4D GPR imaging of a near-terminus glacier collapse feature

Abstract. Recent advancements in drone technology now enable high-density 3D and 4D ground-penetrating radar (GPR) data acquisition over challenging glacial terrain. In this study, we present a drone-based 4D GPR dataset collected over a surface collapse feature near the terminus of the Rhône Glacier, Switzerland. The GPR measurements, repeated four times between July and October 2022, captured the evolution of an air cavity and associated subglacial drainage pathways. Our results indicate that the collapse originated where the main subglacial water channel meanders and merges with a smaller secondary channel, coinciding with a subtle step in bedrock topography. The cavity expanded progressively through a combination of subglacial melt and mechanical failure, leading to thinning of the ice roof and eventual collapse, which manifested at the surface as circular crevasses. Downstream of the feature, the main subglacial channel underwent rapid changes in shape and size over the summer, likely driven by warm air entering from the glacier's portal and enhancing melt at the channel walls. These results highlight the capability of drone-based GPR for capturing detailed, time-dependent changes in glacier internal structure, offering new opportunities for monitoring dynamic glaciological processes in otherwise inaccessible areas.