## 2 Materials and Methods  
Field measurements were made on prepared farmland where the in situ nature of the subsoil and the aggregates are preserved, promoting a study that is devoid of previous anthropogenic influences. The targeted alteration in the soil was achieved by simulating it using tractor movements at several passes on the moderately tilted farmland, where the natural soil particle arrangements are still in place. The tractor has an average weight of about 3000 kg (about 6613.86 lb.), facilitating a relatively deep exertion of normal stress beyond the surface horizon of the soil. The normal stress was induced differentially along different paths from one tractor pass to ten passes across the longitudinal GPR measurement profiles, as illustrated in Figure 1b. The field measurement test site is shown in Figure 1a. A few traverses along the longitudinal axis and also across them were established for the data acquisition, resulting in 13 profiles, as illustrated in Figure 1b. Some soil samples at some points within the test site were also taken for laboratory evaluation on some physical properties for correlation with the geophysical data. GPR geophysical data were acquired using the Mala Pro-Ex unit with an antenna central frequency of 800 MHz, manufactured by MALA Inc., Solna 171 54 Stockholm, Sweden. GPR has been deployed in near-surface investigations in recent years due to its versatility, simplicity of operation, and possibility of real-time interpretation. These attributes and other factors informed the choice of the method for deployment in this study. The GPR technique represents a non-destructive method of investigation that utilizes electromagnetic (EM) waves to capture the image of the subsurface media interactions and the EM energy. The setup of a GPR system consists of three major components: the control unit, the monitor, and the transmitting and receiving antennas. The basics of its operation are that electromagnetic pulse energy is transmitted into the subsurface media, and the corresponding responses in terms of reflections are then measured, which can be viewed simultaneously via the monitor. Details about the principles and techniques of operation are beyond this report and can be found in [17; 18; 19; 20].  
GPR data are considered non-stationary. This is because GPR field data signals are characterized by sudden drops in the amplitude of the waves and the quick disappearance of the important parts of the frequency harmonics [21], which necessitates post-field-data-acquisition processing.  
The field data were analyzed using MATGPR, version 3.1, a GPR data processing software developed by [PERSON] [22], and the following processing routines were performed. The fundamentals of the processing modules are briefly highlighted in the subsequent subsections to present why they are usually applied and how they enhance the raw data. The fact that the wrong use of data processing techniques may mar the results of GPR investigations by eliminating targets informed the choice of these basic processing steps. It is important to note that the steps used in the present data processing are not defaults; it all  
Figure 1: The study test site. (**a**) Soil surface induced with normal stress via tractor (inset) movement (black lines are GPR measurement profiles). (**b**) Illustration of the field measurement profiles.  
depends on the targets of the investigation, the experience of the investigator, and perhaps the details necessary in the outcome of the investigation.