How do englacial radar features appear? Variability of horizons and facies in GPR data of Swiss glaciers

Ground-penetrating radar (GPR) has long been a core tool for glacier investigations, and decades of surveys have created substantial archives of radar observations across a wide range of glaciers. Increasingly, attention is shifting from extracting ice thickness alone to exploiting a broader set of radar signatures (e.g. internal horizons, electromagnetic appearance such as transparent or scattering-dominated regions, and spatial variability) that may contain information on englacial structures. Realizing this potential requires understanding how such signatures manifest in real data, how variable their appearance can be across sites, and what this implies for interpretation confidence.

Here we investigate the variability of englacial GPR features using an archive of airborne and ground-based surveys on Swiss glaciers acquired by the Glaciology and Geophysics Groups at ETH Zurich between 2017 and 2024. The archive spans radar frequencies from 25 to 250 MHz and covers glaciers with contrasting geometries, dynamics, and site histories. To enable consistent description across heterogeneous datasets, we apply an observation-driven, appearance-based organization, informed by radar-facies concepts, classifying features by reflector geometry, continuity and coherence, as well as texture. We describe basal responses, internal layering, channelized features, transparent facies, and scattering-dominated facies, and illustrate each with representative examples from across the archive.

The examples show substantial variability and ambiguity in several features. Basal responses may be discontinuous, split into multiple reflections, obscured beneath scattering-dominated facies, or expressed as gradual facies transitions rather than discrete horizons. Similarly, internal layering varies in coherence, geometry, and continuity. Scattering-dominated facies show pronounced diversity in texture and organization. While it is often interpreted in relation to temperate ice, scattering is an electromagnetic response that is not diagnostic on its own of thermal regime, and a confident thermal interpretation requires independent constraints (e.g. borehole temperatures).

By presenting real-data examples of how radar signatures depart from commonly assumed expressions, we aim to increase awareness of the variability and interpretational ambiguity of englacial GPR features. By doing so, we highlight implications for interpretation confidence and future process-oriented studies supported by complementary observations.