NBSIR 74-388

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

June 1974

Prepared for

U. S. Bureau of Mines

United States Department of the Interior

Pittsburgh, Pennsylvania 15222

Working Fund Agreement HO 133005

NBSIR 74-388

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 Interior

Pittsburgh, Pennsylvania 15222

Working Fund Agreement HO 133005

u s DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director
FOREWORD

This report was prepared by the National Bureau o£ Standards,

Boulder, Colorado, under USBM Contract No. HO 133005. The

contract was initiated under the Coal Mine Health and Safety

Research Program. It was administered under the technical

direction of the Pittsburgh Mining and Safety Research Center

with Mr. Howard Parkinson and Mr. Harry Dobroski acting as the

technical project officers.

This report is a summary of the work completed as part of

this contract during the period January 1973 to June 1974.

This report was submitted by the authors in October 1974.
CONTENTS

Page

1. INTRODUCTION 1

1.1 Background 2

1.2 Mine Description 3

2. MEASUREMENT INSTRUMENTATION 5

3. SPECTRUM MEASUREMENT RESULTS 11

3.1 Introduction 11

3.2 Antenna Sites 11

3.3 Electromagnetic Noise Spectrum Results 12

3.3.1 Introduction 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 20

3.3.7 Underground Workshop -Lunchroom 21

3.3.8 Composite of Worst Case Steady Noise 22

3.4 Pulse Produced with Explosion 22

3.5 Mine Noise Contour Maps 24

3.6 Miscellaneous Measurements 24

3.6.1 Electric Field 24

3.6.2 Measurement of Voltage Between "Roof

Bolts" 25

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 26

i 3.7.2 Magnetic-Field Spectra 3 kHz to 180 kHz 27

4. AMPLITUDE PROBABILITY DISTRIBUTION MEASUREMENT

RESULTS 65

4.1 Introduction and Uncertainties 65

4.2 Measurement Results 67

4.3 RMS and Average Values 69

4.4 Summary Curves 69

5. HOIST-PHONE MEASUREMENT RESULTS 116

6. CONCLUSIONS 121

7. RECOMMENDATIONS 123

8. ACKNOWLEDGMENTS 124

9. REFERENCES 125

10. APPENDIX 126

iv
LIST OF FIGURES

Page

Figure 2-1. Block diagram o£ portable instrumentation 8

Figure 2-2. System for field recording data to obtain

amplitude probability distributions 9

Figure 2-3. Block diagram of laboratory recording system

modified for field use 10

Figure 3-1. Simplified map of Grace Mine where measurements were made 28

Figure 3-2. Expanded map of underground development,

production, and crusher-room areas 29

Figure 3-3. Expanded map of underground workshop and

lunchroom area 30

Figure 3-4. Spectrum of area noise before explosion.

Measurements at development foreman office

with spectral resolution of 78.1 Hz 31

Figure 3-5. Spectrum when crusher is not operating.

Measurements at crusher substation with

spectral resolution of 78.1 Hz 32

Figure 3-6. Spectrum at higher gain when crusher is not

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

Figure 3-7. Spectrum when crusher is operating. Measurements at crusher substation with

spectral resolution of 78.1 Hz 34

Figure 3-8. Expanded spectrum of crusher operating.

Measurements at crusher substation with

spectral resolution of 3.91 Hz 35

Figure 3-9. Spectrum when crusher is not operating.

Measurements at crusher access drift with

spectral resolution of 78.1 Hz 36

Figure 3-10. Spectrum when crusher is operating. Measurements at crusher access drift with

spectral resolution of 78.1 Hz 37

V
LIST OF FIGURES (Continued)

Page

Figure 3-11. Expanded spectrum of crusher operating.

Measurements at crusher access drift with

spectral resolution of 3.91 Hz 38

Figure 3-12. Spectrum of noise at intersection 603 production area with spectral resolution of

78.1 Hz 39

Figure 3-13. Spectrum comparison between steady-state

background noise and LHD noise at entry and

dining area 606E. Spectral resolution is

78.1 Hz 40

Figure 3-14. Spectrum of V-8 diesel LHD. Measurements

at entry and dining area 606E with spectral

resolution of 78.1 Hz 41

Figure 3-15. Expanded spectrum of loaded LHD. Measurements at entry and dining area 606E with

spectral resolution of 3.91 Hz 42

Figure 3-16. Spectrum of operating pneumatic fanhole

drill. Measurements at jumbo drill working

face with spectral resolution of 78.1 Hz 43

Figure 3-17. Spectrum of lowest noise level measured in

Grace Mine. Measurements at Number 2 mine

transfer drift with spectral resolution of

78.1 Hz 44

Figure 3-18. Spectrum of V-10 diesel LHD at Number 2 mine

transfer drift. Spectral resolution is

78.1 Hz 45

Figure 3-19. Expanded spectrum of V-10 diesel LHD at

Number 2 mine transfer drift. Spectral

resolution is 3.91 Hz 46

Figure 3-20. Expanded spectrum after V-10 diesel LHD

left the vicinity. Measurements at Number

2 mine transfer drift with spectrum resolution of 3.91 Hz 47

Figure 3-21. Spectrum of background noise measured at

Number 6 crossdrift substation. Spectral

resolution is 78.1 Hz 48

vi
LIST OF FIGURES (Continued)

Page

Figure 3-22. Spectrum of LHD ' s being refueled 30 meters

distant. Fluorescent light area in shop

office. Spectral resolution is 78.1 Hz 49

Figure 3-23. Expanded spectrum measured at shop office.

