Crosshole radar tomography is a useful tool in diverse investigations
in geology, hydrogeology, and engineering. Conventional tomograms
provided by standard ray-based techniques have limited resolution,
primarily because only a fraction of the information contained in
the radar data (i.e., the first-arrival times and maximum first-cycle
amplitudes) is included in the inversion. To increase the resolution
of radar tomograms, we have developed a versatile full-waveform inversion
scheme that is based on a finite-difference time-domain solution
of Maxwell's equations. This scheme largely accounts for the 3D nature
of radar-wave propagation and includes an efficient method for extracting
the source wavelet from the radar data. After demonstrating the potential
of the new scheme on two realistic synthetic data sets, we apply
it to two crosshole field data sets acquired in very different geologic/hydrogeologic
environments. These are the first applications of full-waveform tomography
to observed crosshole radar data. The resolution of all full-waveform
tomograms is shown to be markedly superior to that of the associated
ray tomograms. Small subsurface features a fraction of the dominant
radar wavelength and boundaries between distinct geological/hydrological
units are sharply imaged in the full-waveform tomograms.