Investigation of Electromagnetic Emissions in a Deep Underground Mine

Douglas F. Scott and Theodore J. Williams

Spokane Research Center, National Institute for Occupational Safety and Health, Spokane, Washington, 

USA

Steven J. Knoll

Coeur Silver Valley, Inc., Wallace, Idaho, USA

ABSTRACT

Highly stressed rock in stopes continues to be a primary safety risk for miners in underground mines because it 

can result in failures of ground that lead to both injuries and death. Spokane Research Laboratory personnel 

investigated electromagnetic (EM) emissions in a deep underground mine in an effort to determine if these 

emissions could be used as indicators of impending catastrophic ground failure. Results suggest that (1) there is no 

increase in the number of EM emissions prior to recorded seismic activity, (2) some EM signals are generated 

during blasting, (3) interference from mine electrical sources mask seismic-generated EM signals, (4) EM 

emissions do not give enough warning (compared to seismic monitoring) to permit miners to leave a stope, (5) the 

distance an EM signal can travel in the rock is between 18 and 40 m, and (6) current data acquisition systems do 

not differentiate between EM signals generated from seismic activity and random mine electrical noise. These 

results preclude monitoring EM emissions as precursors of impending catastrophic ground failure.

1 Introduction

Personnel from the Spokane Research Laboratory 

(SRL) of the National Institute for Occupational 

Safety and Health (NIOSH) conducted field studies to 

record electromagnetic (EM) emissions at the Galena 

Mine, Wallace, Idaho (Figure 1). The goal of the 

research was to determine if EM emissions were valid 

precursors to imminent ground failure. Identifying 

such an association could be useful as an indicator of 

potential rock failure in underground mines. 

Acoustic and EM emissions coincide when certain 

types of rock, especially quartz and quartzite (Nesbitt 

and Austin, 1988), break. Because EM emissions 

travel at the speed of light and acoustic emissions 

travel at the speed of sound, theoretically EM emissions could be a precursor to seismic activity (i.e., 

ground failure, rockbursts). 

Acoustic (including microseismic and seismic) 

monitoring of a rock mass to detect ground movement 

in deep underground mines has been done successfully for several years. Various earthquake researchers have noted that occasionally the number of EM 

emissions would increase prior to a large earthquake. 

EM emissions have been proven to be associated with 

rock failure in controlled laboratory research; however, to date, EM emissions have not been used 

successfully to detect imminent ground failure in deep 

underground mines. 

Fig. 1 Location of Galena Mine, Wallace, Idaho 

2 Previous Studies

Light emissions and low-frequency electrical phenomena associated with seismicity in underground 

mines and earthquakes have been reported for hundreds of years (Brady and Rowell, 1986). Most of this 

work involved the identification of EM emissions as 

an indicator of earthquakes. EM frequencies in both 

earthquakes and laboratory tests ranged from 0.01 Hz 

to 30 MHz. 

2.1 EM Source Mechanisms

• Brady and Rowell (1986) summarized four 

mechanisms that cause light to be emitted from 

fracturing rock: (1) rock fragments frictionally 

heated to incandescence, (2) electrostatic dis-
charge produced by the deformation of piezoelectric minerals or charge separation on fractured 

surfaces, (3) plasmas produced by rapid and 

intense heating of rock material, and (4) excitation of the ambient atmosphere by particle (electrons or positive or negative ions) bombardment. 

Brady and Rowell concluded that the light 

emitted from test rocks in the laboratory was 

caused by excitation of the ambient atmosphere 

by particle bombardment. 

• Zi-qiang et al. (1988) examined three sources of 

light: (1) heat radiation from friction, (2) electrostatic discharges produced by piezoelectric effects 

or charge separation on fractured surfaces, and (3) 

excitation of the ambient atmosphere by particle 

bombardment. Because light emissions were 

observed only at the moment when electrons 

struck air molecules, the authors concluded that 

the most likely source of EM emissions was 

excitation of the ambient atmosphere by particle 

bombardment. 

