1. Introduction
The exploration area is located in the south of Jungar coalfield, and the surface elevation varies from 1,115.4 m to 1,237.4 m (Figure 1). Two thirds of the surface area is covered by thick loess (sand), and the loess thickness varies from 15 m to 120 m; 20% of the area is covered by gangue, and the gangue thickness varies between 3 m and 100 m; the rest are Neogene red bed and bedrock exposed near Dagou.
Figure 1: Stereoscopic diagram of the opography of the exploration area
There are 4 minable coal seams in the area: The thickness of seam 4 varies from 2.30m to 4.70m, and the elevation varies from 720m to 900m; the thickness of Seam 6upper varies from 10.05m to 15.77m, and the elevation varies from 670m to 840m; the thickness of Seam 6 varies from 1.90m to 6.24m, and the elevation varies from 640m to 820m; the thickness of seam 9 varies from 0.85m to 4.75m, and the elevation varies from 620m to 760m. The top interface of Ordovician limestone is 18-25m away from seam 9. According to the original data, the dip is generally less than 4°, there is no large fault, and the occurrence state of limestone is unknown.
The exploration difficulties are as follows: excitation of gangue-covered area, superposition in the same direction under different excitation conditions, imaging of weak reflection signal at the top interface of Ordovician limestone, prediction of fracture zone at the top interface of Ordovician limestone.
2. Treatment solution
The observation system was designed with small track spacing, large array, high covering degrees and wide azimuth, which was beneficial to ensure the effective covering degrees of target strata with different buried depths, and was also beneficial to azimuth superposition and multi-attribute analysis in later data processing and interpretation.
In the data acquisition, the digital geophones were used for reception to ensure the effective reception of weak reflection signals. The area where the gangue filling thickness is less than 15m was selected and drilled by gravel drilling machine. The shot points in other areas were compensated by recovery blasting technology.
During the processing, the gyration wave tomography inversion technology was mainly used for static correction (Figure 2), attenuation of ground roll waves and other low-frequency linear noises in the cross-arranged domain, wavelet consistency and amplitude consistency processing, the 5D regularized interpolation technology, and the anisotropic pre-stack time migration for data processing.
Figure 2: Schematic diagram of the calculated low-velocity zone thickness (black line)
In the process of data interpretation, the time profile interpretation, multi-attribute comprehensive analysis and inversion were combined and used to interpret the faults, fracture development area at the top interface of Ordovician limestone and to predict the occurrence scope of sand bodies at top of coal seams (Figure 3 and Figure 4).
Figure 3: Comprehensive attribute graph of the reflected wave at Ordovician limestone top interface (blue for fracture development area)
Figure 4: Display of the sandstone on the wave impedance profile (The red and the yellow represent sandstone)
3. Construction situation
The work area is 4.206km2, and the area with coverage of 64 times is 1.68km2. 27 survey points, 229 physical test points and 5,286 physical production points were completed. According to the Seismic Exploration General Standard in Coal and Coalbed Methane, the test records were all qualified. There were 3,280 records of the grade A, with a grade A rate of 62.05% and 1,988 records of the grade B, with a grade B rate of 37.11%, thus a qualified rate of 99.66%.
4. Achieved accomplishment
The geological achievements obtained through this exploration are as follows:
(1) The floor undulation pattern of the main coal seams in the exploration area was found out;
(2) A total of 12 faults with drop over 3 m in the exploration area were interpreted;
(3) Two fracture development zones in the Ordovician limestone top interface were delineated;
(4) The occurrence range and the thickness variation trend of medium-coarse sandstone in overlying strata of seam 4 were predicted;
(5) The predicted fracture development area was verified by hydrological drilling data, and the coincidence rate between the predicted occurrence range and thickness variation trend of medium-coarse sandstone and drilling data reached 88%.
5. FAQ
Q1: When the distance between Ordovician limestone and coal seam is small, how to ensure the accurate imaging of limestone top interface?
A: (1) When designing the observation system, it is necessary to design a large receiving arrangement, and the general arrangement length is not less than 1.5 times of the buried depth of the limestone top interface.
(2) The digital geophones were used to receive seismic signals to ensure the reception of weak reflection signals at the top interface of Ordovician limestone.
(3) Anisotropic pre-stack migration was used in data processing to ensure the imaging effect of the reflected waves at the top interface of Ordovician limestone.
Q2: Generally speaking, low SNR and low data frequency occur in the seismic exploration in loess tableland, and the digital geophones receive all the signals. Will the all-digital high-density 3D seismic exploration technology further reduce the SNR and resolution of seismic data?
A: (1) The SNR of seismic data is related to the quality of single-shot data and covering degrees. Generally speaking, the higher the covering degree is, the higher the SNR of seismic time profile is. For signals received by digital geophones, reasonable noise attenuation technology can ensure the SNR of single-shot records, and high covering degree is also beneficial to the improvement of SNR of seismic data. Therefore, adopting all-digital high-density 3D seismic exploration technology in the loess tableland will only improve the SNR of seismic data.
(2) The resolution of seismic data is related to the main frequency and frequency band width of seismic data. Generally speaking, the higher the main frequency is, the higher the SNR of seismic data is. At the same time, the wider the frequency band width of seismic data is, the higher the resolution of seismic data is. Because the digital geophones receive seismic signals in full frequency band, as long as amplitude and frequency preservation processing measures are adopted in the processing process, the frequency band width of seismic data will be wider than that of analog geophones, adopting full digital high-density 3D seismic exploration technology in the loess tableland will only improve the resolution of seismic data.
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