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5 Technologies for Locating Mine Workings Locating underground mine workings in the absence of mine maps is not impossible, but it can be expensive and time consuming. Drilling enough vertical holes on a 3-foot grid will locate virtually any mine workings, but, in addition to the expense, it damages the Earth's surface and creates conduits for underground fluids, including pollutants Mat can degrade groundwater. Because of the time and expense associated with extensive drilling, remote sensing and geophysical methods have been employed to search for aban- doned 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. Because no geophysical technique is capable of performing optimally under all geological and topo- graphic conditions, multiple geophysical techniques may be necessary to reduce the probability for error to an acceptable level. While these methods have proved successful in some cases, drilling is still necessary to confirm interpretations of geophysical and remote sensing data (Table 5.1~. In addition, the absence of evidence of a mine is not evidence of absence of a mine, and there are many opportunities for error in the modeling and geophysical surveys needed to detect voids. To give regional context to a local area, geophysical surveys begin with baseline information. Consequently, the area surveyed with geophysical methods is commonly several times larger than the planned impoundment, particularly for the less expensive methods such as magnetic surveys. Furthermore, during the planning and interpretation stages, geophysical surveys and data collection should be accompanied by geophysical modeling using the tools of physics, geology, engineering, and mathematics (NRC, 2000a). Today, modeling software that runs on personal computers is available for all of the geophysical techniques. Moreover, in-field modeling with a personal computer can often be of use in making evaluations during - 87
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TECHNOLOGIES FOR LOCATING MINE WORKINGS 89 the data collection process, including changing the extent and emphasis of a survey. The appropriate strategy for finding subsurface cavities depends on the size of the target. That size determines the spacing of geophysical measure- ments, which should be sufficient to define subsurface conditions. Decreasing density costs less but may increase the probability of error in the detection of voids. To achieve a suitable detection probability, any site investigation should work from a regional to a local to a site-specific approach; techniques should progress from large-scale and noninvasive (e.g., aerial photography, remote sensing) to detailed site data (e.g., drilling, borehole geophysical methods). Holes should not be drilled until the investigators know what they are looking for, and measurements should not begin without an adequate understanding of the surrounding geology (R. Benson, Technos, personal communication, 2001~. The inclusion of microprocessors in modem geophysical instrumentation has made on-screen, menu-driven geophysical data collection easy for those who have limited geophysical training. Problems can arise when the data are collected with inadequate spatial sampling density or with inappropriate techniques, and these problems can be compounded when individuals with inadequate geophysical education and experience attempt to interpret the data. Even if the data have been collected correctly, inappropriate data processing and interpretation can introduce many pitfalls. Furthermore, no single noninvasive technology works everywhere all the time. Consequently, an interdisciplinary approach is needed to interpret and to integrate multiple geophysical data sets with local geological and engineering data. 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) and in Appendix E. DRILLING There are several ways to drill shallow holes into the Earth (e.g., NRC, 1994~. The simplest is done using an auger that brings soil material to the surface much like a drill bit boring in wood brings wood cuttings to the surface (Hypes, 1995~. Augers cannot penetrate solid rock; they are used primarily to drill in soil, unconsolidated sands and clays, and soft shales. Only the shallowest of coal beds can be reached with auger drills. Most modern drilling at depths of more than 100 feet or so is done using rotary -