• Brady (1996) detected electrical signals in the 

frequency band of 900 to 5,000 Hz using both 

coil and monopole antennas. EM emissions 

coincided with the final failure of an unconfined 

rock sample. Brady then concluded that the lowfrequency signals (900 to 5,000 Hz) recorded 

after rock failure were caused by the rotation and 

vibration of charged rock fragments. His observation was consistent with the hypothesis that no 

low-frequency EM emissions should occur if 

fracturing was confined (thus making particle 

motion impossible). He noted that no emissions 

were evident at frequencies greater than 10 kHz 

and that emissions were evident only in the near 

field and not the far field. 

2.2 Earthquakes

Martner and Sparks (1959) noted electrical potential 

prior to the arrival of seismic waves at the surface of 

the ground. About 30 minutes prior to the arrival of 

main earthquake shocks, Gokhberg and Morgounov 

(1982) recorded EM emissions at frequencies of 27, 

81, and 1.5 kHz and 1.63 MHz. Later, Migunov et al.

(1984) documented EM emissions in the frequency 

range of 0.5 to 50 kHz that were associated with 

seismicity from earthquakes. Fujinawa and Takahashi (1990) observed EM emissions in the 0.01- to 

12-Hz and 1- to 9-kHz frequency bands hours before 

earthquake activity in Ito, Japan. Fujinawa and 

Kumagai (1992) observed ultralow-frequency (0.01 to 

0.6 Hz) to very low-frequency (1 to 3 kHz) electrical 

emissions before, during, and after volcanic eruptions. 

2.3 Laboratory Tests

• Tuck et al. (1976) tested a cube of quartzite 

coupled with a quartz crystal to determine piezoelectric emissions when a 0.5-kg hammer was 

used as a seismic source. They concluded that no 

piezoelectric fabric was found; therefore, it would 

be difficult to use EM emissions for the exploration of ore bodies. 

• Nitsan (1977) fractured quartz crystals, tourmaline crystals, and quartz-bearing rocks and recorded EM emissions in the frequency range of 1 to 

10 MHz. His interpretation of the source of the 

emissions was piezoelectricity. 

• Goncharov et al. (1980) tested several large (0.55 

by 0.55 by 0.65 m) blocks of concrete containing 

pieces of granite by applying load and recording 

both EM and acoustic emissions as the concrete 

failed. They recognized the fundamental problem 

of simultaneously recording both EM and seismic 

emissions and concluded that the number of EM 

emissions decreased as their amplitude increased. 

They also found that the ratio of EM to acoustic 

emissions post-fracturing was 20:1. Prior to fracturing (initial loading), the ratio had been 7:1. 

• In 1981, Bishop studied piezoelectric effects in 

quartz-rich rocks. Using a laboratory-designed 

system, he attempted to prove that the axis of the 

quartz crystals was a factor in EM emissions. He 

found that a relationship existed between EM 

emissions and predictions of the c-axis orientation in quartz crystals. 

• Hanson and Rowell (1982) tested quartzite from 

the Galena Mine. EM emissions peaked sharply 

below 40 kHz on three antennas, leading them to 

conclude that (1) fracture formation coincided 

with EM emissions, (2) EM emissions fell into a 

frequency range of less than 40 kHz, (3) EM 

emissions seemed to be directional, and (4) the 

amplitude of EM emissions seemed independent 

of stress, but not independent of stress drop. 

• Khatiashvili (1984) showed that as the size of 

fractured crystals increased, electrical potential 

also increased. 

• Ogawa et al. (1985) used ball antennas covering 

the frequency range of 10 Hz to 100 kHz to 

measure EM emissions from crustal rocks broken 

in the laboratory. They found that sedimentary 

rocks containing less silica emitted less electromagnetism and concluded that the source 

mechanism for EM emissions was either contact 

or separating electrification and piezoelectrification. 

• Zi-qiang et al. (1988 ) fractured granite in the 

laboratory and found that the most intensive light 
 pulse and acoustic emissions were recorded 

simultaneously at the moment of rock fracture. 

