Crosshole radar techniques are important tools for a wide range of
geoscientific and engineering investigations. Unfortunately, the
resolution of crosshole radar images may be limited by inadequacies
of the ray tomographic methods that are commonly used in inverting
the data. Since ray methods are based on high-frequency approximations
and only account for a small fraction of the information contained
in the radar traces, they are restricted to resolving relatively
large-scale features. As a consequence, the true potential of crosshole
radar techniques has yet to be realized. To address this issue, we
introduce a full-waveform inversion scheme that is based on a finite-difference
time-domain solution of Maxwell's equations. We benchmark our new
scheme on synthetic crosshole data generated from suites of increasingly
complex models. The full-waveform tomographic images accurately reconstruct
the following: 1) the locations, sizes, and electrical properties
of isolated subwavelength objects embedded in homogeneous media;
2) the locations and sizes of adjacent subwavelength objects embedded
in homogeneous media; 3) abrupt media boundaries and average and
stochastic electrical property variations of heterogeneous layered
models; and 4) the locations, sizes, and electrical conductivities
of water-filled tunnels and closely spaced subwavelength pipes embedded
in heterogeneous layered models. The new scheme is shown to be remarkably
robust to the presence of uncorrelated noise in the radar data. Several
limitations of the full-waveform tomographic inversion are also identified.
For typical crosshole acquisition geometries and parameters, small
resistive bodies and small closely spaced dielectric objects may
be difficult to resolve. Furthermore, electrical property contrasts
may be underestimated. Nevertheless, the full-waveform inversions
usually provide substantially better results than those supplied
by traditional ray methods.