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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.
  1. 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.

  2. 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.

  3. Image Interpretation

    1. 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:


      Fig. 1 airborne thermal scanning image

      1. 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"

      2. 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.

      3. 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.

    2. 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.
Results and Discussion
The coal fire area caused by the burning of underground coalseam is a 3-D thermal body spatially. For simple description, w consider the outlet ground surface as the above border surface of the 3-D therma body as well as the ground burning areas as the bottom border surface, and defined the horizontally projected areas of the 3-D body to the above & bottom border surfaces as "Ground Burning Area" Underground Burning Area" (UBA).

  1. Study of Coalseam burning Structure

    1. Thermal anomaly structure of GBA

      Based on the contribution pattern of different levels' radiative temperature in the images, the anomaly structure of GBA determined by thermal image can be classified into four types: Symmetric; Inner-tilted; Outer-titled and Step-bench. Preliminary study shows that, under a certain temporal and spatial condition, the symmetric, inner-tilted and outer-tilted types of thermal anomaly structure exist normally in single coalseam burning areas and indicate the basic feature of ground thermal structure for single seam burning, while the step-bench type exists mostly in multi-coalseam burning areas. Outer tilted type exists in the older burning are, implying that even the strongest burning has moved forward, the fire is still existing in the way strongest burning passed; inner-tilted type appears normally in the less deep burning area where is almost the empty coal face ad even the fire is moving along the strike of coalseam, the burning is not stronger as in coal face.

    2. Burning structure of single coalseam

      For the burning seam which lies horizontally, the profile of its burning structure could be simulated by following curve:

      Y = ARC CtgX


      1.Coal Seam 2.Empty Area by Burning 3.Burning Porfile
      (Arrow indicates the moving derection of fire)
      Fig. 2 Simulating profile of singleseam burning structure

      Fig.2 shows the profile of single seam burning structure. The defined by points c,a,d,b is a dynamic changing profile f burning structure, and is moving forward fire moves toward non-burning part of the seam. During fire moving forward, front part of the profile - ca - goes up because of the oxygen provided by cracks in roof rocks and appears like a wedge, while the back part of the oxygen provided by cracks in roof rocks and appears like a wedge, while the back part of the profile-bd-goes down due to the used-up of burnable materials and burnt ash covering. Within the main burning part-ab, the fire is very strong due to the thicker coal seam, the speedy air flow and less ash covering, thus making a big accumulation of heat, the strong fire there accelerate greatly thermal physical-chemical reaction procedure, and the burning produced heat makes roof rocks be burnt altered and crushed. As the forward moving of point a, the burnt-altered roof rocks collapsed by fire along the empty burnt area in a segment, thus provide good condition for air and oxygen flow, and the accumulated heat can be conducted well through cracks. At that time, a symmetric thermal structure profile can be detected from ground surface.

      Burning structure will change more complexly as the change of incline degree when coal seams are inclined. If inclined degree is less than 30, burning structure changes simply. When coal seam inclines downward, the direction of fire moving goes up correspondently, making ca segment of burning profile be longer while the main burning segment-ab-and back segment-bd --- be shorter and the slope of burning profile be bigger, the inner-tilted type in ground ghermal structure appears. When coalseam is inclined upwards, opposite situation, i.e outer0tilted can be inferred generally based on the ground surface thermal structure interpreted from thermal images and known coal seam attitude from existing geological data.

    3. Multi seam burning structure

      Multi-seam burning structure indicates the burning feature incorporated when two or mote seams burning superimposed spatially (near or completely).

      Step bend type of ground surface thermal structure is the major of underground multi-seam burning structure. The multi peaks in the profile which represent the main burning area of each sea imply the width and burning condition of main burning area for each seam through the width and intensity of each peak in profile, while the intervals between peaks represent the time differences of fire moving forward in each seam. Generally speaking, the fire moving styles of each seam have more or less differences, thus making the feature of burning structure for each could be remained basically, and can be deduced by the clear step-bench type of ground surface thermal structure.

      Some of multi seam burning conditions which are moving faster are more complex. Since the normal burning structure has been destroyed by certain reasons, the ground thermal structure has changed greatly. For instance, single peak with wider width or single peak area with intensities changed notably in it would appear. It si clearly important to study multi seam burning structure more and further works are needs to be done later.

  2. Determination of UBA Boundary

    For the purpose of determining UBA boundary, it is necessary to know first what is the feature of GBA and how to delineate it boundary.

    1. Ground burning Area (GBA)

      GBA can be delineated rather easily based on the integrated analysis of thermal radiative temperature, vegetation effect and boundary cracks.

      According to the analysis of coalseam burning structure model, the rocks above major burning are-ab segment---collapsed continuously as the burnt are (empty burnt are) becoming larger, and vertical cracks created, producing better condition for air flow and heat conducting upto ground surface, thus making the vertical correspondence spatially between major burning are (underground)----a segment---and high temperature area on ground surface. As the coal seam burning continues and the forward moving of point a, the collapsed area in ground surface is also moving forward. Point a can be considered as the underground control point for inner boundary of ground surface thermal anomaly.

