Chris Bellman 
  
USE OF A LANDSAT-TM TIME-SERIES FOR MONITORING EROSION FEATURES IN AREAS 
OF GOLD EXPLORATION, SERRA TEPEQUEM, RORAIMA STATE, BRAZILIAN AMAZON 
Raimundo ALMEIDA-FILHO, Edileuza C. MELO, Yosio, E. SHIMABUKURO 
National Institute for Space Research-INPE, Caixa Postal 515 
Sáo José dos Campos-SP, Brazil 
rai 9 Itid.inpe.br; yosio € Itid.inpe.br 
Working Group VII/2 
KEY WORDS: Remote Sensing, erosion features, renewable resources management 
ABSTRACT 
This paper discusses the use of a Landsat-TM time series for monitoring erosion features caused by the action of 
independent miners (garimpeiros) in the search for gold. Five free-cloud Landsat-TM scenes acquired in March/87, 
October/91, November/94, January/96, and January/99 were used to produce maps showing temporal evolution of 
mined areas in the region of the Tepequém plateau. To produce detailed thematic maps of the degraded areas, images 
were segmented using a region growing approach, followed by the application of a region-classifier algorithm based on 
clustering techniques. According to these maps, eroded areas increased by 388% from 1987 to 1994, comprising 1070 
ha, 2,550 ha, and 4,150 ha, in the 1987, 1991, and 1994 images, respectively. Due to a governmental action, combined 
with the exhaustion of the mineralized alluvial deposits, garimpeiros abandoned the region around 1993/1994. As a 
result, vegetation soon started recovering eroded areas. Images acquired in 1996 and 1999 showed that degraded areas 
decreased to 3,660 ha and to 3,391 ha, respectively. This work shows that Landsat-TM images may be an efficient mean 
for monitoring erosion features, even under the ecological characteristics of the Tepequém plateau, where eroded areas 
have low contrast relative to surrounding open grass fields. Additionally, the image processing procedures permitted to 
save time and to curtail inherent subjectivity involved in the visual interpretation, producing accurate thematic maps of 
land use/cover change classes. 
1. INTRODUCTION 
During the late eighties and early nineties, Roraima State, and particularly the region of the Tepequém plateau, was 
severely exploited by independent miners (garimpeiros) in the search for gold (Oliveira, 1995), resulting in several 
degraded areas along streams. In the search for gold, garimpeiros use high-pressure water jets to wash mineralized 
alluvial deposits. This practice causes removal of the vegetation cover along stream banks, so creating conditions to 
start erosional process in mined areas. In spite of constituting a serious environmental problem, few attention has been 
given to study the impact caused by informal gold mining activities (garimpos) in the Brazilian Amazon. This kind of 
activity causes two major environmental problems. The first one is related to the pollution of rivers and streams due to 
the use of mercury amalgamation for gold recovery. The second problem is the deforestation and erosion along mined 
streams. This second mining-derived problem may be addressed directly by remote sensing techniques. 
Using visual interpretation of Landsat-Thematic Mapper (TM) images, Melo & Almeida-Filho (1996) showed 
preliminarily that mined degraded areas could be mapped under the physiographic conditions of the Tepequém plateau. 
More recently, Almeida-Filho & Shimabukuro (1999) showed that the degraded areas in that region could also be 
identified using orbital radar images acquired by the Japanese Earth Remote Sensing satellite (JERS-1). In this study we 
use techniques of automatic classification applied to a Landsat-TM time series, aiming at to map and to monitor the 
evolution of areas degraded by gold mine activities in the Tepequém plateau region, over a twelve-year period, from 
1987 to 1999. 
2. STUDY AREA 
The study area covers a surface of approximately 150 square kilometers, located in the north-central part of the State of 
Roraima, Brazilian Amazon, close to the border with Venezuela (Figure 1). It includes the Tepequém plateau, being 
defined by the following geographic coordinates: 61° 48” 15°” - 61° 37° 53° west longitude and 3° 50’ 30°’ - 3’ 44°20” 
  
66 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 
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