Abstract
ABSTRACT: The integrated analysis of geophysical loggings for identifying transmissive fractures has had a limited use in Brazil, although a large number of studies have been conducted so far throughout the world. The application of those tools has gained a crescent importance as they are needed for characterizing groundwater contamination in fractured rock aquifers at a great number of industrial sites worldwide and in Brazil. This paper presents the analysis of data collected by caliper, optical televiewer (OPTV), acoustic televiewer (ATV) and flowmeter loggings in four deep supply wells located in the industrial area of Jurubatuba, city of Sao Paulo. Five fracture sets (G1 through G5) were identified based on OPTV and ATV borehole loggings. The main fracture set (G1), NE-striking with low to intermediate dip, is subparallel to the foliation, being at great extent the result of the reactivation of this previous discontinuity. Fractures on ATV and OPTV images were visually classified with regard to flow evidence, and a positive correlation between high flow evidence and significant flow rate, measured by a flowmeter, was identified. On the other hand, the majority of fractures with insignificant, low or intermediate flow evidence are located in depth intervals with no significant flow rate. Fractures that belong to G1 are of major importance for flow, as in 9 of the 16 intervals with significant flow, they are the only fractures present. Data were inconclusive regarding transmissivity of high dip fractures, because few of them were intercepted by the well boreholes. However, there is evidence that subvertical fractures of sets G3 and G5, NE and NW striking, respectively, are also transmissive, which corroborates outcrop observations. Although geophysical loggings are essential to identify the most important segments of boreholes for water input and output, and to collect detailed data of low dip fracture sets at greater depths, the structural geology characterization of fractured aquifers cannot rely on borehole geophysical loggings alone. For proposing realistic conceptual models of the fracture network, data from boreholes should be complemented with data from large rock exposures (quarries), in order to describe geometrical parameters, such as spacing, length, and physical connectivity of low, intermediate and high dip fractures, as well as evidence of flow along individual fractures.
Highlights
The industrial zone of Jurubatuba, located at the southern portion of the city of São Paulo, corresponds to one of the six areas in the state of São Paulo where the groundwater use is restricted (L’Apiccirella 2009), due to the presence of chlorinated solvents in the water of deep supply wells, that extract water from the fractured aquifer (Fig. 1).Until recently in Brazil, studies on fractured aquifers mostly aimed at the location of new wells and were limited to relate their productivity to morphological, lithological and structural data (Fernandes 2008)
Research has started deploying and integrating a number of methods, such as detailed geological mapping and structural surveys, borehole geophysical loggings, hydraulic tests using straddle packers, as well as chemical and isotopic analysis of water sampled at discrete intervals (Alves 2008, Hart et al 2007, Fernandes et al 2011, Wahnfried 2010)
The use of several methods is essential to identify the preferential flow paths in the complex fractured aquifers, the integration of geophysical logging techniques, such as optical televiewer (OPTV), acoustic televiewer (ATV) and flowmeter in hydrogeological studies is still uncommon in Brazil
Summary
The industrial zone of Jurubatuba, located at the southern portion of the city of São Paulo, corresponds to one of the six areas in the state of São Paulo where the groundwater use is restricted (L’Apiccirella 2009), due to the presence of chlorinated solvents in the water of deep supply wells, that extract water from the fractured aquifer (Fig. 1).Until recently in Brazil, studies on fractured aquifers mostly aimed at the location of new wells and were limited to relate their productivity to morphological, lithological and structural data (Fernandes 2008). The deployment of logging techniques in hydrogeological studies have been continuously developed both in fractured crystalline aquifers (Paillet and Ollila 1994, Paillet 1995, Stumm et al 2001, Johnson et al 2002, Johnson & Williams 2003, Pino 2012) and sedimentary aquifers (e.g., Morin et al 1997, 2000, Williams and Paillet 2002). These studies apply, in an integrated manner, both conventional Geol. (2020), 50(1): e20190034 gamma radiation, resistivity, spontaneous potential, water temperature) and advanced loggings (OPTV and ATV loggings, as well as loggings of flow rate and groundwater direction at specific depth intervals, using flowmeter)
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