Potential Profile Magnitude Wolts)

ery 7) 100@ ©2088 «3088 4008
Distance from Origin of Profile (Ft)

Fig. 7. Per-unit scalar potential at 1 kHz for Scenario 1.

as
a2s
a7

28

-0.01

Potential Response (V)

010

20 083 06 023 a2

Frequency (HHz)

(a) Real part

2.18

“2S,

Potential Response (V)

2.08 2.83 06 23 a2

Frequency (MHz)
(b) Imaginary part

Fig. 8. Frequency spectrum of the unmodulated scalar
potentials for Scenario 1.

Figure 8 shows that currents with frequencies above 100
kHz are essentially dissipated in the overburden prior to
reaching the depth of the observation surface. Also
frequencies below the 10-kHz range yield the greatest
Tesponses.

Figures 9, 10, and 11 show the per-unit GPRs for the
conductor segments in the rail and the roof bolts at
frequencies of dc, 10 Hz, and 100 Hz, respectively. A
significant potential difference occurs between the roof bolts
and the rail at the crossover point for de and 10 Hz, as
shown in Figs. 9 and 10. However, this potential difference
vanishes at frequencies of 100 Hz and above, as shown in
Fig. 11. Thus, the potential difference between the
conductor segments is due solely to very-low frequency
components.

An inverse FFT is used to compute the time-domain
GPR in the roof-bolt and rail conductor segments at the
crossover point, and the results are shown in Figs. 12 and 13,
respectively.

COMETS OAR RAE SACO ODA HT 4 AI |

FR of Concetor Natal Cagle? Mere) Woltet

Moy @ 13ee 28083888 (4800
Oiatarce fron Origa f Prfile

Fig. 9. GPR of rail and roof bolt segments at de for Scenario 1.

‘en t) 1808

Distaroe fron Origin of Profile 1)

Fig. 10. GPR of rail and roof bolt segments at 10 Hz for Scenario 1.


TV. CONCLUSIONS

Two scenarios for a simplified model of an abandoned
area of a coal mine were simulated. Rails from the
underground transportation system were used as conductive
structures that are grounded at remote locations, and a row
of 6-ft roof bolts were positioned perpendicular to the rails.
With Scenario 1, lightning was injected directly into the
earth. A steel-cased borehole was added for Scenario 2. A
double-exponential current surge, with a peak value of 84
kA, was used to simulate a lightning strike. CDEGS
software from Safe Engineering Services & Technologies,
Ltd was used for the simulations. CDEGS performed a Fast
Fourier Transform (FFT) to convert the lightning strike from
the time domain to the frequency domain. Current
distributions, scalar potentials, and electromagnetic fields
were computed for selected frequencies at specified
observation points. An inverse FFT was used to obtain time-
domain ground potential rises (GPRs) for specified
conductor segments in the system.

The simulations showed that currents with frequencies
below 10 kHz for Scenario 1 and 30 kHz for Scenario 2
cause the greatest contribution to the scalar potentials in the
mine area. Peak values of 12 kV and 57 kV occurred
between the roof bolt and remote earth for Scenario I and
Scenario 2, respectively.

For both scenarios, the potential differences between the
toof-bolt and rail segments were solely due to very-low
frequency currents, below 100 Hz. Values of 375 V for
Scenario 1 and 15.6 kV for Scenario 2 were calculated.
Given the assumptions and approximations made for
defining this problem, as well as the limitations associated
with any simulations, the authors feel that Scenario 1 does
not provide conclusive evidence that the lightning strike is
capable of initiating an explosion and that further
investigations need to be performed. However, Scenario 2
presents very strong evidence that the presence of a steel-
cased borehole dramatically enhances the possibility of
lightning initiating an explosion in a mine at a 600-ft depth.

