ELECTROMAGNETIC NOISE IN GRACE MINE 

J. W. Adams 

W. D. Bensema 

M. Kanda 

Electromagnetics Division 

Institute for Basic Standards 

National Bureau of Standards 

Boulder, Colorado 80302 

The views and conclusions contained in 

this document should not be interpreted 

as necessarily representing the official 

policies or recommendations of the Interior 

Department's Bureau of Mines of the U. S. 

Government 

June 1974 

Prepared for 

U. S. Bureau of Mines 

United States Department of the lnterior 

Pittsburgh, Pennsylvania 15222 

Working Fund Agreement HO 133005 

U S DEPARTMENT OF COMMERCE, Freder~ck B. Dent, Secretary 

NATIONAL BUREAU OF STANDARDS R~chard W Robert5 D~rectc,r 

CONTENTS 

Page 

1.2 Mine Description------------------------------ 3 

MEASUREMENT INSTRUMENTATION------------------------ 5 

SPECTRUM MEASUREMENT RESULTS----------------------- 11 

3.2 Antenna Sites--------------------------------- 11 

3.3 Electromagnetic Noise Spectrum Results-------- 12 

3. 3.1 Introduct ion- - - - - - - - - - - - - - - - - - - - - - - - - - - 12 

3.3.2 Uncertainties-------------------------- 13 

3.3.3 Crusher Room Area---------------------- 13 

3.3.4 Production Area------------------------ 17 

3.3 5 Development Area- - - - - - - - - - - - - - - - - - - - - - - 19 

3.3.6 Cross Drift Substation----------------- 2 0 

3.3.7 Underground Workshop-Lunchroom- - - - - - - - - 2 1 

3.3.8 Composite of Worst Case Steady Noise--- 2 2 

3.4 Pulse Produced with Explosion----------------- 2 2 

3.5 Mine Noise Contour Maps----------------------- 2 4 

3.6 Miscellaneous Measurements-------------------- 24 

3.6.2 Measurement of Voltage Between "Roof 

Bolts"-----------------.---------------- 2 5 

iii 
CONTENTS (continued) 

Page 

3.7 Intercomparison of Magnetic-Field Noise in 

Different Mines------------------------------- 26 

3.7.1 Summary of 1 to 3 kHz Data------------- 2 6 

3.7.2 Magnetic-Field Spectra 3 kHz to 180 kHz 27 

4. AMPLITUDE PROBABILITY DISTRIBUTION MEASUREMENT 

RESULTS-------------------------------------------- 6 5 

4.1 Introduction and Uncertainties---------------- 6 5 

4.4 Summary Curves-------------------------------- 69 

5. HOIST-PHONE MEASUREMENT RESULTS-------------------- 116 
LIST OF FIGURES 

Page 

Figure 2-1. 

Figure 2-2. 

Figure 2-3. 

Figure 3-1. 

Figure 3-2. 

Figure 3-3. 

Figure 3-4. 

Figure 3-5. 

Figure 3-6. 

Figure 3-7. 

Figure 3-8. 

Figure 3-9. 

Figure 3-10. 

Block diagram of portable instrumentation--- 8 

System for field recording data to obtain 

amplitude probability distributions--------- 9 

Block diagram of laboratory recording system modified for field use ------- ---------- ----- 10 

Simplified map of Grace Mine where measurements were made----------------------------- 2 8 

Expanded map of underground development, 

production, and crusher-room areas - --- - - - - - - 2 9 

Expanded map of underground workshop and 

lunchroom area ------- ....................... 3 0 

Spectrum of area noise before explosion. 

Measurements at development foreman office 

with spectral resolution of 78.1 Hz--------- 31 

Spectrum when crusher is not operating. 

Measurements at crusher substation with 

spectral resolution of 78.1 Hz-------------- 3 .. 2 

Spectrum at higher gain when crusher is not 

operating. Measurements at crusher substation with spectral resolution of 78.1 Hz- 33 

Spectrum when crusher is operating. Measurements at crusher substation with 

spectral resolution of 78.1 Hz-------------- 34 

Expanded spectrum of crusher operating. 

Measurements at crusher subs tat ion with 

spectral resolution of 3.91 Hz-------------- 35 

Spectrum when crusher is not operating. 

Measurements at crusher access drift with 

spectral resolution of 78.1 Hz-------------- 36 

Spectrum when crusher is operating. Meas - 

urements at crusher access drift with 

spectral resolution of 78.1 Hz-------------- 37 
LIST OF FIGURES (Continued) 

Page 

Figure 3-11. 

Figure 3-12. 

Figure '3-13. 

Figure 3-14. 

Figure 3-15. 

Figure 3-16. 

Figure 3-17. 

Figure 3-18. 

Figure 3-19. 

Figure 3-20. 

Figure 3-21. 

Expanded spectrum of crusher operating. 

Measurements at crusher access drift with 

spectral resolution of 3.91 Hz-------------- 38 

Spectrum of noise at intersection 603 production area with spectral resolution of 

78.1 Hz------------------------------------- 39 

Spectrum comparison between steady-state 

background noise and LHD noise at entry and 

dining area 606E. Spectral resolution is 

78.1 Hz ------------------------------------- 40 

Spectrum of V-8 diesel LHD. Measurements 

at entry and dining area 606E with spectral 

resolution of 78 -1 Hz- - -- - - - - -- -- - - ---- - - - -- 41 

Expanded spectrum of loaded LHD. Measurements at entry and dining area 606E with 

spectral resolution of 3.91 Hz-------------- 4 2 

Spectrum of operating pneumatic fanhole 

drill. Measurements at jumbo drill working 

face with spectral resolution of 78.1 Hz---- 4 3 

Spectrum of lowest noise level measured in 

Grace Mine. Measurements at Number 2 mine 

transfer drift with spectral resolution of 

78.1 Hz------------------------------------- 44 

Spectrum of V-10 diesel LHD at Number 2 mine 

transfer drift. Spectral resolution is 

78.1 Hz------------------------------------- 4 5 

Expanded spectrum of V-10 diesel LHD at 

Number 2 mine transfer drift. Spectral resolution is 3-91 HZ----------------------- 4 6 

Expanded spectrum after V-10 diesel LHD 

left the vicinity. Measurements at Number 

2 mine transfer drift with spectrum resolution of 3-91 HZ--------------------------- 4 7 

Spectrum of background noise measured at 

Number 6 crossdrift substation. Spectral 

resolution is 7 8.1 Hz - - - - - - - - - - - - - - - - - - - - - - - 4 8 
LIST OF FIGURES (Continued) 

Page 

Figure 3-22. 

Figure 3-23. 

Figure 3-24. 

Figure 3-25. 

Figure 3-26. 

Figure 3-27. 

Figure 3-28. 

Figure 3-29. 

Figure 3-30. 

Figure 3-31. 

Figure 3-32. 

Figure 3-33. 

Figure 3-34. 

Spectrum of LHD's being refueled 30 meters 

distant. Fluorescent light area in shop 

office. Spectral resolution is 78.1 Hz----- 49 

Expanded spectrum measured at shop office. 

Fluorescent light area. Spectral resolution 

is 3.91 Hz- --------------------------------- 5 0 

Spectrum of LHD passing by the shop lunchroom area. Spectral resolution is 19.5 Hz-- 51 

Spectrum comparison between steady-state 

background noise and LHD noise which is the 

highest spectra measured at Grace Mine. 

Spectral resolution is 78.1 Hz-------------- 5 2 

Spectrum of explosion impulse measured at 

development foreman off ice. Spectral reso - lution is 78.1 Hz ---------------.-- - ------ --- 53 

Contour map showing noise generated from a 

group of detonations comprising a "shot, l1 

as a function of time ---- -- -- -------- ------- 54 

Three-dimensional view of two detonations 

from a larger group comprising a "shot1'--- -- 5 5 

Grace Mine noise contour map for 2 kHz------ 56 

Grace Mine noise contour map for 10 kHz----- 57 

Grace Mine noise contour map for 20 kHz----- 58 

Grace Mine noise contour map for 60 kHz----- 59 

Unprocessed data showing electric field 

strength obtained with an active dipole 

as a function of frequency. Measurements at 

crusher access drift. Spectral resolution 

is 78.1 HZ---------------------------------- 6 O 

Voltage spectrum of roof bolt at crusher 

access drift-------------------------------- 61 

vii 
Figure 4-8. 

Figure 4-9. 

Figure 4-10. 

Figure 4-11. 

Figure 4-12. 

Figure 4-13. 

Figure 4-14. 

Figure 4-15. 

Figure 4-16. 

Figure 4-17. 

APD, 2 MHz, vertical component-------------- 77 

APD, 6 MHz, vertical component-------------- 78 

APD, 14 MHz, vertical component------------- 79 

APD, 32 MHz, vertical component------------- 8 0 

APD, 10 kHz, vertical component------------- 81 

APD, 30 kHz, vertical component------------- 82 

APD, 70 kHz, vertical component------------- 83 

APD, 130 kHz, vertical component------------ 84 

APD, 0.5 MHz, vertical component------------ 8 5 

APD, 1 MHz, vertical component-------------- 8 6 

viii 
LIST OF FIGURES (Continued) 

Figure 4-18. 

Figure 4-19. 

Figure 4-20. 

Figure 4-21. 

Figure 4-22. 

Figure 4-23. 

Figure 4-24. 

Figure 4-25. 

Figure 4-26. 

Figure 4-27. 

Figure 4-28. 

Figure 4-29. 

Figure 4-30. 

Figure 4-31. 

Figure 4-32. 

Figure 4-33. 

Figure 4-34. 

Figure 4-35. 

Figure 4-36. 

Figure 4-37. 

Figure 4-38. 

Figure 4-39. 

Figure 4-40. 

Page 

APD, 2 MHz, vertical component-------------- 8 7 

APD, 6 MHz, vertical component-------------- 88 

APD, 30 kHz, vertical component------------- 8 9 

APD 70 kHz, vertical component-------------- 9 0 

APD, 110 kHz, vertical component------------ 91 

APD, 130 kHz, vertical component------------ 9 2 

APD, 160 kHz, vertical c o m p o n e n t - - - - - - - - - - - - 9 3 

APD, 205 kHz, vertical component------------ 94 

APD, 250 kHz, vertical component------------ 9 5 

APD, 500 kHz, vertical component------------ 9 6 

APD, 1 MHz, vertical component-------------- 9 7 

APD, 2 MHz, vertical component-------------- 98 

APD, 6 MHz, vertical component-------------- 99 

APD, 14 MHz, vertical component------------- 100 

APD, 32 MHz, vertical component------------- 101 

APD, 10 kHz, horizontal E-W component------- 102 

APD, 30 kHz, horizontal E-W component------- 103 

APD, 70 kHz, horizontal E-W component - -- - - - - 104 

APD, 130 kHz, horizontal E-W component------ 105 

APD, 160 kHz, horizontal E-W component------ 10 6 

APD, 250 kHz, horizontal E-W component------ 107 

APD, 500 kHz, horizontal E-W component------ 108 

APD, 1 MHz, horizontal E-W component-------- 109 
LIST OF FIGURES (Continued) 

Page 

Figure 4-41. 

