Analysis of results of seismic reflection wave method
The total working area of seismic exploration in this study is 3.11 km2, which can be divided into the following three areas: the self-operated mining and stripping area of the first mining area of East Open-pit Mine (the first task, with an area of 0.94 km2), the outsourcing stripping area to the west of the self-operated mining and stripping area of the first mining area (the second task, with an area of 0.51 km2) and the coal haulage roadway area to the west of the central part of the southern end of the mine (the first task, with an area of 0.5 Organize and process the data obtained from this 3D seismic exploration. The analysis of the detection data shows that the signal-to-noise ratio is relatively general, the first break wave in the collected data is relatively clear, and some random interference may be generated by large mining equipment and vehicles, with acoustic waves and surface waves as the main interference waves. The quality of the collected data is comprehensively assessed as general.The interpretation of geological structure in this exploration is mainly carried out from three aspects: the inference of goaf, the inference of abnormal area of collapse column and the detection results of seismic evaluation. The comprehensive evaluation is as follows:
The typical time profile of the selected goaf section of small coal mines is shown in Fig. 5. As shown in Fig. 6, in the research area with a total area of 3.11 km2, 9 inferred mined-out areas and 9 suspected mined-out areas were found by using 3D seismic exploration method. It is concluded that the characteristics of mined-out area and suspected mined-out area are the same: on the seismic event profile, the frequency of reflected wave becomes lower, The in-phase axis is disordered and obviously trapped on the horizontal isochronous slice. The difference between inferred mined-out areas and suspected mined-out areas lies in that inferred mined-out areas are clearly reflected in abnormal seismic time profiles, while suspected mined-out areas are not clearly reflected in abnormal seismic time profiles. Illustration of typical inferred goaf and suspected goaf, As shown in Figs. 6 and 7.
Analysis of detection results of transient electromagnetic method
There are 6,900 physical points of transient electromagnetic exploration, with a total area of 2.65 km2, which are mainly carried out in two stages: (1) Self-operated mining area by the working party (in the first stage, there are 2,444 measuring points with an area of 0.94 km2). (2) The stripping area is outsourced to the west of the self-operated stripping area of the first mining area (the second stage, there are 4,456 measuring points with an area of 1.71 km2).
The post-processing process of transient electromagnetic detection includes the following steps: first, preliminary processing of the detected data; Step 2, sift the waveform results to eliminate the influence of useless signals and show the original features of geological signals; Step 3, using special software to transform the detection parameters to obtain the apparent depth, apparent resistivity and other indicators; Step four,Refer to the relevant data of drilling, geology and survey, and correct the elevation and topography; The fifth step is to draw contour map and section map, which can be used for anomaly reliability evaluation, anomaly division and physical parameter statistics. The above process can be simplified into two major steps: the first step,The intermediate results for qualitative analysis (TEM curve type plan) are formed by processing the data collected in the field; The second step is to analyze and interpret the intermediate results to form results maps (apparent resistivity plan and apparent resistivity section).
Because the electrical reflection of mined-out area is compared, when inferring the abnormality of geological body, we should not only consider the continuity of resistivity change, but also consider the particularity of geological, drilling, topography and other conditions, compare with the field geological survey and field exploration records, coordinate the evolution law of plan and cross-sectional drawings, and complete comprehensive and systematic induction and summary.Through the above steps, the detection area is determined, as shown in Table 2 and the detection result is shown in Fig. 8. 223 specific areas have been delineated, and some specific areas have been actually verified with the continuous production.
High damping resistivity method
Due to the limitations of the actual conditions such as site topography, slope contour and flat plate length, this exploration work is only carried out in local flat plate, with 630 physical points completed and the exploration area is about 0.3 km2. See Table 3. The detection results are shown in Fig. 9. The results of high-density electrical prospecting, 3D seismic prospecting and transient electromagnetic prospecting can well verify and complement each other.
Geological drilling exploration
Since the mined-out area was discovered in East Open-pit Mine in 2012, the geological drilling exploration method has been adopted so far. According to the results of underground joint geophysical exploration, 58 drilling holes were arranged in the suspected mined-out area of East Open-pit Coal Mine, and geological drilling exploration was added several times in the later period according to the production needs. Through comprehensive analysis of the production data of small coal mines,The key areas of detection tasks can be obtained, and the utility rate of daily detection drilling is effectively improved.
Distribution characteristics of hidden goaf groups
Through statistical analysis of goaf groups, it is found that the span of goaf is 15–30 m, the maximum span is about 40 m, the vertical height of goaf is 10–15 m, the maximum vertical height is 25 m, the volume of goaf is 500–3500 m3, and the maximum volume of goaf is about 7000 m3.
The representative mined-out areas in the mined-out area group are as follows:
(1)1275-122 drilling hole to expose goaf (1275 flat plate).
According to the results of laser 3D scanning, it can be obtained that the volume of the cavity of the go af is about 4850m3, the maximum height of the goaf is about 10 m, and the minimum distance from the cavity of the goaf to the ground is 26 m. The mined-out area is oval and distributed from southeast to northwest, in which the short axis is about 20 m and the long axis is about 44 m. As shown in Fig. 10a,b.
(2)A-59 drilling to expose mined-out area (1320 flat plate).
According to the results of laser 3D scanning, it can be obtained that the cavity volume of the goaf is about 3400 m3, the minimum thickness of the roof of the goaf from the surface is about 21 m, the major axis of the goaf is about 28.5 m, and the minor axis is about 17.5 m, as shown in Fig. 10c,d.
(3)1290-302 drilling to expose the goaf (1290 flat plate).