go COAL WASTElMPOUNDMENTS methods in which air, water, or drilling mud (often containing high-density barite) is circulated down the center of the drill pipe and back up the outside of the drill pipe to help move the cuttings to the top of the hole. If solid pieces of rock rather than cuttings are required for analysis, the investigator must resort to core drilling, which is usually more expensive than simple rotary drilling. REMOTE SENSING One of the principal uses of remote sensing in coal-related activities is to detect linear features, such as faults and Dacture zones, on scales Tom miles to tens of miles. Delineation and evaluation of such regional linear features is necessary as part of the permit application process for a new impoundment. Remote sensing provides observations of the Earth's surface and shallow (less than 3 feet) 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. For remote sensing to be useful in evaluating subsidence-related problems, spatial resolution on the order of 3 feet or less is required. Measurements from low-flying aircraft, or even from unmanned aircraft, can provide such high-resolution measurements and photography. For additional information on remote sensing applications, see NRC (1995) and Watson and Knepper (1994~. The oldest form of remote sensing is aerial photography. Historic black- and-white aerial photos, often dating from 1930s Works Projects Adminis- tration files, are sometimes available for comparison with modern photography. Modern scanning and digital registration techniques can be used to enter these old photos into a geographic information system format. Historic photos may indicate shafts and other facilities that have been abandoned and covered but not properly plugged, and are also useful for analysis of drainage pattems, detection of geologic conditions, such as landslides and debris flows, and signs of vegetative change, including stress from pollution or from shallow voids. Multispectral scanning samples the field at several different parts of the electromagnetic spectrum (including outside the wavelength range visible to humans) and digitizes the data for later analysis. The spectral bands sampled are selected to provide the most sensitivity to features that can characterize problems of interest. Individual portions of the spectrum are analyzed with digital signal processing techniques and the results are compared with
TECHNOLOGIES FOR LOCATING MINK WORKINGS 91 statistical or physical models or to information extracted from specimen- based laboratory measurements. For regional analysis (tens to hundreds of square miles), the 100-foot resolution satellite-based thematic mapper is a useful tool. It employs six different bands in the reflected spectrum, along with a 400-foot resolution thermal channel. Because of the spatial resolution limitations, the thematic mapper is not useful for the detailed analysis needed for localized problems. Regional studies can also benefit from archival radar and photographic data from national surveys, available from the U.S. Geological Survey's Earth Resources Observation System Data Center. The IKONOS high-resolution satellite, which offers 3-foot resolution, also is useful for mapping and surveying. For localized studies, aircraft systems that use additional spectral channels and better spatial resolution can be employed. Reflectance data have been used to distinguish among geologic units, to detect geologic structures, and to trace linear fractures. They have also been used to indirectly infer lithologic and soil information in vegetated areas based on empirical relations between vegetation and geological conditions. Thermal infrared data can be used to detect exothermic reactions, such as underground coal fires, and to find hydrological features such as springs (NRC, 2000a). 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~. In contrast, passive geophysical techniques measure some natural physical parameters of the Earth, such as minute variations in the Earth's gravita- tional field (see Dobecki and Romig's 1985 review). Physical parameters measured directly during shallow geophysical surveys include: electrical, thermal and stress fields, gravitational and magnetic fields, electrical conductivity, elastic (i.e. seismic) properties, transparency to and polarizability of electromagnetic waves, and natural gamma radiation. These measurements can then be used to infer the permeability, porosity, chemical constitution, stratigraphy, geologic structure, and various other properties of a volume of material near the Earth's surface, including the presence of voids. Geophysical methods can be used to guide exploratory drilling programs, but they cannot be expected to eliminate confirmation drilling.
