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. An important issue, however, is determining how the geophysical
data can be optimally processed and used for maximum hydrological
benefit. Here, we focus on the development of an optimal procedure
for inverting crosshole ground-penetrating radar (GPR) data to characterize
the distribution of hydrological parameters at the Boise Hydrogeophysical
Research Site. Specifically, we develop a novel approach to jointly
invert several intersecting crosshole GPR data sets, which is found
to provide a consistent GPR velocity model that is highly correlated
with complementary neutron-porosity log data. This is an important
prerequisite to using the crosshole GPR data with existing hydrological
measurements for improved flow and transport modeling.