Characterization with geoelectrical methods of fissural porosity and flow pattern in physical models of fractured rock masses

Abstract

Understanding water flow through discontinuities in rock masses is essential for ensuring the safety of engineering works, understanding the transport of contaminants, and directing the prospection and production of water in fractured aquifers. Investigation methods such as boreholes are expensive and not always effective. An alternative can be indirect investigation with electrical geophysical methods, such as Electrical Resistivity (ER) and Streaming Potential (SP), owing to their technical feasibility, low cost, and speed of execution. Although well established, doubts remain about these surveys owing to field limitations. In this sense, simulations in reduced physical models, where the internal and boundary conditions tend to be known, are ideal for testing these surveys. In this study, a rock mass model was developed with vertical fracturing patterns covered by soil; the entire set was saturated, thus reproducing the conditions common to areas of the crystalline basement with a tropical climate. With the use of ER and SP, the objective was to map the preferential flow directions with and without deep tube well influences and estimate fissure porosity. ER results showed an efficiency of 85% in the estimation of porosity, clearly distinguishing the main directions of anisotropy by the fractures. With SP, it was possible to identify flow directions through the fractures and define the geometry of the depression cone caused by pumping a deep tube well. Overall, these findings demonstrate the feasibility of physical models for understanding media with fissural porosity.