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We performed laboratory experiments to investigate the sensitivity of the Spectral Induced Polarization (SIP) method to toluene contamination in clayey soils. We used mixtures of quartzitic sand and montmorillonite as soil samples, artificially contaminated with varying amounts of toluene. Care was taken to quantify the experimental uncertainty resulting from packing since such effects must be quantified if variations in SIP signatures between samples are to be reliably interpreted in terms of the effects of hydrocarbon concentration. The SIP response of all samples following addition of toluene was monitored for a period of 40 days following sample preparation. Stepwise regression was used to examine the statistical significance of correlations between (i) clay content and (ii) toluene concentration and SIP parameters. Both single‐frequency real and imaginary conductivity measurements, along with the integral chargeability, normalized chargeability, DC conductivity and time constant obtained from a Debye decomposition fitting, were examined in this regression analysis. The SIP measurements show a clear time dependence following sample preparation, indicating that samples containing toluene may take significant time to reach an equilibrium electrical response. SIP measurements are significantly related to toluene content shortly after sample preparation, when the expected dependence of SIP on clay concentration is apparently suppressed. However, for the state of electrical equilibrium after 40 days (interpreted to indicate surface chemistry at equilibrium) there is no significant relation between SIP measurements and toluene content; instead SIP measurements are then significantly correlated with clay concentration. The total chargeability, normalized chargeability and relaxation time obtained from the Debye decomposition show no correlation with toluene content, indicating that this procedure, which likely integrates over multiple mechanisms, may not be suitable for understanding relationships between SIP and hydrocarbon contamination. We find only small low‐frequency polarization signals observed in relation to toluene concentration (2 mrad at 0.01 Hz), which initially decreases the interfacial polarization. Unlike earlier works, our results do not support the use of the SIP method as a tool for monitoring toluene contamination in clay soils.

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ABSTRACT. This paper presents the results obtained from surface and borehole geophysical investigations carried out at the Shallow Geophysical Test Site of IAG-USP. The main objective was to characterize the sediments and the top of basement. The area is constituted by sandy-clay sediments of São Paulo Basin (São Paulo and Resend Formations) on the granite-gneiss basement with different degrees of weathering. In the area of studies, three boreholes were drilled for geological and geophysical research purposes (pioneer in Brazil). These boreholes were used to verify the stratigraphy, to collect the sample for laboratory measurements, to calibrate the results of surface geophysics and to carry out well logging. The surface methods used were: seismic (reflection and refraction), electric-resistivity (profiling and RESUMO. Este trabalho sintetiza os resultados de investigações geofísicas de superfície e de poço realizadas no Sítio Controlado de Geofísica Rasa do IAG-USP,visando caracterizar os sedimentos e o topo do embasamento. Aáreaé constituída por sedimentos areno-argilosos da Bacia de São Paulo (Formações São Paulo e Resende) sobrepostos ao embasamento granito-gnáissico com diferentes graus de intemperização. Naárea de estudos, também denominada de campo de provas do IAG, foram perfurados três poços para pesquisas geológicas e geofísicas (pioneiros no Brasil), visando verificar a litoestratigrafia, coleta de amostras de calha, calibrar os resultados e realizar as perfilagens geofísicas. Foram utilizados os seguintes métodos: sísmica rasa (reflexão e refração), eletrorresistividade (caminhamento elétrico e sondagem elétrica vertical) e GPR-Ground Penetrating Radar. A integração dos resultados mostrou uma excelente concordância e permitiu elaborar um modelo geológico-geofísico para este ponto da Bacia Sedimentar de São Paulo, bem como testar, em condições controladas de campo, os diversos métodos geofísicos usados rotineiramente em estudos geológicos, geotécnicos e ambientais.

We present fi ve profi les from electrical resistivity tomography (ERT), with surface constraints and gravity data, in the central uplift of the Araguainha impact structure in central Brazil. The central uplift, the overlying polymict breccias, and decameter-scale impact melt rocks are characterized by contrasting ranges of electrical resistivity. Our resistivity model provides empirical evidence that supports the existing model in which impact melt and breccias resurged toward the crater center in the fi nal stages of the cratering process. On the basis of our results from the fi rst use of ERT in impact cratering studies, we conclude that the deposition and fl ow of impact melt and breccias over the central uplift were infl uenced by the geometry of the lithologic boundaries in the central uplift.