Fluorescent light area. Spectral resolution

is 3.91 Hz 50

Figure 3-24. Spectrum of LHD passing by the shop lunchroom area. Spectral resolution is 19. S Hz-- 51

Figure 3-25. 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 52

Figure 3-26. Spectrum of explosion impulse measured at

development foreman office. Spectral resolution is 78.1 Hz 53

Figure 3-27. Contour map showing noise generated from a group of detonations comprising a "shot,"

as a function of time 54

Figure 3-28. Three-dimensional view of two detonations

from a larger group comprising a "shot" 55

Figure 3-29. Grace Mine noise contour map for 2 kHz 56

Figure 3-30. Grace Mine noise contour map for 10 kHz 57

Figure 3-31. Grace Mine noise contour map for 20 kHz 58

Figure 3-32. Grace Mine noise contour map for 60 kHz 59

Figure 3-33. 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 60

Figure 3-34. Voltage spectrum of roof bolt at crusher

access drift 61

vii
LIST OF FIGURES (Continued)

Page

Figure 3-35. Comparison of magnetic field strengths of

Grace and Robena Mines as a function of

distance from noise source 62

Figure 3-36. Comparison of E-M noise levels near

operating machinery (noise sources) from

four different mines 63

Figure 3-37. Comparison of E-M noise levels along

haulageways in four different operating

mines . Machinery was not operating in

vicinity at times of measurements 64

Figure 4-1. APD, 10 kHz, vertical component 70

Figure 4-2. APD, 30 kHz, vertical component 71

Figure 4-3. APD, 70 kHz, vertical component 72

Figure 4-4. APD, 130 kHz, vertical component 73

Figure 4-5. APD, 160 kHz, vertical component 74

Figure 4-6. APD, 250 kHz, vertical component 75

Figure 4-7. APD, 1 MHz, vertical component 76

Figure 4-8. APD, 2 MHz, vertical component 77

Figure 4-9. APD, 6 MHz, vertical component 78

Figure 4-10. APD, 14 MHz, vertical component 79

Figure 4-11. APD, 32 MHz, vertical component 80

Figure 4-12. APD, 10 kHz, vertical component 81

Figure 4-13. APD, 30 kHz, vertical component 82

Figure 4-14. APD, 70 kHz, vertical component 83

Figure 4-15. APD, 130 kHz, vertical component 84

Figure 4-16. APD, 0.5 MHz, vertical component 85

Figure 4-17. APD, 1 MHz, vertical component 86

viii
LIST OF FIGURES (Continued)

Page

Figure 4-18. APD, 2 MHz, vertical component 87

Figure 4-19. APD, 6 MHz, vertical component 88

Figure 4-20. APD, 30 kHz, vertical component 89

Figure 4-21. APD 70 kHz, vertical component 90

Figure 4-22. APD, 110 kHz, vertical component 91

Figure 4-23. APD, 130 kHz, vertical component 92

Figure 4-24. APD, 160 kHz, vertical component 93

Figure 4-25. APD, 205 kHz, vertical component 94

Figure 4-26. APD, 250 kHz, vertical component 95

Figure 4-27. APD, 500 kHz, vertical component 96

Figure 4-28. APD, 1 MHz, vertical component 97

Figure 4-29. APD, 2 MHz, vertical component 98

Figure 4-30. APD, 6 MHz, vertical component 99

Figure 4-31. APD, 14 MHz, vertical component 100

Figure 4-32. APD, 32 MHz, vertical component 101

Figure 4-33. APD, 10 kHz, horizontal E-W component 102

Figure 4-34. APD, 30 kHz, horizontal E-W component 103

Figure 4-35. APD, 70 kHz, horizontal E-W component 104

Figure 4-36. APD, 130 kHz, horizontal E-W component 105

Figure 4-37. APD, 160 kHz, horizontal E-W component 106

Figure 4-38. APD, 250 kHz, horizontal E-W component 107

Figure 4-39. APD, 500 kHz, horizontal E-W component 108

Figure 4-40. APD, 1 MHz, horizontal E-W component 109

ix
LIST OF FIGURES (Continued)

Page

Figure 4-41. APD, 2 MHz, horizontal E-W component 110

Figure 4-42. APD, 6 MHz, horizontal E-W component 111

Figure 4-43. Field strength excursions between 0.001 and

99% of the time as a function of frequency,

vertical component, development foreman

office 112

Figure 4-44. Field strength excursions between 0.001 and

991 of the time as a function of frequency,

vertical component, crusher substation 113

Figure 4-45. Field strength excursions between 0.001

and 99% of the time as a function of

frequency, vertical component, shop office-- 114

Figure 4-46. Field strength excursions between 0.001 and

991 of the time as a function of frequency,

horizontal E-W component, shop office 115

Figure 5-1. Top view of personnel hoist "A" skip 118

Figure 5-2. Magnetic field strength, dB yA/m, or relative S/N ratio as measured along the

transmission line for the personnel hoist

located in "A" shaft 119

Figure 5-3. Magnetic field strength, dB yA/m, or relative S/N ratio as measured along the hoist

rope for the personnel hoist located in

"A" shaft 120

X
ELECTROMAGNETIC NOISE IN GRACE MINE

by

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

com.municat ions ; 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 magnet ic - 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 • recominendations . 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 [1]

.

