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Geology/Disaster 2
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Application of the Thermal Infrared Remote Sensing Technology in detection and investigation of underground coal fire
Huang Yongfang, Huang Hai
Centre for Remote Sensing in Geology,
Ministry of Geology and Mineral Resources China
Li Yingxi
China Remote Sensing Satellite Ground Station,
Chinese Academy of Science, China
Abstract
This paper describes the study results of an experimental project for the detection and investigation of underground coal fire in a northern China's coalfield by using airborne and space thermal remote sensing data s the main information. During the study, the feature of thermal structure of round surface, and, the burning structures of single coal seam and multi coal seam were analyzed; the methodology of positing and width evaluating for underground coal burning area were presented; the feature of TM6 data, its capability of detecting underground burning area and the method of surface burning area width evaluating by using TM6 was studied, and the experimental value of depth could e detected by thermal remote sensing technology combined with exploration profile.
Introduction
Coal is one of the most important energy resources for many countries in the world. Many areas in north part of China have rich coal reserves, but there is also severe underground coal fire disaster existing due to the arid and semi-arid geographical feature and the long exploration history. Underground coal fire has not only caused the heavy lost of natural resources, but also pollute the nearby ecological environment continuously. The investigation, prevention and cure of coal fire have become a serious problem government should face.
In order to meet the needs of nation's plan and engineering project for underground coal fire control, Centre of Remote Sensing in Geology (CRSG) ahs conducted and experimental project of detection methodology study by using its self-owned Daedal us AADS-230 dual channel air-borne infrared scanner (temperature resolution 0.2°K), combined with TM data received directly by China Remote Sensing Satellite Ground Station. The test site was selected in Ruqige coalfield located in Ningxia Autonomous Region, which is the main production base of "taixi coal" well reputed as the "King of Coal" because of its high quality.
Ruqige coalfield is located in the middle of Helang Mountain with 1400-2400m higher than sea
level, where steep hills and deep valleys interlace. The geological structure is rather simple with well-outcropped bedrocks and strata. The coal seams belong to Jurassic series and in the northeast synclinal structure. The coal seams attitude lies gently (incline nation is 15-25 degree). The coalfield has 7 coal seams with 5 workable among them. The No2 seam has best quality and si the thickest with average thickness 20m, making it shave 60% of total coal reserve in the whole coalfield. It is also the seam with severest coal fire.
Data Acquisition and Methodology
The project was mainly based on airborne thermal scanning data combined with color infrared air photos and complemented by Landsat TM6 data.
- Data Acquisition
Airborne remote sensing data --- high & low altitude color infrared air photos and thermal infrared scanning data --- were acquired by two aircrafts, Twin Otter and Citation-II, and ground radioactive temperature measurement was conducted at the same time with air flying. Some samples of original rocks and burnt rocks were also collected from test site for later laboratory spectrum measurement.
Since there was no usable nighttime TM6 available in the ground station, TM data acquired at 9:30 AM, Nov. 9, 1998 was selected as a space remote sensing data source.
- Image Processing
Color infrared air photos, acquired simultaneously with part of thermal scanning data, were enlarged to 1:6000 and 1:2000 scales for visual interpretation, and a 1:10000 ortho-image map part of the test site was produced also for integrated interpretation. A set of B/W images, coded B/W images &color images and function processed images for coal burning area interpretation were produced from airborne thermal scanning data through DS-1830 ground playback system.
For the purpose of determining coal burning area boundary, area measuring and the need of coal fire prevention & cure engineering designing, airborne scanning data was geometrically corrected by S101 digital image processing system using the above mentioned ortho-image map as reference.
The use of space thermal data was to study and evaluate its capability of burning area detecting. Because the radiative value range of TM6 data is rather narrow, linear stretch of TM6 data was applied before making color compositions of TM(6.4.3), (4,3,2) etc.. Image enhancement and density slicing were also conducted on TM6 image. Since there is a strong interference of solar radiation on daytime TM6 image, a contour map of TM6 radiative value was compiled for the study of extraction of fire area information.
- Image Interpretation
- Establishment of Interpretation marks
Based on the integrated analysis of airborne data, supported by field checks conducted three times in the same seasons of different ears when airborne data was acquired, three major interpretation marks were established:
- Thermal Spectrum effect
Thermal spectrum effect indicates that the interpretable ability of thermal infrared image is going to be worth as the raising of band spectrum. Based on the experimental result, the image of 305mm band was selected to detect "High Temperature Ground Objects" while the image of 8-14mm band was for "Low Temperature Ground Objects"
- Vegetation effect
Vegetation effect implies the sensitive feature of vegetation to ecological environment changes. Vegetation effect can be used to indicate the level of conduction between ground surface and underground coal fire area by cracks, and thus be used for the dynamic change tracing of underground coal fire area and fire condition.
- Boundary cracks
Boundary cracks means the banded cracks between burning area and non-fire area (will be burning as fire moves forward) cause due to the collapsing of roof rock of burnt coalseam and they can be clearly seen on the ground surface. It is clear that although the determining of boundary cracks depends on the integrate analysis of temperature and vegetation changes, boundary cracks is one of important factors for determining the boundary of underground burning area.
- Satellite image interpretation
TM6 single band B/W image, density slicing image and several other color composition images have their own different interpretabilities, for the interpretations of geomorphology, structure and vegetation growth condition, while in TM (6,4,3), (6,7,5) images, which contain detail geomorphological features, a the threshold of TM6 raises gradually, the interference caused by solar radiance is reduced and the information of burning area becomes clear. For day time TM6 image, only those burning areas with large scale and high
temperature can be interpreted from the image directly. Even it is difficult to do direct interpretation id only use TM6 single band image and density slicing image, the thermal anomaly distribution mode of strong and weak burning areas can still be viewed rather clearly through the usage of thermal radiance profile map, indicating they have the capability of reflecting the radiative features of burning area to some extent.
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