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p|N NBS TECHNICAL NOTE 654

***EAU O* h./

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Electromagnetic Noise

in Robena No. 4 Coal Mine
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences a — Applied Radiation " — Quantum Electronics " — Electromagnetics ' — Time

and Frequency " — Laboratory Astrophysics " — Cryogenics 3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis —- Measurement Engineering — Structures, Materials, and Life Safety * — Building

Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government: promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

a Located at Boulder. Colorado 80302.

* Part of the Center for Building Technology.
pnc 1 5 1974

-; Electromagnetic Noise

% in Robena No. 4 Coal Mine

W.D. Bensema

Motohisa Kanda

John W. Adams

Electromagnetics Division

Institute for Basic Standards

[X,b> National Bureau of Standards

Boulder, Colorado 80302

'TedhmcoJ noie- no. &54

Sponsored by

U.S. Bureau of Mines

Pittsburgh Mining and Safety Research Center

4800 Forbes Avenue

Pittsburgh, Pennsylvania 15213

"•rA u o«

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W Roberts. Director

Issued April 1974
National Bureau of Standards Technical Note 654

"Sal. Bur. Stand. (U.S.), Tech. Note 654, 194 pages (April 1974)

CODE1N: NBTNAE

For -ale Im the Superintendent of Documents. U.S. Government Printing Office, Washington, D.C. 20402

(Order b) SI) Catalogue No. CI 3.46:654). 81.50
CONTENTS

Page

ABSTRACT 1

KEY WORDS 1

I. INTRODUCTION 1

1 1 . SPECTRUM MEASUREMENTS 5

A. Noise Measurement Techniques 5

1. Description of Measurement Instrumentation ->

2. Description of Processing Instrumentation 11

a. Laboratory Transcription Process 11

b. Laboratory Digitizing Process 13

c. Spectral Computation and Graphical

Output 14

d. Description of Processing Software 14

3. System Evaluation 18

4. System Calibration 22

B. Noise Measurement Results: Spectra 23

1. Mine Description and Antenna Sites 23

2. Electromagnetic Noise Spectrum Results 25

a. Introduction 25

b. Working Face Area (Location 1) 26

c. At Rail of Haulage Way (Location 2)-- 38

d. Thirty Meters from Rail (Location 3)- 50

e. Nip Point (Location 4) 57

f. Air-split 59

g. Bailey Shaft (Location 6) 62

h. Bailey Shaft (Location 7) 71

i. Summary Plot of Power Line Harmonics- 73

III. AMPLITUDE PROBABILITY DISTRIBUTION MEASUREMENTS 73

A. Introduction 73

in
CONTENTS (continued)

Page

B. Noise Measurement Techniques 75

1. Measurement Instrumentation 75

a. Underground Recording System 78

b. Data Transcribing System 82

c. Data Processing System 86

2. Measurement Technique 90

3. Calibration 93

C. Noise Measurement Results: APD's 96

IV. SPECIAL MEASUREMENTS 136

A. Surface Noise Measurements 136

1. Introduction 136

2. Measurement Techniques 137

a. Spectra 137

b. Amplitude Probability Distributions-- 137

3. Measurement Instrumentation 137

4. Location of Measurements 139

5. Calibration 141

6. Results of Spectral Noise Measurements 141

7. Amplitude Probability Distribution

Results 142

B. Measurements of Voltage Between "Roof Bolts"-- 153

C. Surface-Underground noise coherence tests 159

D. Mine Phone Wire Measurements 162

1. Voltage 162

2. Measured Noise Current 166

E. Trolley Wire Voltage Measurements 166

V. CONCLUSIONS 172

IV
CONTENTS (continued)

Page

VI. RECOMMENDATIONS 173

VII. ACKNOWLEDGMENTS 174

VI I I. REFERENCES 176

IX. APPENDIX--Decoding of Spectrum Captions 177
LIST OF TABLES

Page Table 1. Characteristics of Electromagnetic Interference and Field Strength (EIFS) Meters

(Manufacturer's Specifications) 81

Table 2. Characteristics of Portable Magnetic Tape

Recorder (Direct Record/Reproduce) (Manufacturer's Specifications) 85

Table 3. Characteristics of Laboratory Analog Magnetic Tape Recorder (Direct Record/Reproduce)

(Manufacturer's Specifications) 87

VI
LIST OF FIGURES

Page

Figure 1. Block diagram of portable instrumentation- 6

Figure 2. Block diagram of laboratory transcription

process 12

Figure 3. Block diagram of laboratory digitizing of

dat a 12

Figure 4. Block diagram of spectral computation,

plotting, and printing 15

Figure 5. Map of antenna locations at Robena working

section 24

Figure 6. Spectrum of continuous mining machine in

full operation at face area (location 1)

,

antenna sensitive axis vertical 27

Figure 7. Spectrum of continuous mining machine in

full operation at face area (location 1)

,

antenna sensitive axis, horizontal E-W 29

Figure 8. Spectrum of continuous mining machine in

full operation at face area (location 1)

,

antenna sensitive axis horizontal N-S 30

Figure 9. Three-D plot of spectrum of continuous

mining machine speeding up at face area 32

Figure 10. Expanded frequency scale of spectrum of

continuous mining machine in full operation at face area (location 1) , antenna

sensitive axis vertical 33

Figure 11. Expanded frequency scale of spectrum of

continuous mining machine in full operation at face area (location 1) , antenna

sensitive axis horizontal E-W 34

Figure 12. Expanded frequency scale of spectrum of

continuous mining machine in full operation at face area (location 1) , antenna

sensitive axis horizontal N-S 35

vii
LIST OF FIGURES (continued)

Page

Figure 13. Spectrum of electrically quiet (lunch hour)

mine as measured at face area (location 1) 37

Figure 14. Expanded frequency scale in spectrum of

electrically quiet (lunch hour) mine as

measured at face area (location 1) 39

Figure 15. Spectrum of noise from 13-ton electric

locomotive pulling into mine section,

location 2, antenna sensitive axis , vert ical 40

Figure 16. Spectrum of noise from 13-ton electric

locomotive pulling into mine section,

location 2, antenna sensitive axis

horizontal E-W 41

Figure 17. Spectrum of noise from 13-ton electric

locomotive pulling into mine section,

location 2, antenna sensitive axis

horizontal N-S 42

Figure 18. Expanded frequency scale in spectrum of

13-ton electric locomotive pulling into

mine section, location 2, antenna sensitive axis vertical 43

Figure 19. Expanded frequency scale in spectrum of

13-ton electric locomotive pulling into

mine section, location 2, antenna sensitive axis horizontal E-W 44

Figure 20. Expanded frequency scale in spectrum of

13-ton electric locomotive pulling into

mine section, location 2, antenna sensitive axis horizontal N-S 45

Figure 21. Three-D plot of spectrum of noise from

13-ton electric locomotive pulling into

mine section, location 2, antenna sensitive axis vertical 46

Figure 22. Three-D plot of spectrum noise from 13-

ton electric locomotive pulling into mine

section, location 2, antenna sensitive

axis horizontal E-W 47

vm
LIST OF FIGURES (continued)

Page

Figure 23. Three-D plot of spectrum of noise from 13-

ton electric locomotive pulling into mine

section, location 2, antenna sensitive

axis horizontal N-S 48

Figure 24. Spectrum at location 3, thirty meters from

track, antenna sensitive axis vertical 51

Figure 25. Spectrum at location 3, thirty meters from

track, antenna sensitive axis horizontal

E

_ w 5 2 Figure 26. Spectrum at location 3, thirty meters from

track, antenna sensitive axis horizontal

N

_ s 53

Figure 27. Expanded frequency scale in spectrum at

location 3, antenna sensitive axis vertical 54

Figure 28. Expanded frequency scale in spectrum at

location 3, antenna sensitive axis horizontal E-W 55

Figure 29. Expanded frequency scale in spectrum at

location 3, antenna sensitive axis horizontal N-S 56

Figure 30. Spectrum at location 4, near electric

motor for car-pull hydraulic pump, antenna

sensitive axis horizontal E-W 58

Figure 31. Spectrum at air split, antenna sensitive

axis vertical 60

Figure 32. Expanded frequency scale in spectrum at air

split, antenna sensitive axis vertical 61

Figure 33. Map of antenna locations near Bailey shaft 63

Figure 34. Spectrum at location 6, thirty meters east

of Bailey shaft, antenna sensitive axis

vertical 64

Figure 35. Expanded frequency scale in spectrum at

location 6, thirty meters east of Bailey

shaft, antenna sensitive axis vertical 65

IX
LIST OF FIGURES (continued)

