1. Introduction
In Tongting coal mine of Huaibei Mining Co., Ltd., for the subsequent development and more reasonable mining area layout, it is necessary to explore the geological structure near the fault F5, and use all-digital high-density 3D seismic exploration to solve the above problems.
There are three main difficulties in the project:
(1) There are many obstacles on the surface, so it is difficult to arrange the geophones and shot points.
(2) The wind is strong in spring, which leads to loud noise.
(3) The dip of coal seam is big.
2. Equipment and measures
In view of the above working difficulties, effective measures were taken and the data collection task was successfully completed.
(1) All-digital high-density 3D seismic exploration technology and 18-line and 15-bunch beam observation system were used, because its wide azimuth is beneficial to the formation imaging under obstacles, which is more conducive to migration imaging of coal seams with large dip. High covering degree improved the SNR of the data.
(2) The following effective measures were taken in the acquisition:
① Shot points and geophones were measured, and small explosive holes were used in the village.
Dachen Village and Tongting Town are integrated as a whole, and the whole obstacles accounted for a large proportion. All the geophones in the village were measured and the explosion holes increased in the village to ensure the continuity of underground data. Tongting Coal Mine is located in the northern part of the exploration boundary. In order to ensure that the migration can return to its original position, the method of surrounding excitation and laying the survey line around the mine was adopted to ensure sufficient offset of the edging (Figure 1).
Figure 1: Distribution of the actual geophones (green) and the excitation points (yellow) Crossing the obstacles
② All on-site geophones were buried in the holes to effectively reduce the environmental noise interference. Adequate test was conducted to ensure proper excitation dose and excitation position.
Fig.2 Fusion of the reflected wave attributes of the coal seam 10
③In the data acquisition, asymmetric receiving and observation systems were adopted in local large dip areas.
(3)In the data processing and interpretation, the following measures were taken to improve the imaging accuracy.
Pre-stack time migration technology was used in processing, and attribute technology (Figure 2) and inversion technology (Figure 3) were used in data interpretation, thus realizing fine tectonic interpretation and coal seam thickness interpretation. By using the advantages of the digital geophones, the better imaging of the top interface of Ordovician limestone was realized.
3.Work situation
The scope of this high-density 3D seismic exploration was delineated by 6 turning points, with an area of 2.30 km2. There were 15 3D seismic wiring harnesses, 60 survey lines, 54 physical test points and 6,302 physical production points (thus a total of 6,356 physical points) in the whole area. The records were graded according to the specifications. All of the 54 test records were qualified. There were 6,302 records, including 5,251 records of grade A, with the grade A rate of 83.32%, and 1,051 records of grade B, with the grade B rate of 16.68%.
Fig.3 Inversion profile cross hole 2017-17
4. Achieved achievements
The results of this seismic exploration are mainly reflected in the following aspects:
(1)It is found that the block to the north of F5 is a monoclinic pattern with NE strike and NW dip, with an inclination of 25°-35°, and the block to the south of F5 is a monoclinic pattern with NW strike and NE dip, with an inclination of 30°-46°.
(2)It is found that the faults with a drop over 3m and folds developed in coal seams 32, 7, 82 and 10. A total of 179 faults have been interpreted. Comparing with the structures before exploration, 8 faults were corrected and 171 faults were newly discovered (Figure 4).
(3)The change trend of thickness of the main coal seams was predicted by attribute technology combined with inversion technology.
Figure 4: Comparison of Fault F5 before and after Exploration
(the red is after the exploration, the blue is before the exploration)
5.FAQ
Q1: What on-field measures can be taken to ensure the SNR of data acquisition in spring windy season?
A: (1) Increase the covering degree of field acquisition.
(2) The field geophone is buried in holes.
(3) Ensure proper excitation dose and excitation position.
Q2: What technologies are used to predict coal seam thickness?
A: Through multi-attribute fusion technology and wave impedance inversion technology, combined with borehole data, the thickness of each coal seam in the mining area is predicted.
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