## 2 Materials and Methods  
What constitutes effective data collection is the systematic method of gathering and measuring information from a variety of sources to obtain comprehensive details of the area/subject of interest. Following the review of literature on the different geophysical methods, particularly on their suitability for various projects, terrain, and targets, confidence in the deployment of the GPR was built. The ground-penetrating radar technique is an electromagnetic (EM) geophysical method that utilizes EM in the range of 10 MHz to 3000 MHz [25] as a source of energy that propagates through subsurface media. The transmitter of the GPR system radiates EM pulse energy into the underlying media of investigation and the reflected and refracted energy responses as a result of encounters with buried objects and or a boundary between media of different permittivity levels (dielectric constant). The responses are intercepted and recorded by the receiving antenna (Figure 1a). Details of the principles and theory of GPR are found in [19; 20; 26]. Test measurements were made on a relatively flat grassland loam soil at the Kryspinow area on the outskirts of Krakow, Malopolskie, Poland. Field data were obtained at the beginning of spring of the test year during fair weather conditions, which allowed the field measurements in idealized conditions. Field measurements were conducted using a ProEx model system unit manufactured by MALA Geoscience Sweden (now: ABEM/MALA) with the antenna of central frequency at 800 MHz. The choice of this frequency was based on its short wavelength and thus the resolution it can deliver at a relatively few meters depth of penetration within the unsaturated zone of soil, which was the focus of this study. Field data were acquired in the constant offset (reflection profiling) along the established two transects (A-A1 and B-B1) that are orthogonal (Figure 1b). Field acquisition parameters (Table 1) were input into the GPR system setup, and the entire system unit was mounted, with data acquired by pulling a wheeled antenna along the transect at a walking speed. Measurements were made at different soil conditions, first at their natural state and later when a known volume (25 L and 50 L at different time intervals) of water was added to the soil surface along the transect path, which is relatively flat over a set period. The added water was to simulate the infiltration mechanism within the soil horizons and monitor the infiltration pattern and direction, which theoretically are controlled by potential energy within the matrix of the soil particles. The added water was made at two consecutive intervals of 3 h, and infiltration was allowed for 1 h before GPR measurements were taken. A few multi-offset measurements, namely, wide-angle reflection and refraction (WARR), were also made for EM velocity determination. WARR is a measurement technique that involves keeping the transmitter antenna at a fixed point while the receiver antenna is shifted at varying continuous offsets from the transmitter antenna [19; 20]. WARR data allow for the obtaining of the EM signal travel time as a function of the antenna separation offset distance. Evaluated information can then be used to calculate the velocity of the EM at the point that may facilitate depth determination [16]. After the field data acquisition, the data were edited and presented as radargrams, which represent the stacked plot of the signal amplitude against travel time using REFLEXW version 8.5 software developed by Sandmeier Inc., Karlsruhe Germany [27], as shown in Figure 2a. Figure 1: (**a**) Illustration of GPR system units and EM pulse energy paths. (**b**) Field measurement layout. Figure 2: GPR field data presentation before infiltration: (**a**) processed radargram; (**b**) calculated phase attribute. The field data were subjected to post-acquisition processing to enhance signal-to-noise ratio and distinguish clutters from signals, which aided in providing better interpretations and clarifying targets.