Page

Figure 36. Second spectrum at location 6, thirty

meters east of Bailey shaft, antenna

sensitive axis vertical 67

Figure 37. Second expanded frequency scale in spectrum

at location 6, thirty meters east of Bailey

shaft, antenna sensitive axis vertical 68

Figure 38. Third spectrum at location 6, thirty

meters east of Bailey shaft, antenna

sensitive axis vertical 69

Figure 39. Third expanded frequency scale in spectrum

at location 6, thirty meters east of Bailey

shaft, antenna sensitive axis vertical 70

Figure 40. Spectrum at location 7, near 600 volt dc

supply cables, antenna sensitive axis

vertical 72

Figure 41. Summary of magnetic field strength spectrums observed within Robena Mine 74

Figure 42. Measurement instrumentation systems used

in a) field recording, b) transcribing, j*

and c) APD processing 77

Figure 43. Linearity of EIFS meter 80

Figure 44. Mixer circuit diagram 83

Figure 45. Frequency response of portable analog magnetic

tape recorder 84

Figure 46. Collapsible loop antenna as used in Robena

Mine for frequencies between 10 kHz and

250 kHz 89

Figure 47. APD recording system in operation in Robena

Mine 91

Figure 48. APD data processing system in operation 92

Figure 49. APD, 10 kHz, vertical component 97

Figure 50. APD, 30 kHz, vertical component, Dec. 5,

1972 98
LIST OF FIGURES (continued)

Figure 51.

Figure 52.

Figure 53.

Figure 54.

Figure 55 . Figure 56.

Figure 57.

Figure 58.

Figure 59.

Figure 60.

Figure 61.

Figure 62.

Figure 63.

Figure 64.

Figure 65.

Figure 66.

Figure 67 . Figure 68.

Figure 69.

Figure 70 . Figure 71.

Figure 72

.

Figure 73.

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

APD

70 kHz, vertical component

130 kHz, vertical component

500 kHz, vertical component

2 MHz, vertical component

8 MHz, vertical component

32 MHz, vertical component

10 kHz, horizontal component E-W--

30 kHz, horizontal component E-W--

70 kHz, horizontal component E-W--

130 kHz, horizontal component E-W500 kHz, horizontal component E-.W2 MHz, horizontal component E-W

8 MHz, horizontal component E-W

32 MHz, horizontal component E-W--

10 kHz, horizontal component N-S--

30 kHz, horizontal component N-S--

70 kHz, horizontal component N-S--

130 kHz, horizontal component N-S500 kHz, horizontal component N-S2 MHz, horizontal component N-S

8 MHz, horizontal component N-S

32 MHz, horizontal component N-S--

Page

99

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

10 kHz, vertical component, Dec. 7,

197 2 121

xi
LIST OF FIGURES (continued)

Page

Figure 74. APD, 30 kHz, vertical component, Dec. 7,

1972 122

Figure 75. APD, 70 kHz, vertical component, Dec. 7,

ig72 123

Figure 76. APD, 130 kHz, vertical component, Dec. 7,

1972 124

Figure 77. APD, 500 kHz, vertical component, Dec. 7,

19 72 125

Figure 78. APD, 2 MHz, vertical component, Dec. 7 19 72 126

?

Figure 79. APD, 8 MHz, vertical component, Dec. 7,

19 72 127

Figure 80. APD, 32 MHz, vertical component, Dec. 7,

19 72 128

Figure 81. Field strength excursions between 0.1% and

99% of the time as a function of frequency,

vertical component, Dec. 5, 1973 131

Figure 82. Field strength excursions between 0.1% and

99% of the time as a function of frequency,

horizontal component E-W, Dec. 5, 1973 132

Figure 83. Field strength excursions between 0.1% and

99% of the time as a function of frequency,

horizontal component N-S, Dec. 5, 1973 133

Figure 84. Field strength excursions between 0.1% and

99% of the time as a function of frequency,

vertical component, Dec. 7, 1973 134

Figure 85. Surface recording system 138

Figure 86. Location of surface measurements 140

Figure 87. Spectrum of surface noise, 300 meters from

Blaker shaft, antenna sensitive axis horizontal N-S 143

Figure 88. Spectrum of surface noise, 300 meters from

Blaker shaft, antenna sensitive axis horizontal E-W 144

Xll
LIST OF FIGURES (continued)

Page

Figure 89. Spectrum of surface noise, 300 meters from

Blaker shaft, antenna sensitive axis

vertical 145

Figure 90. Spectrum of surface noise, Bailey shaft,

antenna sensitive axis horizontal N-S 146

Figure 91. Spectrum of surface noise, Bailey shaft,

antenna sensitive axis vertical 147

Figure 92. Spectrum of surface noise, Bailey shaft,

antenna sensitive axis horizontal E-W 148

Figure 93. APD, near Blaker shaft, 10 kHz, surface

vertical component, Dec. 5, 1973 149

Figure 94. APD near Blaker shaft, 30 kHz, surface vertical component, Dec. 5, 1973 150

Figure 95. APD near Blaker shaft, 70 kHz, surface

vertical component, Dec. 5, 1973 151

Figure 96. APD near Blaker shaft, 130 kHz, surface

vertical component, Dec. 5, 1973 152

Figure 97. Field strength excursions between 0.11 and

99% of the time as a function of frequency,

surface vertical component, Dec. 5, 1973- - 154

Figure 98. Voltage spectrum of roof bolts, 3.6 meter

spacing 155

Figure 99. Voltage spectrum of roof bolts, 8.5 meter

spacing 156

Figure 100. Voltage spectrum of roof bolts, 13 meters

spacing 157

Figure 101. Voltage spectrum of 8.5 meter dipole 158

Figure 102. 3D Spectrum at location 2, magnetic field

strength obtained with loop antenna, no

impulses, antenna sensitive axis vertical- 160

xm
LIST OF FIGURES (continued)

Page

Figure 103. 3D Voltage spectrum of roof bolts, 13 meter

spacing, strong impulse observed 161

Figure 104. Surface -underground coherence test results 163

Figure 105. Spectrum of voltage measured between phone

wire and rail at location 2 164

Figure 106. Expanded Spectrum of voltage measured between phone wire and rail at location 2 165

Figure 107. Spectrum of current measured in one phone

wire at location 2 167

Figure 108. Expanded Spectrum of current measured in

one phone wire at location 2 168

Figure 109. Spectrum of voltage measured on 600 volt

dc trolley wire 169

Figure 110. Expanded Spectrum of voltage measured on

600 volt dc trolley wire 170

Figure 111. Spectrum of voltage measured on 600 volt

dc trolley wire, two days later 171

xiv
ABSTRACT

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

an operating coal mine, Robena No. 4, located near

Waynesburg, Pennsylvania. One technique measures noise

over the entire electromagnetic spectrum of interest for

brief tine periods. With present instrumentation, the

spectrum can be covered from 40 Hz to 400 kHz. It is

recorded using broad-band analog magnetic tape, and the

noise data are later transformed to give spectral plots.

The other technique records noise envelopes at several

discrete frequencies for a sufficient amount of tine to

provide amplitude probability distributions.

The specific measured results are given in a number

of spectral plots and amplitude probability distribution

plots. The general results are that at frequencies below

10 kHz, power line noise within the mine is severe.

Impulsive noise is severe near arcing trolleys, and

at lower frequencies near any transmission line. Carrier

trolley phone signals and harmonics are strong throughout

the mine whenever the trolley phone is in operation.

Additional information beyond that included in this

report may be obtained from the authors, who are with

the Electromagnetics Division of the National Bureau of

Standards

Key words: Amplitude probability distribution; coal mine

noise; digital data; electromagnetic interference; electromagnetic noise; emergency communications; Fast Fourier

Transform; Gaussian distribution; impulsive noise; magnetic field strength; measurement instrumentation;

spectral density; time -dependent spectral density.

I. INTRODUCTION

The need for reliable communication systems in coal 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 operational conditions.

During operation, the machinery used in a coal mine

creates a wide range of many types of intense electromagnetic

interference (EMI), and therefore ambient EMI is a major limit inj

factor in the design of a communication system.

The work reported here gives the results of the first

comprehensive measurements of this EMI in critical communication locations such as working sections where miners extract

coal

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. We

made some measurements of each of these parameters, but one

parameter was emphasized, magnetic field strength. There

are several reasons. First, electric field sensors are notoriously insensitive at lower frequencies, and hence probably

will not be useful in any practical 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 reflect currents. Fourth, trolley-wire noise voltages are propagated as transmission line phenomena, are

directly related to transmission line currents, and hence to

magnetic fields induced. 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.