Figure 4-42. 

Figure 4-43. 

Figure 4-44. 

Figure 4-45. 

Figure 4-46. 

Figure 5-1. 

Figure 5-2. 

Figure 5-3. 

APD, 2 MHz, horizontal E-W component-------- 110 

APD, 6 MHz, horizontal E-W component-------- 111 

Field strength excursions between 0.001 and 

99% of the time as a function of frequency, 

vertical component, development foreman 

off ice - - - - - - -- - - -- - - - - - - - - - - - --- - - - - - - - - -- - - 112 

Field strength excursions between 0.001 and 

99% of the time as a function of frequency, 

vertical component, crusher subs tation- -- - - - 113 

Field strength excursions between 0.001 

and 99% of the time as a function of 

frequency, vertical component, shop off ice -- 114 

Field strength excursions between 0.001 and 

99% of the time as a function of frequency, 

horizontal E-W component, shop office------- 115 

Top view of personnel hoist "A" skip- ---- - - - 118 

Magnetic field strength, dB pA/m, or relative S/N ratio as measured along the 

transmission line for the personnel hoist 

located in "A" shaft------------------------ 119 

Magnetic field strength, dB pA/m, or relative S/N ratio as measured along the hoist 

rope for the personnel hoist located in IIAll shaft- --- ----- -------- --- -- -- -------- --- 120 
ELECTROMAGNETIC NO1 SE IN GRACE MINE 

J.W. Adams, W.D. Bensema, and M. Kanda 

Two different techniques were used to make measurements of the absolute value of electromagnetic noise in 

an operating hardrock mine, Grace Mine, located near 

Morgantown, Pennsylvania. Diesel-powered haulage equipment is used in this mine, and the electromagnetic noise 

environment it creates was measured to see how it differs 

from the environment created by electric-powered haulage 

equipment. One technique measures noise over the entire 

electromagnetic spectrum of interest for brief time 

periods. Data are recorded using broadband analog magnetic tape and are later transformed to give spectral 

plots. The other technique records noise amplitudes 

at several discrete frequencies for a sufficient amount 

of time to provide amplitude probability distributions. 

The specific measured results are given in a number 

of spectral plots and in amplitude probability distribution plots. 

Key words: Amplitude probability distribution; digital 

data; electromagnetic interference ; electromagnetic 

noise; electromagnetic pulse (chemical); emergency 

co~.munications; Fast Fourier Transform; Gaussian 

distribution; impulsive noise; magnetic field strength; 

measurement instrumentation; spectral density; timedependent spectral density. 

1. INTRODUCTION 

This report gives data concerning electromagnetic noise 

in a hardrock mine. In this section, background information 

and a brief mine description are covered. In Section 2, 

measurement instrumentation is discussed. In Section 3, 

spectral plots of data are presented. In Section 4, amplitude probability distributions (APD) of magnetic-field noise 

are given. In Section 5, measurement results of attenuation 
and electromagnetic noise on a hoist phone are reported. 

The last two sections (6 and 7) cover conclusions and 

recommendations. 

Only representative samples of the total data measured 

are given in this report. A limited set of data-presentation 

formats have been used. If additional data, or data presentation in other formats, are required, contact any of the authors. 

With the specific permission of the Bureau of Mines, we will 

supply the additional data. A more complete description of 

the measurement systems used is given in the Robena Mine 

report [I]. 

1.1 Background 

The need for reliable communication systems in mines is 

a long-standing problem. For emergency use, when all power 

in a mine is off, the residual electromagnetic noise is no 

problem. However, if a communication system were designed only 

for emergency use, it would have two serious drawbacks. First, 

it would not be ready for immediate use in an emergency; second, 

it would not be of any value during normal operations. Therefore, the Bureau of Mines decided to design a communication 

system that could be used for both emergency and normal operat ional conditions . Also, two-way communication to personnel in a moving 

hoist is desirable for normal operating conditions, and is 

necessary in emergency conditions. 

During operation, the machinery used in mines creates a 

wide range of many types of intense electromagnetic interference (EMI) . This EM1 is a major limiting factor in the 

design of a communication system. 

The work reported here gives the results of comprehensive 

measurements of this EM1 in critical communicat ion locat ions 

where miners extract ore. 
There are several EM1 parameters that can be measured: 

magnetic field strength, H; electric field strength, E; conducted current, I; and voltage, V, between two conductors. 

One parameter was emphasized, magnetic field strength. There 

are several reasons. First, electric field sensors are very 

insensitive at lower frequencies, and hence probably will not 

be useful in any practical low frequency wireless mine communication system. Second, at any air-earth interface, only 

the magnetic field is essentially undisturbed, while the 

electric field is severely reduced. Third, any currents will 

induce magnetic fields, and hence measurement of the magnetic 

field will directly sense currents. Fourth, power line 

voltages are propagated as transmission line phenomena, are 

directly related to transmission line currents, and hence to 

magnetic fields generated. Thus, measuring magnetic field 

strength gives a representative composite picture of noise 

from currents and voltages from most sources, as well as 

measuring the magnetic fields induced by arcing equipment. 

Although magnetic field strength measurements are 

emphasized here, even this one parameter is difficult to 

measure meaningfully. The IEEE definition [2] of magnetic 

field strength, H (magnitude of the magnetic field vector), 

is used in this report. Since there are a multitude of different sources that generate many types of noise, the resultant 

magnetic field strength noise vector is a function of frequency, time, orientation, and location. Small variations 

in these parameters can cause several orders of magnitude 

difference in measured field strength. 

1.2 Mine Description 

The results and data presented in this report are based 

on measurements made on April 24 and 26, 1973, in the Grace 
drills are used for drilling, and nitrates are used for blasting. All haulageways are either through reliable rock or are 

heavily reinforced with concrete and steel. There is a mixture 

of incandescent, mercury-arc, and fluorescent lighting. A 

direct dial phone system is used for point-to-point communication. There is no portable communication system except on 

the personnel cages. 

The temperature and humidity are high, although not 

excessive in most places. 
2 . MEASUREMENT INSTRUMENTATION 

Two measurement techniques were used. The first technique 

covers a large portion of the spectrum as a "snapshot" during 

one relatively short period of time. In three-dimensional 

form, several such "snapshots" can show how drastically a 

signal varies, not only with frequency, but also with time. 

The second technique gives variations over a 20-minute time 

interval as measured in a very narrow band of frequencies. 

Usually, a set of twelve different center frequencies were 

used. Both techniques were used to measure two orthogonal 

components of magnetic field strength. This was done either 

by using two systems simultaneously or by varying the orientation of one system. Both techniques were used in as many 

different locations as possible. 

With the exception of the electric dipole spectral plots, 

all measured noise is reported in absolute quantities (instead 

of relative) to allow others to make effective use of the data. 

For the magnetic field strength measurements, the NBS field 

calibration capability was used with each complete measurement 

system to assure correct calibration [S] . A complication in making these measurements is the mine 

environment, which is generally humid, dusty, and poorly 

lighted. In Grace Mine, a significant fraction of the dust 

was magnetic. We used battery-operat ed, dust -protected gear 

for all of our portable measuring equipment. 

There are two types of noise recorded in the spectral plots, 

and hence two different magnetic field strength parameters are 

required, H and Hd. Results are given as the rms value of one 

component of magnetic field strength, H, versus frequency for 

discrete frequencies, or as one component of magnetic-fieldstrength spectrum density level [2] , Hd, versus frequency for 

broadband noise in the spectral plots. In the amplitude 
channel remained unchanged. The purpose of these two filters 

was to reduce the effect of power line harmonics so that 

receiving system noise could be reduced by increasing preamplifier gain. In some locations where there were fluorescent 

lights (broadband noise sources), this strategy gave only 

moderate improvement, but in some cases the improvement was 

significant . Second, the portable APD measuring system was unchanged 

in design except for the addition of a dust-proof enclosure. 

A procedural change is that about twelve frequencies were 

covered rather than eight, at the expense of eliminating 

measurement of one component of horizontal field strength. 

Thus, the same amount of time was required underground to 

make these measurements as before. 
Third, the system that had been used on the surface was 

changed significantly and was used underground to increase 

the amount of data recorded underground. It is not a minepermissible system. The recorder was equipped to operate 

at 30 ips speed in addition to 15 ips speed. Two channels 

of FM card sets were replaced by two sets of direct record 

cards. This gave an upper spectral limit of 180 kHz rather 

than 20 kHz. These two direct channels also had 10 kHz, 

high-pass filters added for the same reason as mentioned in 

the first system. One FM channel was retained to measure 

the vertical component of magnetic field strength below a frequency of 20 kHz. It did not have a 10 kHz, high-pass filter. 

The recorder was operated mostly at 30 ips. Two direct channels on this third system were used to record data for APD's. 

This gave APD measurements at two underground locations instead 

of one. At 30 ips, this system recorded 24 minutes of time 

instead of 48 minutes at 15 ips, but 24 minutes had been 

determined to be an adequate time for statistical validity 

in the Robena work [l] reported earlier. The data processing 

equipment was unchanged. 
d TRACK 7 (FM) 

MICROPHONE EDGE TRACK 

(VOICE 

Figure 2-1 Block diagram of portable instrumentation. The tape 

recorder's FM tracks are used to record from 40 Hz to 

100 kHz; direct tracks are used from 3 kHz to 320 kHz. 

Systems 2 and 3 are identical to system 1. When the 

direct tracks are used, the 100-kHz low pass filters 

are eliminated, and the amplifier bandwidth is increased 

from 100 kHz to 300 kHz. The microphone is used for 

occasional vocal comments by the operator. 
v COLLAPSIBLE 

LOOP 

ANTENNA 

LOOP 

ANTENNA 

CHANNEL I (FM) 

CHANNEL 2 (DIR) 

11 

j 10 kHz 

- 

HIGH PASS 1 FILTER 

(SENSITIVE AXlS VERTICAL) 

OSCl LLATOR 

CHANNEL 3 (DIR) 

COLLAPSIBLE 

LOOP 

ANTENNA 

BALUN 

b 

EQUIPMENT 

SIMILAR TO 3, CHANNEL 5 (DIR) 

BUT WITH 

ANTENNA ORIENTATION 

FIELD 

STRENGTH 

METER 

10 kh-2mkH2, . 

, ,455 kHz 

LOOP 

ANTENNA 

(SENSITIVE AXlS VERTICAL) 

1 

MIXER kHz 

I 

S 

Figure 2-3 Block diagram of laboratory recording sys tan modified 

for field use. 

BALUN CHANNEL 6(D\R) 

. 

CHANNEL 7 (FM) 

, 

FIELD 

STRENGTH 

METER 

1% kHz-32MHz & 

455kHz , MIXER 

> 

+ 

40kHz 
3. SPECTRUM MEASUREMENT RESULTS 

3.1 Introduction 

In this section of the report, spectrum plots are 

presented and discussed. Most of these plots are of magnetic 

field strength up to 100 kHz. Measurements were made at many 

different locations, and results can be used to characterize 

electromagnetic noise levels generated by most fixed and 

mobile equipment used in this mine. 