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. Hov/ever, 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 operational 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 EMI is a major limiting factor in the

design of a communication system.

The work reported here gives the results of comprehensive

measurements of this EMI in critical communication locations

where miners extract ore

.
There are several EMI 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

3
Iron Mine located near Morgantown, south of Reading,

Pennsylvania. The mine belongs to Bethlehem Steel Company

and produces iron ore in the form of magnetite. The depth

of level 6, where all measurements were taken, is 715 meters

below the surface. The ore body is a large, flat, oval

structure about a hundred meters thick, and is mined by undercutting and allowing the ore to fall into bins called entries.

Air-cooled, V-8 diesel -powered , rubber- tired , Load -Haul -Dump

(LHD) vehicles use front-loading scoops, typically of 5 cubicyard capacity (1 cubic yard = 0.76 m^), to pick up large

chunks of ore, haul the ore to the underground crusher, and

dump it into the crusher ore bin. After the ore is crushed

in the ore crusher, it is transported by conveyer belt horizontally 825 meters, then lifted to the surface by a skip.

Entry is made by a personnel cage in A shaft . This cage

operates in one partition, while two ore skips operate in

another partition of A shaft. B shaft was undergoing

maintenance at the time of the measurements.

There are other types of haulage equipment used in this

mine, but they, too, are diesel powered and rubber tired. Air

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.

4
2. MEASUREMENT INSTRUMENTATION

Two measurement techniques were used. The first technique

covers a large portion of the spectrum a^ 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 [5].

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-operated, 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 H^. 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] , H^, versus frequency for

broadband noise in the spectral plots. In the amplitude

5
probability distributions, results are given as the rms

value of one component o£ magnetic field strength versus

percent of time this value is exceeded. The APD gives the

distribution of the actual instantaneous values only as far

as the measurement-system detector bandwidth will allow the

detector to follow the time variations of the actual magnetic

field. (In this context, noise envelope is sometimes used.)

Thus, the results are applicable for a communication receiver

whose bandwidth is similar to the measurement-system detector

bandwidth.

Three measurement systems were used to make measurements

underground. The three block diagrams are shown in figures

2-1, 2-2, and 2-3. Figures appear at the end of each section

in this report. For a detailed description of these systems,

see a previous report, Electromagnetic Noise in Robena #4

Coal Mine [1] . The changes in the measurement systems as

used in Grace Mine from the way they were in Robena Mine are

as follows:

First, the portable spectral measurement system had

10 kHz, high-pass filters added on two channels while a third

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.

6
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 [1] reported earlier. The data processing

equipment was unchanged.

7
SIGNAL MONITORS

LOUD

SPEAKER

SYSTEM

#1

LOOP

ANTENNA BROADBAND

AMPLIFIER

OSCILLOSCOPE

IMPEDANCE^—

'

TRANSFORMER

10 METERS

OF CABLE

lOO-kHz

LOW PASS

FILTER

SYSTEM

#2

SYSTEM

#3

200^^Hz CRYSTAL

OSCILLATOR

SHORT

r

MICROPHONE

PORTABLE

TAPE RECORDER

(30 ips, record)

TRACK I (FM)

TRACK 2 (DIR)

TRACK 3 (FM)

TRACK 4 (DIR)

TRACK 5(FM)

TRACK 6 (DIR)

TRACK 7 (FM)

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.

8

11-74 -29151
COLLAPSIBLE

ANTENNA

LOOP

ANTENNA

f =1 0kH2-250kHz

455kHz

FIELD

STRENGTH

METER A

BW = 1 .4kHz

MIXER

40kHz

25kHz

OSC

FIELD

STRENGTH

METER

BW=3 .3kHz

455kH2^ MIXER

4 0 kHz.

f =1 50kHz-32MHz

CHANNEL 1

CHANNEL 2

CHANNEL 3 PORTABLE

TAPE

RECORDER

DIRECT

RECORDING

15 IPS

f =100Hz-50kHz

SCOPE

Figure 2-2 System for field recording data to obtain amplitude

probability distributions.

9 IB-7'' -29161
COLLAPSIBLE

LOOP

ANTENNA

BALUN

FIELD

STRENGTH

METER

IOkHz-250kHil

455 kHz MIXER 40 kHz

10 kHz

HIGH PASS

FILTER

BALUN H

LOOP

ANTENNA

(SENSITIVE AXIS VERTICAL)

COLLAPSIBLE

LOOP

ANTENNA

BALUN

FIELD

STRENGTH

METER

l50kHz-32MHz|

455 kHz

O BALUN PREAMP

LOOP

ANTENNA

(SENSITIVE AXIS VERTICAL)

PREAMP

25 kHz

OSCILLATOR

EQUIPMENT

SIMILAR TO 3, O—

BUT WITH HORIZONTAL

ANTENNA ORIENTATION

MIXER

20 kHz

LOW PASS

FILTER

RECORDER

CHANNEL I (FM)

CHANNEL 2 (DIR)

CHANNEL 3 (DIR)

CHANNEL 4 (DIR)

CHANNEL 5 (DIR)

40 kH; Q__ CHANNEL 6 (DIR)

CHANNEL 7 (FM)

30ips

Figure 2-3 Block diagram of laboratory recording system modified

for field use.

