Numerical modeling of borehole georadar data

Abstract

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 tool for probing the shallow subsurface.