3.2 Antenna Sites 

Figure 3-1 is a map of most of level 6 where measurements 

were made. The map has been simplified by the removal of 

overlying (and a few underlying) workings, and is intended to 

show the scale relationship of the entire mine. There is an 

825-meter separation between mining operations near the 

crusher and the vertical shafts where all the ore and personnel 

are transported to the surface. This separation is to prevent 

mining operations and resulting subsidence in and over the ore 

body from interfering with the integrity of the shafts. 

Figure 3-2 is an expanded map of the underground development, production, and crusher-room areas. Figure 3-3 is an 

expanded map of the underground workshop and lunchroom area 

located near the main shafts. Noise spectrum measurements 

were taken in eleven locations, designated by letters A through 

K on figures 3-2 and 3-3. These locations were chosen (after 

consulting with mine personnel) as being locations where men 

are normally found working, and therefore, where communications 

would potentially be used. 
3.3 Electromagnetic Noise Spectrum Results 

3.3.1 Introduction 

When reading values from the spectral plots in this 

report, keep the following points in mind: 

1. Field strength values above the upper roll-off 

frequency and below the lower roll-off frequency are 

not calibrated and are therefore not shown on the 

spectral plots. 

2. The correct units for the spectral peaks are microamperes per meter (pA/m). 

3. The broadband noise between spectral peaks is as 

seen by a receiver having the same bandwidth as the Fast 

Fourier Transform (FFT) spectral resolution bandwidth 

used to compute the spectrum. The correct units for the 

background noise between peaks are microamperes per meter 

per square root x hertz [(p~/m)/m], where x is the 

spectral resolution of the FFT (x equals 78.1 Hz for the 

1-to-100-kHz plots). 

An easy way to obtain the spectral density per (one) 

root hertz for broadband noise is to subtract the required 

number of dB, remembering that the units have now changed to 

(pA/m)/G. For spectra with a resolution bandwidth of 78.1 

Hz, subtract 10 loglO (78.1) or 18.93 dB. 

The Appendix gives the code key used in determining the 

meaning of the numbers in the header block at the top of 

each spectrum. The resolution bandwidth is also given on the 

ordinate of the plots. 

The underground mine workings are divided into three 

parts for discussion purposes, and are called the crusherroom area, the production area, and the development area. 
The crusher room area is defined arbitrarily to be the crusher 

room and adjoining areas within 30 meters. The production 

areas. are those areas producing ore, while the development 

areas are those areas that do not yet produce ore. 

3.3.2 Uncertainties 

The spectra to 100 kHz, to 4 kHz, and to 20 kHz have 

uncertainties of + 1 dB over the following portions of the 

spectra. The 100 kHz spectra are valid either from 1 to 100 

kHz or 10 kHz to 100 kHz as stated or shown. The 40 Hz to 

4 kHz spectra shown in this report have the usual uncertainty 

limits (+ 1 dB) between 100 Hz and 3 kHz. From 40 Hz to 

100 Hz, the uncertainty limits are 2 6 dB. From 3 kHz to 

4 kHz the uncertainty is + 1 dB but the aliased signals are 

less than the specified 60 dB down. The 20 kHz spectra 

have an uncertainty of + 1 dB from 750 Hz to 20 kHz. 

The spectra shown to 180 kHz have an uncertainty of 

+ 2 dB from 3 kHz to 180 kHz. 

3.3.3 Crusher Room Area 

Figure 3-4, upper curve, shows the magnetic field noise 

spectrum received at the antenna location identified as A 

(in figure 3-2). Location A is the development foreman's 

office and is about 6 meters from the corner of the crusher 

room. The lowest curve in this, and in following figures, 

is the receiving system noise. It is included to indicate 

frequency ranges in which system noise may predominate. The 

lower curve is obtained by replacing the antenna with a 

dummy antenna. In figure 3-4, mine noise is higher than 

system noise at all frequencies. The antenna loop was 
the noise at 60 Hz is about 30 dB stronger at C (crusher 

substation) than at A. Figure 3-6 was taken at location C, 

but with higher gain to reduce receiver-system noise. To be 

able to use higher receiver gain, the high-amplitude, lowfrequency noise was attenuated below 10 kHz, and therefore 

the spectrum shows data only from 10 kHz to 100 kHz. 

Figure 3-6 was taken with the crusher not operating. 

Figure 3-7 was taken with the crusher operating. The only 

noise apparently caused by the crusher occurs between 60 kHz 

and 90 kHz as an increase varying from 0 to 8 dB. Interference 

lines are present and are separated by approximately 360 Hz. 
The three locations A, B, and C are all within 30 meters. 

(directly through rock) of the crusher room. The three locations show about the same noise amplitude, and the same rate 

of noise reduct ion (slope) with increasing frequency above 

20 kHz. The slope is measured to be 4.5 dB decrease in noise 

per 10 kHz, i.e., the noise decreases with increasing frequency (-4.5 dB per 10 kHz). These spectra show background 

noise that is present at all times. Superimposed on this 

background are short duration spikes, presumably switching 

transients. This pattern of steady background with superimposed spikes was found at every location. At all other 

locations (other than A, B, and C) however, the amplitude was 

less and the rate of noise decrease with frequency was greater. 

In the immediate vicinity of the crusher room, the noise was 

greatest. The steady-state noise background consisted of a 

train of powerline related spikes averaging 2.78 ms apart. 

This time separation indicates rectification or other fullwave use of three-phase power. A three-phase mercury rectifier for trolley haulage purposes is present elsewhere in the 

mine (more than 250 meters away horizontally and about 90 

meters vertically). As will be shown later, measurements 

made near the 4160 volt lines powering the crusher showed 

much lower noise than the crusher room area. The conclusion 

is that something, as yet unidentified, in the crusher room 

area generated the harmonics of 360 Hz. Noise contour maps 

that confirm this conclusion will be presented later. 

Figure 3-8 shows the spectrum (measured in location C 

with the crusher running) from 40 Hz to 4 kHz. This shows 

the largest amplitude 60 Hz measured in the mine. Features 

of this spectrum are (1) the presence of strong odd numbered 

harmonics, and (2) the absence of strong 360 Hz and its 
harmonics. Strong 360 Hz and harmonics have been associated 

with three-phase full wave rectifiers in other mines measured. 

To cover the amplitude range of the measured signal, the 

abscissa of figure 3-8 shows 130 dB of amplitude range. All 

other spectra use an abscissa with 100 dB of amplitude range. 

Two other areas that were also within 30 meters of the 

crusher room are the crusher access drift (labeled D on 

figure 3-2) and the 603 intersection (labeled E). Figure 

3-9 shows the spectrum measured at location D. The noise 

amplitude is lower at D than at A by 10 to 20 dB below 

1000 Hz, and by about 3 dB between 1500 Hz and 10 kHz. Above 

10 kHz the noise changes with a slope of -6.75 dB per 10 kHz 

of increasing frequency (as opposed to -4.5 dB per 10 kHz at 

location A). Location D was the only area where measurements 

were made on two different days. Figure 3-10 shows the 

spectrum obtained two days later at location D. Although 

the spectrum slope is similar, the amplitude is 3 to 8 dB 

lower on the later day. An exception is a minor "bump" between 12 and 15 kHz where the amplitude is only about 1 to 

2 dB less. The spectrum obtained for a horizontal antenna, 

oriented for maximum pickup, was about 6 dB lower, and is not 

shown. In general, noise fluctuations were less in this mine 

than in other mines where EM noise has been measured. Figure 

3-11 shows the 40 Hz to 4 kHz spectrum measured at location D. 

The one remaining measurement taken within 30 meters of 

the crusher room was taken at intersection 603 (location E) 

and is shown in figure 3-12. Immediately apparent is the 

much faster rate of noise decrease with increasing frequency 

(steeper slope). The slope measures -14.75 dB per 10 kHz. 

The amplitude in the region of 5 to 6 kHz is 1 or 2 dB lower 

than at A (the highest noise location). The noise amplitude 

below 1 kHz is 10 to 20 dB lower. No explanation is offered 

for the variation in slope. 
3.3.4 Product ion Area 

The production areas are defined as areas that are 

currently producing ore. The production area measured was 

the 606E entry labeled F on figure 3-2. This entry was one 

of a series of similar entries lined up 15 meters under the 

ore body where LHD's would load with ore. No power lines 

were in the area. The particular entry where noise was 

measured was in use as a dinner area. The middle curve in 

figure 3-13 shows the background noise measured at location 

F. The background noise at 10 kHz is about 9 dB lower than 

that found at location A. The rate of change of the noise 

with frequency (slope) is -6.0 dB per 10 kHz, slightly 

steeper than at A. At location A, pickup by the antenna with 

the sensitive axis horizontal and oriented for maximum pickup 

(45' to tunnel) varied from 11 dB less at 10 kHz to 14 dB less 

at 35 kHz (not shown) than for vertical pickup. In figure 

3-13, the upper curve shows a type of "high frequency" noise 

produced by a LHD that has passed by. The LHD was out of 

sight around a corner and was about 10 to 15 meters away. The 

fundamental is near 11,900 Hz, and harmonics (up through the 

8th near 96 kHz) are visible. This "high frequency" noise is 

higher than the background by at least 10 dB at 60 kHz. The 

loop antenna with horizontal sensitive axis picked up about 

12 dB less "high frequency" LHD noise than the loop with the 

sensitive axis vertical (not shown). The source of this 

"high frequency" LHD noise is unknown. 

Figure 3-14 shows a much more severe type of noise 

emitted by the LHD at location F. The LHD was passing 

directly by the antenna set in the entry (sensitive axis 

vertical) and was therefore about 2 meters away. The fre- - 

quency of the fundamental component of this noise is about 

425 Hz. This "low frequency" LHD noise is some 6 dB stronger 
therefore be expected to cover all the spectrum above the 

lowest fundamental (i.e., it will leave no permanent holes). 

The noise is presumed to be generated by the alternator on 

the diesel engine. The power is used for headlights, battery 

charging, and other vehicle accessories. The LHD's utilize 

a mechanical torque converter, so a possible noise field from 

a diesel-electric type drive system can be ruled out. The 

LHD's are highly mobile. Level 6 is designed to be 100 percent accessible by LHD. Therefore, any place in the mine will 

be subject to the type of noise shown in figures 3-13, 3-14, 

and 3-15, whenever a LHD is present. 
3.3.5 Development Area 

The development portions of the mine are defined as 

areas not yet producing ore. Measurements were made at three 

locations, lettered G, H and I. Locations G and H were near 

pneumatic rock drills and location I was near the excavation 

of a new underground crusher room. These three locations were 

100 meters, 140 meters, and 200 meters distant from the crusher 

room, respectively. The background noise measured at these 

three areas decreased monotonically as distance from the 

crusher room increased. The noise measured at location G, the 

jumbo-drilling face, is shown in figure 3-16. The noise 

spectrum at location G is 20 dB lower than at location A. 

The slope is the same as at A (-4.5 dB/10 kHz). 

Location H was located near an operating fanhole drill. 