As just mentioned, magnetic field strength measurements

are emphasized, but even this one parameter is difficult to

measure meaningfully. The IEEE definition [1] of magnetic

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

is used in this report. Since there is a multitude of different sources that generate all known types of noise, the

resultant magnetic field strength noise vector is a function

of frequency, time, orientation, and location. Small variations in these quantities can cause many tens of decibels difference in measured field strength.

We used two measurement techniques. The first technique

covers a large portion of the spectrum as a "snapshot" at one

instant of time. In three-dimensional form, several such "snap

shots" can show how drastically a signal varies over a period

of a few seconds. The second technique records variations

over a 20 -minute time interval as a view at one frequency.

We used a set of eight different frequencies. Both techniques

me asure three orthogonal components of magnetic field strength
by using three systems simultaneously or by varying the orientation of one system; both techniques were used in as many different locations as possible within practical time limitations.

Whether the noise signal tends to be Gaussian or impulsive

depends on the number of sources and the distance to each

source

.

With the exception of the 3-D 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 electromagnetic

field calibration site was used with each complete measurement system to assure correct calibration.

A further complication in making these measurements is

the need to have either permissible equipment or to use

explosion -proof enclosures for non -permissible equipment.

The mine environment is generally humid, dusty, poorly

lighted, and without normal electrical power. We used

battery-operated instruments 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 re

quired, H and H , . Results are given as the rms value of one component of magnetic field strength, H, versus frequency for discrete frequencies, or magnetic -field-strength spectrum density

level, H , , [1] versus frequency for broadband noise in the
spectral plots. Results are given as the rms value of one

component of magnetic field strength versus percent of time

this value is exceeded in the amplitude probability distributions (APD's). The rms value of an APD is representative

of the actual peak value only as far as the measurementsystem detector bandwidth will allow the detector to follow

the time variations of the actual magnetic field. (In this

context, noise envelope is sometimes used.) The results are

applicable for a communication receiver whose bandwidth is

similar to the measurement -system detector bandwidth.

Only representative samples of the total data measured

are given in this report. Only a limited set of datapresentation formats have been used. If additional data,

or data presentation in other formats, are required, please

contact any of the authors at the Electromagnetics Division

of the National Bureau of Standards, and with specific permission of the Bureau of Mines we will supply the additional

information

.

II. SPECTRUM MEASUREMENTS

A. Noise Measurement Techniques

1 . Description of Measurement Instrumentation

Figure 1 shows a block diagram of the portable portion

of the spectrum measuring equipment. Systems 2 and 3 are

identical to System 1, so only one system is described. The
SIGNAL MONITORS

LOUD

SPEAKER

SYSTEM

#1

LOOP

ANTENNA BROADBAND

AMPLIFIER

OSCILLOSCOPE

IMPEDANCE

TRANSFORMER

10 METERS

OF CABLE

100-kHz

LOW PASS

FILTER

SYSTEM

#2

SYSTEM

#3

200-kHz 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 1 Block diagram of portable instrumentation. FM

tracks are used to record from 100 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.

IP-UL628 I
receiving antenna used for all measurements is a commercially

available, electrostatically shielded, 11-turn, 78-cm diameter,

air-core loop. A short (40 cm), twin-center-conductor cable

connects the antenna to a transformer which steps up the

impedance of the antenna to match more nearly the input impedance of the receiver. In order to retain transformer response

to about 400 kHz, a compromise between high step-up ratio and

broad bandwidth was required. An impedance step-up ratio of

1 to 50 was chosen, giving a voltage step-up of /5 or 1 to

7.07. Since the transformer output drives a high-gain receiver input, the transformer is shielded. Shielding consists

of a 1/4-inch thick copper enclosure with a layer of high

permeability alloy inside the enclosure and a second layer

outside the enclosure. In addition to serving as a transformer, this portion of the system contains two other

functions. To preclude receiver saturation, a 30-dB attenuator that can be switched in or out is included for use with

very high field strengths. Also, an unbalanced input is

provided for injecting a field calibration signal (1000-Hz

square wave) to assure the system is operating properly when

set up in the field. The step-up transformer is connected

directly to the balanced inputs of the receiver.

A commercially available, broadband, battery-powered

amplifier is used as a receiver. The input impedance is

100 megohms in parallel with 15 picofarads, and the output im-
pedance is 600 ohms. The gain is switch selectable from 10 to

10,000 in 1-2-5 sequence using 1 percent resistors. The

bandwidth is also switch selectable, and the passband usually

is adjusted between 300 Hz and 100,000 Hz with a 6-dB/octave

roll-off outside the pass band for most recordings. For

broad-band recordings, the upper bandwidth limit is increased

to 300,000 Hz.

The amplifier noise is negligible compared to other

sources of system noise at gain settings below 5000. The gain

is selected to provide optimum signal level to the analog tape

recorder (1.4 volts peak). Ten meters of well -shielded (RG-55)

coaxial cable connect the receiver to a low-pass filter. A

100-kHz, low-pass, tt filter is used to greatly attenuate

received signals at frequencies above the 100-kHz limit of the

portable tape recorder.

Two types of signal monitors were used in the mine to

determine the general character of the noise fields and to

assure that the received, bandlimited signal does not exceed

the amplitude capabilities of the tape recorder.

A battery-powered oscilloscope housed in an explosionproof enclosure is the primary monitoring instrument. By observing the oscilloscope, the instantaneous, peak-to-peak

amplitude of all types of noise can be determined readily,

and the gain of the amplifier can be appropriately adjusted.

Also, a battery-powered audio amplifier driving a small

loudspeaker provides audio monitoring of noise in the audio
range. This device is particularly valuable in catching

transient noise phenomena. During the time spent at a particular recording location, it is impossible to monitor the

oscilloscope visually at all times. For example, during lunch

breaks, while changing tape, and when moving the antenna, the

audio monitor is a useful "alarm" to alert the operator to

unusual, otherwise likely undetected events. Once warned of

these events, it is easier to catch them on tape.

The noise signals processed through the receiving portion

of the portable equipment are recorded on the portable, battery

powered, analog tape recorder. The frequency modulation (FM)

mode bandwidth is dc to 100 kHz, and the direct mode bandwidth

is 3 kHz to 375 kHz. Figure 1 shows the assignment of FM and

direct tracks with the systems used. Emphasis is placed on

FM usage because of lower recorder signal distortion. The

recorder weighs about 14 kg and is encased in a dust-proof

enclosure. This recorder is a specially modified version of

a commercially available portable tape recorder. The first

modification is a placement of spark-suppressing diodes across

all relay and motor leads to bring the recorder within the

requirements of Schedule 2G of the Bureau of Mines [2]. This

makes the recorder legally permissible and allows it to be

used in explosive atmospheres. A second modification is a specially built, external, permissible battery box using rechargeable sealed lead-acid batteries. The batteries, a solid-
state current limiter, and a fuse, all in series, are enclosed

in an explos ion -proof enclosure (per Schedule 2G) . A single

battery allows about four hours of recording. The final modification is the addition of a self-contained 200-kHz crystal

oscillator. The output of this oscillator is recorded on track 4 for later use during playback as a reference signal

for controlling the speed of the tape -controlled servo in the

laboratory analog tape recorder. This is the first method

applied to remove flutter, time base error, and sideband

generation. Corbin [3] shows a reduction in sideband generation

of about 20 dB using this method.

A second method of reducing flutter -sideband generation

while using FM mode is to short the input of one channel

(channel 7). This signal is retained through the transcription process and is finally inverted and subtracted from all

FM signal channels during the digitizing process, as will be

described later. This tape recorder is carefully operated and

maintained, as it is the most significant source of system noise

and distortion. Specially spooled, low-noise tape with 700 m

(2300 ft) of tape on a reel is used to increase the recording

time from about 11 minutes to about 14 minutes per reel at 30

inches per second (ips, 1 inch = 2.54 cm). Precautions are

taken to prevent tape stretching and to reduce print -through

.

As recommended by Burnett, Corliss, and Berendt [4] all tape

recordings are retained on the take-up reel. In addition to

10
better tape packing and less print -through , leaving the tape

on the take-up reel precludes the time-consuming process of

rewinding while on site in the mine.

The portable equipment has a total weight of about 40 kg.

Removal of the permissibility requirement would reduce the

weight by approximately one-third.

2 . Description of Processing Instrumentation

a. Laboratory Transcription Process

After the mine visit, the newly recorded tapes are reviewed in the laboratory at the record speed of 30 ips to note

any pertinent recorded vocal comments and to select the data

to be processed.

