The search for brines: GPR markers, proxies, and challenges

GPR is one of the most applicable geophysical techniques to detect brines at shallow depths due to their peculiar electromagnetic characteristics, in particular related to overall high electrical conductivity. However, the range of chemical and physical parameters of brines is very wide and can change with time, thus making their imaging and characterization sometimes challenging. We analyze different GPR datasets collected in continental Antarctica, focusing on amplitude and spectral behavior, also calculating several GPR attributes to detect shallow brine ponds. We show that the geophysical signature of the brines is quite elusive, being sometimes characterized by clear strong reflectors, by extremely high attenuation of the GPR signal with a remarkable spectral shift, or by high scattering. Results are validated by ice coring, direct sampling, and by means of physical and chemical analyses of the brines. We demonstrate the existence of a relevant temporal and spatial variability of the brines, which requires integrated data analyses to achieve a robust GPR data interpretation. The integrated study of amplitude, frequency, and phase behavior is described in detail for different case studies, always trying to get general outcomes and site-independent interpretation criteria.