1. Introduction
The exploration area is located in the south-central part of Longgu Mine. It is the alluvial-proluvial plain of the Yellow River, with flat terrain, densely-distributed villages and rivers and a network of artificial ditches. The thickness of coal-bearing strata is relatively stable, with stable interval between main marker strata, clear lithologic and lithofacies combination characteristics, obvious physical property difference, simple structure, gentle dip angle and deep burial depth of main coal seam (700-1,100m). Technical difficulties in the exploration are as follows:
(1) The Cenozoic is relatively thick (610-720m). Its absorption and attenuation effect on high frequency components of seismic waves is strong, which may reduce the resolution and interpretation accuracy of seismic results.
(2) The surface is complex, the burial depth of the main coal seam is large, and the geological task requires high precision. It is required to interpret faults with drop over 5m, collapse columns with plane diameter over 20m and other geological anomalies.
2. Treatment Solution
(1) For the thick Cenozoic, the treatment solution is as follows:
① Data collection. On the premise of ensuring sufficient energy, small amount of charge is used for excitation to ensure that the obtained wavelet has high frequency and wide frequency band. The recording of the instrument adopts broadband reception, and the high frequency components in the seismic signal are retained to the maximum extent.
② Data processing. In the data processing, the module and parameter tests of static correction, deconvolution and velocity analysis are strengthened to improve the resolution, broaden the frequency band and fully suppress multiple waves.
(2) For complex surface, large buried depth of coal seam and high precision of geological tasks, the solution is as follows:
① Data collection. Optimize the observation system and increase the coverage times to 30 times; Design special observation system for large-area obstacles, monitor the quality in real time during construction, and realize efficient construction at obstacles and good field earthquake data acquisition.
② Data processing. Carefully define the observation system, get the static correction and deconvolution done properly, and ensure the bandwidth and high frequency of reflected waves in the target layer. Use EQDMO technology in the superposition process. Use the denoising and energy balancing module with caution to ensure the authenticity of the data.
③ Data interpretation. Combine superimposed data volume and migration data volume, follow the methods of man-machine combination, time profile and horizontal slice, variance volume and seismic attribute combination. The characteristics of collapse column in time profile are: The same axis of standard reflection wave of coal seam is suddenly interrupted and disappeared, weakened, distorted, deformed, bifurcated and merged, phase transformed with abnormal waves (diffracted waves and side waves, etc.). The collapse column shows regular annular anomalies on horizontal slices and bedding slices. Seismic attribute is another main method to identify collapsed columns. Single attribute can explain the existence of multiple solutions of structural anomalies, and comprehensive attribute has more obvious structural identification effect. The characteristics of collapse column on seismic attribute map are mainly annular geological anomalies (Figure 1).
Figure 1 Reflection of Collapse Column on Amplitude Attribute (Left) and Coherent Body Attribute (Right)
3. Construction Situation
In this construction, a 10L×10S×96T×5R×30-time observation system (Figure 2) is adopted, with a total of 26 swaths and 135 survey lines. There are 10 survey lines in each bundle, with a line distance of 40m and a receiver spacing of 20m. CDP bin is 10m×10m, and the number of full coverage is 30 times. The construction area is 23.85km2, the full coverage area is 12km2, and there are 5,309 physical points, including 5,266 production physical points and 43 test physical points.
Figure 2 Schematic Diagram of Observation System of 10 Shots on 10 Lines
4. Achieved Accomplishment
(1) The occurrence patterns, hidden outcrops and boundaries of coal seams 3 (3upper) and 3lower in the exploration area, and the plane position of the bifurcation and merging lines of coal seam 3 were identified.
(2) The faults in the main coal seam and Cenozoic are explained in detail.
(3) The fluctuation pattern of Neogene bottom boundary and the thickness change of Cenozoic strata in the exploration area are identified.
(4) The change trend of coal seam thickness in exploration area 3 (3upper) and 3lower is predicted.
(5) The fluctuation of the floor of coal seam 16upper are controlled and explained.
(6) Other geological anomalies in the area are explained. A suspected collapse column (Figure 3 and Figure 4) was found in the exploration area, and its plane shape was oval with long axis near SN direction, and the plane area of the coal seam 3upper was 6,140m2. The suspected collapse column was drilled and successfully verified by the Employer.
Figure 3 Reflection of Suspected Collapse Column on Time Profile (Left) and Variance Body Slice Attribute (Right)
Figure 4 Reflection of Suspected Collapse Column on Coherence-instantaneous Amplitude-Drop Value Attribute Fusion
5. FAQ
Q: What are the basis for identifying and judging collapse columns in seismic data interpretation?
A: As on time profile, the same axis of standard reflection wave of coal seam is suddenly interrupted and disappeared, weakened, distorted, deformed, bifurcated and merged, phase transformed with abnormal waves (diffracted waves and side waves, etc.). As on the horizontal slices, bedding slices and attribute maps, there will be regular annular anomalies.
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