Subsurface variability of sandy coastal geosystems revealed by integrated georadar and cone penetrating testing approach: Insights from the Pays-de-Monts coast (Atlantic France)

Beneath dune fields, the subsurface geology of sandy coastal systems can provide valuable insights for understanding coastal environment. While sandy coasts are often described and monitored through surface geomorphology, their internal structure and geotechnical properties remain poorly documented. To address this limitation, a combined ground-penetrating radar (GPR) and cone penetration testing (CPT) approach was applied along the Pays-de-Monts coast (Atlantic France). This multidisciplinary methodology yields complementary information on the internal architecture, stratigraphic variability, and soil behavior of coastal deposits. Two sectors were investigated, both characterized by dune fields underlain by fractured limestone and bordered inland by low-lying marshes. Despite their apparent similarities, the sites reveal distinct features. Results provide new insights into (i) the sedimentary units anchoring the sandy coastline, (ii) the connection with adjacent marshes and low-lying environments, (iii) the relationships between dune systems and underlying deposits, and (iv) soil behavior properties linked to depositional environment or dune types themself. From these findings, two conceptual models are proposed: (1) a prograding sandy coastline with an inland marshward transition overlain by dune field (La Parée Grollier, northern sector), and (2) a channel-fill and spit development sequence overlain by dunes (La Pège-Les 60 Bornes, southern sector). These results demonstrate the value of integrating geophysical and geotechnical methods to improve understanding of coastal geosystems and enabling the identification of site-specific characteristics. More broadly, this study demonstrates and discusses the benefits of integrating GPR-CPT approach for advancing coastal geology and highlights the relevance of local geological models for management and adaptation strategies under climate change. • High potential of coupled GPR-CPT approach to characterize sandy coastal systems. • Buried paleo-shorelines and paleo-channel remain preserved beneath the dune system. • Approach reveals the unexpected transition between sandy shoreface and clay marsh. • A paleo-channel and a spit barrier system are identified beneath the modern dune. • The two geological models enhance the understanding of site-specific characteristics.