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Boundary indentification for burnt coal seams in Xinming spontaneous combustion district of Shenfu Coalfiled, Shaanxi

Feng Fu-cheng
photogrammetry and Remote Sensing Corp. of Coal field P.R.C


Abstract
Black / white aerial photographs with visible light (1:35,000) are used as main information source to fulfil aero-remote sensing survey on scale of 1:50,000 in this project. Distribution of burnt rock and ignition boundary of coal seams in Xin-ming district covered an area of 1300.km2 in shen-fu coalfield, shaanxi had been identified (trapped). The author gives an introduction to the methodology and results of burnt coal area investigation by means of remote sensing. Target of the investigation is an extinct-burnt area. The first task of the study started with basic geological characteristics investigation is to divide marco-lithotype of burnt rocks, determine its forming epoch and make approach to the mechanism of its genesis. The second task of the study is to establish preliminary interpreting indices by means of studying image fratures of burnt rock and ignitation boundary, which is carried out in a known area selected in the light of existed data. The third task of study is to complete the remote sensing investigation for burnt coal boundary all over the district, to replenish, modify, and perfect the original interpreting indices. During the investigation, interpretation was combined with survey; remote sensing was combined with concernments are fulfilled through borehole examination.

Shen-fu coalfield is worldwide famous for its enormous coal-bearing area, abundant reserves, good coal quality and advantageous mining-technical conditions. It has become one of the important coal bases for exploration and development. However spontaneous combustion in most coal seam outcrop iead to such an extensive distribution of burnt coal, which produced a lot of serious difficulties for coal resources exploration and development.

Using conventional drilling method to identify the boundary of spontaneous district usually need a long period and high cost. The effort of magnetic survey may be excellent, but it is often restricted by some economic or technical conditions. Hence, in this project, we applied black/ while aerial photographs with visible light to aero-remote sensing survey on scale of 1:50,000 in Xin-ming district of Shen-fu coalfield, and identified the boundary of spontaneous combustion area. The final result of the work fulfilled accuracy requirements.

General description of the district
Xin-ming district is located at north eastern part of the loessial plateau in north Shaanxi, which is subordinated to Shen-mu county and Fu-gu county of Shannxi province with an area of 1300 km.

The main strata cropped out are Mesozoic including young-ping formation of Triassic (T3y), Fuxian formation (J1f), Yanan formation (J1-2y) of Jurassic, and Cenozoic including Tertiary (N), Quarternary (Q).

Yanan formation with toal thickness of 235m is the main coalbearing strata in the district. It consists of 5-1 coal beds. Minable coal seams are numbered 1-2, 2-2 ,3-1 , 4-3, 5-1 from top to bottom. Thickness of those are respectively 0-1.7m for 1 (with serious regional disintegration), 1.14m for 2-2, 1.92m for 3-1, 0.78 for 4-3 and 3.32m for 5-1 spontaneous combustion, which formed extensive distribution of burnt rocks.

In the district, all of the coal rank belong to long flame coal (high-volatile metalignitous coal) with low ash, low suifur, and low phosphorus, Tectonics in the district is rather simple, strata occur horizontally with dip angle of 2-5.

Investigating methods
The project was started in May 0f 1989 and completed by the end of same year.

The aerial photographs which we used are black/white photos on scale of 1:35,000, which were taken in April in 1976.

Methodology of investigation:
  1. To contrast each other and make analogy from known to unknown .

  2. To interpret repeatedly and all-sidely, to obtain information to the utmost tallied with reality.

  3. To make comprehensive analysis including coal seams analysis, outcrop of burnt rock analysis, stratigraphic and method tectonic analysis; to combin geomorphological analysis with hydrographic net analysis.
According to the principles and methods mentioned above, the research work falls into 3 stages:
  1. To collect geologic for the purposes of analysis and research ; after then , to survey geologic profile, to carry out route geological investigation , and to grasp general geological characteristics and distributional regulations coal series and burnt rocks.

  2. On the basis of basic geological study, to establish preliminary interpreting indices for different geologic bodies in the known area selected in accordance with existed geologic data.

