A method for simulating the three-dimensional morphological evolution of the combustion zone of underground coal fires

Abstract

The present invention discloses a method for simulating the three-dimensional morphological evolution of underground coal fire burnout areas, and belongs to the field of geotechnical engineering geological disaster prevention and control. Sentinel-1A image data is obtained, and the DS-InSAR technology is used to extract the cumulative surface deformation value and the ArcGIS software is used to extract the cumulative surface deformation contour line; according to the stratigraphic parameters, surface temperature, three-dimensional temperature field and cumulative surface deformation contour line data of the area recorded in the data, a three-dimensional morphological prediction model of the underground coal fire burnout area is constructed in a computer using numerical simulation and three-dimensional modeling technology, and the three-dimensional morphological evolution process of the underground coal fire burnout area is inverted, and the formation of the underground coal fire burnout area after the coal seam is burned is simulated, and the three-dimensional morphological inversion result of the underground coal fire burnout area is verified. The method realizes the description of the geometric structure of the underground coal fire burnout area, reveals the morphological changes of the underground coal fire burnout area and its relationship with factors such as surface deformation, and provides a new scientific basis for understanding the spatial characteristics and control factors of underground coal fires.

Description

Technical Field

[0001] The invention relates to the field of geotechnical engineering geological disaster prevention and control, and specifically to a three-dimensional morphological evolution simulation method of an underground coal fire combustion zone. Technical Background

[0002] As an irregular and complex natural phenomenon, the formation process and morphological characteristics of the combustion zone caused by underground coal fires remain difficult to study. The combustion zone formed by coal fires usually presents an extremely irregular shape, which poses a challenge to the establishment and prediction of theoretical models.

[0003] Although coal fire research based on remote sensing technology has achieved certain results in surface temperature and deformation monitoring, most of these studies are limited to the surface level. In fact, the temperature changes and deformation conditions of the surface can often reflect the characteristics of the underground space to a certain extent, such as the three-dimensional morphological characteristics of the underground combustion zone formed by coal fire combustion. However, the research on the synergistic relationship between the surface parameters of the fire area and the underground combustion zone is relatively lacking. Summary of the invention

[0004] In response to the problems existing in the prior art, the present invention provides a method for simulating the three-dimensional morphological evolution of the combustion zone of an underground coal fire. The three-dimensional morphology of the combustion zone of an underground coal fire is simulated by inversion and comparative verification, the spatial distribution, geometric structure characteristics and evolution laws of the combustion zone are described, and the three-dimensional morphological evolution of the combustion zone and its relationship with factors such as surface deformation are revealed, thereby providing a scientific basis and technical support for the effective extinguishing and control of coal fires.

[0005] To achieve the above object, the present invention discloses a method for simulating the three-dimensional morphological evolution of the combustion zone of an underground coal fire, the steps of which are as follows:

[0006] Based on the data of the areas with clear records of underground coal fire burnout, q pieces of Sentinel-1A satellite image data in a certain period of time in the underground coal fire burnout area were selected, and the DS-InSAR technology was used to extract the cumulative surface deformation value of the study area;

[0007] And obtain the stratum distribution, stratum parameters, surface temperature, three-dimensional temperature field, and basic information of the fire zone where the underground coal fire occurs from the underground coal fire air zone occurrence data;

[0008] The three-dimensional morphology of underground coal fire burnout zone is simulated by two methods: three-dimensional morphology inversion and comparative verification, and the spatial distribution, geometric structure characteristics and evolution law of underground coal fire burnout zone are described.

[0009] Among them, the three-dimensional morphological inversion method of underground coal fire combustion zone includes:

[0010] a1. Use ArcGIS software to convert the accumulated surface deformation values ​​into raster data, and extract the accumulated surface deformation contour lines of the underground coal fire burnout area based on the raster data;

[0011] a2. Use the stratum distribution, stratum parameters and cumulative surface deformation contour lines to establish an initial irregular surface 3D model in Rhino software, and use Griddle software to import the initial irregular surface 3D model into FLAC3D software. After importing, the geometric features and physical properties of the irregular surface 3D model are retained, so that FLAC3D can accurately simulate the mechanical response of the coal fire combustion process;

[0012] a3. Establish the formation mechanical parameters according to the rock mechanical parameters recorded in the data along with the temperature, and calculate the temperature of each rock layer in the underground coal fire combustion zone;

