A project developed at Unicamp's Institute of Geosciences (IG) is collecting gravimetric and gamma spectrometric data from the Araguainha dome, the largest meteorite impact crater in South America, measuring 40 km in diameter, located between the states of Mato Grosso and Goiás. The objective is to identify anomalies and develop three-dimensional models of the distribution of mass densities in the subsurface to understand the geological evolution of that area after the meteor impact, which occurred around 250 million years ago. The project, which is financed by the São Paulo Research Foundation (FAPESP), is coordinated by professor Emilson Pereira Leite, from the Department of Geology and Natural Resources at IG.
Gravimetry is a method used to measure the Earth's gravitational field. Gamma spectrometry is a technique used to measure the intensity of radiation emitted by chemical elements, such as potassium, uranium and thorium, found in rocks. According to Emilson, the measurements collected from Araguainha's gravitational field allow us to understand the structure beneath the crater. Gamma radiation measurements allow us to analyze the most superficial area, characterizing the rocks present in the area and the distribution of chemical elements that naturally emit radiation. Gamma radiation is, as a rule, similar to that emitted by x-rays. However, it has higher frequencies that, when exposed to high levels, can be harmful to your health. “Coal mines, for example, need to have radiation levels monitored because coal emits relatively strong gamma radiation due to the high concentration of uranium, which can eventually cause damage to the health of workers exposed to that material daily. As a rule, in craters, emissions are low and there is no risk of harm to health”, highlighted the professor.
The project in Araguainha began in mid-2017 and continues until the end of June 2019. Using data collected in the dome, it was possible to create computational models of physical properties inside the Earth. “This type of study is done to understand the evolution of our planet. There is a slower evolution, related to the movement of tectonic plates and the formation cycle of different types of rocks. And there is the rapid evolution that occurs due to the impact of meteors, which changes the configuration of the rocks and, depending on the size of the impacting body, can change the climate and affect the planet's biodiversity”, pointed out the researcher.
IG professor Alvaro Penteado Crósta was one of the pioneers in surveying and interpreting data from the Araguainha dome in the early 80s. The crater is the subject of several studies inside and outside the country given its age, level of preservation and effects of its impact. “Brazil is a tropical country with a very strong degree of weathering. The erosion process is very intense and the structures exposed on the surface wear out over geological time. The Araguainha crater, however, even though it was formed millions of years ago, is still clearly visible. You can see a large part of its raised edge in a circular shape and the presence of granite rocks in the center, which were hypothesized to have been uplifted after the impact”, highlighted Emilson. There are two cities located inside the crater: Araguainha and Ponte Branca, where the professor and two IG master's students stayed for two weeks in 2017 to collect data for the project. Emilson and one of the students collected additional data over four weeks in 2018.
The group collected gravimetric data and gamma radiation from the entire crater area. Johann Lambert Silva analyzed the gamma radiation part. Marcelle Rose Miyazaki, part of the gravimetric field of the crater core. According to Emilson, who has a background in geophysics, the value of the gravitational field varies depending on some spatial and temporal factors, including the distribution of rock densities inside the Earth. “Making measurements of the acceleration of gravity allows us to understand the distribution of rocks in the subsurface as a function of their densities up to kilometers deep. In the case of Araguainha, our models reach around 2 km,” he said.
Similar studies were carried out, for example, in Cerro do Jarau, on the border between Brazil and Uruguay. According to Emilson, although the geological contexts are quite different, there are similarities between the two studies because in general the craters present a pattern in the gravity field data. “The meteor strikes the Earth's surface, violently pressing down on layers of rock below the surface. When its fragments are ejected after the impact, the deeper layers are brought closer to the surface”, pointed out the professor. What can be understood with the computer models created from the data collected in Araguainha is that layers of rock that are deeper outside the crater area are closer to the surface in the crater area. “This reinforces the meteor impact hypothesis and contributes to other studies that investigate, for example, the intensity of this impact,” said Emilson.
Methodology used
To collect this data, some specific equipment for geophysical studies was used by the IG group, such as the gravimeter, which measures the gravitational field, and the gamma spectrometer, which measures gamma radiation. The gravimeter, for example, makes specific measurements in a process that can last from 5 to 10 minutes. Measurements are taken at points spaced approximately 200m apart, depending on access availability. This covers the entire study area. The information is stored in the equipment's internal memory. The gamma spectrometer is placed against the rock or soil and determines the amount of gamma radiation that is naturally emitted by this material. Next, a numerical conversion is performed to identify the concentration of chemical elements. This type of measurement makes it possible to characterize rock types and carry out analyzes integrated with information from geological mapping.
Dissertation and qualification defended based on the project
Emilson's two students, Marcelle and Johann, produced master's theses with the data they collected in field research in Araguainha. In 2018, Marcelle defended the dissertation “3D gravimetric modeling of the Araguainha impact structure”, in which she mapped the gravitational field of the crater's uplifted core. She extracted information from the Bouguer anomaly, which is a type of gravimetric anomaly that is related to the distribution of mass in the subsurface, and built a three-dimensional model associated with the configuration of the rocks present in the area. Johann has already qualified for his dissertation “Gammaspectrometric characterization of the Araguainha impact structure”, which should be defended this year. With data collected at various points in the crater, the student produced concentration maps of potassium, uranium and thorium, recognizing gamma spectrometric signature patterns of geological units and a possible correlation of anomalies associated with the molten rocks after the impact.
