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OCR for page 87
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
-
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OCR for page 88
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OCR for page 89
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
-
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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
OCR for page 91
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.
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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 fences—can 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
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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_
OCR for page 94
94
500
400
300
200
100
50
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't 40
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30
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COAL WASTEIMPOUNDMEN7S
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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.
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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,
-
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96
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-
._
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
=
OCR for page 97
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
OCR for page 100
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
OCR for page 101
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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 plot—a geophysical log—of 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.
OCR for page 103
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
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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
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Borehole with void Borehole with coal
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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.
