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Coal Resources Available for Development - A Methodology and Pilot Study

By Jane R. Eggleston1, M. Devereux Carter1, and James C. Cobb2
U.S. Geological Survey, Reston, VA 20192, 2Kentucky Geological Survey, Lexington, KY.


In developing a methodology for determining available coal resources, several steps were defined. These are (1) study area selection, (2) data collection and assimilation, and (3) resource estimation. The following sections detail the methodology that was developed for testing in a pilot study.

Area Selection

It might be preferable to study an entire region in the detailed fashion developed in this methodology. However, since we arbitrarily define a region in this study as containing multiple counties and usually several States or parts of States, the time, manpower, and costs required to conduct such a comprehensive study make this approach impractical. It is therefore crucial that the study areas selected be representative of larger areas, so that results can be extrapolated. If a region is divided into segments based on similarity of characteristics such as geology, mining practices, mining history, production patterns, and previous coal resource assessments, areas having similar characteristics can be defined for subsequent purposes of extrapolation.

Geologic conditions may vary throughout a region, and these conditions can be categorized. The stratigraphic position of coal beds in the area is significant, because coals represent a variety of depositional environments through geologic time. These different environments dictate the coal's physical and chemical characteristics, including lateral continuity, thickness, quality, and roof and floor rock types. Regions also vary with respect to the degree of folding and faulting. It is therefore necessary to categorize a region by its stratigraphic and structural features so that no significant geologic characteristics will be ignored in the course of the regional study. At least one study area should be located in each major geologic type area.

Mining practices also may vary within a region, often because of geologic characteristics. Because these variations will influence the restrictions required, mining practices should be categorized. For example, in areas that have gentle topography, strip mining is more prevalent, whereas in areas that have steep topography, contour mining and mountaintop removal are common. In some areas, longwall underground mining is common, whereas in others room-and-pillar is the norm. At least one study area should be located in each different mining area. In many instances, the mining areas and geologic type areas could be the same.

The extent of past and present mining within a region varies and should be categorized in a general manner. Likewise, the reserve base and production of the region should be categorized. Areas that have a larger reserve base and production must be given greater emphasis, although areas that have lesser reserves and production should be represented by study, too. The reserve base has been developed in a number of previous studies, and these published data will provide a relative idea of resource distribution throughout the region. Production figures are published also, usually annually.

Several possible methods of extrapolating the study results from smaller to larger areas that have similar characteristics exist. But it is probably most logical to take advantage of previous resource studies to accomplish this. Once the study is completed for one area, the results can be evaluated and compared to previous resource estimates to obtain percentages higher or lower than previous estimates. These percentages can then be applied to previous resource estimates for the surrounding area to develop an estimate for current available coal for the larger area. In this methodology, however, it is assumed that the previous resource estimates for both the study area and the larger area were conducted in the same fashion, with the same assumptions, level of detail, and geologic knowledge. In addition, of course, the previously discussed geologic and restrictive features must be similar in both the study area and the larger surrounding area. Geological features include lateral continuity of the coal beds, structure (folding, faulting, and dip), and mine-roof stability. Restrictive features include the land-use and technological parameters (as described in the "Introduction") that most impact coal availability. While this extrapolation process is less thorough than performing an intensive study throughout the entire region, it is a workable method that will provide valuable results in a reasonable period of time for a large area.

The size of the study area selected should be a workable size but should have enough variability and information to represent, together with other study areas in the region, a realistic cross section of the region. For the purposes of this study, a 7.5-minute quadrangle was deemed the optimum size for a study area, because it would allow us to apply the necessary detail to accomplish the task in a reasonable period of time. In addition, much of the geologic mapping in the Eastern United States has been done at this scale.

Data Collection and Assimilation

After a study area is selected, data needs must be defined, and the necessary data must be gathered and assimilated. Data needs are categorized in the following way: geologic considerations, past and present mining, land-use restrictions, and technological restrictions. Possible sources of data are shown in table 1. A good data base is the key to development of an assessment of available coal. Time spent talking with local specialists familiar with the area and reviewing appropriate literature is well spent. Supplementary data can be collected in the field when other sources are deemed to be inadequate.

