Geofísica eletrorresistiva aplicada ao monitoramento temporal da percolação de fluidos no interior de estruturas de barragens

Abstract

The importance and impact that dams have on the economic, social, environmental, strategic, and health levels make it necessary to maintain the safety and functionality of these projects. Thus, one way to keep their physical and functional integrity is to perform temporal monitoring through auscultation systems. Several instruments exist for this purpose, including piezometers, pressure gauges, inclinometers, and flow gauges. Geophysical methods are already applied to monitoring dams in several countries. With technological advances, they are becoming more accurate and contributing to the interpretation of data obtained by conventional instrumentation. Among the geophysical methods, the resistivity has great versatility in monitoring dams because it has sensitivity in detecting saturation in the subsurface. Contribute to new methodologies for monitoring dam this work realizes a case study. The chose of case study is the Santa Helena earth dam, located in the city of Mirandópolis, São Paulo, which was monitored monthly from August 2020 to January 2021 with the use of electroresistive geophysics employing the electrical resistivity tomography technique with low-cost equipment for data surveys. Two parallel lines 96 m wide were placed on the longitudinal axis on the dam crest, the spacing between the electrodes was 2 m, which has allowed seven levels of investigation, reaching approximately 7,5 m in depth. As a result, it was possible to see the great influence of rainfall on the measured resistivity values, being very high in the dry months and low in the wet season. In addition, with the aid of internal fluid flow models, it was possible to verify that the system's water percolation behavior is the one expected in a homogeneous dam. The unsaturated surface area increasing as it moves away from the reservoir detected in inversions geophysics by increasing the resistivity in the subsurface from upstream to downstream. Furthermore, the monitoring was able to detect an anomaly of low resistivity positioned close to the surface, interpreted as a piping process caused by possible weakness zones. The construction of a shelter to the adductor pump above the dam crest and the vibrations related to its work might be the reason for the existence weakness zones. The present work demonstrated that even using a limited measuring equipment, it was possible to monitor resistivity variations that may indicate structural problems. It has confirmed that electroresistive geophysical techniques can greatly contribute to dam monitoring systems since their dimensions range can be useful in interpreting the extent of anomalies punctually measured by conventional instruments.