1、翻译部分英文原文Remnant Roof Coal Thickness Measurement withPassive Gamma Ray Instruments in Coal MinesStephen L. Bessinger and Michael G. NelsonAbstruct:Current underground mining practice often requires that a predetermined amount of coal be left on the roof of the mined-out area. The need to leave such c
2、oal occurs on both continuous miner and longwall sections is derived from considerations of ground control, quality control, machine guidance,or simply good operating practice. Efforts at measuring boundary coal thickness have been employed mechanical, nucleonic, and energy adsorption and reflection
3、 methods. The nucleonic methods have found application in operations in the United Kingdom,the United States, the former Soviet Union, and Poland. Natural gamma devices are currently the instrument of choice, and several successful installations exist. The calibration of natural gamma background (NG
4、B) instruments must be carefully maintained,and they cannot be used in areas where a NGB radiation is not present. This radiation is ordinarily present in the fine-grained sedimentary rocks that bound many coal seams.I. INTRODUCTIONModern underground coal mining practice often includes leaving coal
5、on the roof of the mine after mining is completed. Roof coal is often left on continuous miner sections for ground control purposes to prevent the failure of an immediate roof that consists of weak, friable rock. Roof coal may also be left in mines where concentrations of sulfur or ash are higher ne
6、ar the top of the seam to reduce the concentrations of these impurities in the salable product. Control of coal quality in this manner is especially advantageous in mines with longwall sections, where a large fraction of the production originates from one general area of the seam, making blending fo
7、r quality control more difficult.Small amounts of roof coal may also be left for purposes of machine guidance. This practice is common in applications where the coal-cutting machine is to be in an automatic control mode. Longwall face operation in this manner has been demonstrated in the United King
8、dom 1, 2, and similar systems have been tested in the United States 3, 4. Leaving a measured amount of roof coal in such applications makes it possible to guide the shearing machine, keeping it in the seam.Leaving both roof and floor coal can enhance both the performance and reliability of the cutti
9、ng machine by reducing its exposure to the high mechanical stress that is experienced when cutting the rock bordering the seam. This can increase pick life and reduce the wear on all parts of the cutting system 2, 5.The need to leave roof coal leads directly to the need for measurement of the thickn
10、ess of the coal layer left on the roof. Many methods for making this measurement have been investigated. Manual methods, including drilling and borehole inspection, are time consuming and often unreliable.Many instrumental methods have been investigated, including vibration analysis, pick force sens
11、ing, ultrasonic and radar detection, and nucleonic methods, but only the nucleonic methods have been used in actual production. The research conducted by CONSOL Inc. on nucleonic methods will be described in this paper. GAMMA-RAY BACKSCATTER SENSINGThe use of gamma-ray backscatter sensing for machin
12、e guidance was suggested as early as 1958 6. An active nucleonic device for coal thickness measurement was proposed in Great Britain in 1961 7 and designed in 1973 8, 9. In this device, a source of gamma radiation (usually cesium 137 or americium 247) is enclosed in a housing that is positioned near
13、 the surface to be measured. The gamma rays interact with the coal and rock, and are subject to both Compton scattering and attenuation. The backscattered rays are measured by a gamma detector, and coal thickness is calculated from a calibration curve.Several designs of this type of sensor were test
14、ed in England, and a commercial model manufactured by Dowty was tested by CONSOL in West Virginia. A prototype was also tested by NASA in the USBM test mine in Bruceton, PA. In every instance, several problems were encountered. Most significant was the variable effect of the air gap between the sens
15、or and the coal surface. Because of this effect, sensors were designed to operate in contact with the surface, which presented severe difficulties in actual mining operations. In addition, with the low-energy gamma radiation employed, coal thicknesses greater than 200-250 mm (8-10 in) could not be m
16、easured. It was also found that any variation of materials in the boundary coal or the immediate roof could significantly but unpredictably alter the calibration. Finally, the presence of an active radiation source in a typical underground mining environment raised concerns of safety and source control. Because of these problems, gamma backscatter sensors have been generally abandoned in favor of other devices11. NATURAL GAMMA BACKG
