3. Resistivity Characteristics of Geologic Targets

Palacky, G. J.

doi:10.1190/1.9781560802631.ch3

Cited by 353 publications

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“…Thus, the electrical conductivity σ 1 , taken as a parameter, is assumed to be equal to 0.01, 0.1, and 1 mS/m. Common examples of materials with conductivity on these orders of magnitude are permafrost, igneous and metamorphic rocks [5,6]. The numerical evaluation of the field integrals has been performed through the rigorous quasi-analytical procedure described in [7,22].…”

Section: Resultsmentioning

confidence: 99%

“…Such quasi-static field approximations, the summary of which is given by Bannister [8], are subject to the further condition that the half-space is much more dense than air (|k 1 | k 0 ), and this is the reason why they may be referred to as zeroth-order approximations [12,13], whose frequency range of validity strongly depends upon the electromagnetic properties of the material half-space. Due to this feature, the zerothorder formulation fails at extremely low frequencies if the electrical conductivity of the material half-space is very poor, like, for instance, in the case of permafrost, igneous and metamorphic rocks [5,6].…”

Section: Introductionmentioning

confidence: 99%

“…Yet, to date the derivation of closed-form expressions from the complete integral representations for the field components has proven to be a prohibitive task, except for certain special cases which, fortunately, cover many practical applications (EM sounding to detect buried objects, radio propagation, therapeutic heating of tissues) [3][4][5][6][7][8][9][10][11]. Useful simple expressions have been obtained for the case of the vertical electric dipole, under the assumptions that both the source and field points are located on the surface of the medium, and the operating frequency is low enough that k 0 ρ 1, being k 0 the free-space wavenumber and ρ the source-receiver distance.…”

Section: Introductionmentioning

confidence: 99%

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Second-Order Formulation for the Quasi-Static Field From a Vertical Electric Dipole on a Lossy Half-Space

Parise¹

2013

PIER

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Abstract-Improved quasi-static expressions are derived for the timeharmonic electromagnetic (EM) field components excited by a vertical electric dipole (VED) lying on the surface of a flat and homogeneous lossy half-space. An analytical procedure is developed that allows to evaluate the complete integral representations for the fields, once the non-oscillating part of the integrand in the expression of the magnetic vector potential is replaced with its quadratic approximation for small values of the ratio between the wavenumbers in free-space and in the conducting medium. The advantage of the proposed second-order quasi-static approximations resides in the possibility of relaxing the assumption of highly conducting half-space. This makes it possible to overcome the limitations implied by the previously published zeroth-order formulation, whose validity is restricted to extremely low frequencies for poorly conducting media. Numerical results are presented to illustrate the reduction of relative percent error arising from using the improved quasi-static field expressions.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Second-Order Formulation for the Quasi-Static Field From a Vertical Electric Dipole on a Lossy Half-Space

Parise¹

2013

PIER

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“…Na verdade, as assinaturas identificáveis nas imagens com os resultados da resistividade, podem ser detalhadas pelas da condutividade e vice-versa. As imagens de condutividade mostraram pequenas variações nesta propriedade nos litotipos do Greenstone Belt Rio das Velhas, o que parece ser típico de terrenos Pré-cambrianos (Palacky, 1991). As imagens com os logaritmos destes valores detalharam melhor estas assinaturas, visto que em rochas pré-cambrianas as diferenças entre os valores de condutividade são muito pequenas (Palacky, 1991;Silva, 1999).…”

Section: Eletromagnetometriaunclassified

“…As imagens de condutividade mostraram pequenas variações nesta propriedade nos litotipos do Greenstone Belt Rio das Velhas, o que parece ser típico de terrenos Pré-cambrianos (Palacky, 1991). As imagens com os logaritmos destes valores detalharam melhor estas assinaturas, visto que em rochas pré-cambrianas as diferenças entre os valores de condutividade são muito pequenas (Palacky, 1991;Silva, 1999). De acordo com Palacky (1991), as freqüências eletromagnéticas baixas e médias constituem canais melhores para mapear condutores múltiplos e pequenos alvos em profundidade, devido a problemas associados a ruídos.…”

Section: Eletromagnetometriaunclassified

Integração e análise de dados aerogeofísicos por meio da aplicação de técnicas de processamento digital de imagens e classificação não supervisionada: o exemplo do Greenstone Belt Rio das Velhas, quadrilátero ferrífero, MG

Teixeira¹,

Silva²,

Moraes³

et al. 2006

Rev. Bras. Geof.