[0013] a4. Since the combustion temperature of coal fire in different areas is different, high temperature conditions are set on the constructed irregular surface model, and appropriate mechanical parameters are selected according to the changes of dynamic physical and mechanical properties of rock formations caused by high temperature: the weakening coefficient of each rock formation parameter is set, and the weakening coefficient gradually decreases with the increase of temperature. By gradually reducing the weakening coefficient, the mechanical parameters of each rock formation are gradually reduced, which reflects that the mechanical parameters of the rock formation are gradually reduced due to high temperature;

[0014] a5. Under the action of gravity and initial ground stress, the rock layer will cause the surface and the roof of the coal seam to settle, and the descending area can be marked as the plastic area, that is, the irregular three-dimensional shape of the combustion zone of the underground coal fire;

[0015] Among them, the comparative verification methods include:

[0016] b1. Establish a regular three-dimensional model for underground coal fire burning in the area where underground coal fire is not burning;

[0017] b2. Marking the plastic area information in the regular three-dimensional model before the underground coal fire occurs, removing the plastic area from the three-dimensional regular model before the underground coal fire occurs, simulating the collapse of the strata above the plastic area, and simulating the natural deformation of the corresponding surface after the plastic area is removed, simulating the formation of the underground coal fire burnout area after the coal seam is burned;

[0018] The irregular three-dimensional surface model of the underground coal fire burnout zone recorded in a5 is compared with the three-dimensional surface model with the plastic area removed recorded in b2 to verify the correctness of the three-dimensional morphological inversion results of the underground coal fire burnout zone;

[0019] Furthermore, DS-InSAR technology is used to extract the cumulative surface deformation values, and ArcGIS software is used to convert the vector data cumulative surface deformation values ​​into raster data. The cumulative surface deformation contour lines are extracted based on the raster data. The steps are as follows:

[0020] Generate small baseline interferometry atlas: select a super master image from q pieces of Sentinel-1A satellite image data, define the remaining Sentinel-1A satellite image data as auxiliary images, and align and resample all auxiliary images based on the super master image; under the premise of manually screening to ensure that there are no isolated points in the spatiotemporal baseline network, enter the vertical baseline and time baseline thresholds in the corresponding software to construct a small baseline interferometry atlas;

[0021] Use Digital Elevation Model (DEM) to simulate and remove terrain contribution;

[0022] SHP recognition: Determine the similarity between adjacent pixels and the central pixel in the InSAR image through statistical inference. The recognition accuracy of the algorithm directly affects the accuracy of subsequent parameter estimation;

[0023] Optimal phase estimation: The resolution unit of DS is composed of multiple independent scatterers, which will lead to temporal decoherence. Therefore, in order to improve the signal-to-noise ratio of the interferometric phase and increase the density of effective measurement, it is necessary to perform interferometric phase optimization after SHP identification;

[0024] DS point selection: The goodness of fit between the original interference phase and the optimized phase is used as the evaluation index of the optimized phase quality. TC Points with values ​​greater than 0.5 are selected as DS points.

[0025] The calculation formula for goodness of fit is:

[0026]

[0027] Among them, γ TC represents the goodness-of-fit index, also known as temporal coherence; is the original interferometric phase formed by the mth and nth SAR images, θ m and θ n is the optimal phase of reconstruction;

[0028] Surface deformation rate inversion: In the StaMPS software environment, the extracted DS points are 3D phase unwrapped, and the orbit residual error, atmospheric delay error and DEM error are estimated and corrected. Finally, the surface deformation rate information of the DS points is inverted using the least squares estimation.

[0029] Extract the cumulative surface deformation contour lines: Extract the cumulative surface deformation value of each DS point and import it into ArcGIS software. In ArcGIS, convert the vector data cumulative surface deformation value into raster data. Select the Extract Contour Lines command in ArcGIS software to extract the cumulative surface deformation contour lines from the raster data.

[0030] Further, the construction of irregular three-dimensional surface model:

[0031] Irregular modeling using Rhino software: Using the advanced modeling tools of Rhino software, a three-dimensional model of irregular surface is constructed based on the extracted cumulative surface deformation contours and known stratigraphic information;

[0032] Use Griddle to import the irregular model into FLAC3D software: Griddle is an efficient mesh generation and model conversion tool used to convert complex three-dimensional models into a mesh suitable for numerical analysis. Through Griddle software, the irregular surface model created in Rhino is converted into a format that can be processed by FLAC3D software.

[0033] Construction of regular 3D surface model:

[0034] Based on the stratigraphic distribution and stratigraphic parameter data, the surface model is established as a regular three-dimensional model, and the coal seam is a nearly horizontal coal seam.