10
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 manydifferent 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.

11
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 (yA/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 [ (yA/m) / /x Hz ], where x is the

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

1-to-lOO-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

(yA/m)//HT. For spectra with a resolution bandwidth of 78.1

Hz, subtract 10 log-j^Q (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.

12
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 ± 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

13
placed flat on the ground (the sensitive axis was therefore

pointed up-down, i.e., vertically). The noise spectrum at

location B (known as "601 intersection") is similar to

location A above and is not shown.

The electrical substation at the ore-crusher is identified by letter C on figure 3-2. The ore-crusher substation

contains two 500 KVA transformers for stepping down threephase 4160 volts, to three-phase 480 volts. The stepped

down voltage is used by crusher feeders, small conveyers,

fans, and other similar equipment in the area of the crusher.

The ore-crusher (a jaw crusher type) is run by a 1 50 -horsepower

,

three-phase, 4160 volt, 20.3 ampere, wound-rotor motor. The

main contactors for this motor are activated by dc current.

The dc current is supplied by a single-phase, 230 volt,

bridge rectifier. The spectrum of the noise measured in the

crusher substation is shown in figure 3-5. From 20 kHz up,

the spectrum is similar in amplitude and slope to the noise

spectrum at the development foreman's office (location A).

Below 20 kHz several differences appear. At about 13 kHz,

there is a relative maximum, 10 dB higher than at A. Also,

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.

14
The three locations A, B, and C are all within 30 meters

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

of noise reduction (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

15
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.

16
3.3.4 Production 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

C45° 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 frequency of the fundamental component of this noise is about

425 Hz. This "low frequency" LHD noise is some 6 dB stronger

17
than the "high frequency" LHD noise at 96 kHz. The "low

frequency" LHD noise at 850 Hz at location F, is some 40 dB

higher than the background noise at 850 Hz at location A.

At 2550 Hz (the 6th harmonic) the LHD noise is some 20 dB

higher than the background noise at location A. Figure 3-15

shows the 40 Hz to 4 kHz spectrum of the LHD going by

location F. In this spectrum, the comparatively weak

fundamental can be seen. This spectrum covers a real time

period of 1.34 seconds. During this time, the diesel engine

changed its speed so that the alternator interfering fundamental frequency swept over the range from 330 Hz to 410 Hz.

The second harmonic swept over twice the range or 660 Hz to

820 Hz. The third harmonic is not visible. The fourth

harmonic swept over the range from about 1320 Hz to 1640 Hz,

and so on. While not considered necessary, a 3-D plot could

resolve whether the engine speed was increasing or decreasing,

and whether the LHD was approaching or receding. The frequency of this noise was noted in the mine to vary over wide

limits in proportion with engine speed, certainly over a frequency range greater than two to one. This noise can

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.

18
3.3.5 Development Area

The development portions o£ 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.

19
The excavation at location I was being performed by

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

scoop capacity. This LHD would load at I, travel 7 50 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.

20
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 3-2 2 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 powerl ine -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

.

21
Figure 3-24 shows a LHD passing by area K. It was

measured with different instrumentation. This spectrum

extends out to 20 kHz. Spectra taken out to 180 kHz indicate

LHD noise continues at a constant or slightly rising level

from 100 to 180 kHz (not shown).

3.3.8 Composite of Worst Case Steady Noise

Figure 3-25 is a composite of the highest steady-state

background noise (area A) overlain by the highest LHD spectra

measured (area F) . A line (or envelope) drawn over the peaks

of the LHD and background noise may reasonably be used for

highest expected noise levels in Grace Mine. Figure 3-17

shows the lowest noise level measured (area I) , some 65 dB

lower at 1000 Hz to 35 dB lower at 40 kHz.

3.4 Pulse Produced with Explosion

Several impulses associated with blasting in the mine

were recorded. The closest explosion produced the strongest

impulse; the compression wave in the rock and the sound

followed the impulse by a fraction of a second. Figure 3-26

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

22
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 3-2 7 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)

.

23
3 . 5 Mine Noise Contour Maps

To give a better picture of how noise varied as a function of location, several contour maps are presented.

Four frequencies were selected, 2 kHz, 10 kHz, 20 kHz and

60 kHz, and from these data four contour maps were produced.

Each map is produced by selecting a given frequency, say 2 kHz,

and then recording the amplitude measured at each location

in the mine on a map. Lines of constant magnetic noise level

are then drawn on the map. Areas of high or low noise level

can then be more easily located. It is acknowledged that

eleven measurement locations are not enough locations to

provide a detailed map, accurate enough to produce reliable

extrapolation of noise intensities at areas far from locations actually measured. The intent is to give a feeling of

the overall noise distribution using available information.

