DETEÇÃO DE CONDUTORES TABULARES SOB COBERTURA INDUTIVA

Revista Brasileira de Geociências

21(1):82-95, março de 1991

DETEÇÃO DE CONDUTORES TABULARES SOB COBERTURA INDUTIVA
LÚCIA M.C. SILVA* & MARCOS V.G. GALVÃO**

ABSTRACT DETECCION OF HALF-PLANE CONDUCTORS UNDER AN INDUCTIVE SUPERFICIAL
LAYER. The effects of a superficial layer of relatively high induction number on the horizontal - loop electromagnetic (HLEM or Slingram) response to an isolated tilted half-plane are analysed by analog modeling. It was found that the superficial layer produces symmetry in the in-phase component, and therefore makes the tilted body seem vertical. Also, the superficial layer causes amplitude reduction and a clockwise phase rotation of the anomaly, which may result in the inversion of the quadrature component, base level shift of the profiles and appearance of an extra peak in the quadrature. The relations between these effects and the dip, induction number and depth of the half-plane as well as the required loop calibration are showed and compared with those for a superficial layer of weak-to-moderate induction number. The results show that it is possible to find information about the body in the noise, because the effects of the superficial layer are considered as noise. Such a paradox is discussed. Finally, the reported symmetry in the in-phase is analysed in respect to an analogous effect, that can occur for a body in galvanic contact with a conductive environment and it is associated with a current gathering phenomenon, also called negative screening.
Keywords: Overburden, HLEM, Slingram, analog model, scale-model, three-dimensional model, half-plane conductor, tabular conductor, electromagnetic method, EM.
RESUMO Os efeitos de uma camada superficial de número de indução relativamente elevado sob a resposta de um condutor tabular inclinado e isolado, obtida com o sistema eletromagnético de bobinas horizontais coplanares
(HLEM ou Slingram), são analisados por meio de modelamento analógico. Foi observado que a