To obtain tomographic images with the highest possible resolution
from crosshole ground-penetrating radar (GPR) data, raypaths covering
a wide range of angles between the boreholes are required. In practice,
however, the inclusion of high-angle ray data in crosshole GPR inversions
often leads to tomograms so dominated by inversion artifacts that
they contain little reliable subsurface information. Here, we investigate
the problems that arise from the standard assumption that all first-arriving
energy travels directly between the centers of the antennas. Through
numerical modeling, we show that this assumption is often incorrect
at high transmitter-receiver angles and can lead to significant errors
in tomographic velocity estimates when the antenna length is a significant
fraction of the borehole spacing.