An analysis of using ground penetrating radar refraction/reflection methods to investigate possible fractures and cavities in concrete sea walls

In the study we attempt to use a wide angle refraction/reflection method, as well as the conventional reflection radargrams to help identify the fractures and cavities in the concrete sea walls retaining a reclaimed land near the campus of National Taiwan Ocean University. First, we acquired GPR reflection radargrams along nine 30-m-long parallel lines with 1-m apart using three different antenna frequencies, including 38 MHz, 75 MHz, and 150 MHz in the study area. We have determined that the radargram with antenna central frequency of 150 MHz has the best resolution for the possible structures in the sea wall. Second, we collected GPR data at the area adjacent to the previous survey area with four 454-m-long parallel lines. Since the survey lines are 2-m apart, we are able to perform a pseudo 3D model for an area of 454 m x 8 m. We have identified several parabolic structures showing the possible locations of the fractures and cavities. Lastly we conducted wide angle refraction/reflection methods along the 454-m-long line to provide additional information for the existence of the fractures and cavities. First arrival time for the direct wave and the first reflected wave was picked automatically and the data were inverted with the GeotomCG inversion code. With the overlapped images from the reflection radargrams and the inverted velocity tomograms, we have indicated several possible cavities and fractures in the sea wall structures. Future study will focus on the verification of our GPR findings for the potential breaching spots.