Penetration Depth Investigation of L-Band and S-Band SAR Signals into Soils and Hard Ground Surfaces

Understanding the penetrability of synthetic aperture radar (SAR) signals into near-surface materials is a prerequisite for using SAR observations to infer subsurface physical properties. However, direct measurements of penetration depth under controlled material conditions remain limited, especially for comparisons across radar bands, soil water content, sand, and hard ground surfaces. This study provides direct laboratory measurements of L-band and S-band SAR signal penetration using a ground-based SAR system in a microwave anechoic chamber. Unlike penetration depth inversion studies, dihedral corner reflectors were buried at known depths. We identified the depth at which each reflector response became indistinguishable from the background. The reported values represent effective signal penetration intervals under the laboratory geometry. The chamber effectively reduced thermal noise and electromagnetic interference. At an incidence angle of 40°, the L-band signal penetrated 45–50 cm and 30–35 cm in clay samples prepared at 4% and 18% volumetric soil water content (SWC), respectively. The L-band penetration depth was 85–90 cm in dry sand with 3% SWC, about 9 cm in asphalt, 5 cm in gravel, and 2 cm gravel plus 7 cm asphalt in a composite hard-surface layer. The S-band penetration depth was 20–25 cm in loose clay, 15–20 cm in compacted clay, and 15–20 cm in sandy loam under the tested conditions. These results show that SAR penetration depends strongly on wavelength, water content, material type, and compaction/surface condition. We also demonstrate that thin hard ground surfaces can allow measurable L-band penetration. Our findings provide experimental benchmarks for interpreting SAR signals in subsurface parameter retrieval and sand-layer characterization.