Figures 3-29, 3-30, 3-31 and 3-32 show Grace Mine noise

contour maps for frequencies 2 kHz, 10 kHz, 20 kHz, and 60 kHz,

respectively. Every map shows an area of high noise centered

approximately on the crusher room. Note also that the noise

tends to follow the powerlines (through area J) to some extent.

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

nonlinearit ies 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

24
pointed vertically (up-down) and in the direction 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 da> . 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 (10^ ohms) of the

receiver. This effect has been found in other mines.

25
3 . 7 Intercomparison of Magnetic-Field Noise in Different Mines

3.7.1 Summary of 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.

26
3.7.2 Magnetic-Field Spectra 5 kHz to 18Q 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 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.

27
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64
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

[3,4,6].

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] .

65
3. The estimated uncertainty involved in using the portable and the laboratory tape recorders for record and

playback is ± 0.5 dB due to harmonic distortion, flutter,

dropout, cross-talk, etc.

4. The gain instability during measurements, gain

changes between measurements and calibration, and the

non-linearity of electromagnetic interference and field

strength (EIFS) meters and mixers, all combined, contribute ± 0.5 dB uncertainty.

5. The gain instability and non-linearity of the digital

level counter, the tuned frequency converter, the amplifier, and attenuators, all combined, contribute ± 0.5 dB

uncertainty.

6. Connector losses and BNC cable losses, particularly

at higher frequencies above 100 kHz, contribute ± 2.0 dB

uncertainty.

Some additional uncertainty beyond the stated measurement

system uncertainty is caused by the in-mine environment. Care

was taken to provide at least one meter separation from metallic objects wherever possible. However, coal, rock, or earth

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 .

'

66
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 APD's 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

67
6 MHz. Figure 4-20 through 4-42 show the APD's of magnetic

field noise measured at the shop office (location K in

figure 3-3). Figures 4-20 through 4-32 show the APD's of

the vertical component of magnetic field noise measured at

thirteen different frequencies, 30 kHz, 70 kHz, 110 kHz, 130

kHz, 160 kHz, 205 kHz, 250 kHz, 500 kHz, 1 MHz, 2 MHz, 6 MHz,

14 MHz, and 32 MHz. Figures 4-33 through 4-42 show the APD's

of the horizontal E-W component of magnetic field noise

measured at ten different frequencies, 10 kHz, 30 kHz, 70 kHz,

130 kHz, 160 kHz, 250 kHz, 500 kHz, 1 MHz, 2 MHz and 6 MHz.

Air-cooled, V-8 diesel -powered , rubber-tired Load -HaulDump (LHD) vehicles with front -loading scoops of typically

5 cubic-yard capacity (1 cubic yard ^ 0.75 m^) are used to

pick up and haul iron ore to the underground crusher, and

dump the ore into the crusher ore bin. Main sources of EM

noise are considered to be located in the crusher room area

(see Section 3.5). As stated in Section 3.3.3, the crusher

substation contains two 500 KVA transformers for stepping

three-phase voltage down to 480 volts. The crusher itself

is run by a 150 horsepower, 4160 volt, 20.3 ampere, threephase, 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)

.

68
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

00 H

avg / H dp(H)

o and

f OO H

rms

-/ dp(H)

o 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.

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.

4 . 4 Summary Curves

69
Percent of Time Ordinate Is Exceeded

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

April 24, 1973, 10:00 a.m., development foreman office, Grace Mine.

70
Linear by - j iog|Q(- In p)

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

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 Mine.

71
Linear by - y log|Q{-ln p) 40 I II II 1 1 n il I I iiiiiiii II I — I I —> 1

i 1

30

Percent of Time Ordinate is Exceeded

Figure 4—3 APD, 70 kHz, vertical component, 1.0 kHz predetection bandwidth,

April 24, 1973, 11:05 a.m., development foreman office, Grace Mine.

72
Linear by - j logi^t-ln p)

& -50 o

-60

-70 I II ' I' " " " ' ^

" I

'

' '

'

'

1 ^ ^ '

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

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.

73
Linear by - y log|g(-lnp)

-80 ' "I I M ill I I— I I—^ ^—^ '

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

Percent of Time Ordinate is Exceeded

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

April 24, 1973, 12:10 p.m., development foreman office, Grace Mine.

74
-70

-80 ' " II ' " I M I I I i I I I I M I I I I \ \ \ 1

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

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.

75
Linear by - y log|g(-lnp)

Percent of Time Ordinate is Exceeded

Figure 4-7 APD, 1 MHz, vertical component, 1.2 kHz predetection bandwidth,

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

76
Lineor by - y log|Q(-ln p)

Percent of Time Ordinate is Exceeded

Figure 4—8 APD, 2 MHz, vertical con?)onent, 1.2 kHz predetectlon bandwidth,

April 24, 1973, 11:05 a.m., development foreman office, Grace Mine.

77
Linear by - j log|Q(-lnp)

50| II I I I

I I I I

M ill II I I I I—I i—I 1

40

cr

E

Q.

01

o>a

E

o

o

>

o

CD

c

' <bu. *— CO

«L

o

c

o>o

Percent of Time Ordinate is Exceeded

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

April 24, 1973, 11:44 a.m., development foreman office, Grace Mine.

