose of this paper to evaluate the physics of CSEM for an ocean whose electrical thickness is comparable to or much thinner than that of the overburden using the in-line configuration through examination of the elliptically polarized seafloor electric field, the time-averaged energy flow depicted by the real part of the complex Poynting vector, energy dissipation through Joule heating and the Fréchet derivatives of the seafloor field with respect to the subseafloor conductivity that is assumed to be isotropic. The deep water (ocean layer electrically much thicker than the overburden) seafloor EM response for a model containing a resistive reservoir layer has a greater amplitude and reduced phase as a function of offset compared to that for a half-space, or a stronger and faster response. For an ocean whose electrical thickness is comparable to or much smaller than that of the overburden, the electric field displays a greater amplitude and reduced phase at small offsets, shifting to a stronger amplitude and increased phase at intermediate offsets and a weaker amplitude and enhanced phase at long offsets, or a stronger and faster response that first changes to stronger and slower, and then transitions to weaker and slower. These transitions can be understood by visualizing the energy flow throughout the structure caused by the competing influences of the dipole source and guided energy flow in the reservoir layer, and the air interaction caused by coupling of the entire subseafloor resistivity structure with the sea surface. A stronger and faster response occurs when guided energy flow is dominant, while a weaker and slower response occurs when the air interaction is dominant. However, at intermediate offsets for some models, the air interaction can partially or fully reverse the direction of energy flux in the reservoir layer toward rather than away from the source, resulting in a stronger and slower response. The Fréchet derivatives are dominated by preferential sensitivity to the reservoir layer conductivity for all water depths except at high frequencies, but also display a shift with offset from the galvanic to the inductive mode in the underburden and overburden due to the interplay of guided energy flow and the air interaction. This means that the sensitivity to the horizontal conductivity is almost as strong as to the vertical component in the shallow parts of the subsurface, and in fact is stronger than the vertical sensitivity deeper down. However, the sensitivity to horizontal conductivity is still weak compared to the vertical component within thin resistive regions. The horizontal sensitivity is gradually decreased when the water becomes deep. These observations in part explain the success of shallow towed CSEM using only measurements of the in-line component of the electric field.

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Section: Theorymentioning

confidence: 99%

On the physics of frequency-domain controlled source electromagnetics in shallow water. 1: isotropic conductivity

Chave¹,

Everett

Mattsson³

et al. 2016

Geophys. J. Int.

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“…For a more general case, the integrated formulas were derived for the electromagnetic field in the n-layered model of the sea or ocean floor (King et al 1992). Additionally, many excellent works on ELF wave propagation along the ocean floor were carried out by some investigators (Bannister 1984a(Bannister , 1984bEdwards et al 1985;Inan et al 1986;Bubenik and Fraser-Smith 1978;Fraser-Smith and Bubenik 1979;FraserSmith et al 1988;Chave et al 1990). …”

Section: Elf Wave Propagation Along the Ocean Floor And The Marine Comentioning

confidence: 97%

Historical and Technical Overview of SLF/ELF Electromagnetic Wave Propagation

Pan¹,

Li²

2013

Advanced Topics in Science and Technology in China

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In general, very low frequency (VLF) refers to electromagnetic waves with frequencies from 3 kHz to 30 kHz; ultra low frequency (ULF) refers to frequency range between 300 Hz to 3 kHz; the frequency range of 30 Hz to 300 Hz is defined as super low frequency (SLF); and the frequency below 30 Hz is referred to as extremely low frequency (ELF). In some early studies, alternative definitions may have been used with frequency range of 3 Hz to 3 kHz generally referred to ELF. In this book, the emphasis is on SLF (30-300 Hz) and ELF (below 30 Hz) ranges, using the MKS system of units and the time dependency e −iωt . Medium Characteristics of SLF/ELF Wave PropagationDue to the long wavelength, SLF/ELF waves, when excited and propagated on and near the Earth's surface, will cover lithosphere, atmosphere and ionosphere, whose electromagnetic characteristics differ significantly in their propagation paths. The permeability of the atmosphere and ionosphere is approximately μ 0 , the permeability in free space, as well as that of the lithosphere, except in the regions that are rich in iron, nickel, cobalt, etc. Therefore, the permeability of the lithosphere can be also treated as μ 0 , if neither the transmitter nor the receiver is located in mineral-rich areas. The atmosphere is a non-conductive medium with negligible conductivity, whose permittivity is close to ε 0 , the permittivity of free space. While the lithosphere is conductive, the conductivity of sea water σ sea is in the range of 2.5-5.5 S/m, and the conductivity σ g of rock and soil in the range of 10 −2 -10 −4 S/m. The dielectric constant ε r of the lithosphere does not have large effects on the propagation of SLF/ELF waves.The ionosphere is composed of partially ionized gas, which includes electrons, ions, and electrically neutral particles, above 70 km or so from the sea level. The electrons and ions make the ionosphere conductive, thus the majority of the energy of the incident VLF/SLF/ELF waves will be reflected by the ionosphere. When both the transmitter and the receiver are located in the space between the ground and the W. Pan, K. Li, Propagation of SLF/ELF Electromagnetic Waves,

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“…The dipole emits a low-frequency signal into the surrounding media, and the signal is normally recorded by stationary seafloor receivers having both magnetic and electric dipole antennas. The marine CSEM technique was introduced by Cox et al (1971), and has since then been successfully applied to study the oceanic lithosphere and active spreading centres (Young and Cox, 1981;Cox et al, 1986;Chave et al, 1990;Evans et al, 1994;Constable and Cox, 1996;MacGregor and Sinha, 2000).…”

Section: Introductionmentioning

confidence: 99%