• Yamada et al. (1989) also fractured granite in the 

laboratory and recorded EM emissions in the 

frequency range of 80 kHz to 1.2 MHz. They 

concluded that, based on their work, the source of 

EM emissions was not a piezoelectric effect, but 

was related to new surfaces created by cracks. 

• Weimin et al. (1991) fractured quartz, limestone, 

and granite samples and reported that recorded 

EM emissions were a result of rock fractures. 

• Rabinovitch et al. (1995) tested granite and 

recorded EM emissions in the 100-kHz range. 

They also documented EM frequencies of as 

much as 10 MHz in quartz porphyry. Two types 

of EM pulses were noted: “short” pulses of 1 to 3 

microseconds and “lengthy” pulses of more than 

400 microseconds. 

2.4 Theoretical Work

• Rabinovitch et al. (2000) attempted to explain the 

mechanism for EM emissions and concluded that 

following early pore closure, microcracking and 

possibly coalescence occurred, while just before 

peak stress was reached, the rock collapsed. A 

summary of information about the frequency and 

wavelength of EM emissions showed that their 

frequency range was 1 kHz (with a wavelength of 

300 km) to 10 MHz (with a wavelength of 30 m). 

• Goldbaum et al. (2001) identified four distinct 

EM emissions waveforms: short single pulses, a 

short chain of single pulses, an extended chain of 

pulses, and a new group, pulses along baseline 

voltage changes. Significant to their work were 

EM frequencies reaching 25 MHz (formerly 

believed to be only up to 10 MHz). 

• Rabinovitch et al. (2001) continued investigating 

mechanisms for EM emissions and concluded that 

the mechanisms for earthquake EM emissions 

were the same as for microfracturing in laboratory tests. They studied the Gutenburg-Richter 

type and Benioff strain-release relationship for 

earthquakes and found the relationship extended 

to the microlevel. 

2.5 Underground

• Sobolev et al. (1984) tested the value of 

collecting EM emission data as a method of prospecting for quartz veins and base-metal sulfides. 

They detonated explosive charges and measured 

the EM emissions generated by the excited 

minerals. Their tests showed EM signals generated by quartz veins at the Giant Yellowknife Mine 

 (Canada) were in the range of about 8 kHz, which 

was similar to emissions from quartz broken in 

the laboratory. Further tests in a sulfide vein at the 

Sullivan Mine (Canada) also produced EM 

emissions as high as 350 kHz. Their conclusions 

were that quartz and sulfides such as galena, 

sphalerite, and pyrrhotite emit EM waves along 

their grain boundaries. 

• Nesbitt and Austin (1988) recorded seismic and 

EM emissions at a depth of 2.5 km in an underground mine. They found that EM emissions 

preceded the seismic wave. 

• O’Keefe and Thiel (1991) recorded EM emissions associated with blasting in rock quarries in 

Australia and recorded signals in the 20-Hz to 20-

kHz range. 

• Russell and Barker (1991) investigated expected 

EM emission amplitudes in exploration and found 

that the identification of true piezoelectric 

responses was difficult because their data acquisition system recorded both acoustic and piezoelectric signals as part of the same waveform. At 

best, they presumed that a portion of the signal 

collected was piezoelectric. 

• Butler et al. (1994) successfully mapped stratigraphic boundaries using emissions responses. 

They found they could map a boundary between 

glacial till and organic-rich fill by collecting 

emissions waveforms generated by either a 

sledgehammer or blasting caps as a seismic 

source. Their work showed that it was the boundary or interface of the glacial till and the organicrich fill that was responsible for the emissions 

conversion and not the water table. 

• Wolfe et al. (1996) used emissions studies in an 

attempt to identify the depth of the water table. 

Using seismic refraction surveys and dc resistivity surveys in two drill holes as a baseline, they 

showed a consistent depth to the water table as 

compared to the baseline data. Thus their study 

demonstrated that emissions data could be 

acquired in an outwash plain. 

• Russell et al. (1997) used emissions techniques to 

identify quartz and sulfide veins in three underground mines. They were successful in identifying quartz veins, sulfide veins, and the boundaries 

between formations with differing permeabilities 

using data from EM emissions. 