The pneumatic, fanhole drill was very noisy acoustically (the 

operator wore ear protectors). However, since the drill used 

no electrical power, it produced no measurable magnetic noise. 

The spectrum measured at location H is not shown. It is 

similar to that shown in figure 3-16, but is about 10 dB 

lower. 

The lowest noise level measured in the mine was at 

location I. Here, excavation was taking place for a future 

underground crusher room (more exactly, the No. 2 mine transfer drift). This location was 200 meters away horizontally 

and 85 meters lower than the crusher room operating on level 

6. Location I contained a 40-horsepower, three-phase, 480- 

volt ventilating fan. Figure 3-17 shows the noise measured 

at I, the lowest noise level measured in the Grace mine. 

The noise shown is essentially system noise, and as such 

establishes an upper limit to mine noise in quiet conditions. 
The excavation at location I was being performed by 

a huge V-10 diesel LHD with an eight cubic-yard (6.17 m3) 

scoop capacity. This LHD would load at I, travel 750 meters 

to the operating crusher, dump, and return. Figure 3-18 shows 

the spectrum measured with the LHD about 5 meters from the 

antenna. The fundamental frequency here is 535 Hz, with the 

sixth harmonic predominating. The sixth harmonic (at about 

3210 Hz) is some 42 dB stronger than the background noise. 

Again, frequency varied widely with the engine RPM. Figure 

3-19 shows the low frequency portion of the spectrum with the 

V-10 LHD present. Figure 3-20 was taken with the LHD absent, 

with higher gain, (lower system noise) and shows the background 

power-line noise. 

3.3.6 Cross Drift Substation 

A second substation where measurements were made is 

located in the #6 cross drift, identified as J in figure 3-2. 

This substation contains a 300 KVA step-down transformer (4160 

volts to 480 volts) for supplying power to ventilating fans. 

Both circuits feeding power to the crusher room area pass 

through this substation. Figure 3-21 shows the spectrum 

measured at the substation. The spectrum slope is the same 

as the slope measured at location A; however the amplitude 

at J is 10 dB less. Since the amplitude is less at J than 

at A near the crusher, and all the power for the crusher 

room equipment passes through J, it is concluded the primary 

steady-state mine background noise is generated in or near 

the crusher room. The low frequency expanded spectrum is 

similar to that shown for location D; with the exception that 

the 60 Hz fields at location J were about 13 dB more than 

those at D. 
3.3.7 Underground Workshop -Lunchroom 

Figure 3-3 shows an expanded view of the underground 

servicing and fueling facilities for the LHD's. Figure 3-1 

shows the relationship between the rest of the mine and the 

shop area. This area also contains offices, a LHD parking 

area, a lunchroom (with fluorescent lights), ore storage 

silos, and the motor for the main conveyer belt from the 

crusher room. The conveyer-belt motor has a 200 horsepower 

rating and uses 4160 volts. The only rectifier identified 

in this area was a small 110 volt single-phase unit used for 

lighting indicator lamps on the conveyer control panel. 

Measurements were taken in the lunchroom, labeled by 

letter K on figure 3-3. Figure 3-22 shows the spectrum 

measured in the lunchroom. The antenna sensitive axis 

was vertical. With the antenna axis horizontal and oriented 

for maximum pickup, the measured spectrum is essentially the 

same as shown in figure 3-22, with the exception that the 

horizontal spectrum is 2 to 4 dB lower between 2 kHz and 

5 kHz (not shown). In comparing the spectra measured at 

location K and location A, the spectrum at K: (1) is about 

15 dB lower, (2) has about the same slope between 5 and 

40 kHz (except for a bump at 10 to 15 kHz), and (3) exhibits 

aperiodic noise below 5 kHz and between about 40 and 90 kHz. 

We speculate that the aperiodic noise comes from the fluorescent 

lights in the lunchroom. The powerline-related train of 

pulses, averaging 2.78 ms spacing, was observed with the 

portable oscilloscope at the lunchroom as well as at the 

opposite end of the mine near the crusher room. 

Figure 3 -23 shows the low-frequency , expanded spectrum 

measured in the lunchroom. Strong odd harmonics of 60 Hz 

are evident. 
shows the spectrum of the impulse, which was recorded at 

location A a few seconds after the noise background spectrum 

was recorded. Compared to the worst case composite (figure 

3-25), the impulse spectrum amplitude is 13 dB lower at 1 kHz, 

equal at 5 kHz, and up to 7 dB higher from 10 kHz to 90 kHz. 

Figure 3-27 shows a 1.02 second segment of data taken 

during the explosion. The figure is a contour map (produced 

from 157 individual spectra) showing magnetic field strength 

from 1 to 100 kHz as a function of time. The contour interval is 3.33 dB. The figure coordinate axis format is intentionally chosen to be unconventional so that the axes will 
be consistent with the 3-D figure presented later. Figure 

3-27 shows three groups of 5 to 7 individual detonations 

with a single detonation at 1.0 second delay. The final 

detonation may be the beginning of a possible fourth group, 

or single isolated shot used near the roof of the drift. The 

duration of a group of detonations is about 125 to 175 ms. 

The separation between the first detonation of sequential 

groups is 250 to 450 ms. While no positive identification as 

to location and type of blast was made at the time the measurements were made, it is believed the blast was in a small 

3 by 2.1 meter development drift. These blasts typically use 

20 to 24 holes filled with explosives (figure 3-26 shows 

approximately 21 individual detonations, or holes) . The holes 

are shot in concentric "wedges" (or groups) ; the central 

wedge is detonated first, with following wedges delayed by 

multiples of 25 milliseconds. This type of shot is sometimes 

done near the noon hour; data for figure 3-27 was taken at 

11:35 a.m. The possibility that the signature shown came 

from the electrical pulse used in igniting the cap is ruled 

out. All caps in the entire blast are connected electrically 

in parallel and are ignited simultaneously by a single electrical impulse. The required delay is built into each cap. 

The magnetic signature shown in figure 3-27 probably originates 

from the movement of free ions in the expanding plasma produced by the detonating nitrate explosives. 

Figure 3- 28 shows a computer -drawn three-dimensional (3 -D) 

view of the first two "wedges" fired. The time covered is 

450 ms. This shows the inequality in intensity of each hole 

(which is to be expected as each hole is of different length 

and loaded differently). 
3.6 Miscellaneous Measurements 

3.6.1 Electric Field 

An active dipole, 1.93 meters long, was used as a sensor 

on the second day while in area D (crusher access drift). 

This dipole has not been calibrated, nor have any system 

nonlinearities in gain vs. frequency been removed by a 

correction curve. All field strength information is therefore qualitative only. Figure 3-33 shows unprocessed data 

results obtained with the dipole ends pointed across (perpendicular to) the drift. Measurements taken with the dipole 
pointed vertically (up -down) and in the direct ion of (parallel 

to) the drift gave spectra that were 10 or more dB lower than 

figure 3-33. Maximum noise pickup was found a few tens of 

meters away along the drift under some steel reinforcing ribs. 

The noise spectrum has the same shape as figure 3-33 and is 

2 dB higher. 

Because of the relative nature of the electric field 

measurements, the principal conclusions that can be drawn are 

restricted to: (1) an electric noise field exists in location 

D; (2) it is primarily a train of spikes, similar to the magnetic field noise, occurring at an average rate of 360 times 

per second; and (3) the electric field is strongest for horizontal dipole orientation across (perpendicular to) the drift. 

3.6.2 Measurement of Voltage Between "Roof Bolts" 

A single roof-support-bolt measurement was performed at 

location D on the second day. The separation between the two 

bolts was 10 meters, and the voltage was measured using nonshielded copper wire clipped to the bolts. Figure 3-34 

shows the resulting spectrum. No receiver system noise 

curve is available for this spectrum. However, since the 

characteristic 360 Hz harmonic structure extends out to 

about 40 kHz, values out to 40 kHz are probably not obscured 

by system noise. It is not possible to say that the voltage 

measured between bolts was induced by any single mechanism. 

It may be any combination of electric field and magnetic 

field acting on the copper wires connected to the bolts, as 

well as by any potential produced by current flow between 

the bolts. To enhance electric field effects, a measurement 

was made with a single wire (a "monopole"). The resulting 

noise spectrum is 10 to 15 dB higher in the range from 1 kHz 

to 30 kHz than the spectrum shown in figure 3-34. This is 

probably due to the high input impedance (lo8 ohms) of the 

receiver. This effect has been found in other mines. 
3.7 Intercomparison of Magnetic-Field Noise in Different Mines 

3.7.1 Summaryof 1 to 3 kHz Data 

Figure 3-35, lower curve, is a summary of magnetic field 

strength at power-line harmonic frequencies observed within 

Grace. Plotted are the logarithmic averages of the six highest 

powerline harmonics. Average fields at E, F, G, H, and I are 

plotted as a function of distance from the crusher room. 

Point C is crusher room substation data taken with the antenna 

about 3 meters from the control panels. 

The six frequencies chosen are between 1020 Hz and 2940 

Hz, but are not, in all cases, the third through eight harmonics of 360 Hz, which were the frequencies of maximum energy 

in Robena No. 4 Mine. The frequencies chosen are close enough 

to make a valid comparison between noise levels in Robena and 

Grace. 

The Grace noise (as shown by the lower approximate data 

trend curve) is 10 to 20 dB lower than the Robena noise up 

to 100 meters. Beyond 100 meters, the Grace noise falls off 

much faster than the Robena noise. Two factors may contribute 

to this rapid fall off; 

(1) The Robena noise comes primarily from a line source 

(the trolley-rail transmission-line) whereas the 

Grace noise comes primarily from a point source in 

the crusher room. 

(2) The geological geometry and rock types are different. In Grace, propagation is entirely through 

dry diabase rock, 15 to 45 meters below an ore body 

containing 40 percent magnetite and 60 percent 

chlorite. In Robena, propagation is through coal 

several meters thick sandwiched between shale. 
3.7.2 Magnetic-Field Spectra 3 kHz to 180 kHz 

In addition to the low-frequency comparison given in this 

section, spectral plots to 180 kHz taken from several mines 

can give some idea as to relative noise levels in the different mines. The spectra have 2 2 dB uncertainty from 

3 to 180 kHz, but are shown to 200 kHz. Spectra of electrically 

noisy pieces of equipment in four different mines are shown 

superimposed on figure 3-36. There may have been noisier 

equipment, but those selected were: (1) a LHD in Grace Mine, 

(2) a shuttle buggy and continuous miner combination in 

McElroy Mine, (3) an unidentified machine in a longwall section 

in Itmann #3 Mine, and (4) a car pull in Robena Mine (this 

curve extends only to 100 kHz). As might be expected, machines 

which depend on electrical power for basic work force do make 

more electrical noise than diesel -powered equipment which 

only has ancillary electric systems. Also, although the 

spacing between measurement system antenna and source were 

- 3 approximately the same, the spacing makes a crucial (d ) 

difference; therefore these curves are somewhat qualitative, 

even though the measurement uncertainty is less than 2 dB. 

A similar comparison of noise levels in four different 

mines, away from noise sources, but near transmission lines, 

are shown in figure 3-37. Measurement system noise is 

indicated by showing dashed curves over the portion of the 

spectrum where system noise is 1 dB or closer to measured 

mine noise. 

