Liliana Alcazar Diogo

Investigações geofísicas de superfície e de poço no sítio controlado de geofísica rasa do IAG-USP

This paper presents the results obtained from surface and borehole geophysical investigations carried out at the Shallow Geophysical Test Site of IAG-USP. The main objective was to characterize the sediments and the top of basement. The area is constituted by sandy-clay sediments of Sao Paulo Basin (Sao Paulo and Resend Formations) on the granite-gneiss basement with different degrees of weathering. In the area of studies, three boreholes were drilled for geological and geophysical research purposes (pioneer in Brazil). These boreholes were used to verify the stratigraphy, to collect the sample for laboratory measurements, to calibrate the results of surface geophysics and to carry out well logging. The surface methods used were: seismic (reflection and refraction), electric-resistivity (profiling and vertical electric sounding) and GPR-Ground Penetrating Radar. The integration of results show excellent agreement and allow constructing a geological-geophysical model for the site which is at the border of Sao Paulo sedimentary basin. Moreover, the various geophysics methods, commonly used in geological, geotechnical and environmental researches, have been tested under controlled conditions.

Análise do emprego do GPR para estimar o teor de umidade do solo a partir de um estudo na cidade de São Paulo

This paper shows the results of soil moisture content measurements by GPR tests in a study area in Sao Paulo City, Brazil and discuss some aspects related to the accuracy of them. Two different data interpretation methods were used: i) lineal adjustment of the arrival times of ground wave for different antenna separation; ii) time differences between the air and ground wave arrival times at a single antenna separation. We used the RAMAC Mala Geosciences system with 50 MHz, 100 MHz and 200 MHz antennas. The GPR survey was constituted of several multi-offset acquisition, using the WARR (Wide Angle Reflection and Refraction) geometry along of the profile in three different time periods (dry, intermediate and wet seasons). The distance increment between each WARR arrangement and traces was 0.1 m for the 100 MHz and 200 MHz antennas and 0.2 m for the 50 MHz. At the same time of each GPR survey, soil samples, from 0.5 m up to 4 m depth, were collected, in 0.5 m intervals, for water content analysis in laboratory tests. The GPR data showed a good correlation with the laboratory data, but the results from the 200 MHz antennas were the best one. The two data interpretation methods showed different accuracies. The time differences method was less precise due to the time shift in the ground wave arrival time at zero antenna offset that occurred in this survey. These results showed that it is very important to apply a correction factor for this effect, when it is present.

Comparative tests of seismic sources and geophones aiming at shallow reflection seismic investigation in urban areas

Pseudo walkaway noise tests were accomplished in study area in Sao Paulo city, Brazil, in order to evaluate the potentiality of seismic reflection technique in shallow geological/geotechnical investigation in urban environment, where there is usually the presence of different types as well as intensities of noises and the paved surface. Data acquisition was conducted with a 24 channel seismograph with 24 bit A/D converter, 28 Hz and 100 Hz geophones, as well as sledge hammer and seismic rifle as sources. The best results were obtained with: (i) 100 Hz geophones with 0.18 m spike and hammer source with steel plate; (ii) 100 Hz geophones coupled through clay on asphaltic coverage with hammer source applied on asphalt. Laboratory tests were also accomplished in shake table in order to evaluate geophone responses when they were coupled with different types of clay. Tests indicated clay employment is a good alternative to couple geophones, notwithstanding kaolinitic clays are less indicated.