For the initial step of processing, the portable tape

recorder is reconfigured to play back at a tape speed of half

the record speed (15 ips). This is the initial step of frequency range reduction performed to reduce the data bandwidth

ultimately to the requirement of 5 kHz imposed by the digitizer. In this process, all of the original information, including high frequencies, is conserved. The difference now

is that a data signal at 100 kHz lasting, say, 1 second, comes

out at 50 kHz and lasts 2 seconds.

Figure 2 shows the block diagram of the laboratory transcription process. Only tracks 2 and 4 of the laboratory tape

recorder are instrumented to process direct mode recordings.

If both tracks 2 and 6 on the portable recorder contain direct

11
PORTABLE

TAPE

RECORDER

15 ips

(REPRODUCE)

TRACK I TRACK 2 TRACK 3

TRACK 4

TRACK 5 TRACK 6

TRACK 7

DATA

DATA

*-

(ONE

DATA ONLY)

100 kHz SINE WAVE ^

DATA

DATA

SHORT ^

TRACK I TRACK 2 TRACK 3

TRACK 4

TRACK 5

TRACK 7

LABORATORY

TAPE

RECORDER

60 ips (RECORD)

OSCILLOSCOPE

ANY ONE TRACK

SIGNAL MONITOR

Figure 2 Block diagram of laboratory transcription process

This is the first step in frequency range reduction. Newly recorded tape is played back at one- half speed.

LABORATORY

TAPE

RECORDER

3-3/4 ips

(REPRODUCE)

TRACK I (FM)-

•TRACK 3 (FM).

TRACK 5 (FM)-

TRACK 2 (DIR)-

ONE TRACK

AT A TIME

LOW

PASS

FILTER

DIGITIZER

(16,000

SAMPLES/SEC)

AND DIGITAL

TAPE

TRANSPORT

OSCILLOSCOPE

SIGNAL MONITOR

Figure 3 Block diagram of laboratory digitizing of data.

Track 7 inverter output is added to the outputs of

tracks 1, 3 and 5 to reduce distortion. This is

the final step of frequency range reduction.

12

I B- U156<.~1
data, then two runs are made. Note that the crystal oscillator

reference signal (track 4 on the portable recorder) is now 100

kHz. Operating the laboratory tape recorder at 60 ips and recording a servo frequency of 100 kHz meets the Inter-Range

Instrumentation Group (IRIG) standard (according to the manufacturer) for which the manufacturer configured the laboratory

recorder. As with the portable recorder, manufacturer-recommended low-noise tape, tape storage on take up reels, and other

precautions are taken with the laboratory tape recorder.

b. Laboratory Digitizing Process

The laboratory digitizing of data is shown in Figure 3.

The desired frequency range of spectral display must be considered in this step. The considerations of tape recorder

speed versus aliasing, predigitizer filter cut-off frequency

selection, digitizer rate, etc., will not be discussed here.

In general, the laboratory tape recorder is run at 1-7/8 ips

for an upper frequency display limit of 320 kHz (direct mode),

and 3-3/4 ips for 100 kHz and 3 kHz (FM mode) . The most frequently used analysis uses a recorder playback speed of 3-3/4

ips for a frequency display limit of 100 kHz. During this

step the laboratory recorder speed-control servo (including

the motor) is controlled by the crystal oscillator signal

originally recorded at 200 kHz in the field. Most of the

13
accumulated flutter, wow, time base error, and sideband

generation is eliminated. Also, the data are reduced to the

5-kHz frequency range imposed by the 16 ,000 -sample -per-second

maximum sample rate of the digitizer.

c. Spectral Computation and Graphical Output

A 12 -bit conversion system digitizes the data and records

the converted data onto digital magnetic tape for subsequent

computer processing. A study using 6-, 8-, 10-, and 12-bit

data determined the number of bits required. The study indicated that at least 10 bits, and usually 12 bits, are required

to provide enough dynamic range satisfactorily, especially for

the narrow range (750 Hz) analysis.

As outlined in figure 4, a large digital computer is used

to compute the spectra and to print the results on an integral

high-speed cathode -ray tube microfilm plotter". The individual

microfilm frames are subsequently joined to give the spectra

found in this report.

d. Description of Processing Software

Several programs are used in the processing of the digitized data. They are listed here for reference purposes only

without further elaboration: PSPECSET, PSPECZRO, PSPECINP,

PSPECEST, PSPECOUT, PSPECCAP, SPEAKS, FILMGRAF, and DDGRAPH

These, and several other subroutines, were developed primarily

14
DIGITIZED DATA

(TAPE)

MEASUREMENT

PARAMETERS

(CARDS)

FFT COMPUTER

PROGRAM

(CARDS)

MICROFILM

PLOTTER

DATA SPECTRA

' (MICROFILM)

PEAK FINDER

PRINTOUT

—

DATA SPECTRA

(MICROFILM)

MICROFILM

READER

PRINTER

DATA SPECTRA

(PAPER PRINTS)

Figure 4 Block diagram of spectral computation, plotting,

and printing.

15
by L.D. Lewis of the Space Environment Laboratory of the

National Oceanic and Atmospheric Administration, Boulder,

Colorado 80302.

A calling program identified as ELF was written to use

and adapt the software listed above to the needs of this

project. It also serves to adapt the output from the analogto -digital converter to a computer-usable format.

It is outside the scope of this report to describe in any

detail the theory of the Fast Fourier Transform (FFT) or the

internal organization of the software. The program package

is based on a paper by Welch [5] , and more information is

available in his paper and references.

A summary of the processing parameters for the most frequently used bandwidth display (100 kHz) follows:

1. Each printed spectrum (other than 3-D) is an average

of twenty spectra.

12

2. The ensemble length is 2 , i.e., 4096, 12-bit

words. This is equal to 0.008 seconds of real time.

For the 50 percent data overlap factor used, (i.e.,

the first spectrum used samples number 1 through

4096, the second used 2048 through 6144, etc.) each

average of 20 spectra represents 0.08 seconds of

real time

3. Spectral resolution is 125 Hz over the 100,000 Hz

frequency range.

16
4. Cost of computer time per spectrum is about $0.15.

The cost of a plot of 20 averaged spectra is

therefore about $3.00. The 3-D plots use different

parameters, display hundreds of averaged spectra,

and cost about $25.00 each for computer time.

An important improvement achieved in our software is the

virtual elimination of an inherent potential 3-dB error in the

reported amplitude of cw signals. This error arises from the

uncertainty of the frequency of a cw signal, and therefore of

its location within any single 125-Hz spectral resolution

analysis bandwidth. For example, if the cw signal falls on the

dividing line between 125-Hz spectral resolution "cells", its

power would be reported to be 3 dB (1/2 power) below that reported if it were in the center of a cell. By using the rule

that the area (power) under a spectral density curve representing a cw signal is constant (Parsival's Theorem), a correction

to the height of cw signals is applied. This correction has

been demonstrated to give a reduction of the 3-dB maximum

error to less than 0.05 dB . We have named this program "Peak

Finder"

.

The processing software also includes a computer routine

that makes the system present a uniform frequency response

over the bandwidth of interest. This software, a type

of correction curve algorithm, will be discussed under antenna

calibration

.

17
3 . System Evaluation

In order to provide processed data referenceable to absolute field- strength standards, it was necessary to measure

the overall system response and establish that certain error

sources were acceptably small. The system parameters measured

included: receiver gain linearity, dynamic range, system gain

drift, frequency accuracy, and harmonic and intermodulat ion

distortion

The switchable gain settings (providing an amplifier gain

of 10 to 10,000) on the three receiving amplifiers were checked

at 450 Hz and 40 kHz. All amplifier switch settings are

linear in gain within 0.1 dB with one exception. System

number 1 (used mostly for vertical antenna sensitive axis

measurements) gain is 0.8 dB low at a gain setting of 10,000

using the test frequency of 40 kHz.

Instantaneous dynamic range was measured by injecting a

sine wave at the highest amplitude the recorder can tolerate

(1 Vrms) and then reducing the amplitude with a step attenuator

until the signal disappeared into the system noise. Instantaneous dynamic range for the l-to-100 kHz spectra (FM recording technique) is 62 dB using a test signal frequency of

40 kHz. Higher instantaneous dynamic ranges are available for

lower bandwidth spectra, e.g., about 90 dB for 750 Hz spectra.