Future work will address the sensitivity of the model
parameters, such as soil resistivity, depth of overburden, and
diameter of the borehole casing. Simplified simulations
will also be compared with a theoretical model to determine
their level of agreement.

a]

2)

BI)

(4

(5)

(6)

17)

(8]

09}

{10}

V. REFERENCES

Geldenhuys, H.J., A.J. Eriksson, W.B. Jackson, and
J.B. Raath, “Research into Lightning-Related Incidents
in Shallow South African Coal Mines,” Proceedings of
the 21" International Conference of Safety in Mines
Research Institutes, Sydney, Australia, October 1985.

Geldenhuys, H.J., “The Measurement of Underground
Lightning-Induced Surges in a Colliery,” Proceedings
of the Symposium on Safety in Coal Mining, Pretoria,
South Africa, October 1987.

Zeh, K.A., “Lightning and Safety in Shallow Coal
Mines,” . Proceedings of the 23” International
Conference of Safety in Mines Research Institutes,
Washington, D.C., September 1989.

Golledge, P., “Sources and Facility of Ignition in Coal
Mines,” Australian IMM, Illawarra Branch
Symposium, 1981.

Checca, E.L. and D.R. Zuchelli, “Lightning Stirkes and
Mine Explosions,” Proceedings of the 7* US Mine
Ventilation Symposium.

Berger, K., Protection of Underground Blasting
Operations, Chapter 20 ~ “Lightning,” Vol. 2, edited
by R.H. Golde, Academic Press, London, 1977.

Selby, A. and F.P. Dawalibi, “Determination of the
Current Distribution in Energized Conductors for the
Computation of Electromagnetic Fields,” Proceedings
of the IEEE/PES 1993 Summer Meeting, Paper No. 93
SM 427-5 PWRD.

Dawalibi F.P. and A. Selby, “Electromagnetic Fields
of Energized Conductors,” Proceedings of the
IEEE/PES 1992 Summer Meeting, Papet No. 92 SM
456-4 PWRD.

Dawalibi, F.P., W. Ruan, and S. Fortin, “Lightning
Transient Response of Communications Towers and
Associated Grounding Networks,” Proceedings of the
International Conference on Electromagnetic
Compatibility, 1995,

Zipse, D.W., “Lighting Protection Systems:
Advantages and Disadvantages,” IEEE Transactions
on Industry Applications, Vol. 30, No. 5, 1994, pp.
1351-1361.


(£03

%
3
2
> 18
a
3
a
f
@
aace” “dale” 0ade ” “aeie  @ada ” “ease
Time (Millisecond)
@)
(4.03)
2
2
sa
x
3
g
S 18
a
3
2
a3
a
es @ ace aC

Time (Millisecond)
)
Fig. 3. Model of the lightning surge current.

respectively. Figures 4, 5, and 6 show that the three-
dimensional perspectives for the scalar potentials at dc, 10
Hz, and 100 Hz are essentially the same, with peak values of
approximately 0.35 V. Note the small distortion to the
scalar potentials at dc and 10 Hz, due to the presence of the
rail. This distortion essentially disappears at frequencies of
100 Hz and above. Attenuation of the scalar potentials
becomes noticeable at 1 kHz, as shown in Fig. 7, The
frequency spectrums (de to 120 kHz) for the real and
imaginary parts of the unmodulated scalar potentials are
shown in Fig. 8.

“our, 8 1028-2608 «3000 4600
Distance fram Origin of Profile (FI

Fig. 4. Per-unit scalar potentials at dc for Scenario 1.

"ey @  16ea  20ea 3000 4800
Oletence free Origin of Profile

Fig. 5. Per-unit scalar potentials at 10 Hz for Scenario 1.

Potential Profile Magnitude (Volts)

far @ 1628 2600 3800” 4a00
Distance feom Origin of Profile 1)

Fig. 6. Per-unit scalar potentials at 100 Hz for Scenario 1.


