Renato Luiz Prado

Métodos e técnicas geoelétricas rasas na delimitação de área afetada por ruptura em teto de túnel urbano

This paper presents a case study where geophysics was successfully applied to solve a complex problem associated with a tunnel excavation accident in an urban area. It presents and describes the results obtained with geoelectrical methods applied in an emergencial task in a risky urban area in Sao Paulo City. It also compares the techniques and field arrays, ways to present and interpret the data. The employed methods were electroresistivity, induced polarization (IP) and self potential (SP). The objective of the survey was to delineate the extension of a roof failure of an urban tunnel under excavation beneath the Ibirapuera Park in order to orientate the volume of tunnel roof should be treated. Only two days (Saturday and Sunday) were available to carry out the survey and its interpretation. The results were considered satisfactory and brought a significant economy to the final cost of the tunnel. This economy was due to the decreasing of the remediation area from the initial 20 x 20 m (estimated by the engineers) to 3 x 5 m (estimated by the geophysical survey).

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.

Estimativas do teor de umidade empregando o método GPR: uma avaliação comparativa em experimentos de laboratório e campo

Este trabalho faz uma analise das estimativas de teores de umidade obtidas com o metodo Ground Penetrating Radar (GPR) comparativamente as determinadas com os metodos Time Domain Reflectometry (TDR) e gravimetrico. Os dados foram obtidos em dois experimentos diferentes: um experimento controlado em laboratorio buscando reproduzir um meio homogeneo onde foram obtidas as medidas de umidade com GPR (antenas de 1 GHz) e TDR, e outro experimento de campo onde foram obtidos dados com GPR (antenas de 200 MHz) e de amostras de solos do local. Para a obtencao das estimativas a partir do metodo GPR foram analisados os eventos relativos a onda de transmissao direta entre as antenas, onda refratada criticamente e onda refletida em interfaces com diferentes propriedades eletricas.O GPR mostrou-se sensivel as variacoes de umidades presentes nos dois experimentos e apresentou boa correlacao com os dados obtidos com TDR (REQM de0,007 m3m-3) e das amostras (REQM de 0,039 m3m-3).

Single-station ellipticity retrieval and its joint inversion with dispersion curve, for a borehole test site

To obtain the shear wave velocity profile for engineering application to near-subsurface, many geophysical techniques are used. The dispersion curve alone is unable to provide deeper shear wave velocity information for deep soil deposit; therefore, a joint inversion of dispersion curves with the horizontal over vertical (H/V) (i.e., apparently ellipticity) curve is recommended. The H/V curve obtained from the microtremor recording contains a major part of Love wave contribution to the noise wavefield horizontal component. Due to this presence of Love wave fraction, the H/V curve does not completely replicate the ellipticity of Rayleigh wave. In this study, we try to compare the Rayleigh wave ellipticity obtained from the borehole velocity model to the H/V curve obtained at the same locality from seismic ambient noise recording. Two different techniques available for the Love effect minimization are tested and compared with the borehole ellipticity. Finally, the joint inversion of H/V and dispersion curve is made, which shows great correspondence with the previous measurement at the site.

Neotectonics in Marajó Island, State of Pará (Brazil) revealed by vertical electric sounding integrated with remote sensing and geological data

Studies suggest that the Marajo Island has experienced neotectonic activity during its latest evolution. However, there are no data demonstrating the presence of tectonic structures in its shallow subsurface. This work integrates vertical electric sounding, morphostructural lineaments, and geological data aiming to show fault control on the Late Pleistocene-Holocene sedimentation of this area. Resistivity values were related to mud ( 500 Ωm). The latest values were related to the unconformity with lateritic paleosol at the top of the Barreiras Formation. Despite the values ≤ 500 Ωm of both the Barreiras Formation and the Late Pleistocene-Holocene unit, the latter was distinguished along four electric sections due to the presence of this unconformity, combined with the integration of available 14 C, and luminescence ages of Quaternary sediments. The electric sections recorded several places with lateral interruptions of resistivity values within short distances, which were related to faults. The Miocene strata were vertically displaced by normal faults, giving rise to new accommodation space where Late Pleistocene-Holocene sediments were deposited. Fault reactivation was crucial to renew sedimentation in eastern Marajo Island during its latest evolutionary stage.

Métodos geofísicos integrados para a caracterização hidrogeológica dos aquíferos da microbacia Andes, Bebedouro, em São Paulo

Integrated geophysical methods in the hydrogeological characterization of the Microbasin Andes, Bebedouro, Sao Paulo State. This paper presents the results of a geophysical survey integrated to well data for studying hydrogeological characteristics of Adamantina and Serra Geral Formations in a microbasin in the Andes District, in Bebedouro, state of Sao Paulo, Brazil. This area was the focus of studies due to the intense exploitation of groundwater for agricultural use and its relation with the increase of local seismic activity. In this paper, geophysical methods and well information were used, in an integrated way, to get more detailed knowledge of the aquifers and their physical properties. The results show that the approach was effective in imaging geological formations and their geometry study, as well as in the identification of geological structures. An estimation of hydrodynamic properties of granular aquifers can be done by the parameters of Dar Zarrouk and their correlation with data obtained from pumping tests. In this study, it was possible to establish a linear correlation between the transmissivity and transverse resistance, however with some limitations, because the majority of wells were drilled also in volcanic rocks. This procedure can be used to assist the aquifer management, under the aspects of exploration and preservation.

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.