The above instantaneous dynamic range tests were made with

a receiver gain of 100. With a receiver gain of 10,000, signals
30 dB lower can be measured. Higher amplitude signals can be

measured by reducing the receiver gain to 10, and very high

amplitude signals can be measured by switching in the 30 dB

attenuator in the antenna transformer and balun network. The

total of the instantaneous dynamic range and switchable gain

is therefore 170 dB (90 + 30 + 20 + 30). This is the range

of signal amplitude that can be measured with this system for

spectra with a 750 Hz bandwidth.

The system gain stability was measured by calibrating

each of the three systems against a standard H field three

times over a six-month period. Using 20 comparable measurements (i.e., the same system with the same gain, at the same

frequency) made two months apart, the mean system gain increased 0.56 dB with a standard deviation of 0.25 dB . Using

another set of 30 comparable measurements made four months

apart (not overlapping the above two-month period) , the mean

system gain increased 0.44 dB with a standard deviation of

0.21 dB . The highest single gain increase was 1.37 dB for

system 2 over the two-month interval at 100 kHz. This occured

at the highest frequency in a particular band, and was due

largely to variations of filter characteristics which are

dominant at the high end of the pass band.

The frequency accuracy was measured by reading the frequency of WWVB (60,000 Hz) as reported on the "peak-finder"

program computer printout. ("Peak -finder" is the program

that corrects for the minus 3 dB uncertainty mentioned in an

19
earlier paragraph, and also determines frequency). The

average frequency from 110 measurements made over a six-month

period was 59,986.9 Hz (0.022% low) with a standard deviation

of 4.3 Hz (0.00721). (The frequency offset is probably due

to offsets in the crystal oscillators in the tape recorders

or at the digitizer.) Thus, after adding a 0.022% correction,

frequencies can be measured with a standard deviation of

0.0072% (at least at 60 kHz).

Harmonic distortion was measured by injecting a 40 kHz

sinewave. For an input level 10 dB below the full -rated input of the tape recorder (1 Vrms) , the second harmonic is

down 44 dB . For an input of 1 Vrms, the second harmonic is

down 3 5 dB

Intermodulation distortion on the l-to-100 kHz spectrum

was measured by injecting simultaneously two sine waves of

equal amplitude at frequencies of 45 kHz and 55 kHz. The sum

and difference frequencies are down 49 and 46 dB respectively.

System bandwidths and digitizer rates were chosen to

attenuate aliased signals by 60 dB or more. A test 5685 Hz

square wave was injected to test for aliasing. No evidence

of aliasing was found, on this test, nor during the course of

our measurements.

The most significant source of error found to date is

the inaccuracy of the mathematical fit of the system gain correction to the known system response. For the chosen level

20
of effort expended on fitting functions, the largest error

produced by inaccurate fit is estimated to be 1.0 dB

Much of the spectra above 10 kHz in Robena is of such low

noise level as to be obscured by system noise. Subsequent

equipment modifications have allowed selected channels to

attenuate frequencies below 10 kHz so that the 62 dB dynamic

range of the wideband (1 kHz to 100 kHz) system can be used

more effectively. With strong noise components below 10 kHz

attenuated, higher system gain (and therefore lower system

noise as referred to the front end) can be achieved.

Reduction of the system noise level is readily obtainable

for reduced bandwidth recordings (e.g., 100 Hz to 10 kHz). Antenna transformer (balun) design dictates a trade off between

step-up ratio and bandwidth. The transformer with lower stepup ratio but wider bandwidths was selected for use in Robena.

Further reduction of the system noise would be possible

by eliminating the analog tape recorder and substituting a method of high-speed portable digital recording.

It should be emphasized, however, that the noise level of

the recording system is of such low value that the noise data

presented should be satisfactory for many or most system design studies. In cases where the measured noise was equal to

or below the system noise, the system noise floor established

an upper limit which the mine noise did not exceed.

21
4 . System Calibration

The loop antenna calibration is performed by applying a known magnetic field to the receiving loop antenna at the

National Bureau of Standards (NBS) loop calibration facility

located in Boulder, Colorado. The field generated at this

facility has a reported uncertainty of ± 3 percent (± 0.26

dB) [6] over the frequency ranges used.

Strictly speaking, the calibration is dependent on the

surroundings, but the dependence is slight, since the loop is

small compared to a wavelength in all the nearby media. The

highest frequency measured is the fourth harmonic of 88 kHz,

i.e., 352 kHz, with a wavelength of 852 meters. From Faraday's

law, the loop measures the time derivative of the component

of magnetic induction B normal to the plane of the loop,

integrated over the area of the loop. The mathematical definition of H, and a more comprehensive description of the calibration site are given on page 94.

The antenna is not calibrated as an independent component; all instrumentation is calibrated as a system by

using the reduced data from the microfilm plotter as the output indicator. Thus, the system performance, including that

of the software, is measured. The resulting gain corrections

are applied directly to the raw spectra to produce spectra

that are directly readable as absolute field strength.

The current probe used in making the measurements is

calibrated by clamping it around a wire carrying a known

22
current. The transmitting standard loop antenna was used as

the wire with known current. Therefore, the uncertainty of

± 3 percent (0.26 dB) reported for the field [6] also applies

to the current used to generate the field. As with the loop

antenna, the entire system is calibrated by using the microfilm plotter as the output indicator.

The voltage probe used in making the measurements is

calibrated by injecting a known sinusoid into the probe,

again using the microfilm plotter as the output indicator.

The amplitude of the sinusoid is adjusted to a constant value

at each frequency using a commercially available rms voltmeter. The voltmeter accuracy is advertised to be ± 1 percent

(0.086 dB) of full scale. The rms voltmeter was checked on a commercial laboratory voltmeter calibrator (with an advertised

accuracy of ± 0.2 percent) and found to be within specifications

The total system uncertainty is a composite of calibration

field uncertainties and system instabilities. Each has been

discussed, and the user may combine the uncertainties as he

deems best. A total uncertainty of ± 1 dB is felt to be conservative in most cases.

B . Noise Measurement Results: Spectra

1 . Mine Description and Antenna Sites

Measurements were made in the Robena No. 4 coal mine

located near Waynesburg, in southwestern Pennsylvania. Figure

5 shows a map of working section 3-main, 10-right, 2-room near

23
N

1

ROOF BOLTS

A

HYDRAULIC PUMP

FOR CAR PULL

>

13

PHONE 8.5m PHONE WIRE =4=

r8.5n

.. l 3- 6n

t It,

24

A

^#4

^•#2

KK

30m

i

TO

MINE ENTRANCE

-1.5 m MM I M l I I MM I I MM II MM I I Mi ll I MM MM II I I I I I I M l I I I II I f

l IpHU

LOADING AREA /

en

oto

(/) o

cro

NIP POINT HAULAGEWAY #1

AND TRACK

SIDECUT #00

3oen

V}O

cro

ANTENNA LOCATION -A

DRAWING NOT TO SCALE

WORKING FACE

SIDECUT #5 A

Figure 5 Map of working section 3-main, LO-right, 2-room,

111 Robcna No. 4 coal mine as it was on December'

5, 1972 when measurements were being made.

24
Blaker Shaft where the majority of measurements were made on December 5 and 7, 1972. The overburden in this area varies

between 200 and 300 meters. The entire mine, including all

machinery, is powered by 600 volts dc. A combination of ignitron rectifiers and rotary converters is used. All conversion

from ac to dc is done on the surface with the result that no

ac power is brought into this mine.

2 . Electromagnetic Noise Spectrum Results

a. Introduction

When reading values from the 100-kHz spectra in this

report, keep the following points in mind:

1. Note the roll -off frequencies. Values above 100 kHz

and below 1 kHz are not calibrated. Because of this, do

not attempt to read values above 100 kHz or below 1 kHz.

2. The correct units for the spectral peaks are microamperes per meter (uA/m) , since they are narrower than

the spectral resolution of the plots.

3. The broad-band 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 (125 Hz for l-to-100-kHz

graphs) . The correct units for the background noise

between peaks are microamperes per meter per square root x hertz [ (uA/m) //x Hz] , where x is the spectral resolution

of the FFT (x equals 125 Hz for the l-to-100-kHz graphs).

25
An easy way to obtain the spectral density per (one) root

hertz when reading broad-band noise is to subtract the required

number of dB , remembering that the units have now changed

to (yA/m)//Hz". For spectra with a resolution bandwidth of

125 Hz, subtract 20.97 dB , for 62.5 Hz subtract 17.96 dB , and

for 7.81 Hz subtract 8.93 dB

The Appendix gives the code key used in determining the

meaning of the numbers in the block at the top of each spectrum.