Geologic considerations include coal-related information such as coal thicknesses and intervals, lateral extent of coal, outcrops and structure (folding, faulting, and dip), and coal quality. Because this information is the foundation upon which all subsequent restrictions are applied in this methodology, adequate time and effort are required to develop this comprehensive geologic data base.

For each coal bed in the study area, accurate locations of past and present underground and surface mines must be identified and plotted. In some cases, locations already have been plotted by State mining agencies or geological surveys. Even then, however, updating is often necessary. The U.S. Bureau of Mines maintains a microfilm library consisting of maps of abandoned underground mines. In addition, State mining regulatory agencies have maps of recent surface and underground mines, because such maps are required for the mine permitting process. Topographic maps and air photographs also can be used to determine the location and extent of surface mines. Adits of underground mines often are shown on topographic maps, but in localities that have multiple closely spaced coal beds, mines cannot be assigned to a specific coal bed unless additional information is obtained. Also, the lateral extent of the underground mine cannot be determined from locations alone.

Land-use restrictions primarily impact surface mining, although a few may restrict underground mining also. The Federal Surface Mining Control and Reclamation Act of 1977 (Public Law 95-87) defined certain land uses that are protected from surface mining and (or) deep mining. These Federal requirements have now been incorporated into State regulations. State regulations vary but generally maintain the Federal requirements as a minimum. However, variances are given to many of the regulations. It is therefore vital to consider local practices when determining the impact of various land uses on mining in the study area.

The following land-use factors can restrict the mining of coal:

  1. Cemeteries.-Surface mining cannot be conducted through a cemetery; the U.S. Office of Surface Mining Reclamation and Enforcement (OSM) requests mine operators to leave a 100-ft buffer around cemeteries. The locations of most cemeteries are shown on topographic maps.
  2. Streams, Lakes, and Reservoirs.-Surface mining through small streams can be accomplished by rechanneling the streams. Streams that have a mean average flow of more than 5 ft3 /s are under the control of the U.S. Ariny Corps of Engineers, however, and should be considered a restriction on mining. Variances to mine across these streams are difficult to obtain. Streamflow records for streams in the study area can be obtained from the local USGS Water Resources Division offices or by direct measurements of the streams. Although more commonly impacting surface mining, streams and other water bodies restrict shallow underground mining because of the potential for subsidence hazards, which can cause flooding of the mine.
  3. Residences, Towns, and Public Buildings. -Federal law prohibits surface mining within 300 ft of a private residence or public building. But individual homeowners may sign a waiver allowing the mining company to mine up to, or through, their house. Often a house is moved to a new location. Groups of houses and public buildings such as schools, town halls, and hospitals do, however, pose a restriction to surface mining. Only shallow underground mining would be restricted by the presence of towns and buildings, and this restriction is because of the possibility of subsidence.
  4. Historic Sites and Non-Federal Public Parks. -Coal in public parks and historic sites cannot be mined, except in the extremely rare case in which all agencies responsible for managing these parks and sites approve. Locations of parks and historical sites are usually shown on topographic maps, or they can be obtained from local public agencies.
  5. Highways and Railroads. -Roads and railroads can be moved during surface mining, provided such action is cost effective and approved by the responsible agencies or companies. The one exception is federally funded highways, which cannot be mined through or moved for mining. A barrier of 100 ft must be left between the mine and highway.
  6. Powerlines and Pipelines. -No specific laws prohibit mining through these features. Sometimes a 100-ft buffer is left, especially for major line networks, but other times operators move the lines or mine under powerlines and leave islands of coal at the posts. Therefore, some coal may be restricted, depending on local mining procedures and economics.
  7. Federal Lands. -Surface mining is prohibited on lands within the boundaries of the National Park System, the National Wildlife Refuge System, the National System of Trails, the National Wilderness Preservation System, the Wild and Scenic Rivers System (including study rivers), National Recreation Areas, and areas designated as endangered species habitats. Mining is allowed on National Forest land but only if the mining will not interfere with the original purpose for which the land was set aside.
  8. Oil and Gas Wells. -Oil and gas wells restrict both surface and underground mining. In this study, we consider the wells as land-use restrictions when they restrict surface mining and as technological restrictions when they restrict underground mining. A 200-ft buffer is commonly left around the wells, but this buffer zone is sometimes less if the exact location of the well is known. Oil and gas well locations are usually available from State geological surveys.

