Raimundo Almeida-Filho

Digital processing of a Landsat-TM time series for mapping and monitoring degraded areas caused by independent gold miners, Roraima State, Brazilian Amazon

Abstract A 12-year Landsat-Thematic Mapper (TM) time series (1987–1999) was used for mapping and monitoring evolution of degraded areas caused by independent miners (“garimpeiros”) in the search for gold and diamond. Discrimination of target areas was achieved through third principal component images, due to their best enhancement of bare soil areas relative to the surrounding savanna vegetated terrain. A postclassification approach, based on image segmentation/region classification techniques, was used to map degraded areas. This procedure allowed to save time and to curtail inherent subjectivity commonly involved in visual interpretation, producing accurate land-cover change maps. According to these maps, degraded areas comprise 94.4 ha in 1987, 286.4 ha in 1991, and 404.4 ha in 1994. After “garimpeiros” abandoned the region sometime prior to 1994, vegetation soon started recovering mined areas as shown in images acquired in 1995, 1996, and 1999. According to these images, degraded areas decreased to 311.2, 283.7, and 246.2 ha, respectively. Data also indicate that vegetation regrowth is faster near the border of degraded areas, and becomes slower to the center of these areas, where damage process is more intense. Based on the satellite images-derived regrowth rates, it is possible to infer that degraded areas would not be entirely recovered by vegetation by the year 2019.

Using dual‐polarized ALOS PALSAR data for detecting new fronts of deforestation in the Brazilian Amazônia

This study shows the use of dual‐polarized L‐band Synthetic Aperture Radar (SAR) data acquired by the Advanced Land Observing Satellite (ALOS) Phased Array type L‐band Synthetic Aperture Radar sensor (PALSAR) system for differentiation between primary forest and new deforestation fronts. Results showed that the new deforested areas are not unequivocally detected in either of the single bands individually. Although most of the recent deforestation appears brighter than mature forests in the HH band, the identification of new deforestation using only this band depends on the stage of the clearing process. Then, the contribution of the HV band through a normalized difference index (NDI) is important for detection of the new deforested fronts. Results indicate that the method can be used as the basis for an operational programme to monitor deforestation in the Brazilian Amazônia.

Fraction images in multitemporal change detection

The concept of mixed pixels allows the interpretation of remote sensing digital image data at sub-pixel level. Fraction-image data, obtained using the notion of mixed pixels, offer a potentially powerful method to detect changes in land-cover over a given period of time. This study proposes a new approach to detect land-cover changes, using two sets of fraction-image data obtained from sets of multispectral image data acquired at two different dates, over the same area. Changes based on the selected pixel components are then used to generate the fraction-change image data, including both positive (increase) and negative (decrease) changes in each component. The proposed analysis is then performed in the fraction-change space in two different ways: (1) by implementing unsupervised classification methods and (2) by comparing the fraction-change images among themselves. The proposed methodology is tested on two sets of Landsat Thematic Mapper (TM) multispectral image data obtained at two different dates and covering a test area mapped in previous works. Results obtained by the proposed methodology are presented and discussed.

Detecting deforestation with multitemporal L-band SAR imagery: a case study in western Brazilian Amazônia

Applications of L‐band SAR data to map deforestation are generally based on the assumption that undisturbed forests consistently exhibit higher radar backscatter than deforested areas. In this Letter we show that depending on the stage of the deforestation process (slashing, burning and terrain clearing), this assumption is not always valid, and deforested areas may display a stronger radar return backscatter than primary forest. The analysis of multitemporal SAR images, supported by several Landsat Thematic Mapper (TM) images and field knowledge, showed that wood materials left following the deforestation practices function as corner reflectors, causing an initial increase in the radar backscatter, which then subsequently decreases over time as the debris on these fields are removed.

Remote detection of a tonal anomaly in an area of hydrocarbon microseepage, Tucano basin, north-eastern Brazil

Mapping of a tonal anomaly in an area of hydrocarbon microseepage was possible using a Landsat-Thematic Mapper false-colour composite consisting of TM2/3, TM4/3 and of the difference TM2/3-TM4/3, displayed as red, green and blue, respectively. In this false-colour composite, green and yellow express variations in the vegetation cover, whereas hues of magenta are related to terrain conditions representing bleached materials, which occur close to the centre of a soil gas anomaly. Simultaneous occurrence of anomalous soil gas values with magnetic and radioactive minerals suggests that a reducing environment associated with the microseepage phenomenon created the bleached materials, identified as a tonal anomaly in the enhanced TM image.

Quantifying optical and SAR image relationships for tropical landscape features in the Amazônia

This paper discusses the relationship between SAR and optical data for an Amazonian test‐site with different land cover types. L‐band HH JERS‐1 SAR and Landsat TM images acquired few days apart from each other in 1994 and 1997 were analyzed. Landsat TM fraction images (vegetation, soil, and shade) were used to characterize the terrain features in the study area. Based on 220 samples randomly distributed over different land cover types in the fraction and SAR bands, a regression analysis was performed. Consistent results between SAR data and fraction images suggest that L‐band SAR data may be a complementary source of information for mapping land cover changes in Amazônia, especially to monitor deforestation in areas frequently blurred by cloud cover in optical images.