  3. To carry out comprehensive remote sensing investigation and examination for boundary identification of burnt rocks and ignition coal seams.
Research achievements and examination
  1. Basic Geological Characteristics of Burnt Rock
    The long flame coal with extra -thickness in the district is easy to ignite spontaneously under the condition of full oxidation. During the period of coal combustion, the overlaid strata changed gradually into burnt rocks under baking.

    1. Lithotype of burnt rocks (Table 1)
      Three types of burnt rocks shown in Table 1 are usually divided into horizontal and vertical zones cropped out. On the surface, from ravine mouth to ravine end, burnt rocks distribute in zones cropped out. On the surface, from ravine mouth to ravine end, burnt rocks distribute in order of fused-breccial, fragmental, bedded, slaty, and then, unburnt coal. On profile , from coal-seam roof to surface, they distribute in order of the same sequence but without unburnt coal. In most cases, horizontal zones showed as gradual change; and vertical zones showed as sudden varitation

    2. Formation of burnt rock
      The formation of burnt rock in said area could be divided into 4 stages, i.e. baking, procelainization, fusion, and refreezing main factors effected its formation could be summarized as : coal rank, coal seam thickness, oxidized condition , heat resistance of rock, etc. The ignition point of long flame coal in said area is generally some 340. It is easy to ignite spontaneously. By the suthor's study, the thicker the coal seam existed, the higher the burnt metamorphic degree formed; the deeper the ignition effected, the more the topography broken. In the ravine, rock cleavage provided enough spaces for coal ignition, and heat resistance of the rock influence burnt metamorphic degree directly.

    3. Main burnt epoch
      By investigation, the main period when coal ignite spontaneously in said area is before pliocene epoch (N2). In some places, the burning age may be younger, and its influential area was much smaller.

    Table.1 Classification of burnt rock
    Lithotype Classfication and rock name burnt metamorphic degree burnt metamorphism rock characteristics remark
    Color Texture Structure
    Baked Metamorphic Rock bedded burnt rock

    slaty burnt rock    		 Light dehydration light degree

    Oxidation discoloration    		 brownish red,
    brown red,
    light red    		 Relict relict bedded,

    slaty,

    laminated    		  
    fragmental burnt rock moderate oxidation porcelainization.

    Distcouration    		 red lateritic red,
    violet,
    greyish white    		 Porcelainic fragmental
    lamination
    discorder    		  
    Fused Metamorphic Rock Fused
    breccial
    burnt rock    		 Deep fusion
    refreezing    		 brown
    violet
    greyish
    black
    grey
    white    		 recryst
    allised
    fused    		 fused,breccinl

    wrinkly undulant     		 

  2. Image features of Burnt Rock
    According to different image features, we can divide the burnt rocks into 2 distributional types, i.e., cropped and concealed. Its image features are as follows:

    1. Cropped burnt rock
      Appears as deep grey stripe and steep knotty micro-geomorphy without vegetation.

    2. Concealed burnt rock
      Appears as complicated distribution. It could be interpreted comprehensively by analysis of indirect indices and stratigraphy including :

      1. Coal seam cropped spots, minor coal mines, and fire-marginal spots in ravines

      2. Tectonic indices, such as lamination disorder or developed secondary joins.

      3. Micro-geomorphic indices, such as burnt rock of overlaid strata caused by coal ignition , broken geomorphy, varied hydrographic net and dicontinuous ridge ( Fig. 1) caused by rock cleavage, sliding , collapse, etc.

        Burnt rock appears as modern geomorphic landscape, where platform-like slope has a convex front edge and concave back edge (Fig.2 and Fig.3). The front convex part shows outcrop of burnt rock, the median part shows extension of burnt rock, and turning point of the back concave part shows fire marginal spot, namely, the marginal point of coal ignition area. Plane distribution of the platform-like slope geomorphic usually appears as parallel arrangement of a series of ridge from ravine mouth to ravine end, and the length of slope shorten gradually until its disappearance.

        Along the boundary between ignition area and normal area, we can see that the cross-gully meet angle with the dipping direction of slope.