From the results of 3D laser scanning, it can be concluded that the cavity volume of the mined-out area is about 4706 m3 and the distance from the surface is 33 m. Among them, the mined-out area is distributed from southeast to northwest, the major axis of the mined-out area is about 45 m, the minor axis is about 25 m, the vertical height of the mined-out area ranges from 1246 to 12,585 m, and the maximum vertical height of the roof and floor of the mined-out area is about 10 m, as shown in Fig. 10e,f.
(4)1290-303 drilling to expose the goaf (1290 flat plate).
From the results of 3D laser scanning and geological drilling, it can be analyzed that the cavity volume of this goaf is about 3453 m3, the minimum distance from the goaf to the surface is about 34.9 m, and the maximum distance between the roof and floor of the goaf is 10 m. The goaf is distributed in the east–west direction, with the short axis of about 20 m and the long axis of about 48 m, as shown in Fig. 10g,h.
(5)1275-156 drilling hole to expose goaf (1275 flat plate).
The mined-out area is distributed in the northeast direction, with the major axis of about 35 m, the minor axis of about 10 m, the vertical height of the mined-out area ranging from 1,240 to 1,250 m, the vertical heights of the roof and floor of the mined-out area are all about 10 m, and the roof and floor of the mined-out area are nearly horizontal, and the proven volume of the mined-out area is about 1,381 m3, as shown in Fig. 10i,j.
(6)1320 flat plate (1320-183 borehole, 1320-181 borehole and 1320-182 borehole).
From the results of 3D laser scanning and geological drilling, it can be analyzed that the cavity of the goaf has a volume of about 6965 m3 and a large buried depth, and the maximum distance between the roof and the floor of the goaf is 9 m. The goaf is distributed in the southwest-northeast direction with a short axis of about 19 m and a long axis of about 39 m, as shown in Fig. 10k,l.
Based on the above information, the location of the mined-out area is shown in Fig. 11.
Classification and formation mechanism analysis of concealed cavity
The mined-out areas exposed by 3D laser scanning are classified and analyzed. According to the spatial geometry and formation mechanism of mined-out areas, mined-out areas can be classified into three main types. The formation mechanism of goaf and cavity characteristics of goaf can be summarized as follows:
Through statistical analysis, it is found that the discovered mined-out areas are all produced by room and pillar mining in small coal mines. The lithology of overlying rock mass of coal seam is poor. Under the influence of factors such as weathering of rock mass, ground pressure activity and blasting, the damage degree of coal pillar is bound to be serious, the stability of overlying rock mass is constantly decreasing, and the stability of roof above goaf will gradually decrease.Especially, the phenomenon of roof peeling will lead to the upward expansion of goaf and gradually form the geometric shape of “inverted funnel”.
The top of the mined-out area is located above the roof of the coal seam. The span of the mined-out area is not large, and there is no obvious characteristic law in its position and height. There is a large free space in the upper part of this kind of goaf, and the ideal effect can be obtained by blasting control measures.
According to the scanning results, this kind of mined-out area is mainly distributed in the south-central part of the working gang and the mined-out area of Xinjing Mine at the bottom of the ditch. A typical example is the 1320 flat plate A-59 borehole to expose the mined-out area. According to the results of three-dimensional laser scanning, the distance between the roof and the floor of the mined-out area is 25.6 m, the cavity volume of the mined-out area is about 3400 m3, the minimum distance between the roof and the surface is 21 m, the short axis azimuth is 35.7, and the length of the mined-out area.The azimuth angle of the long axis is 129 and the length is about 28.5 m. Arrange the three-dimensional point cloud in the goaf, as shown in Fig. 12.
The cavity of 1225 flat plate 0528 in East Open-pit Mine is also a typical inverted funnel-shaped cavity. The difference is that this cavity has collapsed to the surface. Due to the long formation time of the mined-out area, the degree of rock weathering is high, and at the same time, due to the combined influence of blasting vibration and transportation load in the open-pit mining process, the roof of No.9 coal mined-out area collapsed locally.It is inferred that the north–south span of 9 coal goaf is about 70 m, as shown in Figs. 13 and 14.
According to the existing geological data, in the middle of the east open pit, there are sandy rocks with larger thickness and higher hardness on the roof of the coal seam. After the coal seam is mined, the roof of the goaf formed in this area is not easy to collapse, and can still keep its original shape. Therefore, the characteristics of the goaf formed in this area are as follows: the roof of the goaf is relatively regular and flush,The thickness of the roof is basically the same, so it is relatively simple to judge the drilling depth during blasting. The blasting treatment effect of the goaf is largely determined by the blasting effect of the central cut hole. If the blasting effect of the cut hole is not good, it is difficult to form a free space for blasting above the goaf. Further, the collapse of the rock mass above the goaf is incomplete and insufficient, and a “turtle shell” will be formed on the surface, which may eventually lead to the collapse of large equipment here. According to the scanning results, this kind of mined-out areas are mainly distributed in the middle of the working gang. Typical examples are 129 flat plate 1290-302 borehole exposed cavities, 1290 flat plate 1290-303 borehole exposed cavities and 1275 flat plate 1275-156 borehole exposed cavities.As shown in Figs. 15 and 16.
According to the scanning results, it is found that some mined-out areas can’t be completely detected by a single scanning hole, and the geometric shape of mined-out areas can be fully ascertained only by joint detection and mutual supplement of multiple holes. Generally, this kind of goaf has a large span, and under the influence of rock weathering and blasting, the roof caving of goaf is incomplete. Goaf treatment is relatively difficult. According to the scanning results, this kind of mined-out areas are mainly distributed in the northwest of the working gang. Typical examples are 1320 flat plates 1320-181, 1320-182 and 1320-183, which are drilled to expose mined-out areas, as shown in Fig. 17.