Correction of multi-frequency GPR wave velocity with distorted hyperbolic reflections from GPR surveys of underground utilities

Estimation of Ground Penetrating Radar (GPR) wave velocity and the real part of dielectric permittivity (ε′) play an important role when assessing the condition of buried objects because ε′ is highly affected by moisture and void content in materials. However, errors in velocity occur due to the effect of oblique angles between the alignment of pipelines and GPR traverses during common offset survey. In this paper, field experiments on paving blocks and reinforced concrete were conducted in order to investigate errors caused by the effects of oblique angles on GPR wave velocity. GPR traverses were designed to travel along several oblique angles (θ = 30°, 45°, 60°¸75°, 90°, 105°, 120°, 135° and 150°) relative to the alignment of a ductile iron (DI) pipe. Antennas with various nominal centre frequencies (IDS 200/600, GSSI 400/900 and Sensor & Software 250 MHz) were applied in order to compare the effects. It was found that wider and flatter hyperbolic reflections are obtained and the estimated GPR wave velocity is higher if the included angle between the alignment of the DI pipe and GPR traverse changes from being perpendicular to oblique. The relative error of velocities estimated at oblique angles when compared to that estimated in perpendicular cases can be as much as 44%. Specific steps were taken to correct the errors. It is believed that this study suggests a method whereby the measurement accuracy of velocity estimation for GPR condition surveys of underground utilities can be increased.