To obtain the highest-resolution ray-based tomographic images from
crosshole ground-penetrating radar (GPR) data, wide angular ray coverage
of the region between the two boreholes is required. Unfortunately,
at borehole spacings on the order of a few meters, high-angle traveltime
data (i.e., traveltime data corresponding to transmitter-receiver
angles greater than approximately 50 degrees from the horizontal)
are notoriously difficult to incorporate into crosshole GPR inversions.
This is because (1) low signal-to-noise ratios make the accurate
picking of first-arrival times at high angles extremely difficult,
and (2) Significant tomographic artifacts commonly appear when high-and
low-angle ray data are inverted together. We address and overcome
these two issues for a crosshole GPR data example collected at the
Boise Hydrogeophysical Research Site (BHRS). To estimate first-arrival
times on noisy, high-angle gathers, we develop a robust and automatic
picking strategy based on crosscorrelations, where reference waveforms
are determined from the data through the stacking of common-ray-angle
gathers. To overcome incompatibility issues between high- and low-angle
data, we modify the standard tomographic inversion strategy to estimate,
in addition to subsurface velocities, parameters that describe a
traveltime 'correction curve' as a function of angle. Application
of our modified inversion strategy, to both synthetic data and the
BHRS data set, shows that it allows the successful incorporation
of all available traveltime data to obtain significantly improved
subsurface velocity images.