Ground penetrating radar applications in seismic zonationassessment and evaluation

Resumo

The main objective of the thesis is the evaluation of Ground Penetrating Radar (GPR) as a complementary study in order to support seismic zonation analysis. The evaluation comprises three main aspects: a) the analysis of soils shallow geology in order to determine the effectiveness of the methodology for obtaining valuable details in defining seismic nanozones, applying the methodology in Barcelona, b) the assessment of different buildings structures with the objective of evaluate the information that could be obtained with this methodology, useful for modeling seismic behavior and, c) the analysis and development of new data processing methodologies applied to the previous cases. As a first step, laboratory experiments were designed and developed with sand and gravel, using different patterns and configurations to simulate shallow geological structures of soils, essentially paleochannels and underground streams. Radar data was analyzed by using commercial software and the algorithms developed in this thesis. Numerical simulation was also compared the laboratory results. The second step consists of a survey in Barcelona. The objective was to obtain accurate information about the paleochannels and subterranean streams. This analysis was performed in order to support the nanozonation study of the city. Radar data has been contrasted with passive seismic surveys and borehole. The results allow identifying the characteristics in the radargrams that are associated to the existence of those shallow structures, and also the characteristics associated to the existence of water in underground riverbeds. The most important facts observed in this analysis were: (a) the noticeable changes in the backscattering amplitude as consequence of the scattered energy in areas affected by underground streams and, (b) the abrupt change in the central frequency of the radar signal as consequence of changes in the soil water content. A new algorithm was developed and tested in order to identify these changes. This second evaluation was also contrasted with the studies in other sites, using low frequency GPR antennas. These additional surveys allow us to define clearly the capability the survey to delimitate the principal layers and materials. Numerical simulations were applied in order to define properly the waves trajectories. The third step was focused on the assessment of buildings. GPR was tested and evaluated in the detection of inner elements and creating three dimensional images of non-flat masonry elements. Laboratory tests were carried out in one masonry column, being the test results highly useful in the evaluation of a Modernista building in Barcelona. Radar data was processed in cylindrical coordinates, and results were compared and integrated with other NDT techniques. Other different buildings were also assessed in different cities. In the South of Chile, the methodology was applied to a Museum in a high seismic risk zone. In Barcelona, high frequency antennas were used to define the structural arrangement and material type of the ceilings in the Eixample of the city. The main results and conclusions are: 1) The identification of the backscattering amplitude associated to the existence of shallow geological structures. Until today, random backscattering was considered as noise. In this thesis this phenomenon was identify as an important indicator useful in seismic zonation studies. ...