Off- and on-ground GPR techniques for field-scale soil moisture mapping

•Three GPR techniques are used in an agricultural field to map the soil moisture.•All GPR techniques relatively retrieved similar soil moisture maps.•Surface reflection coefficient method can be also used for on-ground GPR data.•The shallow bedrock makes it confusing to distinguish DGW from on-ground profiles.

Precise measurement of shallow volumetric soil water content (VSWC) is essential in different research fields, including agriculture, hydrology and meteorology. In this research three different approaches were applied for mapping the shallow VSWC of a bare soil at the field-scale, namely, (1) the direct ground wave (DGW) method using on-ground ground-penetrating radar (GPR), (2) the surface reflection coefficient (SRC) method also using on-ground GPR, and (3) the surface reflection inversion using off-ground GPR. Measurements were conducted in an agricultural field in both relatively dry and wet conditions. Soil sampling was performed at different depths (5, 10, 20, 30, 40 cm) for determining the reference VSWC. The on-ground GPR was setup using two transmitting (Tx) and receiving (Rx) 400 MHz bowtie antennas with Tx and Rx offsets of 0.16 and 1.26 m, respectively, thereby setting up a multi-offset system. A vector network analyzer combined with a 0.2–2 GHz TEM-horn antenna located at 1.1 m above the ground was used as off-ground GPR. GPR measurements were performed following more than 10 transects in the entire field and positioning was performed using a dGPS. In addition, on-ground GPR measurements were conducted in the laboratory over a sandy soil layer including 10 different VSWC, ranging 0.02–0.18 cm3·cm− 3, in order to examine the SRC method for the on-ground GPR configurations. The Topp's model (1980) was used in order to transform the relative dielectric permittivity of the soil to the VSWC. Laboratory results showed that SRC method can be confidently used also for the on-ground GPR configurations when the air direct-coupling wave is subtracted from all signals. For the field data, the validation graphs using the ground-truths of the shallowest layer showed relatively reliable VSWC-maps with the RMSE values of 0.042, 0.044, and 0.059 in terms of VSWC for the SRC, DGW, and full-wave methods, respectively. In addition, the existence of a very shallow bedrock made a confusion on the detection of the DGW from the GPR data in the dry condition, but had no effect in the wet condition.