78
Linear by -y log|Q{-lnp)

-90' 'I ' I

' I I'" I I l l ill —iiil ' i " —^ ^—^ '

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

Percent of Time Ordinate is Exceeded

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

April 24, 1973, 12:10 p.m., development foreman office, Grace Mine.

79
-90

-100 '

I I ' ' ' ' '

'

'—^—^—^—

'

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

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.

80
-40 I il ' I I I I I" "I M I I i I

1 I I IHM M I M M M I I I \ 1 \ 1

.0001.001.01 .1 .5 I 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 bandwidth,

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

81
Linear by - j log (-In p)

.0001.001.01 Percent of Time Ordinate is Exceeded

Figure 4-13 APD, 30 kHz, vertical component, 1.0 kHz predetection bandwidth,

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

82
Lineor by - j log|Q(-lnp) 40 I II I I 1 1 n il I I

I I I I I I I I I I I— MM II —\ \—\

1

-60

.0001 .001 .01 .5 I 5 20 30 40 50 60 TO 85 90 95

Percent of Time Ordinate is Exceeded

Figure 4—14 APD, 70 kHz, vertical component, 1.0 kHz predetection bandwidth,

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

83
Linear by - j logi^t-lnp)

-60 I II I ' 'I I " I M M M I M M I I I [ \ \ 1

.0001.001.01 .1 .5 I 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 kllz, vertical component, 1.0 kHz prede taction bandwidth,

April 24, 1973, 4:35 p.m., crusher substation, Grace Mine.

84
Linear by - y logi^l-ln p) 30 I II II 1 1 n il I I iiiiiiii I I I I I I I I I I I I I iii m ii I I I I I I I I I—\ \—\

1

20

-60

-70 '

I I 'I " I I " I I M M M M I I I I 1' I I I I I \ \ ^ 1

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

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.

85
20

Linear by - j log|Q{-ln p)

10

(0

E

a*Q.

Of

k-0)o.E

oo

-10

-20

~ -30 D

a>

CQ

T3

C

a* «-

«>o

0)c

o»o2

-40

-50

-60

-TO

Mine Generated Noise

!

I

-80

.0001.001.01

Percent of Time Ordinate is Exceeded

Figure 4-17 APD, 1 MHz, vertical component, 1.2 kHz predetection bandwidth,

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

86
Linear by - y log|Q(-ln p) 20 rm—I

I II I I— I —I

1

I ,

10

Percent of Time Ordinate is Exceeded

Figure 4-18 APD, 2 MHz, vertical component, 1.2 kHz predetection bandwidth,

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

87
Linear by - j logi^C-ln p) 10 m i l I I I I I I I I I I I I I—iii M i M I —I 1—I 1

-80

-90 "I" II I I

'

'—^ ^—^ '

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

Percent of Time 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.

88
-30

-40 ' II " I ' I '

' " III "

M

I I I I I I I MI I I I M I M 11 I I I \ \ \ 1

.0001.001.01 .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95 98 99 Percent of Time Ordinate is Exceeded

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

April 24, 1973, 11:07a.m., shop office, Grace Mine.

89
Linear by - y log^Qt-lnp)

-70

-80 I ' I I I I II I Il l I I " '

I '

'—^ ^—^ 1

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

Percent of Time Ordinate is Exceeded

Figure 4-21 APD, 70 kHz, vertical component, 1.0 kHz predetection bandwidth,

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

90
Linear by - y log|Q(-ln p)

-50

-60' II ' I

I "' M I I I I I I I MI III M I I M I I I I \ \ \ 1

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

Percent of Time Ordinate is Exceeded

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

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

91
Linear by - y log (-In p)

(/)

a»

0)a.

k.Q.E

o

o

>

o

QD

T3

C

0)

(b iZoa>c

o>a

.0001.001.01 .1 .5 I 5 10 20 30 40 50 60 TO 80 85 90

Percent of Time Ordinate is Exceeded

Figure 4-23 APD, 130 kHz, vertical component, 1.0 kHz predetection bandwidth^

April 24, 1973, 2:15" p.m., shop office, Grace Mine.

92
Lineor by - y log|Q(-lnp)

1

\\

1 -1- -4t

i

t

!

1

1

!

.Mine o(? n e

i rare a iMoise

1 +4-

1 1

- —

RMS

--j

!

1

1 -4-

1

1

1 -

1

-

.0001.001.01 .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95 98 99 Percent of Time Ordinnte is Exceeded

ure 4—24 APD, 160 kHz, vertical component, 1.0 kHz predetection bandwidth,

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

93
-60

-70' II 'I ' I 'l l l l ll I I M M INI M ill I \ \ \ 1

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

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.
Linear by - j log^Qt-lnp)

-90' " " " ' ' """" I I I I I M I I I

1

I I Il i M II M I I M I I I I \ 1 \ 1

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

Percent of Time Ordinate is Exceeded

Figure 4-26 APD, 250 kHz, vertical component, 1.2 kHz predetection bandwidth,

April 24, 1973, 10:27a. m., shop office, Grace Mine.

