1. Introduction
The upper stratum of the exploration area suffered denudation and was cut and damaged by dendritic valleys, with vertical and horizontal gullies and complex terrain (Figure 1). The boundary of this exploration is close to the 3-1 coal goaf, and the surface subsidence cracks near the goaf are obvious, and the surface and shallow seismic geological conditions are complex; The dip angle of strata is small, about 1°, and there are many layers of coal seams with small spacing. The upper coal seam has a strong energy shielding effect on the lower coal seam, and the buried depth of coal seam is shallow (the buried depth of 3-1 coal seam is 83.18-191.45 m), so it is difficult to deploy the observation system. The coal-bearing strata are interbedded by sandstone, mudstone and coal seam. Because of the small difference of wave impedance between sand and mudstone, the reflection intensity of sandy mudstone is weak. There are many obstacles in the area (immigrant villages and burial sites), resulting in uneven coverage of data in some areas, which has a certain impact on the quality of data.
Figure 1 Topographic Map of Exploration Area
2. Mine Water Hazard Treatment Solution
(1) The buried depth of the first coal seam is very shallow locally, and the observation system design follows the principles of small line spacing, small track spacing and high coverage times, and intensified local shot points, ensuring effective coverage times.
(2) Ensure that the excitation point does not enter the goaf, and the receiving point shall avoid the goaf whenever possible to reduce the influence of the goaf on seismic data. This is achieved by simulation calculation.
(3) For large obstacles on the surface, a special observation system is designed to simulate the coverage times determination scheme to ensure the effective reflection information of the stratum under the obstacles.
(4) The difficulty in processing in this area is mainly the static correction caused by complex terrain. This is solved by firstly completing the field static correction using the refraction method, secondly, by the adoption of frequency-division surface consistency residual static correction technology, and thirdly by the use high-frequency reflection data in gradually improving the accuracy of residual static correction. Due to the shallow buried depth of the target layer, the cuttings shall be conducted in less quantity but in more times. Repeated experiments are required to retain the target layer as much as possible and increase the frequency of the shallow target layer as much as possible. At the same time, NMO is used to cut off and keep the roads with flattened near paths and higher frequency as much as possible, thus laying a good foundation for the later speed scanning.
(5) Conduct the structuring and goaf interpretation, following the idea and process of combining workstation with manual interpretation, time profile, horizontal slice and bedding slice interpretation (Figures 2 to 4).
(6) For the problem of coal seam roof sand body prediction, firstly, ensure the low frequency information in seismic data acquisition; in the treatment process, conduct amplitude preservation treatment to ensure the relative amplitude relationship of sandstone and mudstone strata; establish the petrophysical model of sandstone and mudstone formation, and predict the thickness of sand body in 3-1 coal roof using post-stack attribute inversion method (Figure 5).
Figure 2 Reflection of Faults on Seismic Time Profile and Attributes Slice
Figure 3 Reflection of 3-1 Coal Seam Heading and Goaf on Time Profile
Figure 4 Reflection of 3-1 Coal Seam Heading on Attributes Profile
Figure 5 Wave Impedance Inversion Profile
3. Construction Situation
A total of 8 3D seismic swaths, 80 receiving survey lines, 4,437 production physical points and 100 test physical points have been completed in the whole region, with a control area of 5.547 km2 and a construction area of 9.157 km2. According to the rating of Code for Coalfield Seismic Exploration (DZ/T0300-2017), there are 2,534 records of Grade A products, representing a Grade A rate of 57.11%; 1829 records of Grade B products, representing a Grade B rate of 41.22%; 74 waste records, representing a rejection rate of 1.67%. All test records are qualified.
4. Achieved Accomplishments
The floor fluctuation pattern of the main coal seam in the exploration area is identified (Figure 6). The faults with coal seam drop greater than 3 m in the main coal seam are identified. The change trend of thickness of main coal seams is predicted (Figure 7). The lithology distribution of roof and the variation trend of sand body thickness in 3-1 coal seam are predicted (Figure 8).
Figure 6 3-1 Coal Seam Floor Fluctuation Pattern
Figure 7 3-1 Coal Seam Thickness Prediction Plan
Figure 8 Plane Distribution of Sandstone Thickness Trend in 3-1 Coal Seam Roof (red and yellow are sand bodies)
5. FAQ
Q1: How to arrange the observation system considering the shallow buried depth of the first coal seam in this area?
A: Due to terrain, the buried depth of the first coal seam is very shallow locally, and the observation system design follows the principles of small line spacing, small track spacing and high coverage times, and intensified local shot points, ensuring effective coverage times.
Q2: How to improve the accuracy of static correction considering the terrain in this area is complex, and the static correction problem is prominent?
A: First, accurately pick up the first break wave and use refraction static correction method to complete the field static correction; Secondly, use the frequency-division surface consistency residual static correction technology shall to calculate the residual static correction of the data in this area. That is to say, use the middle and low frequency reflection waves to calculate the larger residual static correction, and then use the high frequency reflection data to gradually improve the accuracy of residual static correction.
Q3: How to ensure the coverage times of the target layer for large obstacles on the surface considering the shallow buried depth of the target layer in this area?
A: In the barrier areas such as villages, contiguous graves and farms within the exploration area, the seismic lines and shot points cannot be arranged normally. In order to ensure the basic uniform coverage times, special observation systems are needed to collect data in these areas. For large-area villages, contiguous graves and other obstacles, try to shoot with small amount of explosives in the open space in the obstacle area, minimize the empty road where the geophone can be laid, measure the physical geophone position, appropriately increase the receiving arrays, ensure the distant channel information, and reduce surface wave interference in loess area.
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