The results show that although electromagnetic noise 

levels in Grace Mine are generally somewhat lower than in 

mines with other types of equipment, the levels adjacent to 

noisy sources are comparable to levels near sources in other 

mines in some cases. The noise levels do not decrease monotonically with frequency in this mine. 
Figure 2-2 System for field recording data to obtain amplitude 

probability distributions. 

COLLAPSIBLE 

ANTENNA 

f =10kHz-250kHz 

FIELD 

STRENGTH 

METER 

LOOP BW=1 .4kHz 

ANTENNA 

CHANNEL 1 

CHANNEL 2 

CHANNEL 3 

PORTABLE 

TAPE 

RECORDER 

DIRECT 

RECORDING 

15 IPS 

~~~~~~~5 40kHzc 

d 

f=lOOH~-50kHz 

SCOPE , 

MIXER 

25kHz 

OSC 

FIELD 

STRENGTH 

Hz 1 u 

f =150kHz -3 2MHz 

,455k~~I MIXER 40kHzw 
N Ob 

Figure 3-1 Simplified map 


FREQUENCY, kHz 

Figure 3-4 Spectrum of magnetic field strenpth obtained on a 

* 

to 100 kHz, Grace Mine, underground, development 

antenna sensitive axis vertical, 11:35 a.m., Apr

noise before explosion. Spectral resolution is 7

I2 0 0 2046 20 1 34-001 ' 81.00: ::/34/'3 :8 1: :$ 13 

1.45-003 2 94*001 0 OO*OOO 0 OC.03: 0- . C 

20 43C36 43006 

+- 18 86 4 24 73Goin corr.. ?ec I -!L rot cons\.= 38.4 

w 

2 2~ 60 -J c 4 uL 0 =o f 56. r .z& sGg 40 . ?3 ',< 5 

,=z zg w 5 - -- 30 

5s 5s 3 2 ;7j\-. 

w - 2<;-;1= 20 e 

me c\ Q d 2: - = =- 22 10.. -- 5 

;* ecc 

';s "e ZL sg 0" W 

~2 Ga d * :;j 2 ad ' LZ zs -10 

2~ Ah? 

& 2 -20 O - I-- -* w A :5 z w GSZ-30- 

1 k!i 

CD C'! m 4 ;3 r ZE 05 -40 

C=28 RG= 500(-14dBI CG= : FG= , A&= 52 ... 000*006 . 0. 3386 ! ,4824 005. 

.. 

.. RiNE GEljEhATED NOISE . 

" 

., 

r az 0 10 20 30 40 . jrj EO ii: 
12 o o 204m 20 1.94-001 7 mt4001 10/04173 ta:51 40 47 227 . 1.S-003 2.9mrOO2 0.00*000 0.00*000 20 4300a 4300m 46 86 4 24 73Goin corr., rec.= -6 tot const.= 46.4 

C=28 RG= 200 ( -6dBl DG= 0 FG= 0 AC= 52 

90 t 0004009 

0. 7086 I. 444+CS0. 

50 .. 

40 ,. 

30 #. MINE GENERATED NOISE 

20 *. 

10 O., ~ SYSTEM f NOISE 

- 10 . - : 

0 10 20 30 40 50 60 70 

FREQUENCY, hHz 

Figure 3-5 Spectrum of magnetic field strength obtained on a 

to 100 kHz, Grace Nine, underground crusher subs

sensitive axis vertical, 4:30 p.m, April 24, 1973

operating. Spectral resolution is 28.1 Hz. 
12 0 0 2048 20 1.Y-001 7 11*001 10104171 11 51:O

I %-lo) -4.24408l O.OO*OOO 0 00a000 20 410

E 4% 86 4 24 73Gain corr.. rec.= -26 1

TLI 

5 , 70 W 

a 5g -5 - - ="60. g ;+ 

z. =-g 50 5 '6 -; E" 4E .,% 3d 44 =- -> ;- - 

<< ,= 12 --. I/- 

= yq '@ m iL - LZ z!: 

-= 3s 

- :+< f.: G 20 

- L- 

- - 

-. !+ r- - .-z 

c2 = * 10 L iL 

- = 2 ;=& 5& 

c.2 - g0" yz LL1 ;z '+2 .k! 

., L~ 2 5 -ib 

r- .,< L. 2 > . 

- - - -20 " L, +---=- 

--: - - W -- r n 

< :, t!! 

- - ::- - 

L - . . 30 

- - 

C-26 RG= 20001-26dB1 DG= C FG= 3 AG= 

.. 000+007 . 0.3086 

.. ,, 

.. 

.. 

MINE G

.. 

.' 

r L 

10 20 

Figure 3-6 Spectrum of 

to 100 kHz, 

sensitive a

operating. 
20 .. 

10.. MINE GENER

6.. 

-10'. 

-?O*. 

Figure 3-7 Spectrum of mag

to 100 kHz, Gra

sensitive axis 

operating. Spe
,-=- 

woo ..my 

-w 

m 

u 0 

00 

0" -0, C 

sz 0 

,j "'3 &m 

OF 

(Y(Y 

52 

(D (DO w .- 0 1 

(Y 
Figure 3-9 Spectrum of ma

to 100 kHz, Gr

sensitive axis 

operating. Ex

2 c 0 2 2 I 0: . I. //7 2:2 

0 

I %.001 2.39.0il 4. 04.000 r ::.LC; 20 Ut?

+- 20 86 4 24 73Ga;n corr. , rec. = -20 :o-. 

Y Cz28 RG= lOOO[-iOdPJ Cb= S f6= 5 &= 

2 40~.ooo+oo4 . c. 3ae6 2 4- 2 e 

g so s %z 30' 

zs 

g =Gz 

g .- 20, m 2- MINE GEN

,L 

=o 

-= 55 -10. =a = x? z- 

-0 w= 

g % -20 

ZL zg z=- z= 

0 5 E -30.. 

3% EL Lil Y' 5 ~5 oa 

2 r -40 2- Y4 

, L 0 L.25 FZ-50 2 L.4 5 - 

SYSTEM NOISE 

. 

0 I = 455- 60 , - ... -. .- . -. . -. dm 

zx 

03 0 10 20 
Figure 3-10 Spectrum of magnetic field strength obtained on a l

to 100 kHz. Grace ?,line, underground, crusher acces

sensitive axis vertical, 10:27 a.m., April 26, 1973

operating. Two days later. Spectral resolution is 

12 0 0 2018 20 1.51-001 7 ~10001 10/01/75 I8.$¶:18 58 282 r 

0 I-- 1.95-005 -2 07*001 0. 000000 0 001000 20 15008 15008 

57 86 4 26 73Goin corr., rec.= -20 tot const.= 32.4 W W> 

a= - 301.. Jg d 

g ez -, 20.z z -0C' 

WO I& 

=*- , -= 10 ., 

c3 W W c- 7%- 

bz ye z % - 0 

5 El2 J CT <.'>I- LI LL ~~1s ci 

, Lg C3 :, -10 -0 -- 

-- -a- D" 

- s - g -20.. -3 LL)= L 

zz %kj 5 -- :iz -30 r I-- , & 2% a i2 -40.. 1:- 

52 -5 ne 

-2 z 

2 2 -5"" W 

ZZ 09 LD I--= 

1 -60 'A ZS r E0 CT 

-7Uc 

C=25 RG= 1000 (-20dBl DG= 0 FG= 0 AG= 52 

000+003 , 0.3086 8.984+002. 

MINE GENERATED NOISE . 

.. 

.. 

.. 

" 

cZ 'C 22 313 40 5C 6C 70 

FREQUENCY, hHz 

Figure 3-12 Spectrum of magnetic field strength obtained on a l

to 100 kHz, Grace Mine, underground, intersection 6

tive axis vertical, 12:55 p.m., April 24, 1973. Pr

Spectral resolution is 78.1 Hz. 

I2 0 0 2018 20 1.51-001 7 81*001 10101175 18 56:55 27 :2- *, 

0 1-005 2.550Ol roo0 0 OO*OOO 20 15008 15008 

+- 26 86 4 24 7biL1n corr., rcc.= -20 tot const.= 32.4 

W 

w C=28 RG= 10001-20dBl DL= 0 FG= 0 AG= 52 

2 y , OOOt004 , 

40 0.3086 I. 2644093. a 

=E ds 5 =a 

0 2E %z 30- 

W 25 2g Z, ;z 2OSP MINE GENERATED NOISE 

0 *- 

C3 0 

SYSTEM NOISE ' 

- 

I-- Lo 

g &Z C3 

4 Z = -50 Z 5 " 9s B -60 

1. 

=E 

cL 0 10 20 30 40 50 60 70 

FREQUENCY, hHz 
I2 I 0 2041 20 1.14-001 7.8l.OOt I0104173 II 22 $6 8 

I.%-011 -2.17a001 0.00.000 0 00+000 20 41008 4

7 86 4 24 73Goin corr . rec.= -34 tot con

B C-26 RG= 5000 (-34dBl DG= 0 FG= G At= 52 

u 

Figure 3-13 Spectrum of magne

to 100 kHz, Grace 

sensitive axis ve

LHD passed by. S
Mcd k 

akmu 

E t7.d u 

X cu 

w 

0 :*r2 

z 0 

SX * 

.,-I 

kowh 

WO-dP 

u4 m k 

a, Ccd a0 a, a, mc,cnc 
- - 

g~g~~~~

ISION OkV0 @YO50 U'33 'zHi3d Ull~~i-H311-3~3diiYOU9lt~ 3110 

01 3h11Y13ll BP 'F~ ' 131131 kllSX30 I':llYl"J'IS ~Il?i!3YlS-'\113\j-3Il3N3Y'~ SHll 

ti0 'S313~3n0lkl 313Y3S13 104 ' Y31321 U3d 

ItlIdklVOH'Jlk 3N0 01 3h11V13Y FJP 'h 'H13NIllLS 01314 3113N'JVW SNY 

cd Ucnd 

E cd.4 0 

kXv, 

wuaa 

0 h 
12 0 0 2041 20 1.31~001 7.81.00' !OJ:IJ'3 :B 33 40 

1,s-005 2 63.001 0.00.000 0 0~0000 21 4300

22 86 4 24 73Goin corr., rec.= -34 to

7 

20 k2 5 ?< 

g 5.?5 

4 sg io. 0 cz z- 

- a2 

cr --sq 0 YO -0 - E =A- 2- 

g : g -lG gu - =s gs 

g @ -20 Z 

my - ;: 2 -30 - - zg gE F-; g a= -40 00 mp ZL- Fk2 g 5 g & -59 + La Y 

;L I .5 Y 5 :," 

W , g -1. e -60 0- 2; 

+ L CJ e - U &~ <.'? s? - (0 L? ,.zz d 

2: 5 

c > Y' 92 

<- - ,- 

.. 000+002 . c 3386 

.. 

MIPIE GFNE

., ., ., ., " 

" " 

- 

,n 

10 20 

: .: -= 

Figure 3-16 Spectrum of ma

to 100 kHz, Gr

face, antenna 

Drill operatin

drill bits are 
Figure 3-17 Spectrum of magnetic 

to 100 kHz, Grace M

antenna sensitive ax

line harmonics. Sp
Figure 3-18 Spectrum of mag

to 100 kHz, Grac

antenna sensitiv

V-10 diesel LHD 

w 

S a 5 30 2s 

da 

= 52 zz 20- =s - g =?-? 

g ,z - 10. ez - ?z g 5 -8 - 0. =& F U 52 51~ 

g(e -10.. 

C1 - --G zz > - -.. 2 E 2 eL -20. 

0 

L3 .=> -. 0 - -- , , g -30. L' Ed- G& co L.' g ", 2 kz -40 

W CI, = 5 2, : 1? ..- 6 4 -50- . . Y- :> .! <LC 

2.; -- 

4 =: - .. L Z~.. -60 

o LL! ,n a--- 

- - - -= .r. -70 

C:28 RG=10000(-40dB: DG= 2 iG= : :G: 52 

..000+003 , s.$ce& . 

'' 

" 

2 C 20 .'D 

;p 
n 

t-' 

W 

c 

Y 

0 

rw rtcn z3 ow 

36 0 

0 PC, 

3: .* r N NZ 

oco 

3 @ 

'd @rt 

om 

RMS MAGNETIC FIELD STRENGTH, H, dE RELATIVE TO ONE b!!GROR\!PERE 

PER HETEil, FOR DISCRETE FREGUEhCIES; ill! 

RMS MAGNETIC-FIELD-STRENTH SPECTRUM UENSITY LEVEL, Hd, dB FtEI.hTIYE TO 

GllE MICRGAI;PlRE-PER-tItETEF! PER~~~Z, FOR BtlfihrJ OAiD I!CISE 

r3 ' -r3',.,PCJl--J'=n ooooooo~' 

C 
12 0 0 2048 20 2.69t000 3.91tOOO 04/18/74 23:53:14 51 247 I %-003 9 4+001 0 OO+OOO 0 OOtOOO 20 43008 43000- 

50 109 4 24 73~aih corr., roc.= -40 tot const.- 12.4 

Figure 3-20 Spectrum of magnetic field strength obtained on a 

to 4 kHz, Grace Iline, underground, Number 2 mine tr

2 :22 p.m., April 24, 1973. V-10 LHI) is not in vic

C resolution is 3.91 Hz. 
Figure 3-21 Spectrum of magnetic field strength obtained on a 

to 100 kHz, Grace Mine, underground, Number 6 cros

antenna sensitive axis vertical, 3: 10 p.m., April 

resolution is' 78.1 Hz. 

I2 0 0 204) 20 1.54-001 7.#1+001 10104171 1#:43:29 19 18. 1,s-005 2.06b001 0.0~~00~ 0 ~~10~~ 20 45008 41008 

Z 38 86 4 24 73Goin corr., rec.= -14 tot const.= 38.4 W 

W7 

C=28 RG= 500 (- 14dB) DL= 0 FG= 0 AG= 52 

a ,E 50 ,. 000+005 , 0.3086 4.242+004. 2 4- 

=S 

=o 

eg 40.1 Z 

U;1 E, 29 

<+- 30.p eg w 

-2z z z 7 g. -- - 20 .* MINE GENERATED NOISE 

-0 cJ 

Fe gs 0 0 

z - zcg -10 ,. 

Ee- LA " * 5% -20q. 5% Ed, W cr) ar 

Ls A2 

2 5 -30,. = -B z 00 

er-40. z SW m zg 2z 

a 

SYSTEM NOISE 

0 

- 50 Z 

rT 0 10 20 30 40 50 60 70 

FREQUENCY, hHz 
12 0 o 2048 20 1.34-001 7 e~.oo~ 1olorl73 18:47.07 41 191 

1,s-003 2.92*001 O.OO*OOO 0 001000 20 43008 43008 

0 l-- 40 86 4 24 73Gain corr., rec.= -26 tor const.= 26.4 

C=28 RG= 2000(-26dB) DG= 0 FG= 0 AG= 52 

we 5 2 50, 0004005 . 0.3086 7.752+004. 

W 0- 

2 s'" 

zz 40,. 0 "a Z 

25 siz &- ; 

30 S. 

ow LI1 

l--z 5s J-- 20,. z =, 

zw" E2 ~e m- MINE GENERATED NOISE 

05 

mW 

"L se - e 0 .. =z Ee -0 

Ee -10,. 0 0 Z - w 

fEe- 25 0 ,L UJ 29- -20.. 

25 55% W 'W 

- 5 9% -30q, oa w4 = '7s =z I?= 

== -40., I Zw 

0 I 25 -50 

SYSTEM NOISE . 

e 

r/) 

Z e 0 10 20 30 40 50 60 70 

FREQUENCY, hHz 

