Quantification of ground surface movements on sorted circles using time‐lapse structure‐from‐motion photogrammetry in a karst cave, Slovenia

Sorted patterned ground is among the most striking and characteristic periglacial landforms, and its dynamics provide key insights into cryoturbation processes. Knowledge about patterned ground movements is limited because of observational challenges. Our aim was to use time‐lapse photogrammetry to study patterned ground movements with high temporal and spatial resolution in a karst cave. In our setup, point clouds could be co‐registered on cave walls, and the absence of snow allowed observations throughout the entire freezing period. We first assessed the method's accuracy and limitations, then quantified vertical and horizontal movements, and finally related them to temperature and sediment characteristics. Several cameras controlled by a microcomputer were installed above the patterned ground on a stable cave wall to take photos every 3 h. Cave air and ground temperatures at 5‐ and 25‐cm depth were monitored hourly. Sediment samples from a 75‐cm deep pit were analysed for grain size and radiocarbon ages. The point clouds generated using structure‐from‐motion photogrammetry achieved precisions of up to ±2 mm in this setting. The results showed average vertical movements of up to 50 mm and horizontal movements of up to 15 mm during single freeze–thaw cycles. The movements appear moderately to highly correlated with variations in ground surface temperatures but become uncorrelated when moisture supply is cut off by prolonged freezing periods. The horizontal movements show complex patterns, although the outwards movement component generally prevails during freezing and inwards during thawing. The recorded patterned ground movements are comparable to those observed in other studies on seasonally and permanently frozen ground. The radiocarbon ages indicate an active cryoturbation process, highlighting its presence not only in permafrost areas but also in seasonally frozen ground.