[0035] Furthermore, the steps for selecting appropriate mechanical parameters according to the changes in the dynamic physical and mechanical properties of rocks caused by high temperature are as follows:

[0036] When the temperature continues to increase, the mechanical parameters of the rock formation will weaken, and the higher the temperature, the greater the weakening strength; the formula for the mechanical parameters of rock with temperature is:

[0037]

[0038] In the formula, are the parameter values ​​of different mechanical parameters at the corresponding rock formation temperature; is the initial mechanical parameter; k is the weakening coefficient, the smaller the k value, the more obvious the weakening; T c is the average temperature of different rock layers, T coal is the average temperature of the coal seam.

[0039] Furthermore, the three-dimensional morphology inversion method of underground coal fire burnout zone is used to invert the three-dimensional morphology of underground coal fire burnout zone. The specific steps are as follows:

[0040] Based on the above-mentioned irregular surface model, under high temperature conditions, according to the formula Gradually reduce the mechanical material parameters of each rock formation. When the mechanical material parameters of the rock formation are gradually reduced, plastic areas will appear in the rock formation under the action of gravity and initial ground stress. The plastic area is a direct mapping of the combustion zone. The mapping of coal seam combustion will cause settlement of the ground surface and the upper roof of the coal seam. By continuously reducing the mechanical material parameters of the rock formation, when the upper roof of the coal seam in the simulation result shows a continuous plastic area, these continuous plastic areas are judged as the combustion zone of the coal fire, and the three-dimensional coordinates are saved.

[0041] Furthermore, a comparative verification method is used to verify the correctness of the three-dimensional morphological inversion results of the underground coal fire combustion zone. The specific steps are as follows:

[0042] Establishing a regular three-dimensional model of the underground coal fire before it burns: Establishing a regular three-dimensional model of the ground surface before the coal fire burns, which represents the initial three-dimensional model before settlement;

[0043] Extract the location of the combustion void area: extract the location of the combustion void area obtained by the above-mentioned underground coal fire combustion void area three-dimensional morphological inversion method;

[0044] Excavation of the burnout zone: Import the initial 3D model and the coordinates of the plastic zone into the FLAC3D software; remove the plastic zone in the model to form a cavity, simulate the natural subsidence above the surface of the cavity, and thus simulate the formation of the underground coal fire burnout zone after the coal seam is burned;

[0045] Obtain the three-dimensional model after excavation: After removing the plastic area, the three-dimensional model after the coal fire combustion was obtained; the settlement model was compared with the initial irregular surface model, and the irregular surface three-dimensional model established based on Rhino software and the three-dimensional model after excavation obtained by the forward modeling method of the three-dimensional morphology of the underground coal fire combustion zone were selected. The profile lines of the two irregular models at the same position were compared and analyzed. If the settlement curves of the profile lines of the two irregular models are basically the same, it is believed that this method can accurately invert the three-dimensional morphology of the underground coal fire combustion zone.

[0046] Beneficial effects: The present invention provides a method for simulating the three-dimensional morphological evolution of the combustion zone of underground coal fires. The method is easy to implement and has significant effects, which are mainly reflected in the following three points:

[0047] 1) This method is the first to use numerical simulation technology to simulate the three-dimensional morphological evolution process of the combustion zone of underground coal fire, which is innovative.

[0048] 2) This method only uses numerical data such as formation parameters, surface temperature, three-dimensional temperature field and accumulated surface deformation, so the amount of data is small and the implementation is simple.

[0049] 3) This method simulates the three-dimensional morphology of the combustion void zone of underground coal fires through forward modeling and inversion methods, describes the spatial distribution, geometric structure characteristics and evolution laws of the combustion void zone, and reveals the three-dimensional morphological evolution of the combustion void zone and its relationship with factors such as surface deformation. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a schematic flow chart of the method for simulating the three-dimensional morphological evolution of the combustion void zone of an underground coal fire according to the present invention.

[0051] Figure 2 Schematic diagram of the irregular surface model of an embodiment of the present invention; (a) in the figure shows the extracted surface deformation contour lines, and the red five-pointed star is the geographical location of the Hongliang fire area; (b) in the figure shows the irregular grid of the Hongliang fire area established by the Rhino software; (c) in the figure shows the schematic diagram of the irregular surface model of the Hongliang fire area after grid conversion by the Griddle software and imported into the FLAC3D software.

