High-frequency electromagnetic wave propagation in the vicinity of
boreholes is complex and can be described only partially by analytical
or asymptotic methods. To improve our understanding of the governing
physical processes we present a finite-difference solution of Maxwell's
equations in cylindrical coordinates. This approach allows us to
model the full electromagnetic wavefield associated with a typical
borehole georadar experiment and to assess the adequacy of the ray-based
interpretation methods currently used. We find that the nature of
the medium filling the borehole or changes in the material properties
along the borehole wall may have a major impact on the amplitude
radiation pattern. As a result of waveguide effects, the radiation
pattern of a vertical electric dipole located in a water-filled borehole
is significantly distorted with respect to the corresponding reference
full-space radiation pattern. Even more severe distortions of the
amplitude radiation pattern may result when the emitting or receiving
antenna is located near material boundaries or near the upper or
lower end of the borehole.