## 1 Introduction  
Ground penetrating radar (GPR) has emerged as a vital technology for subsurface investigation [1]. While conventional surveying methods usually include local 1D borehole drilling and manual 2D geophysical surveys (e.g., electrical resistivity tomography), they face limitations in coverage, cost, and time efficiency, especially in challenging terrains [2; 3; 4; 5; 6; 7]. While a foundation design is directly related to the soil-to-rock depth, especially if the soil is disturbed, the depth of the groundwater level (GWL) essentially affects the stability of shallow foundations (consolidation, liquefaction, etc.) [8; 9; 10]. In transportation infrastructure assessment, railway and highway stability also directly depend on these two properties. The numerous applications of GPR for road pavement assessment were reviewed in [21]. For example, the railway/highway vehicle-based GPR studies are held above the ground at a fixed distance, mapping underground infrastructure, including pavement thickness, disturbed soil, soil moisture, and gravel inventories, as was used to assess the condition of Croatia's old railway underground infrastructure [22; 23]. Similarly, in site response analyses, the thickness of the soil column directly affects amplification factors and nonlinear effects, which are critical for understanding how the ground will behave during seismic events [24; 25]. The presence of GWL relatively close to the ground surface significantly impacts the characteristics of ground motions [26; 27; 28; 29; 30]. Although GPR has not yet been used for site response-related applications, knowing the precise location of the soil-to-rock boundary and the WL depth can significantly enhance the accuracy of ground motion prediction models and the design of foundation systems. Traditional ground-based GPR has been used in similar attempts to measure the distributed soil-to-rock depth and GWL [31]. For instance, [PERSON] et al. [6] explored soil water content using electrical resistivity imaging and GPR methods. They compared these findings with variability maps, revealing a promising correlation between the geophysical and hydrological data. [PERSON] et al. [7] employed two GPR methodologies for observing underground moisture: one by evaluating dielectric permittivity from signal attenuation and the other by using Rayleigh scattering to analyze frequency spectrum peak shifts. Both methods showed significant potential for large-scale moisture mapping [7]. [PERSON] et al. [32] used ground-based GPR for mapping nonlinear boundaries between covering clayey soil and natural soil for agricultural purposes. Additionally, [PERSON] et al. [33] utilized land-based GPR to study the 3D sedimentary architecture of recent sediments in the Yun-gang braided river study area in Datong (China), emphasizing the need to characterize unstable, disturbed ground quantitatively. Traditional land-based GPR has already found applications in agriculture in monitoring soil water content during irrigation in clay-disturbed soils [3]. [PERSON] et al. [2] applied ground-based GPR to gain knowledge about the thickness and internal structure of the Oschingen landslide in Germany, detecting near-surface sediment structures despite signal attenuation and reflections. [PERSON] et al. [34] explored the response of various geophysical methods, such as time-domain reflectometry, frequency-domain reflectometry, GPR, remote sensing, and capacitance, to soil moisture content. They highlighted the need to calibrate dielectric models used in measurement techniques based on desired precision and field conditions. This aligns with the conclusions by [35; 36; 37], who noted that integrating GPR with other methods and additional collected on-site information can improve accuracy in estimating soil properties and enhance the investigation of various soil characteristics to understand soil relationships better. [PERSON] et al. [38] investigated the factors contributing to the deterioration of art in the Cave of Altamira, particularly in the Polychrome Hall.