[0052] Figure 3 Schematic diagram of a regular surface model in an embodiment of the present invention; (a) is a regular surface three-dimensional model; (b) is a coal seam cross-section.

[0053] Figure 4 Graph showing the temperature field distribution pattern of the XZ cross section of the combustion-air zone according to an embodiment of the present invention.

[0054] Figure 5 This is a diagram of the plastic region range under the initial rock formation parameters of an embodiment of the present invention.

[0055] Figure 6 This is a cross-sectional view of the plastic region under the initial rock formation parameters of an embodiment of the present invention.

[0056] Figure 7 This is a diagram of the plastic region when the weakening coefficient k=1 according to an embodiment of the present invention.

[0057] Figure 8 It is a cross-sectional view of the plastic region when the weakening coefficient k=1 of an embodiment of the present invention.

[0058] Fig. 9 This is a diagram of the plastic region when the weakening coefficient k=0.5 according to an embodiment of the present invention.

[0059] Fig.10 This is a cross-sectional view of the plastic region when the weakening coefficient k=0.5 of an embodiment of the present invention.

[0060] Fig.11 This is a plastic region range diagram when the weakening coefficient k=0.25 according to an embodiment of the present invention.

[0061] Fig.12 This is a cross-sectional view of the plastic region when the weakening coefficient k=0.25 of an embodiment of the present invention.

[0062] Fig.13 This is a plastic region range diagram when the weakening coefficient k=0.18 of an embodiment of the present invention.

[0063] Fig.14 This is a cross-sectional view of the plastic region when the weakening coefficient k=0.18 of an embodiment of the present invention.

[0064] Fig.15 1 is a forward modeling result diagram of the combustion-air zone of an embodiment of the present invention; (a) is a top view; (b) is a top perspective view; (c) is a front perspective view of the XZ plane; (d) is a cross-sectional view of the XZ plane.

[0065] Fig.16 Schematic diagram of the section line position of an embodiment of the present invention; (a) in the figure is an irregular surface model of the Hongliang fire area; (b) in the figure is an irregular surface model with a step length of 5400.

[0066] Fig.17 1 is a comparison of the section lines (A1-A2) at the same position of two irregular models according to an embodiment of the present invention.

[0067] Fig.18 1 is a comparison of the section lines ( B1 - B2 ) at the same position of two irregular models according to an embodiment of the present invention. DETAILED DESCRIPTION

[0068] The embodiments of the present invention will be further described below in conjunction with the accompanying drawings:

[0069] The present invention discloses a method for simulating the three-dimensional morphological evolution of an underground coal fire combustion zone, the steps of which are as follows:

[0070] 1. Overview of the study area and experimental data

[0071] In this experiment, the Hongliang fire area in Rujigou, Ningxia Hui Autonomous Region, China was selected as the study area; 35 C-band SAR images acquired by the Sentinel-1A satellite from October 2017 to January 2020 were selected, and the acquisition method was the interferometric wide-band mode.

[0072] The precise orbit data uses the precise orbit determination ephemeris data released by the European Space Agency, and the time is consistent with the Sentinel-1A image date.

[0073] The Digital Elevation Model (DEM) data uses SRTM data with a spatial resolution of 30 m measured jointly by NASA and NIMA (https: / / srtm.csi.cgiar.org / srtmdata / ) to remove the terrain phase in time series differential interferometry.

[0074] The topographic and geological data of the coal fire area adopts the "Rujijigou Mine Fire Area Current Status Evaluation and Analysis Report" provided by relevant departments, including the basic situation of the mining area, geological overview, coal seam overview, basic situation of the fire area and the current development status of the fire area.

[0075] The specific flow chart of this method is as follows Figure 1 shown.

[0076] Second, DS-InSAR technology is used to extract the cumulative surface deformation values. ArcGIS software is used to convert the vector data cumulative surface deformation values ​​into raster data. The cumulative surface deformation contour lines are extracted based on the raster data. The specific steps are as follows:

[0077] Small baseline interferometry atlas generation: Through the overall correlation coefficient analysis, the SAR data of 20181124 was selected as the super master image. All auxiliary images were registered and resampled according to the super master image. Under the premise of ensuring that there are no isolated points in the spatiotemporal baseline network, the time baseline threshold is set to ≤60 days and the space baseline threshold is set to ≤300m to construct a small baseline interferometry atlas. DEM is used to simulate and remove the terrain contribution.