      4. Hydrographic net indices, including deepened and widened gully, irregular water system and snakelike strem caused by broken rocks in burnt area. A series of springs and linear leakage could be found along the bottom boundary of burnt rock.

        All the indices mentioned above should be interpreted comprehensively and stressedly.


    Fig.1 ignition boundary of coalbed 5 in hou-yan-liang,
    Yang-huo-pan mine district


    Fig.2 Sketch showing longitudinal section ridge


    Fig.3 ignition boundary of coalbed 3 at eastern ridge of Da-juan-gou,
    Yang-huo-pan mine district


  3. Remote Sensing investigation for Ignition Boundary of Coalbed
    As for extinct burnt district, we have good reason to equate the coal ignition area with the distributional region of burnt rock. If the burnt rock area could be trapped exactly, the extinct ignition and burnt coal area can also be identified easily.

    In the said district, thickness of coal seam trend to thinning northwards, and burnt metamorphic degree trend to decreasing. In the southern and central part of the district, we mainly used interpretation combined with survey; in the northern part, coal ignition boundary was identified by means of both survey and interpretation. According to the difference mentioned above, the said district has been divided into 5 blocks, from south to north , they are in order of Yong-xin, Xinming-dianta (including 3 mine districts, namely, Yan-jia-liang, Sha-gou-cha), Shan-dao-gou, Lao-gao-chuan, Da-chang-han.

    Research results indicated that the ignition boundary of main coal seams 3 and 5 are trapped : most area of 2 , partial area of 4 are also trapped . As to coal1, we have not interpreted owing to its serious erosion and poor distribution.

  4. Examination
    For the purpose of examining our research results, we adopted two methods, i.e., surface geological investigation and correlating with drilling data.

    1. Surface geological investigation
      Examination indicates that the coalbed ignition boundaries which we have trapped possess better reliability and tally with actual situation. All the located errors (stabbing point on black/white aerial photo) for coalbed and burnt rock outcrop are less than 0.3mm, all the line-linked errors are less than 0.4mm. Its accuracy tally with requirement of regulation.

    2. Drilling data
      The boreholes operated during the stage of detail exploration in 2 mine districts and all of the reconnaissance boreholes in the said area are adopted to make correlation after completion of the project.

      The examining results are as follows
      1. Sha-gou-cha mine district
        To examine coalbed ignition boundary with drilling data for main seam 3 (5m in thick) , 5 (2m in thick) in an area of 45 km. Among 29 correlated boreholes, 25 coincide with each other, 4 discoincide. Maximum error amount to 230m, minimum error amounts 50m, 130m on average. The correctness is up 86.2%

      2. Yu-jia-liang mine district
        To examine coalbed ignition boundary with drilling data for 3 (3.7m in thick ), 5 (4.5m in thick ) in an area of 18km. Among 25 correlated boreholes, 21 coincide , 4 discoicide. Maximum error 150m, minimum 50m, and 90m on average. The correctness up to 84%

      3. Boreholes operated in reconnaissance stage
        26 reconnaissance boreholes have been used for examination . Among them, 23 coicide and 3 discoincide . Ad to the error, 200m for maximum, 25m minimum, 92m on average. The correctness takes 88.5%.

      According to the accuracy requirement submitted by shaanxi provincial coalfield exploration company, the results of remote sensing (1:50,000) for burnt rock investigation must be raised 100% than conventional drilling, engineering .The allowable error for ignition boundary tapping is+750m. Hence, all the errors in our project tally with accuracy requirement.
Conclusion
  1. The project results show that using black/white aerial photo to investigate coal bed ignition boundary in extinct burnt area in indeed an exact, reliable and effective method. It provided scientific basis for coal resources exploration and development. It possesses fairly practical and economical significance, and is necessary to spread in similar area in the days to come.

  2. The method combined with remote sensing, geophysical prospecting and drilling for investigating coal ignition boundary is undoubtedly optimum, which would produce evident economical and technical efficiency.

    Owing to the limit of local geological features, scale of aerophotography and other factors, we did'nt exactly interpret the ignition boundary of individual coal bed under the condition of more than two overlapping burnt layers, where one laid on top of another. This is the problem to be studied and discussed further.