Terrain characteristics of a tonal anomaly remotely detected in an area of hydrocarbon microseepage, Tucano Basin, north-eastern Brazil

This study reports the geological characteristics of a tonal anomaly in an area of hydrocarbon microseepage, identified in Landsat-5 Thematic Mapper (TM) images. A diversified dataset was gathered, which included soil gas geochemistry determinations, soil radioactivity intensity and soil magnetic susceptibility measurements, as well as laboratory soil spectra. Data indicated that the tonal anomaly holds a higher concentration of gaseous hydrocarbons in soils than the surroundings, accompanied by enrichment in radioactive minerals. However, there is no definitive evidence of magnetic mineral enrichment inside the tonal anomaly compared with the surroundings. An intense silicification process, characterized by a cover of fine-grained silicified cobbles and boulders, seems to be responsible for inhibiting the development of diagenetic clay mineralization, as indicated by variations in the depth of the 2200 nm soil spectra absorption band.

Mapping land degradation in the Gilbues region, northeastern Brazil, using Landsat TM images

Several areas of land degradation are present in the northeastern part of Brazil, resulting primarily from old practices of vegetation clearing and burning for subsistence farming and grazing. However, no reliable information is currently available on the extent of these areas and whether they are expanding over time. Taking the Gilbués region as an example, this study shows the potential for obtaining this information using multitemporal dual-season Landsat Thematic Mapper (TM) imagery. We used digital image processing, which involved the enhancement of soil-fraction images by segmentation/region classification and Normalized Difference Index (NDI) techniques. The results show highly degraded soils covering an almost continuous area of approximately 600 km2. The data also show that, over the past two decades, land degradation has been concentrated mainly along and in the heads of drainage systems, a pattern that indicates removal of riparian forests, thereby compounding the serious problem of sediment loading in the streams in the area. We show that satellite remotely sensed data can provide valuable information in support of environmental policies for northeast Brazil.

The Serra da Cangalha astrobleme as revealed by ASTER and SRTM orbital data

Structural features caused by the impact of celestial bodies at the surface of the Earth always stir scientific interest. Different from natural geological processes like earthquakes and volcanic activities, these impacts instantaneously release huge quantities of kinetic energy that depends only on the mass and the velocity of the cosmic body. French (1998) postulates that the impact of a celestial body only a few kilometres in diameter can release more energy than the whole Earth releases in hundreds or thousands of years. Like a stone thrown at the surface of a pond, the impact of a cosmic body at the surface of the Earth radiates shock waves outward from the impact point. As a result, a crater is formed in a very short time, with rock being deformed, shattered, pulverized, and melted. These ordinary cosmic events hold not only a geological interest. Scientists postulate that the shock of an asteroid or comet about 65 million years ago might have caused a dramatic change in the evolution of the terrestrial life. The Chicxulub crater in the Yucatán Peninsula, Mexico (Hildebrand et al. 1991, Morgan et al. 1997) is believed to be the site of that event. The impact would have released a large amount of dust as well as CO2 and SO2 gases from the melted sedimentary rocks. The resulting greenhouse effect suppressed photosynthesis and caused dramatic environmental changes at a global scale. The main consequence was the extinction of species, redirecting the biological evolution and favouring the subsequent domain of our ancestors, the mammals. This cover shows the Serra da Cangalha astrobleme, the best preserved impact crater found in Brazil, as depicted by two modern sources of orbital remote sensing data: multispectral images acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and terrain elevation data collected by the Shuttle Radar Topographic Mission (SRTM) (figure 4) . As exemplified in the Serra da Cangalha astrobleme, the use of multiple products generated from highresolution remote sensing multispectral images and radar-derived digital elevation models (now available on a quasi-worldwide basis) can substantially improve structural interpretation of geologically complex terrain.

Location of kimberlites using Landsat Thematic Mapper images and aerial photographs: the Redondao diatreme, Brazil

Abstract Landsat Thematic Mapper images and aerial photographs were used in the detection of kimberlile-derived materials in the Redondao test site. In this area kimberlite-derived soils show a flora constituted mainly by grasses and shrubs, which differ from the surrounding savanna-park (cerrado) vegetation cover. Band-ratio images were able to distinguish kimberlite-derived materials by enhancing areas with different vegetation covers. However, the coarse spatial resolution of Landsat-TM images compared with the spatial variability of the study area, and the removal of topographic shadowing effects on ratio images blurred several landscape features. To increase discrimination, Landsat Thematic Mapper ratio images were merged with digitized aerial photographs through intensity, hue and saturation (IHS) colour transforms. The resulting merged colour composite highlighted the spatial and spectral features of the study area permitting an accurate definition of the kimberlite-derived materials within the Red...