The integration of geophysical data into the subsurface characterization
problem has been shown in many cases to significantly improve hydrological
knowledge by providing information at spatial scales and locations
that is unattainable using conventional hydrological measurement
techniques. In particular, crosshole ground-penetrating radar (GPR)
tomography has shown much promise in hydrology because of its ability
to provide highly detailed images of subsurface radar wave velocity,
which is strongly linked to soil water content. Here, we develop
and demonstrate a procedure for inverting together multiple crosshole
GPR data sets in order to characterize the spatial distribution of
radar wave velocity below the water table at the Boise Hydrogeophysical
Research Site (BHRS) near Boise, Idaho, USA. Specifically, we jointly
invert 31 intersecting crosshole GPR profiles to obtain a highly
resolved and consistent radar velocity model along the various profile
directions. The model is found to be strongly correlated with complementary
neutron porosity-log data and is further corroborated by larger-scale
structural information at the BHRS. This work is an important prerequisite
to using crosshole GPR data together with existing hydrological measurements
for improved groundwater flow and contaminant transport modeling.