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| Home > Publications database > 3-D Electromagnetic Modeling Explains Apparent-Velocity Increase in Crosshole GPR Data-Borehole Fluid Effect Correction Method Enables to Incorporating High-Angle Traveltime Data |
| Home > Publications database > 3-D Electromagnetic Modeling Explains Apparent-Velocity Increase in Crosshole GPR Data-Borehole Fluid Effect Correction Method Enables to Incorporating High-Angle Traveltime Data |
| Journal Article | FZJ-2021-03450 |
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2022
IEEE
New York, NY
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Please use a persistent id in citations: http://hdl.handle.net/2128/30632 doi:10.1109/TGRS.2021.3107451
Abstract: 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, artefacts 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 artefacts, it is common practice to limit the angular aperture to a threshold between 30° to 50°, which reduces the spatial resolution. We apply 3D finite-difference time-domain GPR modelling including borehole fluid and resistive loaded finite-length antenna 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 for a homogenous and 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 average relative error model reduced from 13% to under 9% for the high-resolution inhomogeneous model.
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