92 COAL WASTEIMPOUNDMENTS The geophysical methods used and how they are applied vary according to a project's objectives, resolution requirements, budget, and geological situation. For example, seismic methods are sensitive to the mechanical properties of earth materials but not to the chemical makeup of these materials and the fluids they contain. In contrast, electrical methods are sensitive both to fluids and to magnetic or electrically conductive materials. Usually, multiple geophysical methods offer better answers than any individual method. The difficulty (and sometimes the success) of geophysical surveys is affected by topography. For example, it is easier to collect, process, and interpret geophysical data in the agricultural fields of Illinois than in the steep valleys of West Virginia. In addition to topography, vegetation and cultural features such as buildings, roads with traffic, and fencescan be barriers to geophysical surveys. 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. Electrically conductive contaminants can be tracked using resistivity methods. For example, resistivity would be expected to be more effective for finding mine workings full of polluted water than for detecting mine workings full of air. Under some conditions, these methods can be used to find geological faults and buried valleys but usually not with the precision of seismic reflection techniques. Resistivity surveys are usually cheaper than seismic surveys. Multichannel electrical cables similar to seismic cables have recently been developed to increase the flexibility and the rate of resistivity data collection. Electrical and electromagnetic survey data interpretation often involves mathematical inversion, producing a model that fits the data (e.g., Ellis and Oldenburg, 1994; Zohdy, 1989~. Electromagnetic methods have partially replaced resistivity surveys because equivalent information is obtained faster and without inserting electrodes into the ground. Electromagnetic methods include active methods in which an electromagnetic signal is induced in the ground by human activity, and passive methods in which natural variations in the electromagnetic field of the Earth are analyzed (Appendix E). The induced polarization method is
TECHNOLOGIES FOR LOCATING MINK WORKINGS 93 related to the resistivity method, except that the Earth's delayed response to an induced current is analyzed. The spontaneous potential method measures the natural voltage in the Earth resulting from electrochemical activity. If water in mine workings is reacting with its surroundings, this might be detectable with a spontaneous potential survey. Water in motion through fractured and porous media produces a "streaming potential," which is ~efi~1 in detecting leaks in dams. This technique might also be useful in looking for mine works in which water is flowing. Active electromagnetic methods have become more popular in near- surface geophysical applications (Appendix E). The theoretical basis for, as well as practical background for, electromagnetic methods is provided in McNeill (1990~. These methods have a major advantage over direct-current resistivity because they do not require placing electrodes in the ground. Indeed, the surveys can sometimes be conducted from low-flying aircraft. A recent development in airborne electromagnetics offers the advantages of increased surveying speed and access to polluted, dangerous, or inaccessible areas via small (maximum dimension 3 to 6 feet) unmanned aircraft. 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 magneto- meter surveys in which measurements with precision of one part in 50,000 of the Earth's total magnetic field are made. Although data precision and collection rates continue to improve, magnetic surveying is a relatively mature science. In the future, vector recording of the magnetic field instead of the commonly used total field could be useful. Because coal is relatively nonmagnetic, the removal of coal does not alter the magnetic field very much. Consequently, magnetic surveys are not commonly useful in finding underground mine workings. They could be useful, however, in detecting old cased wells or mine workings that contain metal pipes, cables, rails, or equipment. Figure 5.1 shows the size of a magnetic anomaly that is typical at various distances for common metallic items such as tools and vehicles. Magnetic "radiometry consists of taking simultaneous readings from two magnetometers spaced a few inches to several feet apart and analyzing the difference (the magnetic gradient). Magnetic surveys are also useful in mapping faults, locating magnetic bodies, and estimating the depth to of_
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. Reprinted with permission from Breiner, 1973. Copyright 1973 by Geometries.