[0078] SHP recognition: In order to take into account both the computational efficiency and accuracy of homogeneous point recognition, the HTCI algorithm in the DSIpro software package publicly released by Jiang Mi was used to generate a homogeneous pixel number matrix under the conditions of significance level α = 0.05 and window size [15,15];

[0079] Optimal phase estimation: spatial adaptive filtering and EMI algorithm are used to optimize the phase quality.

[0080] DS point selection: The goodness of fit between the original interference phase and the optimized phase is used as the evaluation index of the optimized phase quality. TC Points with values ​​greater than 0.5 are selected as DS points. The goodness of fit calculation formula is:

[0081]

[0082] Among them, γ TC represents the goodness-of-fit index, also known as temporal coherence; is the original interferometric phase formed by the mth and nth SAR images, θ m and θ n is the optimal phase of reconstruction.

[0083] Surface deformation rate inversion: In the StaMPS software environment, the extracted DS points are 3D phase unwrapped, and the orbit residual error, atmospheric delay error and DEM error are estimated and corrected. Finally, the surface deformation rate information of the DS points is inverted using the least squares estimation.

[0084] Extract the cumulative surface deformation contour lines: Extract the cumulative surface deformation value of each DS point and import it into ArcGIS software. In ArcGIS, convert the vector data cumulative surface deformation value into raster data. Select the Extract Contour Lines command in ArcGIS software to extract the cumulative surface deformation contour lines from the raster data.

[0085] 3. Based on the stratigraphic distribution, stratigraphic parameters and the cumulative surface deformation contour results in the "Rujigou Mine Fire Area Status Evaluation and Analysis Report", establish the irregular surface three-dimensional model and the regular surface model. The specific steps are as follows:

[0086] Construction of surface irregularity model:

[0087] Extract deformation contour lines based on the accumulated surface deformation results obtained by DS-InSAR technology: Extracting contour lines from the accumulated surface deformation data is the first step in building an irregular model. First, import the accumulated surface deformation points into ArcGIS software, convert the point data into raster data, and use the Create Contour Line tool in ArcGIS software to extract the surface deformation contour lines ( Figure 2 (a)). Surface deformation contours show the contours of terrain changes, providing a basis for subsequent modeling and analysis.

[0088] Irregular modeling using Rhino software: Rhino is a powerful 3D modeling software that can be used to create irregular models of the surface. In this step, the Rhino software's advanced modeling tools are used to construct an irregular 3D model of the surface based on the extracted terrain contours and known stratigraphic information ( Figure 2 (b)).

[0089] Use Griddle to import the irregular model into FLAC3D software: Griddle is an efficient mesh generation and model conversion tool used to convert complex 3D models into a mesh suitable for numerical analysis. Through Griddle, the irregular surface model created in Rhino is converted into a format that can be processed by FLAC3D software ( Figure 2 This conversion process ensures that the geometric characteristics and physical properties of the model are preserved, allowing FLAC3D to accurately simulate the mechanical response of coal fire combustion.

[0090] Construction of surface regular model:

[0091] Due to the complex topography of the Hongliang Fire area in the Rujigou mining area in Ningxia, and the lack of relevant 3D terrain scanning information, it is impossible to accurately establish a 3D model. Therefore, in order to facilitate calculation, the initial model before the coal fire is simplified. The simplified 3D surface model and its internal structure are shown in the figure. Figure 3The initial model is based on the "Analysis and Evaluation Report on the Current Situation of Fire Area in Rujigou Mine Area". The coal seam is nearly horizontal. The length (X-axis direction), width (Y-axis direction), and height (Z-axis direction) of the model are 200m, 240m, and 54m respectively, and the plane size of the grid is 4m×4m.

[0092] 4. Selection of formation mechanical parameters in the combustion and void area. The specific steps are as follows:

[0093] Select appropriate bottom mechanical parameters according to the formula of rock mechanical parameters and temperature:

[0094]

[0095] In the formula, are the parameter values ​​of different mechanical parameters at the corresponding rock formation temperature; is the initial mechanical parameter; k is the weakening coefficient, the smaller the k value, the more obvious the weakening; T c is the average temperature of different rock layers, T coal is the average temperature of the coal seam.

[0096] Calculate the temperature of each rock layer. The specific steps are as follows:

[0097] The FLAC3D software was used to simulate the Hongliang fire zone rule model ( Figure 3 ) The temperature field distribution of each rock layer in 2 years, such as Figure 4 As shown, the temperature of each rock layer is: coarse-grained sandstone 149.33℃, medium-grained sandstone 263.91℃, fine-grained sandstone 330.67℃, siltstone 465.83℃, mudstone 743.58℃, coal seam 1000.00℃. Substituting the above formula into the mechanical parameters of each rock layer can be obtained.

