Tomographic inversions of cross-hole ground-penetrating radar provide images of electromagnetic properties of the shallow subsurface and are used in a wide range of applications. Whereas the resolutions of ray-based methods like first-arrival traveltime and first-cycle amplitude tomography are limited to the scale of the first Fresnel zone, full-waveform inversions incorporate precise forward modelling using the full recorded signal for a solution of Maxwell's equation, which results in sub-wavelength resolutions. In practice, the method can be time-consuming in data acquisition and expensive in computational costs. To overcome these expenses, a semi-reciprocal acquisition setup with a reduced number of transmitters and an interchange of transmitter and receiver boreholes instead of a one-sided equidistant setup in either borehole yielded promising results. Here, this optimized, semi-reciprocal acquisition setup is compared to a dense, equidistant, one-sided acquisition setup measured at the field site Krauthausen, Germany. The full-waveform inversion results are evaluated using the checkerboard test as a capable resolution analysis tool to determine resolvabilities. We introduced also a new method of time-zero correction by a cross-correlation of a zero-offset profile with corresponding horizontal traces of each multi-offset gather. The obtained experimental results from Krauthausen combined with the checkerboard analysis indicate the main three-permittivity layers that correspond with different porosities. Also fine-layered structures within these main layers were reliably imaged. We conclude that the use of the semi-reciprocal setup is optimum for acquisition speed, inversion speed and obtained permittivity inversion results. Our results indicate that conductivity results are better for denser transmitter-receiver setups.
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