TECHNOLOGIES FOR LOCATING MINE WORKINGS 95 magnetic earth materials. Such surveys are used to detect variation in the magnetite content of rocks and unconsolidated materials, so they can detect changes in some types of igneous rocks and other geologic structures. They are used also at contaminated sites to measure the perturbation of the Earth's magnetic field caused by buried ferrous metal objects such as steel drums, the ferrous metal waste in landfills, and iron pipes (e.g., Roberts et al., 1990a). Microgravity surveys measure minuscule changes in the gravitational field of the Earth using gravity-meters with a sensitivity of 1 microGal (where 1 Gal is a gravitational acceleration of 1 cm/sec/sec). Readings are made along a profile line or on a grid with typical spacing of 1 to 100 meters (3 to 330 feet). The sensitivity of microgravity measurements is one part per billion in comparison to the Earth's gravitational field. A map of gravity anomalies reflects the lateral density contrasts detectable after removing all known effects that can cause changes in gravity, such as, tide, instrument drift, elevation and latitudinal variation, and terrain. For near-surface geophysical exploration, microgravity surveys some- times are used where a high contrast in density occurs between bedrock and overlying alluvium, or between air and rock in a mine. Minute gravity anomalies can be caused by artificial features such as trenches, tunnels, disposal containers, and incipient subsidence problems (e.g., Roberts et al., l990b; Yule et al., 1998), as well as by geologic features such as cavities, faults, folds, dipping layers, and lateral intralayer heterogeneity. While microgravity methods could be applicable in finding shallow air- filled mine workings, they would not be the first choice for finding water- filled mine workings because the density contrast between the missing coal and the water that replaced it is too small. Figure 5.2 shows the calculated gravity anomaly at the Earth's surface above an air-filled 20-foot-diameter horizontal, cylindrical mine entry. 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), lava-flow tunnels (Watkins et al., 1967), and natural caverns (Rechtien and Stewart, 1975~. Most researchers using seismic techniques for cavity detection cite three phenomena as evidence of a cavity: free oscillations or resonance of the cavity walls, anomalous amplitude attenuations, and delay of arrival time (Cook, 1965; Fisher, 1971; Godson and Watkins, 1968; Robinson and Coruh, -
96 CO - ._ COAL WASTEIMPOUNDMENTS -0.01 -0.03 -n no ~S h~ 26 -0.07 210 105 0 105 210 Horizontal distance from tunnel (feet) FIGURE 5.2 Calculated theoretical gravity anomaly for three air filled 20-foot diameter horizontal cylindrical openings. Lee three curves represent calculations for depths of 26, 52, arid 78 feet to the center of the opening' with the largest negative anomaly associated with the shallowest opening. The background arid instrumental noise for m~crogravity methods limit the absolute value of useful anomalies to about 0.01 mGal. If the cavity were water-filled instead of air-filled, the anomalies would only be one third as large. 1988; Watkins et al., 1967~. In addition, some success in locating water- filled coal-mine cavities at depths of less than 50 feet has been reported using high-resolution P-wave (compressional, i.e., sound waves) reflection seismology techniques (Branham and Steeples, 1988; Miller and Steeples, 1991) in which absence of a normally strong coal-bed seismic reflection indicates a mined-out coal bed. Cook (1965) found that seismic energy transmitted through a cavity and reflected from a deeper horizon gives rise to a seismic amplitude "shadow." Most of the seismic research on coal-mine detection has involved P-wave refraction seismology or S-wave (distortional, i.e., shear waves) reflection seismology. Significant improvements have been made since 1980 in near- surface P-wave seismic-reflection techniques (Hunter et al., 1984; Steeples and Miller, 1990), shallow-seismic refraction interpretation (Lankston and Lankston, 1986; Palmer, 1980), and surface-wave methods (Park et al., 1999; Stokoe et al., 1994; Xia et al., 1999~. Surface-wave phase anomalies might be employed to detect near-surface voids (Rechtien and Stewart, 1975~. Shallow S-wave reflection survey results are reported in the literature =
