1. Introduction
The surface fluctuation of the exploration area is gentle, and the altitude of the whole area is between 950 m and 1000 m (Figure 1). The main purpose of this exploration is to find out the geological structure and coal seam occurrence state of the mining area and provide technical support for safe production. Difficulties in exploration include: (1) The buried depth of target layer in local area is about 100 m; (2) The dip in local area is large, ranging from 25 to 33 degree; (3) The surface sediments are loose, the thickness of Quaternary is 0-8.48 m, with an average of 3.19 m, overlapping unconformity contact with the underlying strata, and has a strong absorption and attenuation effect on seismic waves, which is not conducive to the excitation and reception of seismic waves.
Figure 1 Geomorphologic characteristics of the exploration area
2. Mine water hazard treatment solution
The exploration equipment is French Sercel-428XL all-digital seismograph (Figure 2). DSU1 single-component digital geophones instead of conventional analog geophone were used for data acquisition as they directly outputs digital signals and greatly improve the fidelity of signals.
Figure 1 Geomorphologic characteristics of the exploration area
Figure 2 Sercel-428XL all-digital seismograph
Considering the early observation system, the indoor technical demonstration of the collected parameters and the comprehensive analysis of the field test, it was determined that the 8L×8S×72T×4R×24-time swath observation system should be used for data collection. The main technical measures of field data acquisition were as follows: (1) For the local shallow target layers, the observation system with small receiver spacing, small shot point spacing and small CDP grid was adopted, and the conventional design of 16-fold coverage increased to 24-fold to improve the effective coverage times of shallow burial areas; (2) In areas with large dip, dip excitation method was adopted to properly increase the arrays; (3) Two KZ-28 vibrator vehicle were used for combined excitation, so as to overcome the absorption and attenuation effects of loose sediments on seismic waves.
In the process of data processing, the problem of big difference in amplitude energy between shots and between tracks was solved by the use of the consistent amplitude compensation technology based on energy decomposition model, which improves the amplitude fidelity.
In the process of data interpretation, the combination of time profile and attribute slice was used for structural interpretation to improve the interpretation accuracy (Figure 3).
Figure 3 Dip attribute of Coal Seam A3
3. Work situation
The exploration area is located 8km northwest of Mongolian Autonomous County and Shituoluogai Town, Hobsaer in Xinjiang and is under the jurisdiction of Tacheng District of Yili Kazakh Autonomous Prefecture. The work area of 3D seismic exploration is 7.78 km2, 24-time full coverage area of 4.19 km2 (Figure 4), with 41 test physical points, 3,598 production physical points and total 3,639 physical points. The production records were graded: 2,880 records of Grade A, representing a grade A rate of 80.04%, 718 records of Grade B, a Grade B rate of 19.96%. All test records were qualified.
Figure 4 Schematic diagram of the relative position between the exploration area and the mine field boundary
4. Achieved accomplishment
(1) The occurrence patterns and structural development of coal seams A3, A4 and A7 were identified. It is an asymmetric syncline structure on general, dipping to the west, with a gentle dip of 5 -15 degree in the south wing and a steep dip of 5-20 in the north wing (Figure 5). The maximum dip angle is between 25 and 33 degree in the northwest of the exploration area.
(2) The nature, occurrence and extension direction of faults with a drop of more than 5 m were identified, and faults with a height of 3-5m were interpreted, in total, 14 faults were interpreted (Figure 6).
(3) The burial depth and fluctuation patterns of coal seams A3, A4 and A7 in the exploration area were identified.
(4) Based on multi-attribute interpretation and drilling control, the thickness variation trend of coal seams A3, A4 and A7 in the area was predicted.
Figure 5 Reflection of the syncline on seismic time profile
Figure 6 Reflection of the faults on the seismic time profile
5. FAQ
Q1: What are the advantages of digital geophone compared with analog geophone?
A: As a representative of the new geophone, the digital geophone has the following advantages compared with the analog geophone: (1) The digital geophone has higher resolution than the analog geophone, and has rich interlayer information, good fidelity and slightly higher signal-to-noise ratio, which may improve the energy of high frequency reflected information; (2) The dynamic range is large, which can improve the acquisition accuracy. When the same processing flow and parameters are adopted, the digital geophone has stronger ability to distinguish weak signals in time profile, and can effectively broaden the frequency band of reflected waves in the target layer and improve the dominant frequency, and the data collected by the digital geophone contains more effective information at low frequency, which is beneficial to lithology analysis, as compared with the analog geophone.
Q2: What types of seismic information reflect abnormal seismic attributes related to structures?
A: The seismic information reflecting the anomaly of seismic attributes related to structures include amplitude information, frequency information, phase information and curvature information.
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