[0098] 5. The three-dimensional morphology inversion method of underground coal fire combustion zone is used to invert the three-dimensional morphology of underground coal fire combustion zone. The specific steps are as follows:

[0099] In the above-mentioned surface irregularity model ( Figure 2 ) based on the formula under high temperature conditions Gradually reduce the mechanical material parameters of each rock layer. When the material parameters are gradually reduced, plastic areas will appear in the rock layer under the action of gravity and initial ground stress. The plastic area is a direct mapping of the burnout zone. Coal seam combustion will cause settlement of the surface and the roof above the coal seam. Therefore, plastic areas are used to represent these settlement areas in the established surface irregular model. By continuously reducing the material parameters, when the upper roof of the coal seam in the simulation results shows continuous plastic areas, these continuous plastic areas are judged as the burnout zone of the coal fire. Finally, the three-dimensional coordinates of the burnout zone are saved. This study simulated the initial parameters, weakening coefficient k=1, weakening coefficient k=0.5, weakening coefficient k=0.25, and weakening coefficient k=0.18. The results are as follows: Figure 5-Figure 14 From the results, it can be seen that when the weakening coefficient k = 0.18, the upper roof of the coal seam shows a continuous plastic zone ( Fig.13 , Fig.14 ), and it is believed that the plastic area is the three-dimensional morphology result of the underground combustion space in the Hongliang fire area.

[0100] 6. Use comparative verification method to verify the correctness of the three-dimensional morphological inversion results of underground coal fire combustion zone. The specific steps are as follows:

[0101] Establishing a three-dimensional regular model of underground coal fire before combustion: the settlement surface model of the coal fire in Hongliang fire area before combustion Figure 3 Model shown. This model represents the initial 3D model before settlement.

[0102] Extract the location of the combustion void area: Extract the location of the combustion void area obtained by the above-mentioned underground coal fire combustion void area three-dimensional morphological inversion method ( Fig.13 , Fig.14 ).

[0103] Excavation of plastic zone: Import the initial 3D model and plastic zone coordinates into FLAC3D software. Remove the plastic zone from the model and simulate the formation of underground coal fire burnout zone after coal seam combustion ( Fig.15 ).

[0104] Through the above steps, the three-dimensional model after excavation of the plastic area is obtained according to the cumulative maximum settlement value, as shown in the following figure: Fig.15 The results shown, Fig.15 (a) is a three-dimensional top view of the combustion and void area. Fig.15 (b) is a three-dimensional top view of the combustion and empty area. Fig.15 (c) is a three-dimensional perspective view of the combustion-cavity region in the XZ plane. Fig.15 (d) is a three-dimensional cross-sectional view of the combustion zone in the XZ plane. Different colors in the figure represent the depth of surface subsidence. From the high-subsidence red area in the center to the low-subsidence blue area in the periphery, the change in subsidence gradient intuitively shows the size and intensity of the impact range of the coal seam fire zone.

[0105] Select the irregular 3D model of the surface ( Figure 2 ) and the three-dimensional model after excavation obtained by forward modeling ( Fig.15 (a)), select two profile lines at the same position to verify the consistency of settlement. The specific positions of the profile lines are as follows: Fig.16 As shown in (a) and (b) of Figure 2, the overall comparative analysis shows that the vertical and horizontal profile settlement curves of the two irregular models are roughly consistent, verifying the correctness of the inversion and forward modeling methods of the three-dimensional characteristics of the underground gas void area. From the perspective of quantitative analysis, the average settlement difference values ​​of the same positions of A1-A2 and B1-B2 are 5.86 mm and 4.59 mm, respectively. Fig.17 , Fig.18 ), the standard deviations of the differences were 10.38 and 8.99 ( Fig.17 , Fig.18 ), indicating that the method proposed in this study can invert the three-dimensional morphological characteristics of the burnt-out zone and reveal the synergistic relationship between the surface deformation parameters of the fire area and the burnt-out zone.

