Urban stratigraphy: Infill zones characterization through geophysical methods.

PREZZI, CLAUDIA BEATRIZ; LÓPEZ, RUBÉN; CARLOS VÁSQUEZ; FAZZITO, SABRINA; MARCOMINI, SILVIA

Foz do Iguassu, Brazil

Congreso; 2010 The Meeting of the Americas (2010 American Geophysical Union Joint Assembly Meeting); 2010

The coast of Buenos Aires city (Argentine) is located on the southern margin of La Plata river estuary. The original morphology of this area was completely changed by infilling works. Such works began in 1836 and continue until today. The total infilling surface is of approximately 2054 hectares, with a coast advance ranging between 400 and 1000 m. The anthropic changes imposed on the coast line configuration generated a great variety of problems in building foundations due to the heterogeneous composition of the infilling materials. Taking into account the ongoing urban development and the absence of information about the nature of the zones infilled in La Plata river, we try to determine the type, homogeneity, spatial distribution and thickness of the infilling materials by means of ground geophysical surveys. Such information is vital for the adequate evaluation of the coastal sectors development options, for the corresponding environmental impact assessment and for a proper planning of urban expansion. We surveyed different infilled sectors with the aim of calibrate and test the suitability of distinct geophysical methods for the characterization of the diverse infilling materials. We carried out magnetic, ground penetrating radar (GPR) and electrical resistivity imaging (ERI) surveys. In one of the studied sectors, circular magnetic anomalies ranging between -700 and 500 nT were detected. Such anomalies suggest the presence of demolition materials (beams, concrete blocks with iron rods, etc.) at shallow depths (infill). Maximum infill depths of approximately 10 m were estimated applying Euler deconvolution. In other infilled sector, the magnetic survey detected a pattern which indicated the presence of an underground canal. Such pattern showed an elongated positive anomaly of 1900 nT. In this sector, infill depths of approximately 5 m were calculated through Euler deconvolution. GPR profiles did not generate good results due to high signal attenuation. The presence of clayey wet soil would be responsible of the very limited GPR signal penetration. Only a couple of diffraction hyperbolae were observed. In one of the studied sectors, two ERI surveys were carried out using different electrode spacing and spread lengths. At the top of both sections a 5 m thick layer was observed, which showed a patchy resistivity pattern with values ranging between 35 and 65 ohm.m. This layer would indicate the existence of demolition material infill. Below this layer, an important resistivity diminution is registered (down to 7 ohm.m), probably related to the phreatic zone. Below 20 m depth, resistivity increases up to 50 ohm.m. The obtained results allowed the determination of the thickness, homogeneity and type of the infilling material in the studied areas. To further investigate and characterize the infilled zone along the coast of Buenos Aires city, new magnetic, GPR, ERI and microgravity surveys will be conducted. infill. Below this layer, an important resistivity diminution is registered (down to 7 ohm.m), probably related to the phreatic zone. Below 20 m depth, resistivity increases up to 50 ohm.m. The obtained results allowed the determination of the thickness, homogeneity and type of the infilling material in the studied areas. To further investigate and characterize the infilled zone along the coast of Buenos Aires city, new magnetic, GPR, ERI and microgravity surveys will be conducted. infill. Below this layer, an important resistivity diminution is registered (down to 7 ohm.m), probably related to the phreatic zone. Below 20 m depth, resistivity increases up to 50 ohm.m. The obtained results allowed the determination of the thickness, homogeneity and type of the infilling material in the studied areas. To further investigate and characterize the infilled zone along the coast of Buenos Aires city, new magnetic, GPR, ERI and microgravity surveys will be conducted. infill. Below this layer, an important resistivity diminution is registered (down to 7 ohm.m), probably related to the phreatic zone. Below 20 m depth, resistivity increases up to 50 ohm.m. The obtained results allowed the determination of the thickness, homogeneity and type of the infilling material in the studied areas. To further investigate and characterize the infilled zone along the coast of Buenos Aires city, new magnetic, GPR, ERI and microgravity surveys will be conducted.