GPR of Conchector Neel Magritte? (Mors) alts)

te D
‘ery 8

1@8@ 2808-3080

Distance trom Origin of Profile 0)

4888

Fig. 11. GPR of rail and roof bolt segments at 100 Hz for
Scenario 1.

(E00)
6

Conductor GPR (V)

28 ate 24 EE a8 ace

Time (Millisecond)

Fig. 12. Time-domain GPR of roof bolt at the roof-bolt/rail
crossover point for Scenario 1.

The peak potential at the roof bolt is slightly greater than
that of the rail, but both are near 12 kV. The waveform of
Fig. 13 is subtracted from that of Fig. 12 in order to obtain
the potential difference between the roof bolt and the rail,
with the resulting waveform shown in Fig. 14. The potential
difference has a peak value of 375 V. This voltage is
certainly capable of generating an arc, but is relative small
compared to the two GPRs. Given the assumptions and
approximations made for defining this problem, as well as
the limitations of the software, it is felt that this value does
not provide conclusive evidence that the lightning strike is
capable of initiating an explosion in Scenario 1.

(16.03)
5

©
&
& 3
i a
0.08 ee 024 x 3 0.48 858
Time (Millisecond)
Fig. 13. Time-domain GPR of rail at the roof-bolt/rail
crossover point for Scenario 1.
400
= 350
300
250
i 200
150
100
50
0
C) 04 0.2 0.3 0.4
Time [ms]

Fig. 14. Potential difference between roof bolt and rail for
Scenario 1.

B. Scenario 2

Scenario 2 is essentially the same as Scenario 1, except that
a steel-cased borehole, which extends from the surface to
within one foot of a roof bolt, is included in the simulation
model. As before, a per-unit current of 1.0 + j0.0 A is
injected into the strike point, and the borehole casing. The
frequency spectrums (dc to 120 kHz) for the real and
imaginary parts of the unmodulated scalar potentials are
shown in Fig. 15. Similar to Scenario 1, currents with
frequencies above 100 kHz are dissipated in the overburden
prior to reaching the depth of the observation surface. For
this scenario, frequencies below the 30-kHz range cause the
greatest responses. As expected, the magnitudés of the
unmodulated scalar potentials are significantly larger than
those in Scenario 1. The time-domain GPRs in the roof-bolt


Lightning Propagation Through the Earth and its Potential for Methane
Ignitions in Abandoned Areas of Underground Coal Mines

Thomas Novak, Senior Member, IEEE
The University of Alabama
Department of Civil and Environmental Engineering
Box 870205
Tuscaloosa, AL 35487

Abstract. Strong circumstantial evidence suggests that
lightning has initiated methane explosions in abandoned
and sealed areas of underground coal mines. The Mine
Safety and Health Administration (MSHA) investigated
several of these occurrences within recent years. The
investigated explosions occurred at significant depths,
ranging from 500 ft to 1200 ft. Data from the National
Lighting Detection Network indicate a definite
correlation between the times and locations of the
explosions with those of specific lightning strikes. This
paper addresses the question, “Can lightning cause
potential differences capable of igniting methane-and-air
mixtures at overburden depths at which underground
coal mining occurs?” A mine depth of 600 ft was selected
for this initial study. Computer simulations were
performed, with and without the presence of a metal-
cased borehole extending from the surface to the mine
level. CDEGSTM software from Safe Engineering
Services & Technologies, Ltd (SES) was used for the
simulations.

I. INTRODUCTION

Electrical shock, visible sparking from underground
equipment, premature detonation of explosives, and methane
explosions have been experienced in underground mines
during thunderstorms. These incidents have been
particularly well documented in shallow coal mines in South
Africa [1, 2, 3], with the vast majority occurring at mining
depths of 300 ft or less. In recent years, several methane
explosions in the United States have also been attributed to
lighting. However, these explosions occurred at depths
ranging from 500 to 1200 ft, which are significantly deeper
than any of the incidents experienced in South Africa.