95
Linear by - j log|Q(-lnp)

lOi n il I I I I—II I I —, ,—,

1

0

a: -10

-80

-90 '

' """" ' ' '

'— I " '

I ' I ^ '—^ '

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

Percent of Time Ordinate is Exceeded

Figure 4-27 APD, 500 kHz, vertical component, 1.2 kHz predetection bandwidth,

April 24, 1973, 11:07a.m., shop office, Grace Mine.

96
Linear by - j log J-lnp)

10

10

20

40

50

60

70

80

90

V

-\

M i n A IVI 1 1 1^ enerated Noise

44

'

R^/IS - T~

"1

ise

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

Percent of Time Ordinate is Exceeded

e 4-28 APD, 1 MHz, vertical component, 1.2 kHz predetection bandwidth,

April 24, 1973, 1:00 p.m., shop office, Grace Mine.
Linear by -j log {-In p) -lOi II II I Mi l l I I m il l

I I I I I I I—ni l I I I I I I I I

1 1

>

1

-20

-100

-110 ' 'I ' I " ' ' """" ' I I I I I

1 I I I M il l I I I M I I I \ \ \ 1

.0001.001.01 .1 .5 I 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 Time Ordinate is Exceeded

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

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

99
Linear by - j log|Q{ -Inp)

-20

-30

-50

-60

-70

-80

-90

-100

line Generated Noise r

|-

|-

RMS

A

j

System Noi se

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

Percent of Time Ordinate is Exceeded

Figure 4-31 APD, 14 MHz, vertical component, 1,2 kHz predetection bandwidth,

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

100
Linear by -j log|Q(-lnp)

-no' " " III " I I M I I I I I I I I I— I M il I—\ \—\ 1

.0001.001.01 .1 .5 I 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.

101
-60 I II ' I

' I ' II I M M M I M I I I lll l l ll l I I I M I I I I \ \ 1 1

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

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.

102
-50

-60 I 'I I I I I ll l ll I M I I M M M I I l ll l ll l M il I \ \ \ 1

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

Percent of Time Ordinate 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.

103
Linear by - j log|Q(-ln p) 20

-10

-20

40

-50

-60

-70

-80

—

>

Mi ne

+

-t- -4-

j

> RMS

-4

i

-X i- -X i

1

1 -| i

1 -4 ! \\ -4 i

i -4

-4 1

—t- "4 —

!

|-

1

1

j

1

i-|—

1 1

i

!

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

USCOMM ERL 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.

104
Linear by - j log|Q(-ln p) 20

10

-10

-20

-30

-40

-50

-60

-70

--

F 5

AVG

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

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.

105
Linear by - j iog^J^-\^\ p) 20 I II II I I n il

I I iiiiiiii I I I I I I

I II I I I

'—II I I —I

^

I 1

10

.0001.001.01 .1 .5 I 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.

106
-80

-90' ' I '' I I " iiii'i I ' ' I I

' '

' —^ ^—^ '

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

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.

107
Linear by -j log j-lnp)

01.001.01

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.

108
Lineor by - j logi^l-lnp) 20

10

-20

-40

-50

-60

-70

ll- Ti' 11

||-

n

F

j|-

TT' 11

-

- V

IT

IT -

- i TT

1 MinA fXanarnfar ^ i se

r RMS

AVG

u- -

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

Percent of Time Ordinate is Exceeded

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

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

shop office, Grace Mine,

109
90 ' 11 "

'

'

' '

' '

' ' '

'—^ ^—^ '

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

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 Mineo

110
Linear by - j log|Q(-ln p) 0 I II I I I I I N I I I I III M I I I I I I I I I I I I I I I iillllll I

\ \ \

1

-10

Percent of Time Ordinate is Exceeded

Figure 4-42 APD, 6 MHz, horizontal component E-W, 1,2 kHz

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

shop office, Grace Mine,

111
Oc

ZJ

6^ iH iH

•rl

d

a>

O >-l •H XI JJ d >-( Qj <U o >

4Ju

pH 13 o •H o• O ^ o (U o

•r!

CT' a) (U 14-1 (U ^1 o B o

•rt a; 4-1 o O

QJ

CO

d 4J I-I o (U o o •i-i XI

•H 4-1 0) n o u u d

0)

(J 14-1

N e

ca 3 O

rH

CO o « • >

bC r-l (U C XI

QJ e U •H 4-1 4J 4-1 d

CO 0) 0) d

T) o

iH 4-1

a

(U •H <4-l o <r

P=M o o

CO<rIu

•H

112
I T

S5

O 0> E

E

a, E

o E

O 2 > — 5 CK < 5 -•O

X • o <

-•o

X to

• o

X •<>

u

c

9)

0)

CVJ

I CO oo

I

6^ rH o> •H

<j\ O

•H t3 u

C3 }-4 (0> •

1—

{

o

o a

• CJ o cu Q) d o

cr" J3 § (L) n\ >-i o

o

CD 4J O o o

QJ

UJ U (-< M o

vj •H 1 1 ,_J *ri +J rrt w O u

u § W

o N

f-l QJ O CO 4J • r-l 2

o

0\ •H J-J U fl en m QJ

0) d

-r) o

iH •P

Q) ^

•H o

o o CM

1

<uu

:360

•H

( Sl/\ld ^9\9Ui J8d 8J9duJ00J3!iJU | aA!iO|9J gp )

113
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115
5. HOIST-PHONE MEASUREMENT RESULTS

There are two hoist-phone systems in use. The phone on

the personnel skip in the shaft uses a transmission line (a

single wire terminated in 25 ohms with earth return) installed

specifically for the hoist phone. It operates at 100 kHz.