TECHNOLOGIES FOR LOCA TING MINE WORKINGS 97 (e.g., Goforth and Hayward, 1992; Hasbrouck, 1991), but separating the S- wave reflections from the surface waves that often appear on seismograms at the same time is a problem. Seismic shear waves may be useful for cavity detection because they will not propagate through voids or water-fi~led cavities. Shear wave reflections have also been used to evaluate the resources of a shallow coal seam (Hasbrouck and Padget, 1982~. Geophysical tomography is conceptually and mathematically identical to medical tomography in which three-dimensional X-ray imaging from within the human body is accomplished by computed axial tomography (CAT scan). The technique uses measured travel times or signal strength of many geophysical ray paths through a volume of earth material. Seismic tomography has been used to examine Earth's interior from scales of a tens of feet to thousands of miles (e.g., Clayton and Stolt, 1981; Humphreys et al., 1984). Future seismic applications that merge P- and S-wave refraction information may be useful (Hasbrouck, 1987~. By combining P-wave and S- wave velocities with density readings obtained from gravity surveys or borehole density logs, one can measure the elastic parameters of rocks. When densities and velocities are known, Poisson's ratio, Young's modulus, and the shear modulus can be calculated. When these elastic constants are known, rock types can often be identified and an estimate of pore-space fluid content usually is possible (Domenico and Danbom, 1987~. New opportunities for three-component recording and multimode analysis are a result of decreasing cost and increasing capabilities of seismic hardware designed to collect and process high-resolution, near-surface seismic data. The seismic wave types, generally discarded by classical seismic reflection surveyors during the processing, analysis, and interpretation of data, contain information about the upper tens of feet of the Earth. The capabilities of seismic methods involving depths shallower than 100 feet can be extended by analyzing the near-surface broadband seismic wavefield using three vector components rather than one and by examining multiple types (modes) of seismic waves instead of just P-waves. The principles of in-seam seismic transmission and reflection surveys can be applied to estimate the presence and location of faults (Buchanan et al., 1981; Greenhalgh et al., 1986) and air- or water-filled or collapsed mine workings (Mason, 1981~. In-seam seismic surveys are typically performed in panels surrounding blocks of coal prior to long-wall mining operations. 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
98 COAL WASTEIMPOUNDMENtTS estimate their locations. Brentrup (1970) summarizes more than 200 in-seam seismic surveys, reporting a success rate of 66 percent for reflection and 83 percent for transmission surveys. Since the 1960s, in-seam seismic methods have been applied in coal mines to find disturbances in the coal seams that may pose problems for mining (Schwaetzer, 1965~. Disturbances include faults that offset coal seams and reduce production rates of longwall mining operations, abandoned mine workings that pose a general risk to under- ground activities, and methane zones in which mining activity may trigger explosions. A more extensive discussion of in-seam seismic methods is included in Appendix E. Faults with vertical displacement of one or more seam thicknesses are good reflectors of in-seam seismic waves because of the contrast between the coal and the bedrock material. Water- or air-fled cavities are even better reflectors. In the case of collapsed mine workings, in-seam seismic waves may be scattered rather than reflected, so the reflected waves may not be as coherent as those reflected from a fault or a cavity. Nevertheless, the ampli- tude of the high-frequency transmitted in-seam seismic waves could still be sufficiently decreased to infer We presence of collapsed workings. In-seam seismic techniques can be applied to existing and planned coal-waste impoundments as long as the appropriate coal seam is accessible under- ground. This includes cross-hole tomographic methods, which have proven successful in other applications. The overburden material and surface topography will have no effect on the success of the experiment. Many improvements in seismic equipment and analysis techniques introduced in the last two decades enable a success rate greater than that noted by Brentrup (1970~. The advantages of in-seam seismic technology lie in the two- dimensional propagation of seismic waves in coal seams, and often in accessibility to a coal seam on both sides of an assumed disturbance. In- seam 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. The high-frequency waves are concentrated within the coal seam whereas lower-frequency waves are present within as well as outside the seam (Gritto and Dresen, 1992~. The advantage of absence of surface seismic noise such as wind and road traffic may be countered by mining activity, which might have to be interrupted during the seismic survey within a distance of 0.5 to 1 mile from the survey. However, the problems associated with surface topography are mostly negated.
