(1) borehole observation
It is the most commonly used method at home and abroad for many years to drill holes from the ground and determine the height of overlying strata caving and water-conducting fracture zone by observing the consumption of washing liquid. For example, Cheng Jinquan (2002) arranged two observation lines and five observation wells above the goaf of No.8 mine field of Hebi Coal Industry Group Company. Using the method of drilling flushing fluid consumption, taking the depth and horizon corresponding to the leakage from the normal value to the first peak of the single-hole water-conducting fracture zone as the highest point, the mining-fracturing ratio of the weak roof with an inclination of 23 is measured to be 4.9. The mining-fracture ratio of hard roof is 9.7. Song Jisu (2005) determined that the sudden change of water level, consumption and water injection during normal drilling construction, incomplete core and broken core are the upper limit of water-conducting fracture zone, and the occurrence of drill dropping, sticking and air suction is the upper limit of caving zone, and measured the height of water-conducting fracture zone in 3223, 324 1 and 3222 working faces in Qidong Coal Mine. As a result, the difference between two boreholes in the same working face is 25m. In addition, the observation test of rock movement in borehole was carried out, and the water injection test was carried out in some boreholes. By analyzing the relationship between leakage and rock movement data, the rock movement corresponding to the maximum height of water-conducting fracture zone is determined. For example, Fangezhuang Mine in Kailuan (1964) divides the water-conducting fracture zone with vertical displacement10 mm/m..
With the increase of mining depth, underground inclined drilling gradually replaced ground drilling. 1990, Shandong university of science and technology obtained the national patent (patent No.90 1063487) for the "double-ended water plug" underground elevation observation technology. In the observation system, a water injection probe tube with double-ended sealing capsules is adopted. When the capsule is filled with water or gas at a certain pressure through the pipeline, the capsule expands rapidly, tightly blocking both ends of the well section where the blockage is located, and then blocking the well section through the water injection pipeline. Due to double-ended plugging, downhole inclined holes can be used for drilling. For example, Xiong Xiaoying (1998) used a borehole double-ended plugging leak detector to detect the height of water-conducting fracture zone in Panyi Mine, Huainan. The bedrock is limestone, sandstone and siltstone, and the roof fracture-mining ratio is 10. Gaoyan Method (200 1) has been used to survey the height of Donghuantuo, Tangshan and Qian Ying mines in Kailuan, and achieved good results. Compared with the "surface drilling flushing fluid consumption method", this technology has the following outstanding advantages: ① Small drilling volume and low cost. It can avoid troubles and expenses such as land acquisition and compensation for ground drilling. ② The test accuracy is high, and the permeability is 0. 1L/min, which can be observed repeatedly. ③ It can be used for observation of coal seam mining under lakes, shallow seas and water-bearing alluvium. From the actual effect, the borehole observation accuracy is high, which is often used to verify the caving height and guide height determined by other methods. Its disadvantages are high cost, long period, observation only at local points of borehole, and often failure when primary fractures in rock formation develop.
(2) Geophysical exploration and logging technology
Since 1980s, underground geophysical prospecting technology has been widely used at home and abroad, and modern equipment and technology have been continuously developed and improved. Mine direct current method, geological radar, transient electromagnetic, Rayleigh wave instrument, high-resolution earthquake and microseismic monitoring are all applied to the detection of roof cracking. The combination of various detection technologies forms comprehensive detection, which further promotes the development of roof overburden failure observation. Such as borehole acoustic wave measurement, borehole radio wave perspective technology, borehole electromagnetic wave tomography (CT), borehole acoustic wave CT tomography technology, anti-deformation sounding, borehole rock movement observation, etc.
