overview of 1 mining area

Jining No.2 coal mine is a large-scale production mine. In order to find out the geological structure and coal seam occurrence in No.9 mining area, the mine requires three-dimensional seismic exploration in No.9 mining area. The exploration scope is 1451m long from north to south and 2721m wide from east to west, with a controlled area of 4.1km2.

The strata in the exploration area include Quaternary, Mengyin Formation of Upper Jurassic, Shanxi Formation of Upper Permian, Taiyuan Formation of Upper Carboniferous, benxi formation of Middle Carboniferous and Middle and Lower Ordovician. The coal-bearing strata in this area are Taiyuan Formation and Shanxi Formation, with 27 coal-bearing layers, including 23 coal-bearing layers in Taiyuan Formation and 4 coal-bearing layers in Shanxi Formation. There are 7 layers of minable and partially minable coal seams, and the main minable coal seams are upper 3, lower 3 and upper 16.

magmatic rocks are well developed in the area. The thickness of magmatic rocks in the mining area is 57.2 ~ 136.7 m, and they cover the coal measures strata in the exploration area in the form of bedrock, which is located in the middle and upper Jurassic. The distance from the bottom boundary of magmatic rocks to the top boundary of coal in the main minable seam 3 is generally more than 311m m.

2 Complicated seismic geological conditions

2.1 Shallow seismic geological conditions

The groundwater level in the survey area is generally 2 ~ 6m. Due to the existence of the ancient river bed, quicksand layer is distributed locally, and the excitation conditions are affected. The main lithology of the formation is brownish yellow, light gray green clay, sandy clay and sand layer. The Quaternary system is 1.91 ~ 216 m thick, and its bottom interface is in angular unconformity contact with the underlying stratum. The interface wave impedance is obviously different and the reflection coefficient is large, which can form a group of strong TQ reflected waves and a strong secondary reflected wave, that is, multiple interference waves in this area.

2.2 Seismic geological conditions in the middle and deep layers

The upper Jurassic is relatively simple in lithology, mainly composed of sandstone, with a small amount of mudstone or conglomerate, and there is no obvious wave impedance interface. In addition, the difference with the underlying top boundary is not obvious. Therefore, strong reflected waves cannot be obtained in the Jurassic and its bottom interface. Especially in Jurassic, there are Yanshanian intrusive magmatic rocks, and they intrude into the middle of Upper Jurassic in the form of bedrock. The magmatic rocks are mainly olivine gabbro, hornblende gabbro and pyroxene syenite porphyry. The formation of a shielding layer for the propagation of reflected waves greatly affects the reflected waves in the lower coal seam, so the poor seismic geological conditions in the central part are the main problems existing in this seismic exploration, and effective measures should be taken to solve them. There is a great difference in wave impedance between magmatic rocks and their upper and lower surrounding rocks, forming a group of strong top and bottom interface reflected waves Tr1 and Tr2. 3 upper and lower coal seams are the main minable coal seams in this area, with a buried depth of 716 ~ 819 m. The reflection wave of coal seam has obvious characteristics, strong energy and good continuity, and is the main reflection wave of minable coal seam in this area, collectively referred to as T 3 wave. The thickness of 16 and 17 coal seams is less than 1 m, and the wave impedance is not obvious. Although there are conditions to form reflected waves, the reflected wave energy is weak.

2.3 Analysis and research on complex seismic geological conditions in this area

The shallow and deep seismic geological conditions in this area are good, and the strong shielding of thick magmatic rocks and the influence of multiple waves at the bottom of Quaternary system on the main target layers are the difficulties in 3D seismic exploration in this area. In data collection and processing, effective measures must be taken to overcome the influence of multiple waves and shielding layers. Choose the best excitation horizon, excite with large amount of charge, and reduce the earth filtering and the absorption and shielding effect of magmatic rocks on seismic waves to ensure the propagation energy. For the reception of reflected waves, because there are many stratigraphic interfaces and the target layer is deeply buried, the high-frequency components of effective waves reflected to the ground are lost, so the intermediate frequency detector is used for reception; Adopting large array reception is beneficial to remove the influence of quaternary bottom boundary multiples in processing. In data processing, various appropriate methods are mainly adopted to reduce the influence of multiple waves.

