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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
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).
- Study of Coalseam burning Structure
- 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.
- 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.
- 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.
- 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.
- 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.
- Underground Burning Area (UBA)
- 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)
- 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.
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