At the surface, it is directly coupled. On the skip, a 16-

turn, shielded loop of area 0.24 square meter, couples to the

magnetic field from the transmission line. One edge of this

loop is about 0.2 meters from the single wire of the transmission line. This system works very well.

During normal operation (ore hauling), there is no phone

on the ore skip at the A shaft. One is added during maintenance shift (once a week). It operates at 65 kHz. A

folding, wooden, rectangular core with 30 turns on each side

forms a toroid that couples energy in and out of the hoist

"rope" (2 1/4-inch (5.72 cm) diameter, stranded, steel cable).

In this case, the hoist rope is one wire of the transmission

line; earth is the return, but since the skip is guided with

non-conducting rubber tires on metal guides , ground return is

so poor that a bronze shoe is specially installed for better

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- res is tance ground, communication was not so

good. Standing waves would certainly be probable. This

indicates there may be difficulty in using single-wire (hoist

116
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 toroids 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 line 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.

117
Transmission line

for 100 kHz

hoist phone

Collapsible loop

antennas used for

NBS measurements

Cable from NBS

antennas into skip

through escape hatch

Magnetic- field pickup loop

for hoist phone

3/4 inch (4.45 cm) steel

hoist rope

^/--Escape hatch

Figure 5-1 Top view of personnel hoist "A" skip.

118
MAGNETIC FIELD STRENGTH, dB /lA/m,

OR RELATIVE S/N RATIO IN dB

Figure 5-2 Magnetic field strength, dByA/m, or relative S/N ratio as measured along the transmission line for the personnel

hoist located in "A" shaft.
2,000

2,200

MAGNETIC FIELD STRENGTH, dB ^A/m

OR RELATIVE S/N RATIO IN dB

Figure 5-3 Magnetic field strength, dByA/m, or relative S/N ra

as measured along the hoist rope for the personnel

hoist located in "A" shaft.

120
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 LHD ' s 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 spot" 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 APD ' s 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.

121
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 measurements 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 (1), open, and only moderately

satisfactory for case (2), short.

122
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

.

123
8. ACKNOWLEDGMENTS

Those making significant contributions to this program

are as follows: Laboratory development and field use of measurement equipment, Ed Neisen, Doug Schulze, and Tom Bremer;

data processing, Ann Rumfelt, Nancy Tomoeda, Winston Scott,

Frank Cowley, and David Stearns. Those making valuable but

less time-consuming contributions are Gerry Reeve, Bob

Matheson, Don Spaulding, John Chukoski, Lorne Matheson,

Dave Lewis, and Sharon Foote.

Winston Scott provided much assistance in proofreading,

while Sharon Foote and Janet Becker typed tirelessly through

many versions. Jocelyn Spencer and Barbara Bolton provided

drafting assistance.

Finally, none of this would have been possible without the

excellent cooperation of Paul Vancura, Ray Shucavage, Fred

Eben, Harold Kaley and others at Grace Mine of Bethlehem Steel

Corporation .

'

124
9. REFERENCES

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

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

April 1974.

The Institute of Electrical and Electronic Engineers, Inc.,

IEEE Dictionary o£ Electrical and Electronic Terms,

Std. 100, 1972.

Crichlow, W.Q., et al . Amplitude-Probability Distributions

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

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

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.

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.

125
10. APPENDIX

Decoding of Spectrum Captions

Spectrum captions are generally organized into the £ol- • lowing format:

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

where

MP = Two's power of length of Fourier transform, example,

MP

2 where MP = 12

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, 0

NSA = Number of periodograms averaged, example, 20

NRP = Number of data points processed since spectrum

initialization, example, 4 3008

NPP = Number of data points processed since data initialization, example, 43008.

126
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, 209 (-6 dB)

DG = digitizer gain, example, 0

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

example, 0

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 lO"*)

Standard Error = standard error of curve, example, 0.3162

Spectral Peak = largest spectral peak observed, example,

4.108+003 [4.108 x 10^)

127
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 74-388

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

ELECTROMAGNETIC NOISE IN GRACE MINE

5. Publication Date Tnnp 1 Q74

6. Performing Organization Code

7. AUTHOR(S)

John W. Adams, William D. Bensema, and Motohisa Kanda

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Washington, D. C. 20234

10. Project/Task/Work Unit No. 2768412

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) U. S. Bureau of Mines

Pittsburgh Mining and Safety Research Center

4800 Forbes Avenue

Pittsburgh, Pennsylvania 15213

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) 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. It is recorded using broadband analog magnetic tape and the noise data is 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

amplitude probability distribution plots.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; 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;

f??.^'''.1?':y^..^?j:?^i„?-§^f I'-'-S-^-'-fj:^ strength; measurement instrumentation; spectral den.qity

IB^.T^A-lteL*^'- ^P^^^^'^^nll^ftlJ-

\ZZ For Official Distrib ution. Do Not Release to NTIS

I

' Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

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' Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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