The explosions in the United States took place in
abandoned and sealed areas of underground coal mines. In
some instances, steel-cased boreholes were located in the
vicinity of the explosions. Data obtained from The National
Detection Network were used to determine the number and
magnitude of cloud-to-earth lightning strikes within a 10-
mile radius of the explosion areas at the estimated times of

Thomas J. Fisher
National Institute of Occupational Safety and Health
Pittsburgh Research Laboratory
Cochrans Mill Road
P.O. Box 18070
Pittsburgh, PA 15236

the explosions. An analysis of the data revealed that the
magnitudes of the strikes ranged from 16 kA to 112 kA [5].

Explosions can occur if lightning causes electric sparks
with sufficient energy in a methane/air mixture with
methane concentrations between 5-15% [4]. (The minimum
energy requirement of only 0.3 mJ occurs with a methane
concentration of 8.5%.) Pockets of explosive methane/air
mixtures are not uncommon in abandoned and sealed areas
of coal mines. Lightning-related sparking underground can
result from transient voltage surges on metal structures, such
as conveyors or rails, where small discontinuities occur
within the structure. It is also believed that the dissipation of
lightning in rock strata may cause sparks with sufficient
energy to ignite a methane/air mixture [1].

Lightning can penetrate an underground mine by two
mechanisms — propagation through the overlying strata and
conduction through metallic structures extending from the
surface to the mine [1]. With the first mechanism, a
lightning strike at the surface propagates downward through
the earth in a radial fashion. Analyses of tunneling accidents
in the Swiss Alps show that lightning strikes are capable of
penetrating significant depths of overburden with enough
energy to detonate explosives [6]. The depth of penetration
was shown to be proportional to soil resistivity. In other
words, lightning will penetrate deeper in soils with higher
resistivity. Uniformly elevating the soil’s potential, with
respect to remote earth, by itself may not necessarily create
problems since potential differences are not present in
localized areas. However, large conductive structures that
are grounded at remote locations can distort local current
distributions and result in potential gradients. Geological
faults, although not discussed in this paper, can also
significantly distort current distribution through the
overburden.

The second lightning-penetration mechanism results
from a direct strike to a metallic structure that extends from
the surface to the mine, such as cables, conveyor structures,
water pipes, and borehole casings. The attenuation of such a
strike depends on the surge impedance of the structure and
how effectively the structure is grounded.


a

=
2 a ‘
$
3
2
3
eo
= es
00 3 gs 0.08 axe
Frequency (MHz)
(a). Real part

0.00

05
g 0%
5
a
3
e
nd
s
2 se

ee 0.03 0.06 09 aie

Frequency (MHz)
(b) Imaginary part

Fig. 15. Frequency spectrum of the unmodulated scalar
potentials for Scenario 2.

and rail conductor segments at the crossover point are shown
in Figs. 16 and 17, respectively. The peak potentials in the
roof bolt and rail have dramatically increased to 57 kV and
40 kV, respectively. The potential difference between the
two conductor segments has a peak value of 15.6 kV and is
presented in Fig. 18. Even with the assumptions and
approximations made for defining this problem, the
magnitude of this potential difference provides convincing
evidence that the lightning strike is capable of initiating an
explosion in Scenario 2, depending on the arrangement of
conductors and physical conditions within the mine area.

Conductor GPR (VI

é

Conductor GPR (V)

£83)

24

08 lz 026 0.36 cri ase
Time (Millisecond)

16. Time-domain GPR of roof bolt at the roof-
bolt/rail crossover point for Scenario 2.

£83)
60

m4

ace” Be a2 836 aca O68
Time (Millisecond)

Fig. 17. Time-domain GPR of rail at the roof-

bolt/rail crossover point for Scenario 2.

0 04 0.2 0.3 04

Time [ms}

Fig. 18. Potential difference for Scenario 2.