Figure 3-22 Spectrum of magnetic field strength obtained on a loo

to 100 kHz, Grace Mine, underground, shop office, ant

axis vertical, 330 p.m., April 24, 1973. LHD1s bein

meters distant, fluorescent light area. Spectral res
--. - - - -7 - 0 

0 

OD zooooc3 <Qir,*roc\J=O?F 

31i!6Y flEIVa GYntiU Ur3j '~~"tild gll:ilLl ~Uld-:i~!r!~iYOY3lkl 323 

Oi 3bl!b i38 O? "ti ' 131131 AllS;I!G NI'1iil3'~dl H19~~TiilS-fl14:-3113N"JYki Slld 

110 '~113N3nb31j 313X3Si0 VOj '11313?l ti3d 

3b'3dr('YCYOll.1 3NO 01 3AllV131j 3P 'H ' WXE!dlS (I1314 3113N3VW SWd 
UF: 4J G7@7 

0 0 dd 

kkd0 

4J Ma, v, WkWa) adk 

aa cd 

d 5 ncd 

.d cd k 

'+I -a,+ 

a) k U 

UGda, 

.rl .rl a 4Jz Em 
;,& ' 

!2 I, " t . -' 

9 - 

"Cf 0 

Uw . -z; * 

tt, II 

=- 2 

!! 7- 

CI 

2 I1 

s :2g - 

90 0- . 

- *-. 0 * ;?: " 0:s - 

=:c - . 0.- .5s $ -e 7 0 -* m- ?zg 

: =w 0- 

m ,Yg 

-?a0 QD - 

-%m -- z.- u0 

I 

0 10 20 30 40 50 60 70 80 90 100 

FREQUENCY, kHz 

Figure 3-27 Contour map showing noise generated from a group of 

denonations comprising a "shot ," as a function of time. 
Figure 3-28 Three-dimensional view of two detonations from a 

larger group comprising a "shot. l1 




Q)W k 

au u a 

m cd cdd 
- :2 : : 2:48 2: : 34.:. - S'v:;' :5/:'/-4 '' " '. ' %-..3 5,C2+::: ". ." " .. d... 0: 2. & 26 ;ji.+ ,... . ...... 2: 3 43::ea~n corr. , rec, = -6 tot const. - -6.0 

W Wa CL z-m OW 2CL 

0 ,-E -70 .. 

a !E 

z- Z< -80 +OF 

o= 

w = u E a -90 Z 0 LU 0 LL1L 

a 0 u3+ + -J-IOO.. .A 0 u a=- 2 4 +- ,-- w 

3 g 5 -110 

2 ,E = m~ 

- CLU 

z- -120 ., - u- + 

4" 3 

5 &-130.. 0 

z- m, u3 

5 cL -140 

i l 'i 2 

30 110 53 €9 70 

Figure 3-34 Voltage spectrum obtained with a monopole attached to 

10 meter separation, 10 kHz to 100 kHz, Grace Mine, u

11:35 a.m., April 26, 1973. Spectral resolution is 7
70 - 

v, MINING MACHINE 

(ROBENA) 

50- 

40 - P c' 30 - LOAD -HAUL- DUMP 

(GRACE) 

20 - 

lo - APPRO

0- 

-10 - 

-20: I 1 I 1 

I 2 3 45 

DISTANCE TO NEARE

DISTANCE TO CRUS

Figure 3-35 Comparison o

Hnes as a f
om 

I-W W 

WU 

>Z 

WW 

I-= 

au 

me W XII-LIJ 

a= 

Z I 

We 

em 

I-n 

m I 

FREQUENCY, kHz 

Figure 3-36 Comparison of E-M noise levels near operating machinery fr

four mines. Vertical magnetic-field components are shown. 

Broken sections of the curves represent system noise. 
3SION aN 8 at10118 1104 'ZH~SL~ 1136 tt313W-113d-3113dWWOt13IW 3N0 ' I 01 3AIlW13tt 8P H '13A31 AlISN3a WnI133dS H19N3111S-al3IJ-3113N9WW SWI 
4. AMPLITUDE PROBABILITY DISTRIBUTION MEASUREMENT RESULTS 

4.1 Introduction and Uncertainties 

Statistical representations are required since the 

variations of field strength are, in general, random. The . amplitude probability distribution (APD) of the received 

noise envelope is one of the most useful statistical descriptions of the noise process for the design and evaluation of 

a telecommunications system operating in a noisy environment 

[3,4,61 

By plotting the cumulative APD on Rayleigh graph paper, 

one can show clearly the fraction of time that a noise envelope 

exceeds various levels. Rayleigh graph paper is chosen with 

scales such that a Rayleigh distribution (i. e. , envelope distribution of Gaussian noise) plots as a straight line with 

slope of -1/2. Noise with rapid large changes in amplitude 

(e . g . , impulsive noise) then has a much steeper slope, 

typically -4 or -5, depending on the impulsiveness of the 

noise and the receiver bandwidth. 

With the exception of the roof-support bolt measurements, 

all APD measurements are reported in absolute quantities. 

The estimated limits of error for the APD noise measurements are + 5 dB. Several sources of error that are critical 

to the overall accuracy of our measurements are listed below: 

1. Use of a discrete, digital level counter (levels are 

6 dB apart) contributes + 1-dB quantization error limit. 

One-decibel step attenuators are used to achieve the 

+ one decibel. 

2. The system, i.e., recording, data transcribing, and 

data processing, has a calibration uncertainty of + 0.5 

dB [3]. 
jp, , , 

was sometimes immediately adjacent to a loop antenna. In all 

observed cases, this had no effect at frequencies up to 1 MHz. 

Above 1 MHz, earth and other reflections did in some cases 

cause + 1 dB variations, even with a shielded, balanced loop 

antenna. An estimate is that an additional + 5 dB uncertainty 

might be advisable. However, due to the complexity of the 

shielded loop in the mine environment, this uncertainty cannot be rigorously bounded without substantial additional 

analysis. 
4.2 Measurement Results 

APD measurements were made on April 24, 1973, during 

operation in the Grace Iron Mine located near Morgantown, 

south of Reading, Pennsylvania. Descriptions of Grace Mine 

are given in section 1.2. APD measurements were made at 

three locations. The first set of APD measurements of eleven 

different frequencies was made at the development foreman 

office, which is identified as A in figure 3-2. The second 

set of APD measurements of eight different frequencies was 

made at the electrical substation of the crusher, which is 

identified as C in figure 3-2. In these two sets of APD 

measurements, only the vertical component of magnetic field 

was measured. The third set of APD measurements was made at 

the shop office, which is identified as K in figure 3-3. 

Here two orthogonal components of magnetic field were measured: 

the vertical component of magnetic field was measured at 

thirteen different frequencies and the horizontal component 

(E-W) was measured at ten different frequencies. 

Predetection bandwidth is either 1 kHz or 1.2 kHz as indicated on each APD. 

Figures 4-1 through 4-11 show the APDts of magnetic 

field noise measured at the development foreman office (location A in figure 3-2). Only the vertical component of magnetic field was measured at eleven frequencies ranging from 

10 kHz to 32 MHz. These frequencies are 10 kHz, 30 kHz, 70 

kHz, 130 kHz, 160 kHz, 250 kHz, 1 MHz, 2 MHz, 6 MHz, 14 MHz, 

and 32 MHz. Figure 4-12 through Figure 4-19 show the APD's 

of magnetic field noise measured at the crusher substation 

(location C in figure 3-2). Again only the vertical component of magnetic field was measured at eight frequencies, 

10 kHz, 30 kHz, 70 kHz, 130 kHz, 500 kHz, 1 MHz, 2 MHz and 
phase, wound-rotor motor. Crusher feeders, conveyers and 

other similar equipments in the area of the crusher are run 

from 480 volts with typical currents of 400 amperes. Thus 

the general nature of noise measured at the electrical substation of the crusher and at the development foreman office is 

essentially a train of power line related pulses as mentioned 

in Section 3.3.3. The shoulder in the APD curves taken at 

frequencies from 10 kHz to 130 kHz results from this train of 

pulses. In other cases where the train of pulses was not 

present, this shoulder is not present (see APD curves taken 

at frequencies above 130 kHz). 
4.3 RMS and Average Values 

The APD's are integrated to give average and rms values 

of the field strength, according to the equations 

H 

avg = - 1 H dp (H) 

0 

and 

where H represents the magnetic field strength of the noise, 

and p is the probability that the measured field strength 

exceeds the value H. These quantities are also dependent upon 

the measurement bandwidth, the length of the data run, and 

possibly other parameters. Finite series are actually used 

for the numerical integration. The rms and average values so 

arrived at are identified on each graph and are time averages 

(23 minutes) of these time-dependent parameters. If the tapes 

are played into ordinary rms-reading meters, the meter readings will vary 10 to 20 dB over fractions of a second, since 

the averaging time constant is, of course, much less than 23 

minutes. The rms value is directly relatable to noise power. 

With these wide variations of field strength with time, the 

most suitable presentations are statistical ones. 

4.4 Summary Curves 

Excursions of field strength between 0.001 and 99 percent, 

as well as rms and average values, are shown in figures 4-43 

through 4-46 for magnetic field noise on April 24, 1973. The 

predetection bandwidth for these APD measurements either is 

1 kHz or is normalized to 1 kHz. Some long-term fluctuations 

in values occur because of different operating conditions 

during different times of the day. 
Percent of Time Ordinate is Exceeded 

Figure 4-1 APD, 10 kHz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, 10:OO a.m., development foreman office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-2 APD, 30 kHz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, 10:36 a.m., development foreman office, Grace Mi 
-. 

m xO'd 0 uum 

Magnetic Field Stren
Percent of Time Ordinate is Exceeded 

Figure 4-4 APD, 130 kHz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, 11:44 a.m., development foreman office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-5 APD, 160 Hz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, 12:lO p.m., development foreman office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-6 APD, 250 kHz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, 12:40 p.m., development foreman office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Lgure 4-7 APD, 1 MHz, vertical component, 1.2 lcHz predetection bandwidth, 

