1. Introduction
Caojiatan Mine Field is located in the northeast of Ordos Plateau, the northern end of Loess Plateau in northern Shaanxi and the southeast edge of Maowusu Desert. Most of the areas are typical aeolian sand dunes and sandy beaches, with good vegetation development and large topographic relief. The thickness of coal-bearing strata in the area is relatively stable, the strata occurrence is gentle (only about 1°), and the structure is simple. The burial depth of main minable coal seams is generally 200-500 m. Technical difficulties in the exploration are as follows:
(1) Because the surface overburden in the exploration area is dry and loose, the absorption and attenuation of effective signals, especially high-frequency information, is strong, resulting in the attenuation of seismic wave energy.
(2) The task requires high accuracy, and it is required to identify faults with a drop of more than 5 m and interpret goaf and heading of old kiln.
2. Mine Water Hazard Treatment Solution
Adopt all-digital high-density three-dimensional seismic exploration technology.
(1) Instruments and equipment: Ue 428XL digital seismograph. DSU1 digital geophone has the characteristics of wide frequency band, large dynamic range and basically straight phase characteristics, and can receive more reflected wave information from low frequency to high frequency, providing massive data for interpretation of small structures and lithology analysis.
(2) Determination basis of observation system:
① Adopt small track distance (10m) and small binning (5×5m) to improve vertical and horizontal resolution and ensure the accuracy of implementing small faults.
② Apply wide azimuth angle and high coverage times (64 times) to ensure the accuracy of longitudinal velocity analysis and anisotropy research.
③ Roll one geophone line at a time, as this can improve the accuracy of static correction coupling, improve the consistency of offset and azimuth distribution, and reduce the collection footprints.
(3) Key treatment techniques
Based on careful analysis of the characteristics of the original data, highlight high fidelity and amplitude-preserving processing, maintain the dynamic characteristics of seismic signals, widen the effective broadband of seismic signals as much as possible, and ensure the resolution of small structures.
Azimuth processing technology can properly solve the problems of azimuth anisotropy and different seismic wave propagation velocity. Select reasonable azimuthal gathers (select survey lines with equal maximum longitudinal and transverse offsets), and then carry out azimuthal velocity analysis and anisotropic migration. Azimuth processing combined with high-density data can judge strata and faults more accurately and improve the prediction accuracy of goaf and heading.
(4) Comprehensive interpretation method
Diffraction wave appears on the seismic superposition time profile of the heading, but converges to a point on the corresponding migration time profile. There is a small range of discontinuity or concave deformation in the same direction axis of coal seam reflection wave (Figure 1).
Figure 1 Reflection of Heading on Time Profile
Seismic attribute interpretation can complete the changes of seismic waves that cannot be distinguished and recognized by naked eyes, and highlight the changes of underground structures, fissures and lithology. A variety of attributes sensitive to geological anomalies, such as fractures and fractures, are mathematically calculated, and the comprehensive attributes thus obtained have more obvious structural recognition effects. The single attribute and comprehensive attribute of the heading are obviously reflected, showing linear anomaly (Figure 2 and Figure 3), and two headings with a distance of 25m in some sections can also be clearly reflected, which is consistent with the actual heading position.
(a) Drop attribute (b) Dip attribute
Figure 2 Reflection of Heading on Single Attribute Diagram of Coal Seam Reflection Wave
(a) Dip-drop-reflection coefficient attribute fusion; (b) Instantaneous amplitude-instantaneous frequency-instantaneous phase attribute fusion
Figure 3 Reflection of Heading on Comprehensive Attribute Plan of Reflection Multi-attribute Fusion of Coal Seam
3. Construction Situation
The construction adopts 16L×4S×72T×1R×64-fold observation system, with a line distance of 40m, a track distance of 10m, a shot row distance of 45m, and a horizontal-vertical ratio of 0.875. A total of 70 swaths and 85 survey lines are laid, with a construction area of 22.19km2, a multifold coverage area of 20.16km2, a full coverage area of 15.38km2, and 42,405 production physical points (41,440 design production physical points), 320 test physical points (31o design test physical points). In the production records, the Grade A rate is 79.89%, the Grade B rate is 20.10%, and the rejection rate is 0.01%. All test records are qualified.
4. Achieved Accomplishment
The results of this 3D seismic exploration are mainly as follows:
(1) A total of 25 faults have been interpreted, including 2 faults with a drop ≥5m, 23 faults with a drop < 5m and zero fault with a drop ≥10m.
(2) The burial depth and undulation form of main minable seam are controlled, and the folds with amplitude greater than 20m and axial length greater than 500m are controlled.
(3) The developed heading is interpreted, and the coincidence degree is high.
5. FAQ
Q: What are the interpretation methods for small faults and geological anomalies?
A: In addition to the conventional manual interpretation of time profile breakpoints, the horizontal slice of variance body, bedding slice, ant body crack tracking and multi-attribute fusion technology can be used to comprehensively predict the plane distribution of faults, collapse columns, mined-out areas, old kiln headings and geological abnormal bodies, which are further confirmed by the results of profile interpretation.
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