• Frid (1997b) concluded that EM emissions in coal 

mines lay in a narrow band from 30 to 150 kHz. 

He used 100 kHz as the most convenient frequency while examining EM emissions (1997a). 

He also concluded that the higher stress associated with rock near mine workings increased 

natural EM emissions. 

• Frid (1999) used EM emissions to delineate stress 
 in coal seams. He measured EM activity during 

borehole drilling and found that a hole nearing a 

stress peak excited a sharp increase in EM 

activity. 

• Frid et al. (2000) continued their work in the 

laboratory and attempted to correlate EM emissions with crack dimensions. They found that the 

amplitudes of EM emissions and their changes 

with loading were independent of both tensile and 

shear failure and that they were dependent only 

on the area of the entire crack. 

• Frid (2001) recognized the value of using EM 

emission criteria to forecast rockburst hazards in 

coal mines by using the limiting value of broken 

coal volume, mine working width, coal seam 

thickness, and coal elastic properties. 

• Sines and Knoll (oral conversation, 2000) used a 

data acquisition system to collect both seismic 

and EM emissions on the 4600 level of the Galena 

Mine. They sampled at a rate of about 7,200 

samples per second (a Nyquist frequency of 3,500 

Hz) using two monopole antennas 12.5 and 15.2 

m long. They used no filters to eliminate lowfrequency emissions and found numerous triggers 

on the EM antenna, which were initially thought 

to coincide with seismic activity. However, when 

the EM and seismic waveforms were analyzed, 

they found that most of the seismic emissions had 

actually preceded the EM emissions, which is 

physically impossible. Further evaluation of the 

collected waveforms showed that most of the EM 

emissions were caused by mine cultural noise, 

including the opening and closing of air doors 

(60-Hz solenoid), locomotive activity, and chute 

loading. 

• Butler et al. (2001) conducted field studies at the 

Brunswick No. 12 Mine in Canada in an attempt 

to link EM emissions with seismic activity and 

also to delineate sulfide ore. They used various 

antennas covering a range of frequencies from 1 

Hz to 4.5 MHz. They found that broadband EM 

emissions with frequencies up to 800 kHz could 

be induced by seismicity and blasting. However, 

results did not confirm that EM emissions 

preceded seismicity. 

• Vozoff (2002) attempted to demonstrate the use 

of EM monitoring as a warning system for roof 

failure in a large coal seam in Australia. He 

collected three complete datasets and concluded 

that of the three, one set coincided with a roof fall 

and was correlated with EM activity, one set 

might have had a “weak correlation at best,” and 

one set had no EM correlations with roof falls. 

3 Technical Approach

As noted above, many researchers have attempted to 

capture EM emissions before, during, or after ground 

failure (i.e., rockbursts) in underground mines. 

However, to date, none have conclusively linked rock 

breaking underground with EM emissions. The 

following work describes the methods and results of a 

study at the Galena Mine. It builds on the work of 

previous researchers, but uses new methods in an 

attempt to capture EM emissions from either blasting 

or rockbursting. 

The equipment used included an ESG data acquisition 

system, ULTRAQ,1

 capable of sampling up to 10 

million samples per second (Nyquist frequency of 5 

million samples per second) on four channels and a 

Pentium 166 computer. The system was enclosed in a 

box to keep it as clean and dry as possible; fans were 

installed to keep air moving in the box. Voltages for 

triggering the system could be set as low as 1 mV. 

Figure 2 shows the setup for data acquisition. 

Two monopole antennas constructed of solid copper 

wire and enclosed in plastic pipe sealed at both ends 

were used to collect EM emissions data. The first, 91 

m long and having a resonance frequency of 821 kHz, 

was inserted into a drill hole extending from the 5500 

level down toward an active stope (Figure 3). The 

second, 3.8 m long and having a resonance frequency 

of 19,737 kHz, was suspended from the roof above 

the data acquisition system about 91 m from an active 

stope. 

