Selma Isabel Rodrigues

O sítio controlado de geofísica rasa do IAG/USP: instalação e resultados GPR 2D-3D

The data interpretation from shallow geophysical survey can be complex because the uncertainty about subsurface targets. To reduce the ambiguity and help in the interpretation of geophysical data, the Shallow Geophysical Test Site of IAG-USP (Sitio Controlado de Geofisica Rasa-SCGR do IAG-USP) was installed inside the campus at USP. Different targets at various depths were installed along seven lines. The targets buried in the Test Site simulate targets found in geotechnical, archaeological, and environmental studies. In this article, the construction methodology of the Test Site and some GPR results along a line characterized by empty metallic tanks are presented. The geophysical characterization in controlled field condition allowed the calibration of geophysical responses due to known targets, such as, metallic tanks. 250 MHz GPR profile showed an excellent agreement with a GPR 2D model determined by finite difference in time domain method. Fk-migration improved the resolution and the spatial position of the metallic tanks. GPR 3D results, such as 3D cube, fence diagrams, and depth slices, were very important to improve the characterization of the targets in subsurface. SCGR-IAG revealed itself as important tool for teaching, research and extension activities in university related to Geophysics, and it will be very important to our community. With the installation of this Test Site, an important step was done in searching the State of Art in Shallow Geophysics in Brazil, were the development of news equipment, modeling and inversion software could be tested and its response calibrated.

Radarfacies and sedimentological analysis: Study of sedimentary substrate from an archaeological site (shell mound), southern Brazil

Integrated results of GPR (Ground Penetrating Radar) and sedimentological analysis are presented for the Jabuticabeira II archaeological site (shell mound), Santa Catarina, Brazil. By means of radarfacies identification, this study aims to delimit the archaeological site and differentiate the coastal depositional systems that compose its substrates. For these purposes, available models of the temporal–spatial distribution of depositional systems in the area were used and sedimentological analysis (granulometry, quantification of heavy minerals and clay-mineral characterization) were performed on samples spaced at 0.5-m vertical intervals in auger drills cutting the identified radarfacies. GPR data were obtained along a radial grid, which allowed the rapid mapping of a large area (several hundred square metres). The results allowed to characterize an archaeological layer, the soil and two sedimentary layers (palaeolagoon and aeolian) in the substrates under and around the site. The high porosity and the grain size, cementation and heavy mineral segregation contrasts along wind-controlled laminations are for the reflector sharpness in the aeolian deposits. The archaeological site settlement mostly overlies the palaeolagoon, which was a newly emerged land during the epoch of occupation and is situated on the margins of aeolian deposits that formed in the region after the Holocene maximum flooding. This configuration reinforces the sambaquis occupation model of south-central Santa Catarina during the Holocene, strongly controlled by the proximity to lagoon bodies. The resolution of the data in this study was sufficient to advance our understanding of the regional sedimentary evolution and its relation with sambaqui occupation.

GPR applied to map Jabuticabeira-II coastal sambaqui archaeological site (Brazil)

In this work, some GPR results obtained in Jabuticabeira-II coastal sambaqui archaeological site, located in Jaguaruna region, Santa Catarina state, southern Brazil, are presented. Sambaqui is a word in Guarani (Native South American language) that means: Tamba - shells, and Qui - conic mound, or shell mound. GPR profiles present interesting anomalous reflectors most likely related to archaeological interest targets. GPR depth slices exhibit anomalies with high amplitude probably related to point reflectors. In order to give greater reliability for real result interpretations, 2D FDTD numerical modelling was accomplished, and it shows good agreement with real data obtained in the field. Then, real and synthetic results results as reference guide to archaeological excavation activities. Archaeologist from Universidade de Sao Paulo excavated Jabuticabeira-II site after GPR results analysis. Horizontal reflectors were related to sambaqui structure, and its bottom, characterized by rich archaeological layers as well as water table level. Anomaly typified by hyperbolic reflection pattern was associated with a concentration of carbonatic shells, from 3160 to 2850 cal yBP. GPR results were very important for archaeological research in development allowing adjusting excavation, and guarantee this research success.