The resolution bandwidth is given on the ordinate of the plots.

b. Working Face Area

Figure 6, upper curve, shows the magnetic field noise

spectrum received at the antenna location identified as 1 (in

figure 5). The lower curve in this, and in following spectra,

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 6, mine noise is higher than system noise at all frequencies. Note that the system noise varies with gain when it

is expressed as equivalent input noise. The antenna loop was

placed flat on the ground (the sensitive axis for near fields

was therefore pointed up-down, i.e., vertically). The antenna

position was approximately 10 meters behind the back end of the

advancing continuous mining machine (continuous miner). One of

the two shuttle cars was positioned immediately behind the miner

26
«

-•»

o CJ

B X ~

2 w o * 3 -C lo O m coo — > o

•*J 1- c

4-» O

G n a N 0) ~- WO « — o

O O , ~

3 O

o _: ld ti H u-j

IT, •r.

OC » 3

rt rt o

e c f~ C ac Vi o ^« O -M O

c -a E n c

3 3

>- c

4-> o - u o o

O —c C -H

x i- v. 3 o -* c coo <u o c -

> 4-J H -h e 3

+j —

o —

H 3 *j O

o jr: o o U 'J O o C O O c

< e i- v.

•H

Pn

3SI0NQNVaQV0H9y0J ,zH^lyA H3d H313W-H3d-3H3dWV0y3IW 3N0

01 3M1V13H 8P

' P H

' 13A31 UISN3Q W(M33dS H13N3M1S-Q13IJ-0I13N3VW SNH

HO ! $3I3N3(103HJ 313M0S10 UOJ 'N313W H3d 3H3dWV0H0IW 3N0 01 3M1V13H 8P

l H 'H19M3M1S 013'J 0I13N0VW SWM

27
and was receiving coal. The miner power and water cables were

on the floor about 2 meters away from the antenna. Probably a large majority of the energy received by the antenna was

radiated by the power cable. The highest spectral peaks of

field strength, 48 dB uA/m , 45 dB uA/m, and 46 dB uA/m, etc.,

at frequencies of 1.67 kHz, 3.33 kHz, 5 kHz, respectively,

etc., were due to rotating electric machinery. These peaks

were slowly shifting upward in frequency as will be shown later

in 3-D plots. The peaks were caused by an electrical circuit

being closed and opened 1.67 thousand times per second, producing that number of small impulses per second. Commutator

bars rotating under a brush on a dc motor were the probable

source

Figures 7 and 8, upper curves, show noise signatures picked

up by two other orthogonal antennas. The loop antennas were

standing on edge with the horizontal antenna sensitive axis

directed E-W in the first case and N-S in the second case. All

three noise signatures shown in figures 6, 7, and 8 were taken

simultaneously using the three antennas and three separate

tracks on the tape recorder. At least three conclusions can

be made about these figures. First, the horizontal sensitive

axis noise received is lower by 10 to 30 dB than the vertical

sensitive axis noise. Second, the horizontal sensitive axis

noise reaches a relative minimum in the region of 32 kHz and

then rises 10 dB or so to an apparent maximum in the region of
cd o

c >:

CO C3 -

4-> C

-C O

O J3o

j: 3:

+-" M 00 - 2 <u ~

l- ^ - » 4-1 >— tn o g ^° ^ o o UJ '-I

3SI0NQNVaQV0HO0J' zH^lyA H3a y313W -«3d -3H3dWV0M01W BNO

01 3M1V13H 8P

' P H 'TO! UISN3Q WfM03d$ H13N3H1S-Q13IJ-0I13N9VW SWM

HO •83ION3n03MJ 313H0SI0 N0J 'H313W H3d 3H3dWV0N0IW 3N0 01 3M1V13M 8P *H 'H19N3H1S Q13IJ 0H3N0VW M

^ lo r

n) —So Uh -yi 3 GJ —

o -^ 13 rt c

o > o s- '-J

O +-* (U (/) u o

: o E o •

» Cfl C </", Cl

a

»- C.

*j o

u o

<u —< c = c o

CI, tH <D 'H .^ * S. n E *J £ ir. in

CD

•H

29
3SI0N 0NV8 0V0H9 HOJ

' 2H9gl/

V

M3d d313W-H3d-3H3dl1V0H0IW 3N0

01 3M1V13H 9P

' P H '13A31 UISN3Q W(M33dS H13N3H1S-QBJ-0I13N0VW SWH

MO 'S3ION3n03yj 313H0SI0 M03 'H313W H3d 3d3dWV0H0IW 3N0 Oi 3AI1V13H 9P

l Hl H13N3dlS 01313 0I13NDVW SWH

30
80 kHz. These are the only two spectra taken in Robena No. 4

that show this broad rise. Possibly it is necessary to be

close to the noise source to observe this. A third observation

is the extension of spectral features, 5-kHz wide, up to 100

kHz. These features are shown to be shifting upward in frequency with time in the 3-D display shown in figure 9. The

machine producing the noise was increasing its mechanical speed

during the time covered, thereby causing the frequency of output noise to increase. Figure 9 displays a frequency range of

21 kHz, and there are four visible repetitions of the feature

that is about 5-kHz wide. Since these features move in frequency with time, the peaks of the noise at any given time

could appear at any frequency. Therefore, when considering

reading noise values at a particular frequency, the highest

adjacent values probably should be the ones used. Keep in

mind also that the noise presented here is continuous in

nature. Short -duration impulses were observed earlier that

exceeded the noise levels presented in the plots so far. Some

typical examples of impulses will be given later.

Figures 10, 11, and 12 are the same spectra as shown in

figures 6, 7, and 8, but the frequency scale is expanded.

These spectra have a 7.81 Hz resolution bandwidth. Field

strengths can be read from these spectra in the range from

100 Hz to 3 kHz; outside this range they are not calibrated.

Figure 10 shows that there is some broadband noise present be31
FREQUENCY , KH

Figure 9 Three -i"i spectrum of magnetic field strength obtained or. a loop antenna, 74 kHz to 95 kilz, Robena

No. 1 mine, face area. Time progression presentation showing continuous mining machine noise increasing in frequency as a function of time.

Spectral resolution is 1 kHz. Relative amplitude

i • ;iioiv n .

32
12 2048 20 1 . 34*000 7.81*000 11/09/73 14:40:11 8 32 1.95-003 -5.94*000 0.00*000 14 94 12 5 73Gain

0.00*000 cor r. , rec. = DG= FG=

0.3086

20 43008 43009 -20 tot const. = 31.9

-1 AG= 52

1.558*006

Figure 10

FREQUENCY, KHz Spectrum of magnetic field strength obtained on

a loop antenna, 100 Hz to 3 kHz, Robena No. 4 mine, underground, face area (location 1). An- tenna sensitive axis vertical. Continuous

mining machine in full operation. Lower curve

shows receiver system noise. 11:55 a.m., Dec.

5, 1972. Spectral resolution is 7.81 Hz.

33
12 ct>49 20 1.34*000 7.81*000 11/09/73 14:41:34 1.95-003 -V 35*001 0.00*000 0.00*000 20 43008 16 94 12 5 73Gair\ core, rec.= -20 tot

1043006 const. = 31

42

C=34 RG=

.0004006

1000(-20dB DG= FG= - 0.3086

AG= 52

2.578*005

ex: ^ . ~

1 Cj>

C3»

- C.J? - c-O

i— ce:

I— O—

c/5 u-i

MINE GENERATED NOISE

CO

FREQUENCY, KHz Figure 11 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, face areu (location 1). An- tenna sensitive axis horizontal E-W. Continuous

mining machine in full operation. Lower curve

shows receiver system noise. 11:55 a.m., Dec.

5, 1972. Spectral resolution is 7.81 Hz.

34

lfr-74 -702~|
12 2048 20 1.34*000 7.81*000 11/09/73 14:40:53 9 1.95-003 4.46*001 0.00*000 0.00*000 20 43008 43008 15 94 12 5 73Gain corr. , rec.= -20 tot const. 1 C=34 RG= 1000(-20dB) DG= FG= -1 AG= 52

.000+005 0.3086 4.751+004

37 31.9

MINE GENERATED NOISE

1— rr:

l_i_l ^ cs> c_3

CJ> \ 2=.

«=e LU

CO

U-J oz:

an CO

FREQUENCY, kHz Figure 12 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robcna Xo. 4 mine, underground, face area (location 1).

Antenna sensitive axis horizontal ,\'-S. Con

tinuous mining machine in full operation. Lower

curve shows receiver system noise. 11:55 a.m.

Hcc. 5, 1

:J 7 2 . Spectral resolution is 7.8L Hz.

35

I B-?'- -701 I
tween the commutator -generated noise peaks. Figures 11 and 12

show that even with face machinery in operation, powerline

harmonics are predominant in this frequency range for horizontal antenna sensitive axis orientation.

