1. Introduction
This 2D seismic exploration is a sub-project of Geological Exploration Fund Project for Xinjiang Uygur Autonomous Region titled as “Coal resources pre-survey in the west of No.2 district of Tuokexun Keerjian coal mining area, Xinjiang”. The pre-survey area is about 9.59 km long from the east to the west, 6.45 km wide from the south to the north, with an area of 61.38km2. The main purposes of the exploration are to preliminarily acquire the tectonic outline of the pre-survey area, to know the distribution range of coal-bearing strata, and to provide parameter holes and coal searching holes. Difficulties in exploring the pre-survey area: (1) The near-surface stratum is extremely dry and loose, and is mingled with a large amount of gravel, which has a strong absorption and attenuation effect on seismic waves, resulting in the attenuation of seismic wave energy; (2) The exploration area is large and there are only two known boreholes in the area and few geological data; the three main survey lines were arranged at a space of 4 km, which was large, only 1 tie line was arranged, and the characteristics of reflected wave group change greatly, so it was difficult to identify the characteristics of the reflected waves in coal seam and to calibrate the target layer; (3) Te seismic geological conditions of the shallow layers are complex, there is no water table, it is difficult to drill holes, and it is impossible to select ideal excitation position, so it is difficult to obtain ideal excitation effect by using explosive source excitation.
2. Treatment solution
The work was done by the reflected wave method of the longitudinal waves and based on the principle from the known to the unknown. The seismic data were acquired by the Aries digital seismograph made in Canada and 60Hz geophone with a data recording format of SEG-Y, the instrument preamplifier gain of 24dB, the sampling interval of 1ms, and the recording length of 2.0s.
KZ-28 vibrator vehicle were used for combined excitation to overcome the attenuation of seismic waves caused by loose gravel layer and drifting sand (Figure 1).
Figure 1: KZ-28 vibrator vehicle
The data were processed in combination with the existing geological data and the understanding of the pre-survey area by insisting on high fidelity as the processing goal and amplitude preservation as the key, selecting the targeted processing modules, optimizing the processing flow and carefully choosing the processing parameters. After processing, the acoustic waves, surface waves and random interference were suppressed well. The main reflected waves in seismic time profiles were complete, with strong energy, good continuity, high SNR, high resolution and clear and reliable seismic geological phenomena.
In the process of data interpretation, synthetic seismic records were made by using borehole data on the survey line to calibrate the reflected waves of coal seams. The No.4-2 coal seam corresponding to T4-2 wave in the seismic time profile showed two strong phases, with obvious wave group characteristics, good continuity and continuous tracking, which is the standard reflected wave in the area. The No.1 coal seam corresponding to T1 wave showed two strong phases, with obvious wave group characteristics and weak local reflected wave energy, most of which could be tracked continuously (Figure 2). Track the calibrated reflection waves of coal seams, thus connecting the underground geological target strata corresponding to the reflection waves on the seismic time profile. By combining manual interpretation with workstation interpretation, the seismic reflection wave intensity phase contrast, cross section contrast and comprehensive contrast were used for interpretation to improve the data interpretation accuracy.
Figure 2: Calibration of the reflected wave position of the coal seams by the synthetic record
3. Work situation
There were 4 survey lines in this 2D seismic exploration (Figure 3), among which 3 main survey lines were arranged at a space of 4 km, namely DZ12, DZ16 and DZ24. The tie line was DL1. The total length of the survey lines was 33.86 km, with a total length of 25.20 km for 60-time coverage, a controlled area of 61.38 km2, 1,455 physical production points and 45 physical test points (thus a total of 1,500 physical points). The production records were graded. There are 1,074 grade A records, with a grade A rate of 73.81%, and 381 grade B records, with a grade B rate of 26.19%. Test records were all qualified.
Figure 3: Schematic layout of 2D seismic exploration project
4. Achieved accomplishment
(1) It was preliminarily found out that the pre-survey area is generally a monoclinal structure with a northward dip, and the stratum strike is roughly EW-, and the dip is generally 17°-35°.
(2) It was preliminarily found the occurrence of No.4-2 and No.1 coal seams. Coal-bearing areas are mainly distributed in the middle and the east of the pre-survey area, and the buried depth of No.4-2 coal seam is 0-1,100m, and the occurrence area is about 18.28km2; the buried depth of No.1 coal seam is 0-1,550 m, and the occurrence area is about 22.73 km2.
(3) The structures in the area were preliminarily identified, in which there are 9 interpretation breakpoints, 1 combined fault, mainly reverse fault, and 7 isolated breakpoints (Figure 4).
Figure 4: Reflection of the coal seam reflected waves and fault on the seismic time profile
(4) Three suggested holes were provided for drilling, which provided reliable geological basis for the arrangement of next drilling work. The provided parameters of suggested holes were verified by drilling and agreed well.
5. FAQ
Q1: How to calibrate the reflected waves of a coal seam?
A: During the interpretation of seismic data, artificial synthetic record is made based on the known borehole data on the survey lines by the natural gamma (GR) curve, and are compared with the seismic time profile of the through hole to calibrate the reflection waves of a coal seam or its related marker bed and determine the coal seam position. In the synthetic records, the corresponding relationship between coal seam and reflected wave can also be verified by layer abstraction test.
Q2: Compared with the explosive source, what are the main advantages of vibrator vehicle?
A: (1) The characteristics of seismic signals generated by a vibrator vehicle are known, and the signal spectrum and amplitude are controllable within a certain range. From the perspective of seismic signal excitation, there is great potential to improve the quality of seismic data.
(2) The vibrator vehicle-related records can suppress some environmental noise effects and have high SNR.
(3) The maximum output force of vibrator vehicle widely used in seismic exploration is about 20-30t, and its output energy is adjustable. When scanning the quake, most of the energy of the vibrator vehicle is used to generate the seismic elastic waves introduced into the earth, and the damage and influence on the environment is far less than that of the explosive source, so it can be used in residential areas and other explosion-forbidden areas.
(4) When using vibrator vehicle for seismic exploration in difficult hole-forming areas, the construction efficiency is high and the cost is low.
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