3 methods of field work

according to the analysis and study of shallow and deep seismic geological conditions in this area, it is very important to do a good job of testing before construction in this area. Therefore, point and line tests are designed to determine the best acquisition parameters and choose the best construction scheme. The experiment follows the principle of combining point with line, combining point with line and changing single factor.

point test to determine the excitation conditions. Dosage test: In wells with different depths, the dose contrast test is carried out to determine the excitation dose. Well depth test: the explosive amount determined by the test is used to excite in wells with different depths, and the excited explosive amount is determined. Through analysis and comparison, the explosive charge is 2 ~ 3 kg, and 12m and 14m are used in different sections of well depth.

fig. 1 Time profile of test line received by geophones with different frequencies

In line test, each receiving point of test line is simultaneously received by 41Hz, 61Hz and 111Hz geophones, and the initial stack profiles with three different frequencies are obtained (fig. 1). As can be seen from fig. 1, if the 61Hz intermediate frequency geophone and combined reception are selected, the target layer of main coal seam reflection wave in time profile has high resolution, strong continuity and clear breakpoints. Selection of arrangement mode: the test line adopts midpoint firing and 72 channels are received. During data processing, the time profiles of 72 channels, 48 channels, 36 channels with midpoint firing, 36 channels with receiving end firing and 24 channels with five different arrangement modes are obtained through different channel extraction processing. By analyzing these five initial overlapping profiles, it can be seen that 36 channels with midpoint firing are effective in overcoming multiple wave interference and magmatic shielding.

According to the experimental analysis, geological conditions of the survey area and requirements of geological tasks, the following working methods are adopted:

Observation system type: beam-shaped 8 lines and 8 guns, with intermediate excitation; Number of channels received: 288 channels;

number of receiving lines: 8; Receiving track distance: 21m；; Receiving line spacing: 41m.

geophone combination: six 61Hz geophone strings are used.

4 processing methods

according to the seismic geological conditions and geological task requirements in this area, digital processing is based on the principle of "high resolution, high fidelity and high signal-to-noise ratio", and at the same time, the development characteristics of multiples are emphatically analyzed, and the influence of multiples is removed by various methods, and finally a satisfactory three-dimensional time data volume is obtained.

in view of the above processing objectives, in data processing, the following links are mainly grasped for repeated testing, and the correct processing flow and the best processing parameters are selected.

(1) Establish a correct geometry library and a static calibration library.

(2) It is an important link in data processing to do speed analysis carefully.

(3) The rational use of deconvolution can strengthen the high-frequency information and improve the vertical resolution.

(4) Pre-stack partial migration (DMO) and post-stack one-step migration technology are adopted to improve the lateral resolution and spatial imaging effect.

(5) Choose a suitable method to remove multiples, so as to remove the influence of multiples in new strata on coal seam reflection wave.

4.1 multiples processing

The multiples developed in this area are generated by the bottom boundary of Quaternary, and the velocity is low, which is consistent with Quaternary, and has obvious velocity difference with coal measures strata, so it is easy to attenuate. After a lot of experiments in F-K domain and τ-p domain, the CDP gathers are selected for τ-p transformation and F-K filtering, and the multiples are attenuated to the greatest extent by taking advantage of the velocity difference and the slope of multiples compared with the reflected waves in the target layer, so as to highlight the effective waves and achieve the purpose of removing multiples (Figure 2).

due to the reasonable processing flow and parameter selection, and the methods of τ-p transform and F-K filtering, the influence of multiple waves and other interference waves is eliminated, and the quality of time profile is improved.

fig. 2 comparison of multiple-removed wavefront and multiple-removed gathers

4.2 time profile quality

The computer shows that 1118 time profiles can be obtained, including 469 east-west profiles; There are 649 north-south profiles. According to the 41m×81m grid, the time profile is randomly selected, with ***114 pieces, totaling 237.18km. According to the requirements of regulations, the class I profile is 153.53km, accounting for 64.73%, and the class I+II profile is 211.37km, accounting for 89.12%. The time profile quality is relatively good. The profile quality is higher than the requirements of the regulations, which lays a reliable foundation for completing geological tasks.

5 geological achievements

In this 3D seismic exploration, the ups and downs of coal measures strata and the development of secondary folds have been found out; The distribution and law of faults are deeply studied, and faults over 5m are identified by combining drilling data and roadway data. The faults or breakpoints with a drop of 3 ~ 5m are explained. At the same time, the change trend of coal thickness in the third floor and the third floor is predicted and studied, and good results are obtained.

in structural interpretation, there are 1 faults: fault F 61; Seven faults were modified: Balipu fault and Balipu fault branch 2, F35, F36, F37, F58 and F59；; 38 newly discovered faults: 2 with a drop of > 11 m; 5 pieces of 5 ~ 11 meters; There are 19 faults of 1 ~ 5m and 12 small faults of about 3m. The occurrence of Balipu fault is strictly controlled, as shown in Figure 3.

fig. 3 structural comparison of 3 coal seams before and after 3D seismic exploration

This 3D seismic exploration has successfully completed various geological tasks stipulated in the agreement through meticulous work in collection, processing and interpretation and reasonable technical measures. The data of three-dimensional seismic exploration results provide reliable geological basis for the rational layout of coal mining face in the mine, and have achieved remarkable economic and technical benefits.

(This article was published in the supplement of Coalfield Geology and Exploration in 2115)