As herein defined, technological restrictions primarily impact underground mining. However, depth and thickness of a coal bed may be considered as technological restrictions to surface as well as underground mining. Technological restrictions are based on the limitations of mining safety, costs, and equipment. The following are some technological factors that can restrict mining.

  1. Coal-Bed Depth and Thickness. -Technological factors can limit the minimum surface-minable coal-bed thickness because certain equipment does not have the flexibility to separate thin (generally 14 in or less) coal beds from the surrounding rock. Surface-mining depth is limited because certain equipment or combinations of equipment are not designed for deep pits. Most underground mining is limited to coal beds greater than 28 in thick because of equipment and manpower requirements; mines are generally within 1,000 ft of the surface because production costs increase with an increase in depth.
  2. Coal Beds too Close to Another Bed or Mine. -Beds that are close (usually 40 ft or less vertically) to an abandoned underground mine or another coal bed that is more likely to be mined are restricted from mining (sterilized) because of safety concerns. In mining close to an abandoned or active mine in the same coal bed, a barrier of at least 100 ft is generally required for safety purposes.
  3. Geologic Conditions that Impact Mining. -Unstable roof or floor rock can be a restriction if safety and cost factors adversely impact mining. If a coal bed is known to split, pinch out, or be faulted, it could be considered a restriction on mining, depending on local practices. Steep dip of the coal bed sometimes limits mining because the equipment is designed for relatively flatlying beds. An exception is in areas such as the Anthracite region, where the mining technique is especially designed for steeply dipping beds.

In summary, technological factors can limit the minability of coal in a variety of ways. Because local mining practices and geologic conditions vary regionally, regional differences must be taken into account in quantifying available coal. Mining engineers working for local companies and regulatory inspectors, who regularly visit the mines and are familiar with local conditions, are a good source of additional information.

Coal quality can influence the marketability of coal and therefore the likelihood of mining particular coal beds. Because of EPA's sulfur emission standards for coalburning powerplants, high-sulfur coal is not as marketable as low-sulfur coal. Other coal-quality factors, such as ash, moisture, and sodium and chlorine content, also have important effects on coal marketability. In southern West Virginia a few coal beds have high levels of inertinite macerals, which inhibit complete and rapid burning of the coal in power generation boilers. This coal-quality parameter restricts the coal as a competitive fuel source. Mine operators and coal marketing representatives usually have the best information on problems associated with coal quality, unique to the area, that impact marketability. State geological surveys have files of coal-quality data, and, if data are limited, they can be supplemented by collection and analysis of channel samples of coal beds in the study area.

Resource Estimation


The three basic elements essential to coal resource calculations are (1) coal-bed thickness, (2) specific gravity of the coal, and (3) the size of the area to be included in the tonnage estimate. Thickness is determined by measurements from coal-bed exposures at the surface (outcrops), from boreholes, and in coal mines. The number and spacing of the thickness measurements are major determinants of the degree of reliability of the estimate. Specific gravity is a measure of the weight factor of a coal and may be determined from individual coal analyses in the area involved. This measurement is employed mainly for mine development studies. However, a table of the average specific gravity for each coal rank in the United States has been established and is generally used for large-area coal resource estimation in this country. For bituminous coal, the average specific gravity is 1.32, or 1,800 tons per acre-foot; this figure is used for coal availability studies in the central Appalachian region. Areal measurements in this study were accomplished by computer calculation of the digitized areas.

The methodology for coal resource calculations developed in this study follows the Coal Resource Classification System of USGS Circular 891 (Wood and others, 1983). The basic criteria set forth by Circular 891 were modified slightly for timeliness in this study and are as follows.

  1. Only coal in beds greater than or equal to 14 in thick is included as a resource. Coal in a bed less than 14 in thick is excluded.
  2. Coal resource tonnages are reported in thickness increments of either 14 to 28 in or greater than 28 in.
  3. Three overburden categories are reported: 0 ft to surface-minable limit, surface-minable limit to 1,000 ft, and greater than 1,000 ft. Surface-minable limits may be 0 to 100, 0 to 200, or 0 to 300 ft, whichever would most closely follow local practice. The remainder are considered potential underground-minable coals.
  4. Coal resource estimates are reported in the following categories of assurance or reliability: measured (including coal 0 to 0.25 mi from point of thickness measurement), indicated (0.25 to 0.75 mi), inferred (0.75 to 3 mi), and hypothetical (greater than 3 mi).