      For the area correspondent to bad segment, because the roof rocks collapsed mostly, producing an open system where the fire became weaker and extinguished quickly and the heat could be conducted spreadly, making it be very difficult to determine the correspondent boundary of d point on ground surface, so the determination of outer boundary for GBA is rather at will.

      In many cases, for instance, the roof rock is rather thick, there is not clear thermal anomaly on ground surface for ca segment, it may be related to the limited empty space there, the shorter vertical distance roof rocks collapsing pass, eh smaller cracks, the shorter burning time period and the limited heat accumulation.

    2. Underground Burning Area (UBA)

      1. UBA width calculation for single seam

        The with of UBA for single seam can be calculated by following equations : -

        Lca = = Sca x L x COS Q ------------------------(1)

        Lab= = Sab x L x COS Q ------------------------(2)

        Ldb = = Sbd x L x COS Q -----------------------(3)

        Where
        L------------- thickness of burning seam, m.
        Q------------- inclinenation angle of coal seam, degree
        S---------------width coefficient
        Lca, Lab, Lbd-------- horizontal projected width for ca, cb, bd segment respectively, m.

        The whole width of burning seam, Lt, can be calculated by following equation:

        Lt = (Sc+Sab+Sbd) x L x COS Q ---------------------(4)

      2. Estimation of UBA for multiseam situation.

        Since the burning profile in this case changes greatly, the UBA width can only be estimated by following equation :

        Lt =(Sca + Sbd) x L x cos Q + ab ----------------------(5)

        Where ab-----------total width of high temperature anomaly area in the ground surface correspondent to the segment a in profile, impiles the spatial overlay of widths for each seam's major.

  3. Estimation of Detectable Depth

    As we know generally, it is almost impossible to detect directly the depth and location of UBA by only the thermal information extracted from thermal images, but if we ignore the radiative background caused by normal ground thermal flow and the day-night change of solar radiative energy on the ground, it is possible to consider that the thermal radiative energy in GBA depends on the thermal energy conducted up to surface from UBA, and a close correspondent relationship exists between them. The depth study of remote sensing potential for detecting underground coal fire through the analysis of thermal structure and the determination of thermal anomaly in images, combined with know geological and exploration data, is definitely necessary and interested.

    According to the existing exploitation in burning area, No2 coal seam, which is burning severally, has 160m depths mostly and average depth 86m. The "depth" here means the distance from coal seam roof to ground surface. Actually, these GBA correspondent to the o 2 burning seam all he thermal anomaly equal or exceed the setting of HI reference source (BB2) in air-borne thermal IR scanning images, indicating the depth value detectable by it could be deeper.

    Preliminary study shows that the detectable depth is depend on many factors, such as thickness of coal seam, the burning time period, the basic features of roof rocks, etc. in this case, since burning coal seam is very thick , the roof rocks are mainly sandstone and there are many cracks, the detectable depth is much bigger than the value reported in abroad reference.

  4. GBA Delineation and Width Estimation Using Satellite Data

    During the creations of TM (6.4.3) or (6,7,5) color compositions, a method of threshold compression segmentally was used to determine GBA boundary from the intensity anomaly in TM6 image where has been delineated as GBA by airborne thermal image interpretation. Experimental results show that the boundary anomaly area after intensity compressed to be half of the previous could be consider as the boundary of GBA.

    The reason of selecting half compressed intensity to determine boundary of GBA is related to the high correlation of TM data, the effects of mixed-pixels and the way of TM6 data creation (interpolated from 120m to 30m resolution). The integrated effect is that the width of GBA in TM6 image has been increased notably comparing to the true width, that is, the integrated effect looks like positive.
Summary
A part from the preliminary study of some basic technical problems, this experimental investigation has got remarkable result sin standardized procedure establishment and in engineering application.
  1. The two temporal situations of underground coal fire in the test site before 1986 and within 1986-1988 have become clear, the new and old burning areas are delineated, the total loss of coal resources for severest fire in each burning area were predicted. All results have been used as one of basic data & information sets for the drafting of "The engineering design of underground coal fire prevention and cure project in Ruqige coalfield".

  2. Besides verifying TM6 data's capability of detecting burning area detected by airborne thermal scanning data, two new burning areas have been found newly in TM6 image of 1988.

  3. A se of apply able techniques & procedures for underground coal fire detail investigation by using mainly airborne thermal IR was established while the method of using satellite thermal data as the reconnaissance tool was studies also.

  4. The results have greatly promoted the carry outing of large-scale investigation for underground coal fire by using remote sensing technology in north part of China.
Acknowledgement
The authors wish to thank prof. Zeng Shaomign and Mr. Zhou Fuzhen very much for their valuable helps in conducting and paper drafting. Thanks also to the project participating colleagues from CRSG, Ruqige coalfield and the Professional Design Institute, Ministry of Railway.

References
  1. Wang Guoqin, song Dexiang: "Coal Geography Worldwide", Business Publishing House, 1987.

  2. Helan Coal Geology Exploration inc.: "Detail Exploration Report for Ruqige Coalfield", 1966.

  3. Robert N. Colwell et al: "Manual of Remote Sensing", Second Edition, American Society of Phorogrammetry, 1983.