- freely available
Energies 2014, 7(12), 8483-8507; https://doi.org/10.3390/en7128483
2. Mining and Geological Conditions
2.1. Location and Surface Morphology of the Study Area
2.2. Mining and Geological Conditions of the 6203# Coalface
|Sequence||Lithology||Thickness (m)||Depth (m)||Remarks|
|1||Aeolian sand||22.0||22.0||Aquifer located at the base|
|6||Packsand||10.6||74.3||Main roof (Key stratum)|
|7||Silty mudstone||3.5||77.8||Immediate roof|
|8||Coal||5.0||82.8||6# coal seam|
3. Method of Radon Detection on the Surface
3.1. Steps for Radon Detection on the Surface
3.2. Layout Schemes of the Measuring Points (MP) and Evaluating Points (EP)
3.3. Layout of the Detective Cup (DC)
3.4. Sizes of the Borehole
4. On-Site Radon Detection Trial and Measured Data Analysis
4.1. On-Site Radon Detection Trial
4.2. Measured Data Analysis
4.2.1. Experimental Error Evaluation
|EP||Average value||Standard deviation σ||Coefficient of variation|
4.2.2. Measured Data Analysis
|Date||Advance distance (m)||Date||Advance distance (m)|
|25 July||8.8||2 August||84.8|
|26 July||18.3||3 August||93.7|
|27 July||27.2||4 August||100.3|
|28 July||38.6||5 August||109.1|
|29 July||47.9||6 August||117.7|
|30 July||57.6||7 August||127.6|
|31 July||66.3||8 August||137.0|
|1 August||75.9||9 August||147.5|
5.1. The Source of Radon
5.2. The Mechanism of Radon Migration
- Radon diffusion and convection. Fliigge and Zimens first proposed that radon migration was caused by the diffusion effect in 1939 [37,38]. They suggested that radon migrates from areas of high concentration to areas of low concentration, a migration based on the Fick Theorem. Fleisher and Mogro Campero were the first to propose that radon migration was driven by the convection effect in 1979 [39,40]. Radon has been reported to travel considerable distances under the convection effect. However, many researchers have found that the combination of diffusion and convection is a viable migration mechanism for radon over distances of hundreds of meters [41,42].
- Relay transmission. It may be radium to transmit radon and radon to transmit radon, also may be to rely on other matter or natural force. There are two types of transmission: two-step relay and multi-step relay. The authors think that the action of relay transmission may interpret the long migration of radon [47,48].
- Infinite source model. The build-up of stress prior to an earthquake or the release of the stress after the event can cause the change in strain field within the earth. This field is expected to extend to long distance if the earth acts as an elastic continuum. Wherever the ground is squeezed, the pore spaces are narrowed non-linearly and radon gas will be extruded. Wattananikorn et al.  considers that an earth model in which an infinite source is overlain by an overburden of thickness which contains no radon source.
- He-Rn cluster. Jia et al.  found that radon and most of its daughters and parents are decaying bodies that are able to radiate α-particles. After deceleration, the 4He combine with 222Rn and its daughters and parents to form He-Rn atomic clusters. When the buoyancy of air is greater than the gravity of the clusters, the self-ascending phenomenon occurs.
5.3. The Principle of Radon Detection Technique
5.4. Requirements for Future Research
- A series of laboratory tests are necessary in order to investigate the effect of uncertain factors, such as strata types, moisture content, and mineral particle size, on radon concentration in strata, especially reduce the degree of influence due to environmental factors that can affect the radon variability. This is the most elementary study for radon detection technique.
- How the mining-induced effect affects the movement of radon through strata? Although some numerical models of radon migration in soil have been proposed, there are few published studies available. Therefore, numerical modeling analysis is necessary to study the mining-induced effect on radon movement, in particular on its upright migration through underground multilayer strata. This study is of fundamental significance to the radon detection technique.
- More on-site detection work on mining-induced fractures in overlying strata by using radon technique on the surface should be undertaken. These studies should consider the effect of various mining and geological conditions for quantitative study on the development law of mining-induced fractures in overlying strata.
- If the conditions are allowable, the multi-parameter information should be synthetically considered. In recent years, some studies have investigated the detection of other forms of energy (e.g., acoustic emission, electromagnetic emission and neutron emission) as a reliable method for the monitoring of fracture phenomena and environmental protection against seismicity. For example, Carpinteri et al.  observed that energy emission during the failure process, clearly indicating charge redistribution, and neutron bursts, necessarily involving nuclear reactions, by subjecting quasi-brittle materials such as granite rocks to compression tests. Borla et al.  measured the electromagnetic pulses generated during micro-cracking of rock specimens by a dedicated loop antenna sensitive up to MHz, then took into account the relationship between electromagnetic emissions, neutron emissions and seismic activity, finally considered that it would be possible to set up a sort of alarm system that could be at the base of a warning network. Therefore, we should consider using the detection of other forms of energy together with radon measurement for our research.
- Due to its properties, the radon migration process is relatively intricate and the various mechanisms suggested for its transport through the overlying strata are not fully understood. However, the diffusion and convection effects are widely considered to be the primary mechanism of radon migration, which lays the theoretical basis for the relevant research.
- The on-site radon detection trial conducted in the 6203# coalface demonstrates that the first weighting step is about 60 m, and the average periodic weighting step is about 20 m. These values are consistent with the observation results of underground mine pressure. In addition, it is inferred that the influence coverage of the advanced abutment pressure is equal to 30 m.
- Although this study is based on specific detection conditions and limited measured data, the results suggest that the radon detection of mining-induced fractures from overlying strata to the surface is feasible. The radon detection technique may not directly help to prevent from any damage, as the damages (e.g., rock fracture, roof failure) already have happened during underground coal mining. Nevertheless, based on the detection results, the corresponding control measures of mining techniques (rapid advance, reducing mining height, localized backfill, grouting reinforcement, etc.) can be performed to reduce the development of the mining-induced fractures for the mitigation of safety issues as well as the adverse environmental consequences. Therefore, the new detection method should be generalized to account to the caused damage for the coal companies in western mining areas in China, which is also the long-term aim of this study.
- In order to apply the radon technique to the detection of mining-induced fractures in overlying strata with greater certainty, further studies are needed in future, such as a series of laboratory tests, numerical modeling analysis, more on-site detection work, and multi-parameter information consideration.
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