TECHNOLOGIES FOR LOCATINrG MINE WORKINGS Ground-Penetrating Radar 99 In many places, ground-penetrating radar has become the method of choice for exploring the upper several feet of the Earth's subsurface (e.g., Daniels, 1996; Davis and Annan, 1989~. Ground-penetrating radar employs a source of microwave radiation that is radiated into Me Earn via an antenna at a known time. From the time measured for the wave to echo back to a receiving antenna at the surface, one can calculate the depth to various layers in the Earth, once an accurate wave-propagation velocity has been measured. Velocity determination is a critical factor and can be done by measurements on samples, by one-way travel time in a borehole, or by fitting a least- squares hyperbola to an observed-travel time versus horizontal-distance curve (Tillard and Dubois, 1995~. 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. However, the Earn environments in which the two techniques perform optimally tend to be mutually exclusive. Ground-penetrating radar works best in dry media in the absence of clays or other electrically conducting earth materials because electromagnetic radiation cannot penetrate into electrical conductors (e.g., moist clay). In contrast, seismic waves transmit well through moist clays but are rapidly attenuated in dry sand. Ground-penetrating radar imaging works at depths of 30 to 60 feet under favorable conditions, but may fail at depths of less than a 3 feet if clays or other conductive materials are present near the Earth's surface. Nuclear Magnetic Resonance Magnetic resonance imaging was developed for medical diagnosis as an outgrowth of physicists' nuclear magnetic resonance experiments (Knight et al., 1999~. Proton nuclear magnetic resonance, which responds to the state of hydrogen nuclei in the ground, is of interest to geoscientists. Proton nuclear magnetic resonance might be used to detect water-filled cavities because of its sensitivity to hydrogen nuclei. However, owing to the immature state of research in this field, referring to the geophysical use of nuclear magnetic resonance as "imaging" may be premature (Appendix E). ._
100 COAL WASTEIMPOUNDMENTS Radio-metric Techniques Radiometric methods measure emissions from radioactive isotopes. The techniques can be used to sense the presence of radioactive contaminants or to explore for radioactive ores. Specific isotopes that occur within 3 to 6 feet of-the Earth's surface can be identified by spectral gamma methods, which may also be useful for locating radioactive hazards, such as natural radon gas sources. Nielson et al. (1990) review natural gamma and radon emana- tion methods. Methods that measure anomalously high radon concentrations show promise for identifying abandoned underground mine workings. Research using this method was pioneered by geologists looking for buried geologic features such as faults (FIeischer and Mogro-Campero, 1979; Heirendt, 1988), and the method has since been adapted to locate abandoned coal mines (Misquitta, 1989~. Radon is thought to be concentrated in the voids left by mining, then released to the surface by way of subsidence fractures that result from mine collapse. The method is based on the concept that the alpha particle emission that occurs during the radioactive decay of radon will leave impressions on high-density plastic detectors and that the number of impresssions on the detectors can be directly correlated with standardized radon concentrations (Figure 5.3~. Anomalously high radon concentrations may correlate with the portion of the profile above the mine voids. Further refinement and development of this method could lead to a cost-effective, noninvasive screening method for detecting abandoned underground mines. Measurement of soil gases or gases that emanate from the ground is commonly used to detect buried wastes or containers. For example, photo- ionization detectors are hand-held instruments that detect gasoline deriva- tives or additives from underground, leaking storage tanks. Natural gas utilities and transmission companies also use similar instruments ("sniffers") to detect gas leaks. The technical literature is sparse regarding the innovative use of gas monitoring to detect old mine workings in coal seams. 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! to identify where mine voids with accumulated gas are connected to the surface by natural or subsidence-induced fractures. Detection of anomalous gas concentrations may not alone be indicative of mine voids, but this method could prove useful as a screening tool for a drilling or geophysical investigation of a specific area. The method could reduce costs while helping to focus more
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102 COAL WASTEIMPOUNDMENTS quantitative methods such as drilling, and may aid in the success in detecting unmapped, mined-out areas adjacent to waste impoundments. Borehole Geophysical Methods Borehole geophysics is done by lowering a long sensing tool (called a sonde) attached to a cable into the drill hole (Daniels and Keys, 1990; Hearst et al., 2000) (Appendix E). The geophysical information is relayed to the surface via the cable, and the data are recorded at the surface for later analyses. The resulting data provide a plota geophysical logof various geophysical parameters as a function of depth in the hole. To detect underground mine workings, Borehole geophysics can com- plement drilling by examining the area around the Borehole to a radius of 3 feet or more. Also, if two or more boreholes are available, cross-hole tomographic analyses can be performed with several of the Borehole geophysical methods. Borehole geophysical measurements offer the best resolution and decrease the effects of near-surface signal attenuation, formation heterogeneity, and some types of "noise." Many near-surface geophysical technologies obtain a degree of ground-truth from such borehole measurements. Small-diameter versions of the logging tools used in the petroleum industry have been developed for near-surface investigations. Near-Surface Geophysical Research Near-surface geophysical methods should have a bright future in the coal mining industry. 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. Theoretically, the potential is great for widespread use of seismic techniques for detection of voids, such as underground coal mines. However, currently available two-dimensional seismic reflection and refraction methods have met with limited success. Extending the usefulness of the seismic method for void detection will require new, state-of-the-art techniques that utilize more of the wavefield than seismic P-wave reflection alone.
