ÁREA ELEMENTAR REPRESENTATIVA PARA MEDIDAS DA DENSIDADE E POROSIDADE DO SOLO USANDO TOMOGRAFIA COMPUTADORIZADA DE RAIOS GAMA

Abstract

Computed tomography (CT) is a technique that provides images of sample cross sections of porous media. The technique has shown efficient to investigate different soil physical phenomena since its first use in soil science in 1982. One of the most widely investigated issues in physics applied to porous media regards the definition of representative elementary areas (REA) to measure the soil physical properties. REA is related to the minimum area of a sample needed to represent its characteristics of interest. The objective of this work was to define REAs to be adopted for density and porosity measurements in clod samples of a clay soil. The equipment used was a first generation tomograph with 241Am source, with millimetric resolution (1.1 x 1.1 mm2). The photon detection was made with a plane NaI(Tl) solid crystal scintillator. Clods (18 samples) were also submitted to the clod method (standard method) in order to determine its. In order to reconstruct tomographic images, the program Microvis developed by Embrapa/CNPDIA was used. Concentric square areas with sizes ranging from 1.2 to 1162.8 mm2 were selected. To define REA for the following criteria were adopted: average value relative deviation between the last and each of the remaining areas not superior to 5%, 4%, 3%, 2% and 1%; and 2) that at least three consecutive areas cannot differ among themselves, using the variation criterion in item 1. The measurements through the clod method presented variation coefficient of 3.8%. Thus, it was considered that the REA was reached at the 4% variation criterion. For the criterion is similar, but the relative deviation considered cannot be superior to 10%. The REA defined for measurements corresponds to 640.1 mm2. On the other hand, for the REA found was 882.1 mm2. So, in order to carry out representative measurements of these two physical properties the REA must be at least 882.1 mm2. Key-words: Representative Measurements. Tomographic Images. Porous Media.