High-frequency electromagnetic (EM) wave propagation phenomena associated
with borehole georadar experiments are complex. 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 EM wavefield associated with
borehole georadar experiments and to assess the adequacy of ray-based
methods currently used to interpret the observed amplitudes. Our
results indicate that because shallow boreholes are often water filled,
the finite length of the boreholes as well as changes in material
properties along a borehole wall can have major effects on the amplitude
behavior of borehole georadar data. As a result of waveguide phenomena,
the radiation pattern of a vertical electric dipole source located
in a water-filled borehole may be distorted significantly with respect
to the corresponding reference radiation pattern in a homogeneous
medium. Even greater distortions of the radiation pattern result
when the electric dipole source is located near material boundaries
or near the upper or lower end of a borehole. This study indicates
that some of the basic assumptions of conventional ray-based amplitude
tomography often are not fulfilled for borehole georadar data and
that the derived constraints on the attenuation and conductivity
structure should be regarded as qualitative in nature. The algorithm
and the results presented in this study do, however, offer the perspective
to alleviate some of these inherent problems and thus help to make
ray-based georadar attenuation tomography a more reliable and effective
toot for probing the shallow subsurface.