April 24, 1973, 10:36 a.m., development foreman office, Grace Mine. 
Linear by - loc~,~(-In p) 

Percent of Time Ordinote is Exceeded 

Figure 4-8 APD, 2 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 11:05 a.m., development foreman office, Grace Mine. 
Percent of Time Ord inote is Exceeded 

gure 4-9 APD, 6 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 11:44 a.m., development foreman office, Grace Mine. 
Percent of Time Ordinote is Exceeded 

Figure 4-10 APD, 14 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 12:10 p.m., development foreman office, Grace Xne. 
Percent of Time Ordinate is Exceeded 

Figure 4-11 APD, 32 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 12:40 p.m., development foreman office, Grace Mine. 
.0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

Percent of Time Ordinate is Exceeded 

Figure 4-12 APD, 10 kHz, vertical component, 1.0 kHz predetection bandwi 

April 24, 1973, 3:00 p.m., crusher substation, Grace Mine. 
-10 

-20 

-30 

-40 

D001.001.01 .I 5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

Percent of Time Ordinate is Exceeded 

.gure 4-13 APD, 30 kHz, vertical component, 1.0 kIIz predetection bandwidth, 

April 24, 1973, 3:30 p.m., crusher substation, Grace MLne. 
Percent of Time Ordinate is Exceeded 

Figure 4-14 APD, 70 kHz, vertical component, 1.0 kHz predetection bandwic 

April 24, 1973, 4:05 p.m., crusher substation, Grace Mine. 
c 

? 

fi 

-30 

0 

Q4 .- 

LL 

C) .- 

C 

Q, & -40 

0 

z 

-50 

-60 

,0001 D01.01 .I 5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

. Percent of Time Ordinate is Exceeded 

Figure 4-15 APD, 130 Mlz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, 4:35 p.m., crusher substation, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-16 APD, 0.5 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 3:00 p.m., crusher substation, Grace Mine. 
Fig 

Percent of Time Ordinate is Exceeded 

;ure 4-17 APD, 1 MHz, vertical component, 1.2 kHz predetection bandwi 

April 24, 1973, 3:30 p.m., crusher substation, Grace Mine. 

dth, 
c 

f! 

C 

V) 

- 50 2 

Q, .- 

LL 

0 .- 

C 

u 

c 

p -60 

r" 

-70 

- 80 

.0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 '- 99 

Percent of Ti me Ordinate is Exceeded 

Figure 4-18 APD, 2 MIz, vertical component, 1.2 kHz predetection bandwi 

April 24, 1973, 4:05 p.m., crusher substation, Grace Mine. 
Percent of Ti me Ordinate is Exceeded 

Figure 4-19 APD, 6 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 4:35 p.m., crusher substation, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-20 APD, 30 kHz; vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, ll:Oj'a.m., shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

-gure 4-21 APD, 70 kHz, vertical component, 1.0 kHz predetection bandw 

April 24, 1973, 1:00 p.m., shop office, Grace Mine. 

idt h, 
Percent of Ti me Ordi note is Exceeded 

Figure 4-22 APD, 110 mz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, 5:15 p.m., shop office, Grace Mine. 

91 
Figu 

Percent of Time Ordinate is Exceeded 

.re 4-23 APD, 130 kHz, vertical component, 1.0 kHz predetection bandwidt 

April 24, 1973, 2:15' p.m., shop office, Grace Mine. 
Percent of Time Ordinnte is Exceeded 

Figure 4-24 APD, 160 kHz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, lQ73, 4:45 p.m., shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-25 APD, 205 kHz, vertical component, 1.0 kHz predetection bandwidth, 

April 24, 1973, 5:33 p.m., shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-26 APD, 250 kHz, vertical component, 1.2 kHz predetection band"' 

April 24, 1973, 10:27a.m., shop office, Grace Mine- 
Percent of Time Ordinate is Exceeded 

Fig ;ure 4-27 APD, 500 kHz, vertical component, 1.2 kHz predetection bandwi 

April 24, 1973, ll:07a.m., shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-28 APD, 1 MHz, vertical component, 1.2 kHz predetection bandwi 

April 24, 1973, 1:00 p.m., shop office, Grace Mine. 
.0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

Percent of Time Ordinate is Exceeded 

Figure 4-29 APD, 2 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 2:15 p.m., shop office, Grace Mine. 
Percent of Ti me Ord i no te is Exceeded 

Figure 4-30 APD, 6 MHz, vertical component, 1.2 kHz predetection bandwi 

April 24, 1973, 4:45 p.m., shop office, Grace Mine. 
-80 

- 90 

- 100 

-110 

.0001.001 .01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

Percent of Time Ordinate is Exceeded 

pigure 4-31 APD, 14 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 5:15 p.m., shop office, Grace Mine. 
.0001.001 .01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

Percent of Time Ordinate is Exceeded 

Figure 4-32 APD, 32 MHz, vertical component, 1.2 kHz predetection bandwidth, 

April 24, 1973, 5:33 p.m., shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-33 APD, 10 kHz, horizontal component E-W, 1.0 kHz 

predetection bandwidth, April 24, 1973, 3: 35 p.m. , shop office, Grace Mine. 
Percent of Time Ordinote is Exceeded 

Figure 4-34 APD, 30 kHz, horizontal component E-W, 1.0 kHz 

- predetection bandwidth, April 24, 1973, 12:18 p.m., 

shop office, Grace Mine. 
1WUH* IHI 

Percent of Time Ordinate is Exceeded 

Figure 4-35 APD, 70 kHz, horizontal component E-W, 1.0 kHz 

predetection bandwidth, April 24, 1973, 1:36 P.m. 

shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-36 APD, 130 kHz, horizontal component E-W, 1.0 kHz 

predetection bandwidth, April 24, 1973, 3:00 P.m. 

shop office, Grace Mine. 
- 50 

-60 

-.- .+.. -- , .t-,..+ . . 1 + + 4.*# 

8, 

I;,, :.i 1 + ..;. - .- + 

I , - . . ,.- . . .: ..+' . , - , , : - . - ....; ;: i ; / 1 . ... . / ; I 

. . . . 4 r- , . , . -. I I-: j * '"1 "it ! I I L. ' . . '1. ..- - C. .... 2 ' . . , I , .. :;Li.::* 1 - ij - t -- 

8. ! -?- , - ,~.+ 2 

, I I::-- i 

' 

! ~ , , . , ~ . .,., ,!I 1 I ? 11 4. .I.. . - . , 

. ,.., I 1, 

. . . , , t-. > , -4 .- , 1 : . . : ';I! I! 1 I 

,, ! ,I Ic ; / ; ' ,i :,:I , ,., -80 I 

,0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

Percent of Time Ordinate is Exceeded 

Figure 4-37 APD, 160 kHz, horizontal component E-W, 1.0 kHz 

predetection bandwidth, April 24, 1973, 4:15 p.m., 

shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-38 APD, 250 kHz, horizontal component E-W, 1.2 kHz 

predetection bandwidth, April 24, 1973, 3:35 p.m., 

shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-39 APD, 500 kHz, horizontal component E-W, 1.2 kHz 

predetection bandwidth, April 24, 1973, 12:18 p.m., 

shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-40 APD, 1 MHz, horizontd component E-W, 1.2 kHz 

predetection bandwidth, April 24, 1973, 1:36 p.m., 

shop office, Grace Mine. 
Percent of Time Ordinate is Exceeded 

Figure 4-41 APD, 2 MHz, horizontal component E-W, 1.2 kHz 

predetection bandwidth, April 24, 1973, 3:00 p.m., 

shop office, Grace Mine. 


X,, 

0 

0 

N 

I 

I 

- 

0 

- 

N 

I 3 

0 

0 

cO 

I 

- - - - - 

7 - - - 

- - 

n 

-- 8 h - 

8 8 - Q, 

0 - 

w 

Q, 

V 

- = m= 

- E oE -0 - -; CA ; .E - oz>.- - zaa z - 

- xaoa - 

- 

- - 

- 

- 

- - 

- - 

- 

- 

- - 

- 

- - 

- 

- 0 

- - 

- - 

- - 

- - - 

- 

- - 

'xX I I 0 L CD 0 0 0 0 0 d CU CU d W 

I I I 

- I 1 


grounding during maintenance. Without the ground path through 

the metal shoe, communication is very poor and is not possible 

over a large portion of the 2200 foot path. Some hasty experiments performed in a shop at the mine indicated a 50 dB 

increase in signal with a resonant capacitive return path or 

40 dB increase with a low-resistance return path. With the 

metal shoe making good connection while the skip is stopped, 

communication was reported to be good -- we did not have the 

chance to observe this first hand. For any conditions not 

providing a low-resistance ground, communication was not so 

good. Standing waves would certainly be probable. This 

indicates there may be difficulty in using single-wire (hoist 
rope) transmission lines with ground return unless special 

attention is given to providing low-resistance ground returns. 

A side note is that these clamp-on, wooden frame toroidk are 

tuned (fairly broadband) for a somewhat higher frequency than 

65 kHz, and some slight improvement might be obtained with 

different tuning. 

Measurements of magnetic-field noise and of attenuation 

. of signal level were made on the hoist-phone system on the 

A shaft. A separate-wire, terminated transmission 1 ine was 

used; the noise and signal levels were relatively uniform, 

indicating no standing waves and little attenuation. The 

signal-to-noise ratio was over 70 dB. 

Magnetic-field noise and magnetic-field signals were 

both measured every 200 feet while the hoist ran from 0 to 

2200 foot depths. Measurements were made next to the carrier 

phone pick-up loop and next to the hoist rope. Antennas were 

positioned as shown in figure 5-1. The results for the transmission line are shown in figure 5-2 and the results for the 

hoist rope are shown in figure 5-3. Noise and signal are 