Based on the work by Hanson and Rowell (1982), the 

ideal antenna length for EM data collection (considering EM frequencies of about 107 kHz) would be 

 

1

The mention of specific products and manufacturers does 

not imply endorsement by the National Institute for 

Occupa-tional Safety and Health. 

Fig. 2 Schematic of data acquisition 

s stem
Fig. 3 Plan view of downhole antenna 

about 700 m, which compared closely with EM frequencies (100 kHz) obtained in laboratory experiments from breaking rock. A “wound” antenna was 

also developed in the laboratory; however, this 

antenna failed to pick up EM waveforms. 

Initially, filters were not installed; however, both 

high-pass (2.56 kHz) and low-pass (102.4 kHz) filters 

were used after the first tests to trap a “range” of frequencies and eliminate any triggers that might be 

related to common electrical interference in the mine 

in the 60- to 120-Hz range. After several data sets 

were collected, use of the low-pass filter was discontinued, and only waveforms with frequencies above 

2.56 kHz were collected and analyzed. 

In an effort to eliminate “grounding” problems in the 

mine stemming from the mine's ac power source, an 

independent dc power source was tried. However, the 

“noise” generated by the dc power source was too 

intense and automatically triggered the system. 

Therefore, the mine's ac power was used to operate 

the data acquisition system. 

EM emissions decay at a rapid rate in rock (table 1), 

so attenuation was also a concern. As frequency increases, the distance the waveform can travel from the 

EM source to the antenna decreases. Therefore, if the 

frequency of an EM emission is high (2.56 MHz), the 

distance from the antenna to the EM emission source 

would be 3.5 m, with virtually no energy left. 

However, if the EM emissions were at a much lower 

frequency (500 Hz), the distance between the antenna 

and an emission source would have to be nearly 79 m, 

with about 64% of the energy left. 

Table 1 Attenuation of EM emissions (resistivity 

of the rock at 125 ohm/m) 

Frequency (kHz) Skin depth 

(m) 

Attenuation (% 

of energy left) 

5 79.1 64.2 

10 55.9 53.5 

20 39.5 41.3 

40 27.9 28.6 

80 19.8 17 

160 13.9 8.2 

320 9.8 2.9 

1280 (1.28 MHz) 4.9 0.08 

2560 (2.56 MHz) 3.5 0 

Different settings were configured in an attempt to 

record EM emissions. The EM source was a striker 

commonly used for igniting barbecue grills. The 

striker emits a short EM response. 

5 Results

5.1 Waveform Identification

Figures 4 and 5 show waveforms collected with the 

system. Figure 4 is a waveform collected from the air 

door solenoid (60 Hz), and figure 5 shows an EM 

waveform where the striker was used as a source. The 

two waveforms are distinctly different. The latter EM 

waveform has a high-amplitude spike followed by 

smaller spikes originating from the striker. The air 

door waveform has a large spike that is followed by 

closely spaced decaying spikes. The air door waveform (and all electrical noise and grounding effects) is 

always characterized by four to five closely spaced 

spikes following an initial spike. 

5.2 Downhole Antenna without Filters 

Several different voltages, ranging from 10 mV to as 

much as 1 V, were tested as triggers; 50 mV was 

selected as the final triggering threshold at a sampling 

rate of 1 MHz. Data were collected for 5 days. The 

number of events triggered ranged from 900 per 24-

hour period to as many as 1,589 per 24-hour period, 

with an average of 1,236. All the events were 

identified as electrical mine noise; no EM activity 

occurred during blasting. 
5.3 Downhole Antenna with Filters

Test voltages ranged from 10 mV to 1 V. All voltages 

from 400 mV or less automatically triggered the data 

acquisition system; therefore, the trigger threshold 

was set at 400 mV at a sampling rate of 1 MHz. Data 

were collected for 21 days. The number of events 

recorded per 24-hour period decreased when the 

filters were in place and ranged from 281 per 24-hour 

period to as many as 581 per 24-hour period, with the 

average being 332 events per day. As with the unfiltered antenna, the events were identified as mine 

electrical noise, and no EM activity was recorded 

during blasting. 