This paper addresses both lightning propagation
methods. A simplified model of an abandoned area of a coal
mine is created. Rails from the underground transportation
system are used as conductive structures that are grounded at
remote locations, and a row of 6-ft roof bolts is positioned
perpendicular to the rails. A double-exponential current
surge is used to simulate a lightning strike and is injected
into the earth at the surface. The CDEGS software first
performs a Fast Fourier Transform (FFT) to convert the
lightning strike from the time domain to the frequency
domain. Current distributions, scalar potentials, and
electromagnetic fields are then computed for selected
frequencies at specified observation points. This
information provides insight into the frequency response of
the earth and associated metal conductors. Finally, an
inverse FFT is then used to obtain time-domain ground
potential rises (GPR) for specified conductor segments in the
system. Computational methods for the CDEGS software
can be found in references [7], [8], and [9].

I. MODEL DEVELOPMENT
A. Physical Model

The physical model is structured to create a situation
where the GPR (ground potential rise) can be calculated at
two nearby conductor segments to determine if a significant
potential difference exists. A worst-case scenario would be
if one conductor segment were part of a tire-mounted mining
machine whose frame is tied to the safety ground bed on the
surface, which could be a few miles from the equipment
location. However, the explosions occurred in sealed areas,
and cables and conductors are not permitted to extend
beyond this area. Therefore, a more realistic situation was
selected in which the two conductor segments reside in a rail
and a roof bolt, with the roof bolt being located directly
above the rail. Fig. 1 depicts the situation to be modeled and
consists of a partially caved area of a coal mine. The plan
view of Fig. 1 shows a 4000 ft by 4000 ft area to be
modeled. The rail system spans the area in the x direction,
while the roof bolts span the area in the y direction. A 500-ft
length of the rail system on each end is located under caved
material, while the remaining center portion of the track
entry remains open. Thus the rails are essentially grounded
at remote locations, with respect to the roof bolts. The roof-
bolt entry remains open so that the roof bolts are located a
few feet above the rails in the z direction, as shown in the
side view of Fig. 1. The side view also shows the strike
point where the voltage surge enters the earth, directly above
the crossover point of the rails and the roof bolts.

The actual mode! was simplified to reduce the number
of conductor segments, which in turn reduces the simulation
time. (The maximum length of a conductor segment was set
at 10 ft, which is less than one-sixth the wavelength of the
highest frequency expected. Even with the simplified
model, processing time can exceed 24 hr for each simulation

when run ona PentiumTM III computer.) Figure 2 shows the

simplified model for the situation defined in Fig. 1. A
single, cylindrical rail, with the same cross-sectional area of
a typical 60-lb/yd rail, is used instead of two separate rails.
Since the portion of the CDEGS software used for this study
does not permit modeling of void areas in the-earth, such as
mine openings, some approximations have to be used to
model this situation. To accommodate this limitation, the
3000-ft center portion of the rail is modeled as a coated
conductor, with a 1-ft thick coating. The coating is assigned
the same resistivity and permitivity as air. The 500-ft end
portions of the rail are left uncoated in intimate contact with
the soil. Again, to reduce the number of conductor
segments, only a single row of roof bolts is used. Steel
conductors, 6-ft long with a 5/8-in. diameter, are used to
model the roof bolts. The steel conductors for the rail and
the roof bolts are assigned a relative resistivity of 17 and a
relative permeability of 300. A copper rod, driven three feet
into the earth, is located on the surface directly above the
crossover point of the roof bolts and rail. This rod is used
for injecting the lightning surge into the earth for Scenario 1.
An observation surface is positioned horizontally, between
the roof bolts and the rail. The intersecting points of the
observation grid are used to calculate the scalar potentials in
the soil.

Typical overburden consists of many layers of various
types of strata, and the resistivity of each layer can vary
dramatically. The composition of overburden is site
specific, and discontinuities and geological faults can affect
its electrical properties. However, to make the problem
manageable, a uniform layer of soil, with a 400 Q-m
resistivity, is used to model the overburden.

