Maxwell curl equation datuming for GPR test of tunnel grouting based on Kirchhoff integral solution

In two-dimensional (2D) ground penetrating radar (GPR) data, the reflection from the detection targets in depth are severely obscured by the strong scattering generated from near-surface non-target structures. For using GPR as a geotechnical non-destructive testing device, how to eliminate the strong scattering caused by near-surface rebars in the tunnel liner to image and assess the grouting condition behind tunnel liner is still an unsolved problem. This study proposed a method for the reconstruction of deep GPR images, termed the Maxwell curl equation datuming. To eliminate the deleterious effect caused by near-surface diffractive scattering, we have redefined the reference surface to an actual geologic interface by using Maxwell curl equation datuming methodology based on Kirchhoff integral solution. Maxwell curl equation datuming procedure can redefine the reference surface into deeper horizon on which the GPR transmitters and receivers appear to be located. Case studies were presented for synthetic examples and real GPR data for assessments of tunnel grouting. The results show that the datuming technique based on Maxwell curl equation, is able to eliminate the strong scattering related to near-surface rebars in tunnel liners, and improve the quality of deeper images beneath the tunnel liners. The Maxwell curl equation datuming is also applicable to other GPR testing situations that depend on the elimination of scattering effects caused by near-surface structures.