Evaluation of the transient electromagnetic geophysical method for stratigraphic mapping and hydrogeological delineation in Campos Basin,Brazil

Abstract

ABSTRACT. The applicability of the Transient Electromagnetic (TEM) geophysical method for stratigraphic mapping and hydrogeological delineation has been tested in terrestrial part of the Campos Basin in Brazil. The study area is located within the fluvial plain of Paraíba do Sul River, at the W edge of the continental portion of the Basin, geologically consisting of Tertiary and Quaternary sediments and a gneissic basement. Around 32 central loop TEM soundings were made with 10 or 20 m-sided transmitter loops, to map the conductive and/or resistive zones in the subsurface, which were correlated with lithology from a drilled borehole in the test area, whose location was indicated by the present study. In general, the one-dimensional (1D) models constructed for the TEM field data suggest a 3-layer resistivity structure characterized by a resistor-conductor-resistor pattern. The shallow section of 100-500 ohm.m and 50 m average thickness is interpreted as correlating with clay and shaly sand formations. The intermediate layer, with thickness around 20 m and resistivity less than 10 ohm.m, corresponds to clay. The basal resistor of 10-100 ohm.m at depths of more than 60 m at most sites is interpreted as the combined signature of the basal conglomerates, shaly sands and crystalline basement. Comparison with borehole data shows that the zone corresponding to the conductive geoelectrical units contains many thin layers of clay and sand that cannot be individually resolved by the TEM data and that there are no thick aquifers in the investigated area. In addition to that the shaly sand sediments have iron rich clays, which pollute groundwater, deeper resistive layers formed by conglomerates are considered the main aquifers of this area.Keywords: transient electromagnetic, inverse one-dimensional interpretation, Campos Basin, stratigraphic mapping, hydrogeological delineation. RESUMO.A aplicabilidade do método geofísico Transiente Eletromagnético (TEM) foi testada na parte terrestre da Bacia de Campos, Sudeste do Brasil, no mapeamento estratigráfico e no delineamento hidrogeológico. Aárea do estudo, situada na borda W da porção continental da bacia e dentro da planície fluvial do Rio Paraíba do Sul, consiste geologicamente de sedimentos Terciários e Quaternários sobre um embasamento gnáissico. Os trabalhos de campos consistiram de 32 sondagens com o arranjo loop central, com uma extensão lateral 10 ou 20 m. O objetivo central do levantamento foi o de mapear as zonas condutoras e/ou resistivas em subsuperfície, as quais foram correlacionadas com a litologia de um poço perfurado naárea estudada, cuja localização foi indicada a partir deste estudo. Em geral, os modelos unidimensionais (1D) resultantes da interpretação dos dados de campo sugerem uma estrutura de três camadas, a qualé caracterizada por uma intercalação resistorcondutor-resistor. A seção rasa, de 100-500 m de resistividade e uma espessura média de 50 m,é interpretada como estando constituída por formações areno-argilosas.A camada intermediária, com espessura em...

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