Ground penetrating radar (GPR) models of the regolith and water reservoir of an underground dam in the Brazilian semiarid region

Abstract

Underground dams supply water for smallholders in semiarid regions. The aim was to use a ground penetrating radar (GPR) to detect and map the depth to the regolith, validate the size and position and calculate the water reservoir volume of a future underground dam. A GPR survey using a 450 MHz antenna was conducted across an intermittent stream bed in a 0.85-ha area in northeastern Brazil. Nine soil trenches were opened, and the depth to the regolith was recorded at each site. The radargrams were pre-processed, migrated, and the features corresponding to the top of the regolith were delineated in the radargrams. The regolith and the terrain surface were interpolated and plotted in 3D, and the underground water reservoir volume was estimated. The depth to the regolith ranged from 0.81 to 1.60 m at the soil sites, with larger values observed along the intermittent stream thalweg, and close to the dam at the lowest part of the terrain. The radargrams transverse to the slope suggest that the future dam should be 45 m longer and centered at the thalweg position, 22 m south of the initially proposed location. This would increase the water reservoir by 50% to a total of 6 million L. The GPR allowed to properly locate the future underground dam, visualize its shape in 3D and estimate its water reservoir volume, with minimal need of soil sounding.