Claims

1. A method for simulating the three-dimensional morphological evolution of underground coal fire combustion zone, characterized in that: Here are the steps: According to the data of underground coal fire burnout area with clear records, q pieces of Sentinel-1A satellite image data in a certain period of time in the underground coal fire burnout area were selected, and the DS-InSAR technology was used to extract the cumulative surface deformation value of the study area; And obtain the stratum distribution, stratum parameters, surface temperature, three-dimensional temperature field, and basic information of the fire zone where the underground coal fire occurs from the underground coal fire air zone occurrence data; The three-dimensional morphology of underground coal fire burnout zone is simulated by two methods: three-dimensional morphology inversion and comparative verification, and the spatial distribution, geometric structure characteristics and evolution law of underground coal fire burnout zone are described. Among them, the three-dimensional morphology inversion method of underground coal fire combustion zone includes: a1. Use ArcGIS software to convert the accumulated surface deformation values ​​into raster data, and extract the accumulated surface deformation contour lines of the underground coal fire burnout area based on the raster data; a2. Use the stratum distribution, stratum parameters and cumulative surface deformation contour lines to establish an initial irregular surface 3D model in Rhino software, and use Griddle software to import the initial irregular surface 3D model into FLAC3D software. After importing, the geometric features and physical properties of the irregular surface 3D model are retained, so that FLAC3D can accurately simulate the mechanical response of the coal fire combustion process; a3. Establish the formation mechanical parameters according to the rock mechanical parameters recorded in the data along with the temperature, and calculate the temperature of each rock layer in the underground coal fire combustion zone; a4. Since the combustion temperature of coal fire in different areas is different, high temperature conditions are set on the constructed irregular surface model, and appropriate mechanical parameters are selected according to the changes of dynamic physical and mechanical properties of rock formations caused by high temperature: the weakening coefficient of each rock formation parameter is set, and the weakening coefficient gradually decreases with the increase of temperature. By gradually reducing the weakening coefficient, the mechanical parameters of each rock formation are gradually reduced, which reflects that the mechanical parameters of the rock formation are gradually reduced due to high temperature; a5. Under the action of gravity and initial ground stress, the rock layer will cause the surface and the roof of the coal seam to settle, and the descending area can be marked as the plastic area, that is, the irregular three-dimensional shape of the combustion zone of the underground coal fire; Among them, the comparative verification methods include: b1. Establish a regular three-dimensional model for underground coal fire burning in the area where underground coal fire is not burning; b2. Marking the plastic area information in the regular three-dimensional model before the underground coal fire occurs, removing the plastic area from the three-dimensional regular model before the underground coal fire occurs, simulating the collapse of the strata above the plastic area, and simulating the natural deformation of the corresponding surface after the plastic area is removed, simulating the formation of the underground coal fire burnout area after the coal seam is burned; The irregular three-dimensional surface model of the underground coal fire burnout zone recorded in a5 was compared with the three-dimensional surface model with the plastic area removed recorded in b2 to verify the correctness of the three-dimensional morphological inversion results of the underground coal fire burnout zone.

2. The method for simulating the three-dimensional morphological evolution of the underground coal fire combustion zone according to claim 1 is characterized in that: DS-InSAR technology is used to extract the cumulative surface deformation values. ArcGIS software is used to convert the vector data cumulative surface deformation values ​​into raster data. The cumulative surface deformation contour lines are extracted based on the raster data. The steps are as follows: Generate a small baseline interferogram: select a super master image from q Sentinel-1A satellite image data, define the remaining Sentinel-1A satellite image data as auxiliary images, and align and resample all auxiliary images based on the super master image; On the premise of ensuring that there are no isolated points in the spatiotemporal baseline network by manual screening, the vertical baseline and time baseline thresholds are input into the corresponding software to construct a small baseline interference atlas; Use Digital Elevation Model (DEM) to simulate and remove terrain contribution; SHP recognition: Determine the similarity between adjacent pixels and the central pixel in the InSAR image through statistical inference. The recognition accuracy of the algorithm directly affects the accuracy of subsequent parameter estimation; Optimal phase estimation: The resolution unit of DS is composed of multiple independent scatterers, which will lead to temporal decoherence. Therefore, in order to improve the signal-to-noise ratio of the interferometric phase and increase the density of effective measurement, it is necessary to perform interferometric phase optimization after SHP identification. DS point selection: The goodness of fit between the original interference phase and the optimized phase is used as the evaluation index of the optimized phase quality; TC Points with values ​​greater than 0.5 are selected as DS points. The calculation formula for goodness of fit is: Among them, γ TC represents the goodness-of-fit index, also known as temporal coherence; is the original interferometric phase formed by the mth and nth SAR images, θ m and θ n is the optimal phase of reconstruction; Surface deformation rate inversion: In the StaMPS software environment, the extracted DS points are 3D phase unwrapped, and the orbit residual error, atmospheric delay error and DEM error are estimated and corrected. Finally, the surface deformation rate information of the DS points is inverted using the least squares estimation. Extract the cumulative surface deformation contour lines: extract the cumulative surface deformation value of each DS point and import it into ArcGIS software, convert the vector data cumulative surface deformation value into raster data in ArcGIS, and extract the cumulative surface deformation contour lines from the raster data by selecting the Extract Contour Lines command in ArcGIS software.