TECHNOLOGIES FOR LOCATING MINE WORKINGS 103 A large portion of the information contained within shallow, high- resolution seismograms is not used or emphasized in standard two- or three- dimensional reflection surveys. Cavity detection is a fundamentally different problem than those addressed in standard petroleum exploration. Although the reflected portion of the wavefield may yield the highest resolution information about void location, other portions of the wavefield may provide key constraints in the detection and interpretation phases of a survey. Thus, a variety of different processing, modeling, display, and interpretation methods should be investigated to determine whether it is possible to exploit seismic wavefields uniquely altered by the presence of a shallow subsurface tunnel. No seismic method appears to be uniformly applicable in the highly variable near-surface geology of different tunneling environments. Each method will probably be of use in some specific geological environment (Tables 5.2 and 5.3~. Sidebars 5.1, 5.2, and 5.3 illustrate case histories where seismic reflection has been used to locate mine workings. The mine workings cause a shadow effect, which decreases signal strength for reflection from layers below the coal (Sidebar 5.1~. The void presence in Sidebar 5.2 causes the coal bed reflection to disappear. At present the immense computational resources required limit the full waveform inversion of both seismic and ground-penetrating radar data to small data sets. When computing costs have decreased sufficiently, these inversions may become commonplace. One caveat, however, is that the inversion process treats noise with the same reverence that it treats data. 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 methods- from model airplanes carrying microsensing devices to robots roving the ground over hazardous or polluted areas. Automatic emplacement of geophones (Steeples et al., 1999) may significantly improve cost- effectiveness of near-surface seismic surveys. All geophysical techniques could benefit from improved precision, resolution, and bandwidth. Data processing would benefit from faster and more robust methods, especially if the ambiguities and uncertainties that plague data interpretation could be decreased. By combining robotics, automation, expert systems, and miniature aircraft it may be possible to decrease costs and improve productivity. The efficient and timely transfer of technology from developers to users and potential beneficiaries could be enhanced through continuing education programs. -
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106 COAL WASTEIMPOUNDMENTS SIDEBAR 5.1 Case History: Coal Void Schematic Seismic reflection methods have been used successfully in a few cases to detect old coal mine workings. This diagram depicts schematic seismic- reflection data superimposed on a hypothetical geological cross-section containing a coal mine void. 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. While the coal represents a strong seismic reflector, the absence of coal (i.e. the mine void) results in the absence of the strong reflection as illustrated by the two seismic traces that pass through the void without producing a reflection. The seismic data are processed and displayed such that each seismic trace represents seismic-wave motion as a function of time, as if the seismic source (such as a small explosion) and the seismic receiver (a geophone) were located at the same point on the Earth's surface. Hence, the reflections with blackened peaks occur at a time on the seismogram that represents travel from the Earth's surface downward to the coal seam and then back to the Earth's surface. Coal i~ Schematic seismic-reflection response to a coal-mine void. = \ Void
TECHNOLOGIES FOR LOCA TING MINE WORKINGS 107 SIDEBAR 5.2 High-Frequency Seismic Case History In some cases it is possible to infer the presence of mines from decreased signal strength of seismic reflections from layers beneath the coal, which in this instance is about 550 feet deep. The vertical axis is in seconds of ~vo-way reflection time, and the coal reflection is present at about 0.13 seconds. In contrast to the Pittsburg, Kansas, example (Sidebar 5.3), it is not possible to distinguish individual rooms and pillars in this figure. It is possible to see the general location of the mined area and to define the mine boundaries to within about 65 feet (\/\laters, 1987~. The location of the mine near the center of the seismic section is indicated by a "faded" area except where the coal reflection is present. The coal reflection also has a lower frequency appearance in the mined area than in the unmined area. Surface Mined area 60 55 ~548~. I ~l~l,~ CO By .05 o a) .1 U' a) .15 .20 50 45 41 35 30 25 20 15 10 5 . . . . . . . - - Q a) Seismic detection of mine works at 548-foot depth. 