Cross-hole radar tomography is a useful tool for mapping shallow
subsurface electrical properties viz. dielectric permittivity and
electrical conductivity. Common practice is to invert cross-hole radar
data with ray-based tomographic algorithms using first arrival
traveltimes and first cycle amplitudes. However, the resolution of
conventional standard ray-based inversion schemes for cross-hole
ground-penetrating radar (GPR) is limited because only a fraction of the
information contained in the radar data is used. The resolution can be
improved significantly by using a full-waveform inversion that considers
the entire waveform, or significant parts thereof. A recently developed
2D time-domain vectorial full-waveform crosshole radar inversion code
has been modified in the present study by allowing optimized acquisition
setups that reduce the acquisition time and computational costs
significantly. This is achieved by minimizing the number of transmitter
points and maximizing the number of receiver positions. The improved
algorithm was employed to invert cross-hole GPR data acquired within a
gravel aquifer (4-10 m depth) in the Thur valley, Switzerland. The
simulated traces of the final model obtained by the full-waveform
inversion fit the observed traces very well in the lower part of the
section and reasonably well in the upper part of the section. Compared
to the ray-based inversion, the results from the full-waveform inversion
show significantly higher resolution images. At either side, 2.5 m
distance away from the cross-hole plane, borehole logs were acquired.
There is a good correspondence between the conductivity tomograms and
the natural gamma logs at the boundary of the gravel layer and the
underlying lacustrine clay deposits. Using existing petrophysical
models, the inversion results and neutron-neutron logs are converted to
porosity. Without any additional calibration, the values obtained for
the converted neutron-neutron logs and permittivity results are very
close and similar vertical variations can be observed. The full-waveform
inversion provides in both cases additional information about the
subsurface. Due to the presence of the water table and associated
refracted/reflected waves, the upper traces are not well fitted and the
upper 2 m in the permittivity and conductivity tomograms are not
reliably reconstructed because the unsaturated zone is not incorporated
into the inversion domain.