Figure 13 shows the noise received at the face area during

lunch hour (quiet time) and in the absence of any electrical

machinery (the miner was parked elsewhere) . In this case the

system noise (primarily the tape recorder) limits sensitivity

to about -20 dB relative to 1 yA/m. Figure 14 shows the same

spectrum as Figure 13, but with the frequency scale expanded.

The first three powerline harmonics (of 360 Hz) were stronger

during lunch hour than when the equipment was present and

working. In the region between 1080 and 2880 Hz, the 3rd

through 8th harmonics of 360 Hz are found. The 360-Hz noise

is generated when 3-phase 60-Hz ac power is rectified to provide mine operating dc power. Technically, without any filtering, the mine power is pulsating dc (pulsating at 360 pulses

per second) . The levels of the harmonics are 41 dB yA/m for

the 3rd harmonic, 36 dB for the 4th, 32 dB for the 5th, 25 dB

for the 6th, 12 dB for the 7th, and 9 dB for the 8th. The loga

rithmic average (average dB value) of these six harmonics is

26 dB yA/m. These values are obtained from a computer printout from the program "Peak Finder".

36
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37
c. At Rail of Haulageway (Location 2)

Figures 15, 16, and 17 show the three orthogonal components of noise recorded at antenna location 2, which is 1.5 m

from the nearest rail in the main haulageway (refer to figure 5 for antenna location). The time is 2:35 p.m., between shifts,

with a medium-sized (13 ton) locomotive arriving pulling a man-trip car with the second shift complement of workers.

Apparent on these three plots are the higher levels of 360 Hz

and associated harmonics next to the trolley, as compared to

their levels on figure 10 during lunch hour at the face. Figures 18, 19, and 20 show expanded spectra of this event. From

figure 20 (antenna sensitive axis pointing towards trolley

wire) the harmonic magnetic field strengths are 73 dB yA/m

for the 3rd harmonic, 69 dB for the 4th, 65 dB for the 5th,

59 dB for the 6th, 48 dB for the 7th, and 42 dB for the 8th.

The logarithmic average strength of harmonics 3 through 8 is

59 dB uA/m. The logarithmic average strength of harmonics

3 through 8 at the face (vertical moment) was 26 dB yA/m.

Subtracting the two averages, we find that field strengths

in the haulage way, between 1080 Hz and 2880 Hz, on a logarithmic basis, are higher by 33 dB, a power ratio of

about 2000 to 1. Later we will show evidence of much higher

field strengths near main dc power feed lines.

Figures 21, 22, and 23 are 3-D plots showing how the

noise changes as a function of time. The first portions of

38
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FREQUENCY, KHz Figure 14 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, face area (location 1).

Antenna sensitive axis vertical. Quiet (lunch

hour) , no electrical equipment in the area. Lower curve snows system noise. 11:32 a.m.,

Dec. 5, 1972. Spectral resolution is 7.81 Hz.

39

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MINE GENERATED NOISE

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FREQUENCY, KHz Figure 18 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, crosscut No. 7, 1.5 m from

rail, antenna sensitive axis vertical, 2:35 p.m.

Dec. 5, 1972. Thirteen-ton locomotive pulling

miners into section. Spectral resolution is

7. 81 Hz.

43

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5 C=34 RG= 10( 20dB) DG= FG= -1 AG= 52

000*009 0.3086 4.115*008

FREQUENCY, KHi

Figure 19 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, crosscut No. 7, 1.5 m from

rail, antenna sensitive axis horizontal E-W,

2:35 p.m.. Oec. 5, 1972. Thirteen-ton locomotive pulli ;g miners into section. Spectral

resolution is 7.81 Hz.

44

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FREQUENCY, KHz

Figure 20 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, crosscut No. 7, 1.5 m from

rail, antenna sensitive axis horizontal N-S,

2:35 p.m., Dec. 5, 1972. Thirteen-ton locomotive pulling miners into section. Spectral

resolution is 7.81 Hz.

45

ff-lU62U 1
Figure 21 Three-D spectrum of magnetic field strength obtained on a loop antenna 74 kHz to 95 kHz, Robena

No. 4 mine, underground, crosscut No. 7, loop

antenna, antenna sensitive axis vertical, 1.5

meters from nearest rail. Thirteen- ton locomotive pulling miners into section. Spectral reso- lution is 1 kHz. Relative amplitude is shown.

46

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48
these plots were shown in the six preceding spectra (antenna

location 2). Here, the frequency region between 74 kHz and

95 kHz is selected for analysis over about a 1/2 -second

period with 1-kHz spectral resolution. Immediately apparent on

two of these graphs are impulses of varying strength. These

impulses are caused by the approaching 13-ton locomotive. The

strongest impulses are again received by the antenna with

horizontal sensitive axis oriented N-S (pointed toward the

trolley wire). This is not too surprising, as the trolley

wire and rails probably act to transmit the noise to some

extent. The vertical scale on the 3-D plots is incremented

in 10-dB steps. The height of the strongest impulse is about

25 dB above the noise floor. The noise floor here represents

the receiver noise, and for the gain settings used for this

measurement this floor is about 20 dB above 1 uA/m. The impulse can then be inferred to be about 45 dB uA/m. These

pulses are typical and should not be considered as maximum

field strengths encountered. The APD information given elsewhere in this report will give more accurate information on

distributions of pulse amplitude for the measurement bandwidth.

Note that for each 3-D graph, the information shown during the

interval of time, to 80 ms , is averaged for presentation in

the corresponding 2-D spectra in figures 15, 16, and 17.

These three figures (21, 22, 23) also illustrate the

necessity of measuring three orthogonal components simultaneously

49
to obtain the field at any given point in the mine . For example,

if only one horizontal component (fig. 22) were measured, the

impulses present at this point (as shown in figures 21 and 23)

would have been entirely missed.

Other measurements made at antenna location number 2,

including roof -bolt voltage, trolley-wire voltage, and phone

line current and voltage, will be discussed later in section

IV, Special Measurements.

d. Thirty Meters from Rail (Location 3)

Antenna location number 3 is 31.5 meters from the track

in crosscut number 7, or 30 meters farther away from the track

than location number 2. Figures 24, 25, and 26 show the three

orthogonal components of noise received at location number 3.

All three orthogonal components generally show 20 to 30 dB

less noise than received at location 2, in the frequency region

below 10 kHz. The 88-kHz trolley phone shows up clearly at

32 dB uA/m for the vertical sensitive axis, and 25 dB yA/m and

23 dB yA/m for horizontal sensitive axes E-W and N-S, respectively. The trolley phone signal received on a vertical sensitive axis antenna is 7 and 9 dB higher than the signals received on the two horizontal sensitive axis antennas.

Figures 27, 28, and 29 show expanded spectra for the above

location 3. Power-line harmonics are strongest on the antenna with the sensitive axis horizontal N-S, pointing toward

the trolley wire and tracks.

50
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53
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0.3086 5.466*004

MINE GENERATED NOISE

FREQUENCY, KHz Figure 27 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, crosscut No. 7, 31.5 m from rail, antenna sensitive axis vertical, 5:14 p.m. Dec. 5, 1972. Spectral resolution is 7.81 Hz.

54

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MINE GENERATED NOISE

FREQUENCY, KHz Figure 28 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, crosscut No. 7, 31.5 m from

rail, antenna sensitive axis horizontal E-W,

5:14 p.m., Dec. 5, 1972. Spectral resolution

is 7.81 Hz.

55

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12 2041 20 1.54*000 7.11*000 11/19/73 15:03:15 t 22

1.55-003 5.49*001 0.00*000 0.00*000 20 43001 43001 5 95 12 5 72Goin corr. , rec.= -6 lot const. = 45.9

C=34 RG= 200 ( -6dB) DG= FG= -1 AG= 52

1.000*007 0.3086 1.696*006

MINE GENERATED NOISE

FREQUENCY, KHi

Figure 29 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, crosscut No. 7, 31.5 m from

rail, antenna sensitive axis horizontal N-S,

5:14 p.m., Dec. 5, 1972. Spectral resolution

is 7.81 Hz.

56

mi669"
e. Nip Point (Location 4)

The intersection of the main haulage and crosscut 9

contains the "nip point," associated "power sled," and "carpull," and is a central point of activity. The shuttle cars

use this area to load (transfer) coal into the train cars. As

the cars are filled, they are moved forward under the control

of an operator using a large hydraulic car-pull to move in

empty cars for filling. Antenna location number 4 is one meter

away from the electric motor that drives the hydraulic pump for

the car-pull. The antenna is located between the motor and the

track in an area of almost constant occupancy by the car-pull

operator. The antenna sensitive axis is horizontal E-W.

