Characterizing Sagging and Collapse Sinkholes in a Mantled Karst by Means of Ground Penetrating Radar (GPR)

Sinkhole development created by diverse subsidence mechanisms (suffosion, collapse, sagging) constitutes the main hazard in most karst terrains. Precisely mapping the limits of sinkholes and inferring the subsidence mechanisms are critically important for the effective mitigation of sinkhole risk. Gathering this information commonly requires the application of subsurface investigation methods, such as geophysics or trenching. Here, we analyze the potential of ground-penetrating radar (GPR) for sinkhole characterization in covered karsts. Extensive GPR surveys have been conducted across two buried active sinkholes of contrasting genetic types (collapse vs. sagging) in the mantled evaporite karst of Zaragoza city, NE Spain. Data were acquired with 100 MHz and 50 MHz unshielded antennas and similar to 180 MHz shielded antennas. Interpretation of the processed GPR profiles allowed us to map reliably the boundaries of the sinkholes, characterize their internal geometries (deformation style), infer the subsidence mechanisms, and estimate subsidence magnitudes. The suitability and limitations of the GPR technique in covered karsts are illustrated considering different sinkhole types and sizes, as well as data acquired with different antennas (shielded versus unshielded and various frequencies).