Ground penetrating radar (GPR) data has proven to be a powerful tool for non-invasively estimating the electrical properties of the shallow subsurface. Petrophysical relationships allow them to be converted into hydrological parameters such as soil water content (SWC) and porosity. In the presence of thin layers, complex wave phenomena can occur that are usually ignored in standard processing. Recent developments in modeling tools and computation power allow us to include detailed modeling in advanced inversion algorithms, such as full-waveform inversion (FWI). In this thesis, GPR wave phenomena occurring due to the presence of boreholes in crosshole tomography and the presence of permittivity gradients in surface GPR data acquisition will be investigated. First, these phenomena need to be identified and understood so that they can be effectively included in advanced inversion algorithms. Using the finite difference time-domain solver gprMax, various models with different borehole radii and different permittivity distributions are evaluated. Additionally, a resistor loaded finite-length antenna model is built to compare its radiative properties with a point dipole source in crosshole GPR applications. The results reveal that the differences between these two antenna types are small, but not equal. The dominant factor that influences the radiated wavefields is found to be the properties of the surrounding medium. For increasing vertical offsets, elongated wavetrains are observed, which are probably caused by waveguiding effects of the water-filled boreholes. The permittivity gradient models for surface GPR reveal scenario-dependent phenomena, notably the presence of a lower halfspace ground wave and dispersive wave propagation for increasing and decreasing permittivity gradients, respectively. Overall, the results suggest that integrating parameters into the FWI workflow, which account for surface gradients, could be beneficial and lead to improved imaging capabilities.