GPR Backscattering Intensity Analysis Applied to Detect Paleochannels and Infilled Streams for Seismic Nanozonation in Urban Environments

Seismic microzonation of urban areas is used to be determined from few soils' response measurements in each area. In consequence, results can be considered correct only in the case of possible depth-dependent soils, being the existence of lateral soil changes the cause of imprecision. Ground-penetrating radar (GPR) could be a useful tool to determine, previously to the passive seismic measurements, the location of geological structures. Moreover, depending on the ground materials, the GPR energy is randomly backscattered. Consequently, the background noise in the radar scans increases. Therefore, the analysis of the amplitude of the noise could be a useful method to determine changes on the ground characteristics. The analysis of the background GPR amplitude noise is tested in two radar lines, crossing the city of Barcelona. The results show significant differences in the background noise amplitude in the A-scans that could be used to define zones in the city depending on the noise level. These changes on the amplitude are associated with the backscattered energy as a consequence of soil characteristics. Hence, the analysis of the variation in the background noise amplitude allows defining the possible location of subterranean streams, paleochannels, and other structures crossing the plain of the city. Radar results are also compared to ambient vibration measurements, using the spectral horizontal-to-vertical (H/V) quotient, and to historical and contemporary information. The application of the methodology underscores the ability of the evaluation of the backscattering associated with the background noise in the scans in the detection of sharp geological changes in sedimentary deposits.