Li Jun (1997) measured the weak roof caving zone and caving zone in Tiebei Mine of Zhalainuoer Mining Bureau with high resolution seismograph. The error of depth interpretation is less than 0.5m, and the result is 1.3 and 8. 1 1. The working principle of high-resolution seismograph is that when seismic waves encounter an interface with non-zero impedance difference during stratum propagation, refracted waves and reflected waves will be generated at the interface. By receiving refracted wave or reflected wave, we can know the lithologic information and structural information of stratum interface. Because of the loose rock in the goaf caving area, there are gaps, and the wave impedance of air and water in the gaps is quite different from that of surrounding rock, and the top interface can form a "bright spot" of longitudinal wave reflection; However, the elastic modulus of water and gas is zero, so there will be reflection "dark spots" on the shear wave profile, and the lithologic interface and gas (liquid) interface can be judged by the comparison between shear wave and shear wave. However, because the top of caving zone is composed of some irregular rock blocks, and the interface distribution is irregular, the reflection formed by them is some diffusion, which leads to the sudden change of seismic reflection wave group. The reflection formed by caving zone is mainly manifested in the destruction of horizontal reflection layer by caving zone, the change of seismic phase and the diffraction phenomenon of local development.
In Gordonstone Coal Mine, Australia (1994), the long-wall working face with a width of 250m and a length of 1500 ~ 2000 m in Germancreek, the main coal seam, was monitored by microseismic monitoring technology. The roof fracture law of event response is consistent with the theoretical analysis, which well reflects the roof fracture height. The main principle is that after the underground coal seam is mined, the overlying strata will lose their support and produce stress imbalance. When this stress reaches a certain level, it will suddenly release, leading to roof rock fracture or caving, thus generating a large number of microseismic signals. These signals are similar to natural earthquakes, but the energy is weak and the propagation range is small, so people generally can't feel them, so they are called "microseisms". There are two kinds of cracks in the roof of coal seam, namely tensile cracks and shear cracks. Microseismic monitoring can calibrate the shear fracture zone. Shandong Coalfield Geology Bureau (1998) introduced Australian coal mine microseismic monitoring system, and carried out "two zones" monitoring test on 4320 working face of Xinglongzhuang Coal Mine of Yankuang Group Company. Taking the event-intensive area as the fracture zone, the buried depth of coal seam in the working face changes greatly due to the influence of coal seam dip angle. The monitoring results show that the shallower the buried depth of coal seam, the smaller the development height of the two zones may be. No.3107 working face in Luxi Coal Mine (Wang Huajun, 2006) used microseismic (MS) monitoring technology to monitor the microseismic signals of overburden failure for a long time, located 92 events collected by geophone, and took the height of microseismic event intensive area as the height of water flowing fracture zone, which achieved satisfactory results.
Han Xushan (200 1) uses acoustic amplitude images and acoustic time difference images formed by acoustic logging to identify separation zones, fracture zones and caving zones by identifying sedimentary rhythm and sequence changes.
During the trial mining of Group A coal in Kong Ji Mine (1998, 1999)-250m horizontal West No.2 and No.4 mining areas, the height of water-conducting fracture zone of roof strata and the failure depth of floor in the working face after the mining of Group A coal were found out through acoustic CT detection test. Its working principle is: using the characteristics of slow propagation speed, large energy consumption of rock mass sound, small amplitude and strong absorption of high-frequency components of spectrum when sound waves propagate in damaged loose rock mass, the height of water-conducting fault zone is detected by measuring the sound wave changes in overlying strata before and after mining. The principle is similar to acoustic CT detection. Radio wave perspective detection and electromagnetic wave CT tomography technology of rock mass between holes can also be used to perspective rock mass in some water-conducting fractures, which has certain reference significance.
As a reference value, the caving height detected by geophysical exploration technology often needs to be demonstrated or verified according to actual geological conditions and production conditions. According to the data analysis in recent years, microseismic monitoring can not only dynamically observe the failure characteristics of roof overburden, but also better reflect the whole picture of fracture zone and distinguish caving zone from fracture zone. The observation results are consistent with the theoretical analysis and have a good application prospect.