3. The method for simulating the three-dimensional morphological evolution of the combustion zone of underground coal fire according to claim 1, characterized in that: Construction of irregular 3D model of the surface: Irregular modeling using Rhino software: Using the advanced modeling tools of Rhino software, a three-dimensional model of irregular surface is constructed based on the extracted cumulative surface deformation contours and known stratigraphic information; Use Griddle to import the irregular model into FLAC3D software: Griddle is an efficient mesh generation and model conversion tool used to convert complex three-dimensional models into a mesh suitable for numerical analysis. Through Griddle software, the irregular surface model created in Rhino is converted into a format that can be processed by FLAC3D software. Construction of regular 3D surface model: Based on the stratigraphic distribution and stratigraphic parameter data, the surface model is established as a regular three-dimensional model, and the coal seam is a nearly horizontal coal seam.

4. The method for simulating the three-dimensional morphological evolution of the combustion zone of underground coal fire according to claim 1, characterized in that: The steps to select appropriate mechanical parameters according to the changes in the dynamic physical and mechanical properties of rocks caused by high temperature are as follows: When the temperature continues to increase, the mechanical parameters of the rock formation will weaken, and the higher the temperature, the greater the weakening strength; the formula for the mechanical parameters of rock with temperature is: In the formula, are the parameter values ​​of different mechanical parameters at the corresponding rock formation temperature; is the initial mechanical parameter; k is the weakening coefficient, the smaller the k value, the more obvious the weakening; T c is the average temperature of different rock layers, T coal is the average temperature of the coal seam.

5. The method for simulating the three-dimensional morphological evolution of the combustion zone of underground coal fire according to claim 4, characterized in that: The three-dimensional morphology inversion method of underground coal fire combustion zone is used to obtain the three-dimensional morphology of underground coal fire combustion zone. The specific steps are as follows: Based on the above-mentioned irregular surface model, under high temperature conditions, according to the formula Gradually reduce the mechanical material parameters of each rock formation. When the mechanical material parameters of the rock formation are gradually reduced, plastic areas will appear in the rock formation under the action of gravity and initial ground stress. The plastic area is a direct mapping of the combustion zone. The mapping of coal seam combustion will cause settlement of the ground surface and the upper roof of the coal seam. By continuously reducing the mechanical material parameters of the rock formation, when the upper roof of the coal seam in the simulation result shows a continuous plastic area, these continuous plastic areas are judged as the combustion zone of the coal fire, and the three-dimensional coordinates are saved.

6. The method for simulating the three-dimensional morphological evolution of the combustion zone of underground coal fire according to claim 5, characterized in that: The comparative verification method is used to verify the correctness of the three-dimensional morphological inversion results of the underground coal fire combustion zone. The specific steps are as follows: Establishing a regular three-dimensional model of the underground coal fire before it burns: Establishing a regular three-dimensional model of the ground surface before the coal fire burns, which represents the initial three-dimensional model before settlement; Extract the location of the combustion void area: extract the location of the combustion void area obtained by the above-mentioned underground coal fire combustion void area three-dimensional morphological inversion method; Excavation of the burnout zone: Import the initial 3D model and the coordinates of the plastic zone into the FLAC3D software; remove the plastic zone in the model to form a cavity, simulate the natural subsidence above the surface of the cavity, and thus simulate the formation of the underground coal fire burnout zone after the coal seam is burned; Obtain the three-dimensional model after excavation: After removing the plastic area, the three-dimensional model after the coal fire combustion was obtained; the three-dimensional model after the coal fire combustion was compared with the initial irregular surface model, and the irregular surface three-dimensional model established based on Rhino software and the three-dimensional model after excavation obtained by the forward modeling method of the three-dimensional morphology of the underground coal fire combustion zone were selected. The profile lines of the two irregular models at the same position were compared and analyzed. If the settlement curves of the profile lines of the two irregular models are basically the same, it is believed that this method can accurately invert the three-dimensional morphology of the underground coal fire combustion zone.

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