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This work estimates and discusses the effects of the rainforest on the airborne gamma-spectrometric surveys, taking as case study an area in the center of the Rondônia State, Amazonia, northern Brazil, where wooded and deforested areas are frequently juxtaposed. The control of the wooded areas is made using Landsat satellite images, by the calculation of the Normalized Difference Vegetation Index (NDVI), which allows distinguishing between areas with low and high concentration of green leaves. The results show that, compared to the NDVI values for non-vegetated areas, there is an attenuation of the mean signal of this index in the rainforest areas corresponding up to 36% of the Total Count of Radiation (TC), 12% of potassium (K), 37% of equivalent thorium (eTh), and 29% of the equivalent uranium (eU) concentrations. In our interpretation, part of this effect might be caused by the radiation shielding of the biomass in rainforest areas. However, as a secondary effect for the gamma-spectrometry data processing, the canopy trees are considered on the Digital Elevation Model being used as a surface to calculate the distance between the source of radiation and the sensor, which causes a bias in the processing, underestimating the real flight height to the ground surface. This last effect could have been avoided if there were a Digital Terrain Model available for the area, which should be considered for the data processing. Improving the understanding about this phenomenon, an increase in the quality of the signal-to-noise ratio of airborne gamma-spectrometric data used for the quantitative land modeling may be achieved for regions where the
presence of rainforests is significant such as the Amazon region northern Brazil.
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