Utilizing electromagnetic spectrum for subsurface void detection—case studies

Abstract

Three field projects are presented to demonstrate the application of electromagnetic spectrums for subsurface void detections. It was found that when the void is air-filled, a large negative peak will appear in the waveform, since the dielectric constant of air is much lower than for any surrounding material. Coring and drilling were used for verification purpose. Void sizes from 50 mm to 2 m were successfully detected in this study by using 400 MHz ground-coupled penetrating radar. There is a strong correlation for the arrival times and the size of the void. When the size of the voids increased, the arrival times for the top of voids become longer (e.g., approximately 8 to 10 %, depending on the size of voids). There are inversions and significant negative reflections in the ground-penetrating radar (GPR) image where the defect (air-filled void) is. In contrast, water-filled voids were detected and confirmed in case study 3 with positive peaks. The electromagnetic spectrum was able to locate voids in time. Otherwise, the voids would have grown in size and probably caused the pavement to collapse, which may have caused human casualties and property loss.