A study of the GPR’s direct wave

The powerful signal of the direct wave (DW) has an impact on ground penetrating radar (GPR) signals by saturating the early time recorded signals and also, by possibly masking detection and complicating interpretation of reflections from shallow buried targets.In this paper, we investigate the spatial distribution of the DW signals recorded by a GPR receiving (Rx) antenna. We aim to study whether there is an advantageous positional configuration of the transmitting (Tx) and the Rx antenna pair, where the direct wave is recorded with the least possible amplitude, showcasing the reflections of targets lying at shallow depths.For this purpose, we performed static field tests as well as synthetic measurements in a reflection common offset (CO) mode around a Tx antenna. The Rx antenna recorded the GPR signals in three concentric circles of various radii (i.e., varying the Tx/Rx separation), using a specific angular step and varying the Tx/Rx polarization each time. In the field, Wide Angle Reflection Refraction (WARR) data were also collected to determine the GPR wave velocity in the upper medium. The synthetic data were produced using a three-dimensional (3D) finite-difference time-domain (FTDT) modeling tool. Observed and synthetic data were analyzed and compared to study the behavior of the DW around the Tx antenna when the factors of the Tx/Rx distance, their angular position and their relative polarization/orientation are changing.