The accurate location of aquiferous fracture zones in granite beneath a > 50 m thick weathered mantle in semi-arid regions is a major hydrogeological problem. It is expected that the zone of intensive fracturing will be more susceptible to weathering and thus be characterized by the thickest development of saprolite, a good electrically conductive target for deep-probing electromagnetic systems. The single-loop transient electromagnetic (TEM) technique is well known to have the capability for detecting concealed steep mineralized targets in mining environments and can be adapted to this hydrogeological problem. We propose that combining the conventional frequency-domain horizontal-loop electromagnetic (HLEM) and single-loop TEM is an effective practical approach to locating concealed aquiferous fracture zones. In the supporting case studies presented here, we deployed multifrequency HLEM profiling (with 50 m transmitter-receiver separation) and TEM soundings with contiguous 10 or 20 m sided loops along the survey lines in a granitic terrain affected by deep (> 50 m) weathering in northeast Brazil. A somewhat layered structure consisting of resistive hardpan/leached zone, conductive saprolite, and resistive basement is identifiable in the typical TEM depth sounding data. We obtained coincident HLEM and TEM anomalies at all the sites, enabling a relatively straightforward selection of potential drilling positions. Simple resistivity-depth transformation of the TEM data was done for each site, yielding an approximate section from which drilling depths were estimated. All of the boreholes located were successful. Although our results appear to indicate that the single-loop TEM method could be used independently for borehole sitting in deeply weathered granitic terrains and that the weathering profile over granite can be mapped using TEM depth soundings of appropriate observational bandwidth, we recommend a joint electromagnetic approach for optimal well sitting.

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ABSTRACT.In the present work we present a case history where the Controlled Source Audiomagnetotelluric -CSAMT method have been applied to unveil the geological structure of Regis, a kimberlite pipe in Minas Gerais state, Brazil. The kimberlite outcrops were discovered in the late 1970's by surface geological mapping.Three CSAMT profiles, two striking north-south and the third striking east-west were acquired crossing the kimberlite crater. The Stratagem CSAMT sounding stations, with electrodes and coils laid in the classical cross configuration. Typical site spacing spanned from 50 to 100 m along each profile. The frequency range spanned from 10 Hz to 100 kHz, adequate for shallow investigations (depths less than 500 m). Shallow conductive anomalies were identified and associated to the main kimberlitic body. The present work subsidized the location of two successful drill holes within the studied pipe and allowed a proposition of a new geological model for the pipe.Keywords: kimberlites, CSAMT, 2-D inversion. RESUMO.No presente trabalho apresentamos um estudo de caso onde o método CSAMT (audiomagnetotelúrico com fonte controlada) foi aplicado para determinar a assinatura geoelétrica da estrutura geológica do kimberlito Régis, localizado e conhecido no Estado de Minas Gerais, Brasil. Afloramentos desses kimberlitos foram mapeados na década de 1970. Três perfis CSAMT, dois com direções norte-sul e um leste-oeste foram adquiridos ao longo do corpo. As estações CSAMT foram adquiridas com eletrodos e boninas dispostos na configuração clássica em cruz. O intervalo típico entre estações variou de 50 a 100 m ao longo de cada perfil. A faixa de frequência utilizada foi de 10 Hz a 100 kHz, adequadas para investigações rasas (profundidades menores que 500 m). O presente trabalho deu subsídio para a locação de dois furos de sondagens dentro do pipe estudado e permitiu a proposição de um novo modelo geológico para o kimberlito.Palavras-chave: kimberlitos, CSAMT, inversão 2-D.

We present a new interpretation of the basement immediately below the pre-Silurian unconformity in the mid-western part of the Parnaíba cratonic basin. Regional seismic reflection data, constrained by well and gravity data, have revealed the remnants of a pre-Silurian basin named the Riachão basin. It has a north–south trend and is about 120 km wide with a minimum area of c. 35 000 km2 and a maximum thickness of 4 km. Seismic interpretation shows the Riachão basin is defined by erosional unconformities at its top (the pre-Silurian unconformity) and base (the top of the pre-Riachão basement). The basin fill is formed of three stratigraphic sequences – Riachão I, II and III – separated by seismic discontinuities and laterally bounded by monoclinal folds and basement uplifts related to thrust faults. Exploration wells show that the Riachão sediments are Ediacaran to Lower Cambrian, clastic and a minimum of 1 km thick, unconformably overlying a heterogeneous metamorphic basement. The mechanism of formation of the basin is unclear. However, our seismic stratigraphic interpretation suggests that Riachão II and III are remnants of marine, clastic, foreland basin sequences of opposed polarity, related to late Neoproterozoic tectonics. The banded, high-amplitude, Riachão I sequence is interpreted as a carbonate platform preceding the main subsidence and clastic input of the Riachão basin.

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The deep structure beneath the basalt‐covered sediments of Paraná basin is not well understood. Magnetotelluric (MT) broadband (1,000 to 0.001 Hz) data recorded at 296 selected stations along three new survey lines were first inverted in three dimensions to reveal the local scale resistivity structure of the central part of the basin where there are three deep exploration wells with resistivity logs for model validation. The 3‐D MT inversion model correctly identified the resistive (>100 Ωm) flood basalts and postvolcanic cover layer, the underlying electrically conductive (~10 Ωm) sediments, and the highly resistive (>1,000 Ωm) crystalline basement at the well locations and provides a reliable average crustal resistivity structure consisting of ~20–24 km thick highly resistive upper crust and ~20 km thick conductive lower crust above a resistive mantle. Subsequent basin‐scale joint inversion of highest quality MT data sets (261 stations) and geomagnetic depth sounding array data imaged two prominent subparallel belts of N‐S and NNE‐SSW trending steep conductors in the deep crust and upper mantle, one located directly beneath the present‐day axis of thickest sedimentary deposits in the basin and another near the coastal location where dolerite dyke swarms intrude the outcropping Precambrian basement. Other roughly orthogonal bands (E‐W to NW‐SE) of linear crustal conductors radiate from these two main belts. The resulting networks of steep conductors at upper to midcrustal level seem to mark the fossil plumbing system of Paraná magmatic province and are interpreted as preexisting shear zones exploited by the voluminous Early Cretaceous magmatism in the region.

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