5.4 Suspended Antenna without Filters

We concluded that mine electrical grounding in the 

rock created voltage readings up to 1 V that were not 

EM related. We then tried a second approach, which 

was to suspend an antenna from the back where the 

antenna could not be grounded. 

Trigger voltages ranging from 10 to 30 mV were 

tested at a sampling rate of 3 MHz and a trigger 

threshold of 30 mV. Data were collected for 5 days. 

The number of events ranged from two per 24-hour 

period to as many as 24 per 24-hour period, with a 

total number of 52 and an average number of 10 

events per 24-hour period. As before, these events 

were identified as mine electrical noise. 

System settings were then changed to a sampling rate 

of 4 MHz and a trigger threshold of 25 mV. Seventythree events ranging from one per 24-hour period to 

as many as 43 per 24-hour period were recorded over 

a 9-day period, with an average number of eight 

events recorded per 24-hour period. These events 

again were classified as electrical noise. 

Using a sampling rate of 10 MHz and a trigger 

threshold of 25 mV, the system recorded 107 events. 

Over a 7-day period, the number ranged from two per 

24-hour period to as many as 24 per 24-hour period, 

with an average per day of 15. Again, waveforms 

Fig. 4 Air door (electrical noise) waveform. 

Fig. 5 EM emissions from striker 
activity occurred during blasting. However, the result 

was a marked decrease in the amount of mine 

electrical noise recorded by the data acquisition 

system compared to the 24-hour period of sampling 

(with or without filters) in the downhole. 

5.5 Suspended Antenna with Filters 

Using information from work by Hanson and Rowell 

(1982), we installed a high-pass (2.56 kHz) filter that 

allowed EM waveforms with frequencies above 2.56 

kHz to be collected. EM emissions were recorded for 

17 days at a sampling rate of 10 MHz and a 25-mV 

trigger threshold. Twenty-eight waveforms were collected, of which nine occurred during blasting (Figure 

6). This was important because only waveforms associated with blasting were possibly associated with 

seismicity or rock breaking. Figure 7 summarizes the 

number of events collected using different methods. 

Fig. 6 EM event recorded during blasting 

Fig. 7 Average number of events per day 

6 CONCLUSIONS

Cultural noise associated with EM emissions in a 

deep underground mine can be caused by, but is not 

limited to, blasting, drilling, motors, air doors, 

ventilation fans, shaft noise, chute activity, power 

tools, welding, power surges, and water pumps. 

Seismic activity can also be a source of EM 

emissions. A grounding effect caused by the mine's 

power source creates as much as 1 V of electrical 

interference in the mine rock. The effect of such 

interference on an antenna installed in a drill hole 

Average Number of Events Per Day

0

200

400

600

800

1000

1200

1400

Downhole

antenna/no

filters

Downhole

antenna with

filters

Suspended

antenna without

filters

Suspended

antenna with

filters

Antenna Used

A

v

era

g

e

N

u

m

b

er of

E

v

e

nts
was enough to trigger the data acquisition system; 

therefore, nearly all mine electrical noise was 

recorded and mixed with possible “true” seismicgenerated EM emissions. Various trigger voltages and 

the use and nonuse of the low-pass and high-pass 

filters provided a wide range of data. The best EM 

data came from the suspended antenna using a highpass filter. 

However, results to date suggest that (1) there is no 

increase in the number of EM emissions prior to 

recorded seismic activity, (2) some EM signals are 

generated during blasting, (3) interference from mine 

electrical sources masks true EM signals, (4) EM 

emissions do not give enough warning (compared to 

seismic monitoring) to permit miners to leave a stope, 

(5) the distance an EM signal can travel in the rock is 

between 18 and 40 m, and (6) current data acquisition 

systems do not differentiate between EM signals 

generated from seismic activity and random mine 

electrical noise. In summary, these results preclude 

monitoring EM emissions as precursors of impending 

catastrophic ground failure. 

Acknowledgments

The authors wish to thank Harry Cougher (general 

manager, Galena Mine) for access to the mine. 

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