shown in dB relative one microampere per meter, while signalto-noise (S/N) ratio is in dB; all the measured magneticfield strengths are primarily a measure of the currents in 

each wire, as these currents are the nearest and hence 

dominant sources of magnetic field energy. 

There are two key observations. One is that with a lowloss, terminated transmission line, there are no standing 

wave patterns, and levels vary only slightly within reasonable 

bounds indicating very low attenuation. The second is that 

the noise also is nearly uniform with depth. Some variations 

were noted near working levels, but not more than 10 to 15 dB, 

and with the very high signal-to-noise ratio that prevails, 

that size of variation would create no problem. 
through escape hatch 

Figure 5-1 Top view of personnel hoist "A" skip. 
MAGNETIC FIELD STRENGTH, dB pA/m, 

OR RELATIVE S/N RATIO IN dB 

Figure 5-2 Magnetic field strength, d~p~/rn, or relative S/N ratio as 

measured along the t,ransmission line for the personnel 

hoist located in "A" shaft. 
MAGNETIC FIELD STRENGTH, dB pA/m 

OR RELATIVE S/N RATIO IN dB 

Figure 5-3 ?fag-tic field strength, d~~~/rn, or relative S/N ratio 

as measured along the hoist rope for the personn,el 

hoist located in "A" shaft. 
6. CONCLUSIONS 

The spectral plots show a wide range of magnetic-field 

noise levels at different locations. In general, the noise 

levels decrease with frequency, although if there is a nearby 

dominant noise source, there are many variations that are 

characteristic only of that source. 

The most important observation is that the magnetic-field 

noise levels in this mine are about 10 to 50 dB lower than in 

most coal mines. There are two exceptions. First, near the 

crusher-room substation, where heavy current is flowing, there 

are high power-line harmonics. Noise at higher frequencies 

is also relatively high. Second, the LHD1s generate high 

fields anywhere in the mine they happen to be. Even near 

these "hot spot" areas, electromagnetic noise levels are 

significantly lower than near similar "hot spot1' areas in coal 

mines. 

Noise contour maps at 2 kHz, 10 kHz, 20 kHz and 60 kHz 

show a definite continuous source in or near the crusher 

room. Some type of fast - rise-time , full -wave, three-phase 

current pulse is present. Spectral lines produced by this 

source are separated by approximately 360 Hz. Since the 

separation is approximate, the source may be an arc. 

There are electromagnetic impulses generated by chemical 

explosions. These create short-duration peaks higher (at 

frequencies above 5 kHz) than levels generated by other 

equipment in the mine. 

The APD1s also indicate relatively lower noise levels 

for horizontal and vertical fields at locations away from the 

crusher substation. However, noise variations with frequency 

are not monotonically decreasing as had been observed either 

in coal mines or for surface locations. The time variations 

are random; impulsive noise is present a lower percentage 

of time in Grace Mine than in coal mines, particularly at 

lower frequencies. 
The hoist-phone measurements showed a high signal-tonoise ratio, practically no standing waves, and no measurable 

loss over the 670 meter (2200 feet) depth at the 100 kHz 

operating frequency, for the system with a separate, terminated cable that runs down the hoist shaft. The "ground" 

return path is mostly through metallic cables, pipes, etc., 

and therefore is not significantly affected by earth parameters. This is a common situation. 

Although direct observations or measurenlents were not 

made on another hoist using only the hoist cable and either 

(1) no termination (open circuit condition), or (2) a brass 

shorting plate (short -circuit condition) , reported operation 

was not satisfactory for case (I), open, and only moderately 

satisfactory for case (Z), short. 
7. RECOMMENDATIONS 

The measured electromagnetic data in this report should 

be compared with similar data from coal mines. A different 

type of haulage locomotion (diesel rather than dc electrical) 

power is used in this mine; there are significant differences 

in the electromagnetic environment produced. 

If a mobile (to LHD vehicle) communication system is to 

be designed, some shielding of electrical components on the 

LHD's may give high improvements relative to costs. 

Where a separate personnel hoist is operated, e.g., the 

ore skip and the personnel skip are not the same, and hence 

where falling ore is not going to damage a special transmission line that can be run down the shaft, use of a separate 

transmission line appears very attractive. Where the ore and 

personnel skip are the same, practical considerations require 

the skip rope (cable) to be one path of a transmission line, 

while earth and/or metallic pipes and cables can provide the 

other path. In this case, standing waves will be a problem 

of unknown severity, since there is no reliable termination. 

Elimination of the noise source in the crusher room 

would lower continuous in-mine background noise significantly. 

Hardwire, dial telephones are used in this mine for 

point-to-point communication; this system works well for all 

locations where phones are available. In less static situations such as exist in working areas of coal mines, such a 

system might not always be practical. 

I 9. REFERENCES 

[I] Bensema, W.D., Kanda, M., Adams, J.W., Electromagnetic 

Noise in Robena No. 4 Coal Mine, NBS Tech. Note 654, 

April 1974. 

[2] The Institute of Electrical and Electronic Engineers, Inc., 

IEEE Dictionary of Electrical and Electronic Terms, 

Std. 100, 1972. 

[3] Crichlow, W.Q., et al., Amplitude-Probability Distributions 

for Atmospheric Radio Noise, NBS Monograph 23, 1960b. 

[4] Thompson, W.I., 111, Bibliography of Ground Vehicle Communications and Control, AKWIC index, Report No. DOTTSC -UMTA- 71 - 3, July 1971. 

[5] Taggart, H.E. and Workman, J.L., Calibration Principles 

and Procedures for Field Strength Meters (30 Hz to 1 GHz), 

NBS Tech. Note 370, March 1969. 

[6] Spaulding, A.D. and Disney, R.T., Man-Made Radio Noise - - 

Part 1: Estimates for Business, Residential, and Rural 

Areas, OT Report 74-38, June 1974. 
10. APPENDIX 

Decoding of Spectrum Captions 

Spectrum captions are generally organized into the following format : 

First line: MP NDT NZS NDA NPO RC DF date, time, frame, serial, 

where 

NDT = Detrending option, example, 0 (dc removed) 

NZS = Restart spectral average after output, example, 0 

(restarted) 

NDA = Data segment advance increment, example, 2048 

NPO = Number of spectra averaged between output calls, 

example, 20 

RC = Integration time in seconds per spectra, example, 0.168 

DF = Resolution bandwidth, spectral estimate spacing in 

hertz, example, 62.5 

Date = Date of computer processing, example, 03/21/73 

Time = Time of computer processing, example, 15:06: 34 

Frame= Frame set number, example, 10 

Serial = Film frame serial number, example, 42. 

Second line: DTA DA(1) DA(2) DA(3) NSA NRP NPP, where 

DTA = Detrending filter parameter a, example, 0.00195 

DA(1) = Detrending filter average, K=l, example, 59.4 

DA(2) = Detrending filter average, K=2, example, 0 

DA(3) = Detrending filter average, K=3, example, O 

NSA = Number of periodograms averaged, example, 20 

NRP = Number of data points processed since spectrum 

initialization, example, 43008 

NPP = Number of data points processed since data initialization, example, 43008. 
Third line: RUN, SESSION, MONTH, DAY, YEAR Gain corr., rec. = 

tot. constr. =, where 

Run and Session = the title of the portrayed frame identifying 

the digitizing session and run number, 

example, 21 83 

Month, Day, Year = date data were recorded in the mine, 

example, 8 25 73 

Gain corr. rec. = receiver gain correction, example, -6 

tot. const. = constant gain correction of entire system, 

example, -46.4 

Fourth line: C =, RG =, DG =, FG =, AG =, where 

C = correction curve used with data, example, 25 

RG = receiver gain and accompanying correction in dB added to 

the data, example, 200 (-6 dB) 

DG = digitizer gain, example, 0 

FG = filter gain in dB, often rounded to nearest single digit, 

example, 0 I 

AG = absolute gain correction added to data, example, 52 

Fifth line: Top of Scale, Standard Error, Spectral Peak, where 

Top of Scale = largest scale marking for computer drawn 

graph, example, 1.000+004 (1.0 x lo4) 

Standard Error = standard error of curve, example, 0.3162 

Spectral Peak = largest spectral peak observed, example, 

4.108+003 (4.108 x lo3) 
several discrete frequencies for a sufficient amount of time to provide amplitude 

probability distributions. 

The specific measured results are given in a number of spectral plots and 

amplitude probability distribution plots. 

i I 

17. KEY WOKIIS (six to twelve entries; alphabetical order; capitalize only the first letter of the 

rlanle; separated by semicolons) Amplitude probability distribution; coal mine noise; digital 

data; electromagnetic interference; electromagnetic noise; electromagnetic pulse 

(chemical); emergency communications; Fast Fourier Transform; Gaussian distribution; 

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