1. Introduction
According to the previous data, the dip of strata in the exploration area varies from 10° to 30°, and faults are developed. According to the exposure data, the accuracy of past 3D seismic exploration is less than 70% for faults with a drop of more than 10m, less than 50% for faults with a drop of 5-10m and less than 10% for faults with a drop of 3-5m, which seriously affects the layout of the working face and the stope. Therefore, the technical difficulty of exploration is how to effectively improve the recognition ability of small faults.
2. Equipment and methods
(1) Observation system
According to the characteristics of large dip of coal seam in the exploration area and wide variation range of buried depth of the target layers, the main mined coal seam 32 was divided into three zones with respective buried depth of less than 350 m, 350-700m and 700m and above, and three high-density 3D seismic observation systems of 16L×8S×80T×1R×64, 16L×8S×128T×1R×64 and 16L×8S×160T×1R×64 were designed to account for the effective covering degree of target layers with different buried depths.
(2) Acquisition equipment
All-digital geophones were used for reception to improve the effective resolution of reflected waves. The digital geophones are characterized by the high sensitivity, the wide dynamic range, the wide receiving frequency band, no phase distortion and the good amplitude fidelity, which can effectively improve the resolution of record obtained from a single shot.
(3) Key processing technologies
The attenuation of seismic wave was compensated by spherical diffusion compensation and surface induced amplitude compensation, so that the energy of seismic wave truly reflects the real situation of underground medium; three velocity analyses and iterations of residual static correction were carried out in data processing to ensure the accuracy of velocity analysis; the method of pre-stack time migration was adopted to ensure the resolution and imaging accuracy of data.
(4) Interpretation method
The functions of color display, magnification and arbitrary line display of workstation interpretation software were used for the tectonic interpretation of the fine time profiles, and multi-attribute fusion analysis technology was used for comprehensive tectonic interpretation (Figure 1 and Figure 2) to improve the accuracy of the tectonic interpretation; the roadway exposure data, drilling data, stacking velocity and the like were used to construct a fine velocity field so as to ensure the elevation accuracy of coal seam floor.
Figure 1: Gaussian curvature attribute of coal seam 32
3. Work situation
A total of 24 wiring harnesses, 31 survey lines, 9,936 physical production points and 71 physical test points were completed in the whole area, with an actual control area of 4.46km2. According to the Seismic Exploration General Standard in Coal and Coalbed Methane, the test records were all qualified; there were 7,746 records of grade A, with the grade A rate of 77.96% and 2,132 records of grade B, with the grade B rate of 21.46%, thus a qualified rate of 99.42%.
4. Achieved accomplishment
(1) Comparison before and after exploration
Comparing the all-digital high-density 3D seismic exploration with the conventional 3D seismic exploration, for faults with a drop of over 10m, 24 faults were discovered, 16 faults were corrected and 20 faults were basically consistent; for faults with a drop of 5-10m, 27 faults were discovered, 5 faults were corrected and 6 faults were basically consistent; for faults with a drop of 3-5m, 52 faults were discovered, 1 fault was corrected and 12 faults were basically consistent. It can be seen that the recognition ability of high-density 3D seismic exploration for the small faults with a drop of 3-5m has been greatly improved.
(2) Verification
The working face 3404 in No. 34 mining district has been extracted, with a mining area of 0.16km2, and 32 faults had been exposed, in which there were 3 faults with a drop over 10m, 2 faults with a drop of 5-10m, 5 faults with a drop of 3-5m, and 23 faults with a drop less than 3m. According to the standard that the swing error of fault plane position does not exceed 15m, the verification rate of fault interpretation by conventional and all-digital high-density 3D seismic exploration was counted respectively (Table 1).
Table 1: Summary of the faults exposed by mining and detected by exploration in working face 3404
Drop | Actually exposed | Conventional 3D seismic interpretation | High density 3D seismic interpretation | ||
Qty. | Accuracy rate | Qty. (Nos.) | Accuracy rate | ||
Over 10m | 3 | 2 | 66.67% | 3 | 100% |
5-10m | 2 | 1 | 50% | 1 | 50% |
3-5m | 4 | 0 | 0 | 3 | 75% |
Less than 3m | 23 | 0 | 0 | 2 | 8.70% |
(3) Typical seismic profiles of the mining district
Figure 3: Reflection of fault NDF146 on the conventional (left) and high-density 3D seismic (right) time profiles (with drop of 8m)
Figure 4: Reflection of fault GF27on the conventional (left) and the high-density 3d seismic (right) time profiles (with drop of 4m)
It can be seen from Figures 3 and 4 that the small faults are basically not reflected in the conventional time profile, resulting in the omission of interpretation. However, the distortion phenomenon on the high-density 3D seismic time profile is obvious, which provides a solid foundation for fault identification. In addition, it can be seen from Figure 3 that the actual apparent drop of the fault is smaller than the actual drop, mainly due to the fact that the fault is a reverse fault along the strike under steep dip.
5. FAQ
Q1: What problems shall be paid attention to in the interpretation of small faults in Huaibei area?
A: Small faults are well developed in Huaibei area. Apart from the conventional reflection wave distortion phenomenon, according to the actual exposure of the miner in this area and other exploration areas, when the reflection wave of coal seam (or the auxiliary layer near the reflection wave of coal seam) showed similar deflection phenomenon or a slight mutation similar to the occurrence and frequency, most of them weree the reflection of small faults. At the same time, the apparent drop of faults along the strike under steep dip angle is often smaller than the actual drop, which should be carefully compared in interpretation, without letting go of any "clues", otherwise it is easy to cause omission.
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