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. This study compares data sets obtained from three geophysical probes used for measuring rock resistivity. A detailed analysis was carried out to determine which of these probes provides the best option in terms of identification of coal-seam thickness. The resistivity probes used were guard log (GL), single-point resistance (SPR), and induction log (IND) probes. To aid the comparison among the different logs, two other geophysical logs were included: natural gamma (NG) and optical televiewer (OPTV), which helped to identify the coal seams. In the specific case of OPTV, when the situation is favorable, it identifies the coal seams and checks the vertical resolution of the other logs due to its excellent vertical resolution. In this study, analyzed data were obtained from four different coal deposits, namely B3, Calombo, Cerro, and Seival. All the coal deposits mentioned are located in the state of Rio Grande do Sul, Brazil. In all, 13 boreholes with recovered core samples are presented and discussed. The geological descriptions of the 13 boreholes are the main information used as a control parameter in comparisons with the geophysical logs, while OPTV images served as an alternative to verify the vertical resolution of the logs. At the end of the analysis, it was found that the guard log probe tends to offer better performance in the aspect of lithological identification, demonstrating a greater capacity in comparison with the others in terms of vertical resolution. Statistically, it presented the closest estimated value of coal-seam thickness in relation to that provided by geological description of the recovered core samples. Keywords : geophysics, coal, resistivity, OPTV. RESUMO. Este estudo compara registros de perfilagem geofisica obtidos por tres sondas que medem a resistividade de litologias. Uma analise detalhada foi feita para determinar qual destas sondas proporciona a melhor opcao em termos da identificacao de espessuras de camadas de carvao. As sondas de resistividade utilizadas foram: guard log , single point resistance e induction log . Para auxiliar na comparacao entre as diferentes sondas, outros dois registros geofisicos foram incluidos: radiacao gama natural e optical televiewer (OPTV), os quais ajudaram a identificar os estratos de carvao. No caso especifico do OPTV, quando a situacao e favoravel, ele permite identificar os estratos de carvao e verificar a resolucao vertical dos demais registros, em funcao da excelente resolucao vertical. Neste estudo, foram analisados dados obtidos em 4 depositos de carvao diferentes: B3, Calombo, Cerro e Seival. Todos os depositos de carvao mencionados situam-se no estado do Rio Grande do Sul – Brasil. Ao todo, 13 furos de sondagem com recuperacao de testemunhos sao apresentados e discutidos. As descricoes geologicas dos 13 furos de sondagem sao as informacoes principais utilizadas como parâmetro de controle nas comparacoes entre os perfis geofisicos, enquanto que as imagens OPTV serviram como uma alternativa para verificar a resolucao vertical dos perfis geofisicos. Ao final das analises, verificou-se que o dispositivo guard log tende a ter o melhor desempenho no aspecto de identificacao litologica, demonstrando uma capacidade superior em relacao aos demais arranjos em termos de resolucao vertical. Estatisticamente, ele apresentou as estimativas de espessurasmais proximas em valor, em relacao as espessuras proporcionadas pelas descricoes geologicas de testemunhos de sondagem. Palavras-chave : geofisica, carvao, resistividade, OPTV.

COMPARATIVE ANALYSES OF GEOPHYSICAL LOGS OF RESISTIVITY AS APPLIED TO COAL DEPOSITS