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. This is accomplished by conducting hydraulic packer tests in boreholes drilled into the coal (Harrow and LeCain, 1991; Minns, 1993~. Tested intervals where the permeability of the coal is significantly higher than the statistical range for confined coal seams would be suspected of having void space within the smaller surface volume of unconfined coal. The increased stress could result =
108 COAL WASTEIMPOUNDMENTS SIDEBAR 5.3 Case Study: Cavity Detection The principle illustrated in the schematic diagram was used successfully to detect abandoned mine workings in an industrial park at Pittsburg, Kansas (Branham and Steeples, 1988~. In this case, the coal seam was about 3 feet thick at a depth of 33 feet. 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. The geological cross section above the seismic section shows the geological interpretation that was supported by too boreholes, one of which hit a mine void and the other, the coal seam. ~ 0 Qua `,, 5 ~ Void Coal Void 0 ~ 1 C cat _ Hi: 15 NE Seismic section Borehole with void Borehole with coal O ~ Distance (feet) o 33 - 66 99 132 Seismic detection of mine void at 33-foot depth. From Branham and Steeples, 1988. Courtesy of Society for Mining, Metallurgy, and Exploration, Inc. cone of influence of the injected fluid; this could be used to discern indirectly the coal outcrop barrier or seam's width. It should be noted that the interpre- tation of these data is complicated because the coal remaining to form the outcrop barrier or pillars would be under greater loading stress than the mmined coal seam due to the increased overburden pressure distributed on a
TECHNOLOGIES FOR LOCATING MINE WORKINGS 109 in the expansion of the cleat void spaces, effectively increasing the hydraulic characteristics of the coal. Therefore, additional data for coal under high lithostatic pressure should be collected and evaluated for feasibility and development of this method. Research should be performed to ascertain whether hydraulic testing has merit as a cost-effective and accurate method to aid in determining the extent of coal outcrop barriers and coal voids in mines adjacent to coal waste impoundments. SUMMARY One of the critical tasks in site characterization is ruling out the presence of voids. Invasive drilling programs can provide the necessary information. However, they may compromise the hydrological integrity above the mine, and their cost is often significant, both economically and environmentally. Well-planned and appropriate use of geophysical techniques can often help to minimize the amount of drilling required to detect mine voids. However, no single geophysical technique will work at all depths in all types of geology. From a practical standpoint, steep topography compounds the difficulty in collecting, processing, and interpreting geophysical data when surface methods are used, but these effects are minimized when borehole, cross-hole, and in-seam methods are used. In addition, trees and cultural features such as fences can impede geophysical data collection, processing, and interpretation. 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. The committee also concludes that geophysical techniques have been underutilized in the coal-mining industry and could benefit from additional research. The committee recommends that demonstration projects using modern geophysical techniques be funded, and that the results be widely conveyed to the mining industry and to government regulatory personnel through workshops and continuing education. Continuing education could include the opportunity to attend short courses and seminars that present the latest technology along with case histories to support its use. The committee notes that much more work has been done using geo- physical techniques on coal field problems than is indicated in the literature. Since a large amount of the work is proprietary or involved in litigation, little has been published. The committee notes that publication of case histories on this work would be desirable.
