3-D Electromagnetic Modeling Explains Apparent-Velocity Increase in Crosshole GPR Data-Borehole Fluid Effect Correction Method Enables to Incorporating High-Angle Traveltime Data

For high-resolution crosshole ground-penetrating radar (GPR) tomography, a wide-range of ray path angles are required, including transmitter-receiver pairs with high-angles. However, artifacts have been observed in the inverted GPR tomograms when high-angle data were incorporated in ray-based inversion (RBI) tomography due to not well-understood increasing apparent velocities for increasing ray-angles. To reduce these artifacts, it is common practice to limit the angular aperture to a threshold between 30° and 50°, which reduces the spatial resolution. We apply 3-D finite-difference time-domain GPR modeling including borehole fluid and resistive loaded finite-length antenna (FLA) models to study the increase of apparent velocity with increasing ray path angle. This study shows that the strong refraction of the electromagnetic waves at the borehole interface between water and subsurface is one of the reasons for these not well-understood phenomena. We introduce a novel borehole-fluid effect correction (BFEC) that relocates the transmitter and receiver positions to the location, where the refraction is occurring to remove any influence of the borehole such that the remaining traveltimes can be inverted using an RBI. BFEC improves the estimated apparent-velocity (relative permittivity) values and enables the incorporation of wide-angle ray paths resulting in more accurate tomograms. We verify the BFEC with a simple layered subsurface model and a realistic synthetic model. By applying curved-ray RBI without and with the BFEC, the subsurface structures are reconstructed with more details for the BFEC data and the average relative error model reduced from 13% to below 8% for the high-resolution inhomogeneous model.