Detecting brine zones in salt deposits with the ground penetrating radar (GPR) for safety assessments of underground waste disposals

The presence of brine is one of the principal hazards of underground waste disposal. A clear reflection at a brine reservoir would be expected when carrying out ground-penetrating radar (GPR) measurements because of the high contrast in the electrical conductivity between dry rock-salt and brine. However, this is not the case as many previous measurements have shown, and as is also demonstrated by some of our GPR measurements. The lack of a distinct reflection at higher frequencies can be explained by the presence of a gradient zone leading to a frequency-dependent reflection coefficient. Therefore, measurements at two frequencies (25 and 100 MHz) were carried out in a potash salt-mine with a massive brine reservoir. When comparing the energy distributions of these measurements, the area of the brine reservoir shows a frequency-dependent reflection coefficient as opposed to areas without brine. Several finite-difference forward calculations using different frequencies and models deduced from drill-cores from the area of the brine reservoir are presented. They demonstrate that brine zones are detectable by comparing different frequencies if they are not enclosed by a sharp interface but by a gradual increase in the brine content.