Computer Resource Calculations

The initial steps required in calculating resources for this study include collection of data points for coal thickness, elevation, and quality parameters (where available); correlation of beds; determination of the approximate specific gravity; selection and delineation of the land-use and technological restrictions; and preparation of outcrop maps. Once these initial steps are completed, data entry begins. Data entry is by far the most time-consuming aspect of the study, but it is essential that data be entered correctly and stored in clearly documented fields. All subsequent work by a variety of users derives from this basic data base.

The point source data, including coal-bed thickness, elevation, location, lithology, and chemistry, are digitally recorded and stored in their appropriate stratigraphic and geochemical data bases. The coal-bed outcrop, the mined areas, and most of the land-use and technological restrictions are drawn on base maps, and each is digitized, labeled, and stored in its individual data base. Once the data have been entered, checked for errors made during entry, and corrected, the user may begin to generate derivative maps. Data-point maps are plotted to display the number of points and spread of the basic information. Gridded files of coal thickness, structure, and quality are generated, and the isopachs, structure contours, and isopleths of chemical values are plotted. The computer-drawn lines may then be modified, if necessary, to follow the user's interpretation of the thickness, elevation, and chemical character of the coal.

The depth of coal from the surface may be generated from a file of digital surface elevations. The National Mapping Division of the USGS has produced Digital Elevation Models (DEM's) covering about one-third of the United States. Fortunately, DEM's are available for most of the Appalachian basin. When the DEM is used, computer grid-to-grid subtraction of the elevation at the top of the bed from the surface elevation creates a grid of the overburden, which can be contoured to derive the depth-of-burial (overburden) lines. Once coal thickness and overburden maps have been generated, individual lines (14- and 28-inch coal-bed thickness, 200- and 1,000-foot overburden) can be selected and stored for future use.

As previously stated, the areas covered by land-use restrictions and some of the technological restrictions may be plotted on base maps and digitized. Most of the areas affected by technological restrictions, however, are readily generated by the computer. Barrier pillars of coal, left for safety purposes around active or abandoned coal mines, may be created as buffers at the required distances from the digitized boundaries of the mine. For underground mining, the interburden between beds is determined through grid-to-grid subtraction of the top of the lower bed from the base of the upper bed. Where the two beds occur within less than the restfictive distance, a determination is made as to which of the two coals would most likely be mined, largely based on coal thickness. These restriction lines are saved and stored with the other computer-derived restfictions for coal-bed depth and thickness.

At this point, the user has all of the line files necessary for coal resource calculation: outcrop, coal-bed thickness, overburden thickness, surface and deep mines, land-use and technological restrictions, and parameters for quality. Given the weight factor, the computer will then calculate the amount of original, mined and lost in mining, restricted, and available coal resources for each coal bed in the prescribed thickness and overburden categories.

First, the original coal resource is calculated by thickness and overburden categories to quantify all of the coal in the ground prior to mining and prior to application of restrictions. Next, the resources are calculated for the surface and deep mines and subtracted from the original resource to determine the amount of coal left in the ground after mining-the remaining resource. Each of the land-use and technological restrictions is then combined individually with the remaining coal resource to ascertain the amount of coal that will be restricted from future mining by each type of constraint.

Many of the restrictions overlap one another, as is the case with towns/streams, thin interburden, and previous mining above or below a coal bed. Therefore, to avoid duplication of restrictions before the available resource is calculated, all of the land-use restrictions must be combined into one land-use restrictions file, and, likewise, all of the technological restrictions must be combined into one technological restrictions file for each coal bed. Care must be taken also not to overlap or duplicate land-use and technological restrictions, especially in the surface-minable area. The resultant restriction files are finally excluded from the remaining coal resource to derive the amount of coal resource available for development in each of the prescribed thickness and overburden categories.

While noncompliance with respect to sulfur dioxide potential is not considered a restriction to mining for the coal availability studies, an estimate of compliance coal was included in this study because it is a factor in determining marketability for most of the power-generating coals in the central Appalachian region.

When the original, remaining, available, and available compliance coal tonnages have been determined for the study area, these results can be related to previous estimates and extrapolated to the larger, similar area defined earlier in the study when the study area was selected.


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