4D imaging of a near-terminus glacier collapse feature through high-density GPR acquisitions

Abstract. Recent advancements in drone technology have introduced new possibilities for high-density 3D and 4D ground-penetrating radar (GPR) data acquisition over alpine glaciers. In this study, we present a 4D dataset acquired over a near-terminus collapse feature at the Rhône Glacier in Switzerland. The survey covers an area of approximately 100 m x 150 m, consists of over 100 parallel GPR lines with a lateral spacing of 1 m, and was repeated four times between July and October 2022. The glacier’s rough surface made such high-resolution and high-density surveying impossible with conventional acquisition methods, highlighting the advantages of the drone-based GPR system. The GPR data provide insights into the formation of the collapse feature as well as the evolution of associated glaciological structures. Our analysis suggests that the collapse initiated where the main subglacial water channel meanders and merges with a smaller secondary channel, coinciding with a small step in bedrock topography. After initiation, the subglacial cavity expanded through a combination of ice melting and mechanical failure, with ice lamellas detaching from the cavity roof. This process led to a progressive thinning of the roof, contributing to further instability. At the surface, these subsurface processes manifested as concentric circular crevasses, ultimately culminating in the collapse of the cavity roof. The GPR measurements also reveal the rapid temporal evolution of the main subglacial channel downstream of the cavity. During the observed summer, the channel underwent significant changes in both shape and size, which we attribute to the advection of warm air from the glacier’s large portal and the resulting increase in melt at the channel walls.