Combining electrical resistivity tomography and passive seismic to characterise the subsurface architecture of a deeply weathered lateritic hill within the Avon River Critical Zone Observatory

Lateritic landscapes are structurally complex systems formed through intense chemical weathering under tropical paleoclimates. These profiles are found in stable, low-relief landscapes across tropical, subtropical, and Mediterranean climates, particularly between 35°N to 35°S. Their vertical structure reflects long-term shifts in climatic, hydrological, and tectonic conditions, offering a valuable "memory" of past environmental changes. Despite their environmental and economic significance, lateritic landscapes remain underrepresented in CZ research, a bias compounded by the concentration of Critical Zone Observatories in the Northern Hemisphere, where shallow, truncated profiles prevail due to glacial erosion. This underrepresentation limits our understanding of long-term CZ processes and how they have shaped subsurface architecture.

This study investigates the subsurface architecture of a lateritic hillslope at the Avon River Critical Zone Observatory (AR-CZO) in Western Australia. Prolonged subaerial weathering since the Cretaceous, followed by mid-Miocene aridification, has created a stratigraphically complex regolith hillslope shaped by weathering, erosion, and colluvial deposition. To resolve the structural complexity of this hillslope, we applied a multi-method geophysical approach, combining electrical resistivity tomography (ERT), horizontal-to-vertical spectral ratio (HVSR) passive seismic methods, and borehole observations. ERT captured fine-scale stratigraphy, delineating the pallid zone, saprolite, and duricrust, while HVSR resolved broader interfaces, such as the duricrust-bedrock boundary and the base of the colluvial deposit.

The results reveal how landscape position influences CZ structure. The hilltop is capped by a duricrust that transitions downslope into an erosional surface, where the pallid zone of the lateritic weathering profile is exposed at the surface. At the foot slope, approximately 11 m of colluvial sediment has accumulated from the erosion of the hillslope material. Bedrock depth estimates differed between methods, with ERT indicating depths of 23 m on the slope and 32 m at the foot slope, while HVSR revealed deeper depths of 31 m and 39 m, respectively. The discrepancy highlights the limitations of ERT in saline environments, where conductivity masks key interfaces, while HVSR’s broader resolution provides more reliable bedrock detection in such conditions. Together, these methods reveal a laterally variable weathering profile that responds to shifts in landscape position, erosion, and deposition.

The complementarity of ERT and HVSR underscores the value of a multi-method geophysical approach for resolving the structural complexity of lateritic CZs. Our conceptual model demonstrates how weathering, erosion, and colluvial processes shape the structure of a deeply weathered hillslope, while also providing a transferable framework for characterizing saline, regolith-dominated systems. Given their depth, age, and capacity to preserve past climatic and tectonic conditions, lateritic CZs offer a vital opportunity to enhance global understanding of long-term CZ evolution. This research addresses the Northern Hemisphere bias in CZ science, highlights the underexplored role of stable, deeply weathered landscapes, and underscores the need for future comparative studies to understand the drivers of heterogeneity in subsurface architecture across CZs worldwide.