Coal Waste Impoundments Risks, Responses, and Alternatives (2002) / Chapter Skim
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5 Technologies For Locating Mining Workings
5 Technologies For Locating Mining Workings
Pages 87-110
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From page 87... ...
Because of the time and expense associated with extensive drilling, remote sensing and geophysical methods have been employed to search for abandoned coal mines (e.g., Branham and Steeples, 1988; Miller and Steeples, 1991, 1995~. The objective of geophysical surveys is to provide descriptive information about the physical characteristics of a three-dimensional volume of earth material, including the presence of voids.
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From page 89... ...
This chapter offers an overview of the site characterization techniques that can be utilized to locate abandoned underground mines. A more detailed description of these techniques is available in a guide for selecting surface geophysical methods issued by the American Society for Testing and Materials (ASTM, 1999)
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From page 90... ...
subsurface to complement information from mapping and geophysical methods. Because the measurements are commonly made from a moving airborne platform, a large area can be examined quickly and cost-effectively.
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From page 91... ...
. GEOPHYSICAL METHODS Noninvasive active geophysical methods used to search for coal mine voids employ artificial electrical, electromagnetic, or mechanical energy to examine the shallow subsurface of the Earth (Sharma, 1997; Ward, 1990~.
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From page 92... ...
Furthermore, because of the need to obtain regional geophysical background information, the area needed for a geophysical survey may greatly exceed that owned or leased by a mining company, in which case rights of ingress and egress for geophysical measurements may be a serious issue. Electrical Resistivity and Electromagnetic Methods Resistivity techniques sense the electrical properties of the material through which a current passes.
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From page 93... ...
However, airborne surveys also have disadvantages, including limited separation between the source and receiver coils and a higher noise level caused by the movement of the coils through the Earth's magnetic field (Blakely, 1996; Nabighian, 1988, 1991~. Potential Field Methods Buried metal objects such as steel drums are often found with magnetometer surveys in which measurements with precision of one part in 50,000 of the Earth's total magnetic field are made.
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From page 94... ...
94 500 400 300 200 100 50 u, 't 40 in o 30 20 10 5 4 : COAL WASTEIMPOUNDMEN7S ~ \\ \ \\ \\ AN If. No \ go ~3 \~ Anon \\ \ \~m Feet 2 4 6 10 20 \\ ,\~\ 40 60 80 100 150 200 Centimeters 100 200 400 600 8001000 2000 30004000 Distance from magnetometer— FIGURE 5.1 Example of magnetic anomaly at various distances for common metallic items such as tools and vehicles.
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From page 95... ...
Near-Surface Seismic Methods Seismic research has met with limited success when conducted to detect cavities resulting from abandoned subsurface coal mines (Fisher, 1971; Hasbrouck and Padget, 1982) , salt-solution mining (Cook, 1965)
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From page 96... ...
in which absence of a normally strong coal-bed seismic reflection indicates a mined-out coal bed.
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From page 97... ...
Seismic-wave transmission surveys are set up to test the transmissivity of the coal seam by deploying seismic sources along one face of a coal panel and placing geophones along the opposing face. If disturbances are inferred from the transmission experiment, a seismic reflection survey may be used to
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From page 98... ...
The advantages of in-seam seismic technology lie in the twodimensional propagation of seismic waves in coal seams, and often in accessibility to a coal seam on both sides of an assumed disturbance. Inseam seismic methods allow the use of higher frequencies and broader seismic bandwidths than surface seismic methods, and these offer better resolution of features of interest.
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From page 99... ...
In geometrical concept, ground-penetrating radar and seismic reflection are similar. Ground-penetrating radar data can be displayed in a format identical to that used for seismic sections.
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From page 100... ...
In the case of underground coal mines, the cracks induced by mine collapse and subsequent subsidence create fractures that can allow for accumulated gases to escape from mine workings to the overlying soil or ground surface. Methane gas is often associated with coal seams, and thus, portable gas detectors could be useful as a screening too!
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From page 101... ...
101 (God)
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From page 102... ...
Many near-surface geophysical techniques are still developing rapidly; though limitations imposed by steep terrain must be addressed. Today, for example, ground-penetrating radar data are collected using a single receiving antenna, but using multiple antennae could enhance ground-penetrating radar capabilities in much the same way that the seismic reflection method was revolutionized by common-midpoint surveying in the 1960s.
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From page 103... ...
When noise is present in shallow seismic or ground-penetrating radar data, a data-inversion routine may produce artifacts related to its attempt to invert the noise. Automation could improve very near-surface geophysical methodsfrom model airplanes carrying microsensing devices to robots roving the ground over hazardous or polluted areas.
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From page 104... ...
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From page 106... ...
A coal seam usually produces a strong seismic reflection from its top and bottom interfaces with surrounding rock, such as shale. The reflections from the top and bottom of the coal are represented by waves with blackened peaks that can be followed by eye in a coherent fashion from one seismic trace to another.
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From page 107... ...
Courtesy of CONOCO, Inc. HYDRAULIC TESTING A potential technique that may aid in determining the extent of a coal outcrop barrier or coal seam is to test a questionable area hydraulically.
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From page 108... ...
The first two coherent blackened peaks on the seismic section represent seismic refractions rather than reflections. The third and fourth blackened peaks represent the coal reflection, which is absent where the seismic survey passed directly above the abandoned mine workings.
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From page 109... ...
Multiple geophysical techniques may be necessary to reduce the probability for error to an acceptable level; drilling is required for confirmation. The committee concludes that geophysical techniques are useful in some cases in coal mine void detection, especially the use of seismic surface waves, seismic reflection, ground-penetrating radar, and electrical resistivity methods.
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