José Domingos Faraco Gallas

Contaminação por chorume e sua detecção por resistividade

Resistivity geophysical techniques were applied closed to the Londrinas county sanitary landfill, allowing the detection and mapping of leachate contamination plume. The study area lies on the Parana Basin which is locally represented by basaltic rocks of the Serra Geral Formation. A correlation between lower resistivity values and the supposed leachate contaminations was stablished, once the electrical conduction, under natural conditions, occurs mainly in an ionic way, which are normally presented in the leachate. The main contamination flows in a general SE direction. An unexpected finding of this work was the detection of another flux direction of contamination plume to SW, which flows trough horizontal fractures or alteration surfaces of basaltic contacts.

O método do potencial espontâneo (SP): uma revisão sobre suas causas, seu uso histórico e suas aplicações atuais

This paper synthesizes the results of a compilation and integration of the most important papers and textbooks dealing with the self potential method (SP), also taking into account the author experience on the subject. Furthermore, it introduces survey and processing techniques as well as presentation and interpretation modes. A short description of the SP generation processes, related to both mineral exploration and ground water flow studies, is presented. In the former case, the usually negatives SP anomalies are ascribed to the presence of massive sulphide bodies (electrical conductors). In the case of environmental or engineering studies, the main application of the SP method is to determine the sense of the ground water flow. The occurrence of noises, and the way of eliminating or minimizing them, are considered. The distinct applications of the SP method as well as the expected results from different situations and possible interpretations are also discussed.

Métodos e técnicas geoelétricas rasas na delimitação de área afetada por ruptura em teto de túnel urbano

This paper presents a case study where geophysics was successfully applied to solve a complex problem associated with a tunnel excavation accident in an urban area. It presents and describes the results obtained with geoelectrical methods applied in an emergencial task in a risky urban area in Sao Paulo City. It also compares the techniques and field arrays, ways to present and interpret the data. The employed methods were electroresistivity, induced polarization (IP) and self potential (SP). The objective of the survey was to delineate the extension of a roof failure of an urban tunnel under excavation beneath the Ibirapuera Park in order to orientate the volume of tunnel roof should be treated. Only two days (Saturday and Sunday) were available to carry out the survey and its interpretation. The results were considered satisfactory and brought a significant economy to the final cost of the tunnel. This economy was due to the decreasing of the remediation area from the initial 20 x 20 m (estimated by the engineers) to 3 x 5 m (estimated by the geophysical survey).

Desenvolvimento e testes de mini-sonda Wenner para avaliação de contaminações rasas

The evaluation of soil and groundwater contamination has been an important task in the geosciences field and pushing on the methodological improvement. The contamination extension is normally investigated using geophysical methods or boreholes and chemical analysis. Otherwise, several contamination problems are at low depths (less than 0.5m) allowing the easy access to the contaminated region. Therefore, a low-cost and simple system (mini-probe Wenner) was developed and tested and it is aimed at low depth resistivity measurements. The system is formed by a current source (AC) and a PVC probe which contains a Wenner array with a electrode spacing of 1 cm. Laboratory tests show that the current source works adequately in the resistance range normally associated with contaminated materials while the system tests (including the current source and probe) indicated apparent resistivity values very similar to the resistivity values calculated using the equation proposed by Archie, in the 1.5 to 57 Wm interval. Preliminary field tests showed that the mini-probe is easily applicable in sandy and sand-silty soils while in clayey or hard soils is difficult its introduction. Therefore, interesting applications to the proposed system are the study of agricultural areas or the study of contamination in predominantly sandy lithologies.

Eletrorresistividade 3D: uma avaliação preliminar da técnica

This paper shows the results of a 3D resistivity survey conducted to assess the potential of this acquisition technique in shallow geophysics studies. Twenty-five electrodes arranged in a square mesh (4m × 4m) were used for data acquisition (pole-pole array with 1-m spacing between electrodes). The results were compared with two different systems of data acquisition and confirmed the techniques capacity to map variations in 3D resistivity. The data acquisition also confirmed that the coverage of wider areas with this type of test will only be achieved with multi-electrode equipment and automated acquisition, given the time required to perform all the measures involved in this type of survey.

O arranjo pólo-dipolo como alternativa ao dipolo-dipolo em levantamentos 2D de eletrorresistividade

This study shows the results of a near-surface investigation conducted to compare pole-dipole with dipole-dipole array. The survey was carried out in a single profile, 40 meters long. In the dipole-dipole array, the inter-electrode spacings used were 2 m and 1 m. In the pole-dipole array, spacing was 1 m. The data collected from the dipole-dipole array were combined into a single section and then compared with the results of the pole-dipole array. The investigation levels used in each array were selected in order to achieve, a priori , the same depth in the subsurface. The corresponding apparent resistivity pseudosection (field data) and the model (obtained by an inversion procedure) were very similar, proving that the pole-dipole array is a possible alternative to the dipole-dipole array, since it provided similar results for resolution and depth of investigation, as shown in the final models.

Resistividade e polarização induzida (IP): modelagem analógica

The aim of this study is to develop an analogical modeling methodology for IP and resistivity. Simple geometrical cylindrical and tabular shapes were built using standard cement, quartzitic sand and graphite. The experiment was carried out with various amounts of graphite in order to measure the IP response for different model geometries. The results are discussed in terms of IP response as a function of geometry, graphite content, depth and strike of the models.