A steel-cased borehole is used in Scenario 2. The
borehole, not shown in Figs. 1 or 2, extends from the strike
point on the surface to within a foot of a roof bolt. The
borehole casing was modeled as a 6-in diameter pipe with an
interior diameter of 5 in. Similar to the rails, the casing was
assigned a relative resistivity 17 and a relative permeability
of 300.

B. Lightning Surge

A magnitude of 84 kA was selected for the lightning
strike. This value is 75% of the highest value obtained from
the National Lightning Detection Network data. The
lightning surge was modeled as a current source with the
following double exponential function:

I(t) = 1, |e" -e-*'| ay

with
In = 85.69 kA,
a@= 1.42 x 10', and
B= 4.88 x 10°.


bt s00n >< 3000

ft ————— Pl 500 8 >}
: aS

be 5002 >————— 30008 ————>!« 5000 >|
Side View

Fig. 1. Partially caved area of a coal mine to be modeled.

These values result in a rise time of 1.2 ps to a peak
current value of 84 kA. The surge decays to 50% of its peak
value (42 kA) at 50 1s, as shown in Fig. 3a. Figure 3b
shows that the waveform essentially decays to zero at 600
Ws. This type of waveform is typically used for modeling
lightning strikes [10].

TI. COMPUTER SIMULATIONS

The CDEGS software first uses a forward FFT to
decompose the time-domain lightning surge of Fig. 3 into its
frequency spectrum. It then selects a finite number of
frequencies from this spectrum, based on the
electromagnetic field response in the frequency domain.
More frequencies with finer steps are selected in the regions
where rapid changes occur. Electromagnetic fields are
computed for defined observation points at each selected
frequency to obtain the frequency spectrum of the fields.
Finally, an inverse FFT is applied to the frequency spectrum
of the computed electromagnetic fields, at the defined
observation points, to yield the time-domain responses of the
fields [9]. Simulations are performed for two scenarios.
Figure 1 shows the physical model that is simulated in

b¢ 500 ft >i¢——-—— 3000 —————>/ 500 ft >!
pessgenigussegenegiiiripsnsgy ne deers : vy

°
°
130
@o
: Poi Be :
Stee! Rail : Po ifs
- = 8
—' 2 «3
rs t 3 °
Uniform Soil °
font PoE °
°
°
deessdeescheesdo steeds @ee i
Observation Surface
Plan View
ike Point
Surface Strke Poin
} _
Uniform Soil
e
6-f Steel Roof Bolts
Observation Surface

—_—<«
Stes! Rail ingulated Coating
ke 500 ft >«———— 30008 —————>Ht 500 ft >

Side View
Fig. 2 Simulation model for Fig. 1.

Scenario 1. Scenario 2 is identical to Scenario 1, except that
a steel-cased borehole is placed from the surface to within a
foot of a roof bolt. The borehole is in intimate contact with
the overburden for its entire length.

A. Scenario 1

Figure 2 depicts the model for Scenario 1. The
frequency spectrum of the lightning surge in Fig. 3 ranges
from dc to the mega-hertz range. Therefore, the model's
unmodulated frequency response is first investigated. As an
illustration, a per-unit current of /.0 + 0.0 A is injected into
the strike point at the following frequencies: de, 10 Hz, 100
Hz, and | kHz. Scalar potentials, based on the per-unit
current, are calculated for each frequency at the intersecting
points of the observation surface, illustrated in Fig. 2. The
observation surface is a horizontal grid, located between the
rail and the roof bolts, and consists of 81 profiles, with 81
points per profile. This arrangement results in a total of
6561 observation points, spanning the 4000 ft x 4000 ft area
with a 50-ft spacing between adjacent points in the x and y
directions. Figures 4, 5, 6, and 7 show the system’s response
to the per-unit current at the specified frequencies,