We used shallow seismic surface waves and multioffset ground penetrating radar (GPR) to investigate the vadoze zone at a sandy soil site in Grugliasco (Turin), Italy. The two geophysical methods yielded coincident high resolution imaging of the upper six meters of the subsurface. We identified three distinct horizons along a GPR common offset profile and corresponding shear wave and radar wave velocity profiles. Seismic surface wave velocities are sensitive to soil shear strength whereas radar velocities are related to soil volumetric water content.
Near surface geophysical methods offer the potential to characterize remotely properties of the heterogeneous shallow subsurface environment. Of interest to environmental and hydrologic studies is imaging of subsurface structure and lithology, and determination of fluid content. Seismic methods respond to the mechanical properties of the subsurface whereas GPR responds to subsurface dielectric properties. Although mechanical and dielectric properties of media are not directly liked by laws of physics, joint use of high-resolution seismic and GPR reflection methods has provided improved imaging of the shallow subsurface (Cardimona et al., 1998; Baker et al., 2001; Sloan et al., 2007). We evaluate the joint use of seismic surface wave methods and multioffset ground penetrating radar for characterization of the vadoze zone at a sandy soil site. The dispersive properties of seismic surface waves are used to determine soil shear wave velocity structure (e.g. Heisey et al., 1982; Xia et al., 1999; Socco and Strobbia, 2004; Ivanov et al., 2009) whereas velocity analysis of GPR reflection multioffset data is used to provide estimates of soil volumetric water content (Topp et al., 1980; Davis and Annan, 1989; Greaves et al., 1996).
Site Description Geophysical data were acquired at the University of Turin, Department of Agriculture test site in Grugliasco, Italy. The shallow subsurface consists of aeolian sands with porosity ranging between 0.35 and 0.4. Two main soil horizons are recognised in the area (Comprehensive Soil Classification System): the A-horizon of 80 % sand, 14 % silt and 6 % of clay (porosity 0.4) has an average depth of 1.2 m; the C-horizon (95 % sand) ranges from 1.2 m to 2.5 m (porosity 0.35) (Godio and Ferraris, 2005). The water table is at a depth of approximately 20 m. In the past decade, various geophysical studies have been undertaken at this site. Godio and Ferraris (2005) employed electrical resistivity tomography and time domain reflectometry (TDR) to monitor over time a controlled infiltration experiment and to assess near surface heterogeneity. Ground Penetrating Radar We acquired GPR data using an IDS system operating at 100 and 200 MHz frequencies. A 27 m long, single fold, continuous acquisition mode reflection profile (approximately 100 traces/m) at 200 MHz frequency was collected along the multifold GPR and seismic transects. Multioffset radar data were acquired in wide angle reflection-refraction (WARR) mode using denser gather spacing of 1 m at the center of the transect (at positions 10 m to 20 m) and wider spacing at the transect edges.