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ABSTRACT. Poor outcrop and deep weathering characterize the Rio das Velhas Greenstone Belt in the southeastern from Brazil. This paper summarizes the use of high-density airborne survey for mineral exploration studies based on interpretation enhancements of magnetic, radiometric and frequency domain electromagnetic data using image-processed methods and an unsupervised classification. The generated products provide new insights and an excellent tool for mapping and trace individual lithological units, improving the information content of the single geophysical channels. The geophysical images were processed using different combinations. The best product was the analytical signal amplitude and inclination integrated by IHS transformation. Information extracted from this image maps the geology and lineament patterns at both regional and local scales. The K-means technique using ten classes was also applied to the geophysical data. These results enhance the lithologies mapped by the field geologists at the 1:100.000. Also shows important host rocks and different gold mineralized geological domains. Such domains host the known gold mineralization, illustrating the utility of these techniques to improve the geological knowledge in the study area.Keywords: Digital Imaging Processing, gold mineralization, Rio das Velhas Greenstone Belt. RESUMO.Afloramentos escassos e um intemperismo acentuado caracterizam o Greenstone Belt Rio das Velhas no sudeste do Brasil. Este artigo sumariza a utilização de dados aerogeofísicos de alta densidade de amostragem aplicadosà exploração mineral baseado no realce e na interpretação de dados magnéticos, radiométricos e eletromagnéticos no domínio da freqüência através de métodos de processamento digital de imagens e classificação não supervionada. Os produtos gerados forneceram novos insights e uma excelente ferramenta para mapeamentos dos diferentes litotipos, melhorando o conteúdo da informação presente nos canais individuais. As imagens geofísicas foram processadas utilizando diferentes combinações. O melhor produto obtido foi a integração via IHS da amplitude e da inclinação do sinal analítico. Informações extraídas desta imagem mapeiam tanto a geologia quanto lineamentos na escalas regionais e locais. Aplicou-se a técnica de classificação não supervisionada conhecida com média K aos dados aerogeofísicos. O resultado realça litologias mapeadas por geólogos de campo na escala 1:100.000. Ela mapeia também rochas hospedeiras e diferentes domínios associados com a mineralização aurífera. Tais domínios são conhecidos por hospedar a mineralização aurífera, ilustrando a utilidade destas técnicas para enriquecer o conhecimento geológico daárea estudada.Palavras-chave: Processamento Digital de Imagens, mineralização aurífera, Greenstone Belt Rio das Velhas.

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Subsurface imaging reveals a confined aquifer beneath an ice-sealed Antarctic lake

Dugan

Doran

Tulaczyk

et al. 2015

Geophys. Res. Lett.

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Liquid water oases are rare under extreme cold desert conditions found in the AntarcticMcMurdo Dry Valleys. Here we report geophysical results that indicate that Lake Vida, one of the largest lakes in the region, is nearly frozen and underlain by widespread cryoconcentrated brine. A ground penetrating radar survey profiled 20 m into lake ice and facilitated bathymetric mapping of the upper lake basin. An airborne transient electromagnetic survey revealed a low-resistivity zone 30-100 m beneath the lake surface. Based on previous knowledge of brine chemistry and local geology, we interpret this zone to be a confined aquifer situated in sediments with a porosity of 23-42%. Discovery of this aquifer suggests that subsurface liquid water may be more pervasive in regions of continuous permafrost than previously thought and may represent an extensive habitat for microbial populations.

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3. Resistivity Characteristics of Geologic Targets

Cited by 353 publications

References 0 publications

Second-Order Formulation for the Quasi-Static Field From a Vertical Electric Dipole on a Lossy Half-Space

Second-Order Formulation for the Quasi-Static Field From a Vertical Electric Dipole on a Lossy Half-Space

Integração e análise de dados aerogeofísicos por meio da aplicação de técnicas de processamento digital de imagens e classificação não supervisionada: o exemplo do Greenstone Belt Rio das Velhas, quadrilátero ferrífero, MG

Subsurface imaging reveals a confined aquifer beneath an ice-sealed Antarctic lake

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