Figure 30 shows the spectrum obtained with the car-pull motor

operating. The car-pull operates intermittently every few

seconds, for a few seconds, while a shuttle car is unloading.

The maximum spectrum value is at 78 dB uA/m at 1000 Hz, drops

to 47 dB uA/m at 10 kHz, and is down to 25 dB uA/m at 3 kHz.

This spectrum contains no spectral peaks due to brush

noise, which is unusual for a dc motor. The sound produced

in the audio monitor while recording contained no whine. Whine

usually is associated with brush-produced spectral peaks. No

explanation is apparent for this absence of peaks. Note that

the field strength measured here at antenna location 4 is the

highest field produced by a single machine. However, higher

field strengths are measured at multiples of power line frequencies in cuts containing primary mine dc power cables.

57
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Other spectra taken at antenna location #4 showed

the 88-kHz mine phone signal strength as 42 dB yA/m.

£. Air-split

An area considered important for a communication substation in the event of an emergency is an "air-split," that

is, an area of the mine where two streams of fresh air converge or diverge. One air-split (not shown on report mine maps)

in Robena No. 4 mine is at the intersection of 3-main with

10-right about 1 mile from the working section previously discussed. Figure 31 shows the spectrum taken at the air-split

with the antenna (sensitive axis vertical) about 1 meter from

the nearest rail. Figure 32 shows the expanded spectrum. In

the region between 1080 and 2880 Hz the 3rd through 8th harmonics of 360 Hz are apparent. The levels of the harmonics

are 80 dB yA/m for the 3rd harmonic, 76 dB for the 4th, 73

dB for the 5th, 68 dB for the 6th, 60 dB for the 7th, and 49

dB yA/m for the 8th harmonic. The logarithmic average strength

of harmonics 3 through 8 is 67.8 dB yA/m. This compares with

26.0 dB yA/m at the face and 59.4 dB yA/m (for harmonics 3

through 8) in the section haulage way. Apparently larger mine

dc supply currents are flowing through cables near this intersection with correspondingly higher (8.4 dB) logarithmic

average powerline harmonic noise. Above the 8th harmonic (see

figure 31), mine noise continues to decline, having a value

59
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MINE GENERATED NOISE

FREQUENCY, \h

Figure 32 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, antenna sensitive axis ver- tical, 1:11 p.m., Dec. 7, 1972. Air Split.

Spectral resolution is 7.81 Hz.

61

TT 16601
41 dB yA/m at the 18th harmonic (648 kHz) , 31 dB yA/m at the

25th harmonic (9 kHz) , and going into the system noise of

about 19 dB yA/m at about 16 kHz.

g. Bailey Shaft (Location 6)

An entirely different area of the Robena No. 4 mine near

Bailey Shaft, about 10 -km away, was measured. Figure 33 shows

the features near Bailey Shaft. The overburden in this area

is 184 m (605 ft.). Bailey shaft is an open shaft carrying

fresh air for ventilation, a water pipe, and two heavy mine dc

power cables. The bottom of the shaft is several hundred

meters from the nearest rail haulage. Thirty meters east and

10 meters south of the bottom of Bailey shaft is a pump room

containing a dc motor-driven pump for pumping water out of

Robena No. 4. Figure 34 shows the vertical sensitive axis

spectrum taken for an antenna location (number 6) thirty meters

east of the bottom of Bailey shaft. Figure 35 shows the expanded spectrum. The logarithmic average of the amplitudes

of harmonics 3 through 8 is 66.1 dB yA/m. The harmonic amplitudes remain fairly constant (between 58 and 70 dB yA/m)

out to the 12th harmonic (4 . 32 kHz). Beginning with the

13th harmonic, a dramatic and steady decline in harmonic amplitude occurs. The 18th harmonic (6 . 48 kHz) is down to 37 dB

yA/m. The last clearly recognizable harmonic (before receiver

noise becomes predominant) is the 25th (9 kHz) with an ampli62
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FREQUENCY, KHz Figure 35 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, antenna sensitive axis vertical, 2:45 p.m., Dec. 7, 1972. 30 m east of the

bottom of Bailey shaft. Spectral resolution is

7. 81 Hz.

65

H-U1607~l
tude of 22 dB yA/m. Note that the logarithmic average amplitude is about the same as that found at the air-split (logarith

mic average of 3 though 8 is 67.8 dB yA/m). The noise

measurements of primary interest are the ones taken at times

of maximum noise, since potential communications systems must

operate through this noise. To illustrate the variability of

the noise spectrum with time, figure 36 shows the noise spectrum taken a few seconds later. Figure 37 shows the expanded

spectrum. The logarithmic average of the amplitudes of

harmonics 3 through 8 is 55.8 dB yA/m, or lower by 10.3 dB

Figure 38 shows the vertical sensitive axis noise spectrum

taken 27 minutes later at the same location. Figure 39 shows

the expanded spectrum. The logarithmic average amplitude of

harmonics 3 through 8 is about 45 dB yA/m, or lower by 21 dB.

Figure 38 also shows the amplitude of the 88-kHz trolley

phone as being 46 dB yA/m. The signal probably is propagated

along the 600-volt dc line supplying power to the water pump

motor about 10 meters away.

Because of the remoteness of Bailey shaft, recording

equipment sufficient for only one channel of data was carried

to the location. To measure the three orthogonal components

of noise at Bailey, the antenna had to be placed serially in

three different orientations. A set of three measurements

taken three minutes apart produced logarithmic -average noise

amplitudes (of harmonics 3 through 8) of 47.2, 35.0, and 49.2

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FREQUENCY, KHz

Figure 37 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, antenna sensitive axis vertical, Dec. 7, 1972. 30 m east of the bottom of

Bailey shaft. A few seconds later. Spectral

resolution is 7.81 Hz.

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FREQUENCY, KHz

Figure 39 Spectrum of magnetic field strength obtained on

a loop antenna 100 Hz to 3 kHz, Robena No. 4 mine, underground, antenna sensitive axis vertical, Dec. 7, 1972. 30 m east of the bottom of

Bailey shaft. 27 minutes later. Spectral

resolution is 7.81 Hz.

70

H- 1U596
dB yA/m for horizontal N-S, horizontal E-W, and vertical antenna sensitive axes, respectively. Typically, the vertical

sensitive axis is strongest. The spectra are similar to

those shown previously and are not included here.

h. Bailey Shaft (Location 7)

A final magnetic field strength measurement was made at

antenna location 7, 30 meters in another direction from

the bottom of Bailey shaft in a cut carrying the primary 600

volt dc supply cables for a large portion of the mine. The

antenna sensitive axis is vertical, and the antenna is placed

about 2 meters from the cables carrying heavy currents. The

primary purpose of this particular antenna placement is to obtain a coherence of variations in noise simultaneously on the

surface and underground. The results will be discussed in

section IV, A. Absolute field strengths are obtained from this

measurement by the usual calibration and correction techniques.

The spectral analysis bandwidth (62.5 Hz) and the frequency

range (300 Hz to 40 kHz) are different than past data analyses.

Figure 40 shows the field strength measured. The field strengths

are unusually high. The logarithmic average of harmonics 3

through 8 is 98.2 dB yA/m. Subtracting 26.0 dB yA/m (the

logarithmic-average harmonic strength measured at the face

during quiet time) gives a 72 -dB range in magnetic field

strengths for these power line harmonics in Robena No. 4 mine.

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This large range corresponds to an amplitude ratio of 5000 or

a power ratio of 25,000,000. In summary, a coal mine can be very

quiet or very noisy (electrically), depending on where measurements are taken relative to operating electrical cables and

equipment

i. Summary Plot of Power Line Harmonics

Figure 41 is a summary of magnetic field strength at

power-line harmonic frequencies observed within Robena No. 4.

Plotted are the logarithmic averages of harmonics 3 through 8

of 360 Hz (i.e., 1080 Hz through 2880 Hz). Impulsive noise

is not shown. For comparison, the two diamond-shaped points

show equipment -generated noise.

III. AMPLITUDE PROBABILITY DISTRIBUTION MEASUREMENTS

A. Introduction

The amplitude probability distribution (APD) of the received noise signal magnitude is one of the more useful

statistical descriptions of the noise process for the design

and evaluation of a telecommunications system operating in a noisy environment [7, 8, 9].

By plotting the cumulative APD on Rayleigh graph paper,

one can show clearly the fraction of time that the noise exceeds various levels. We use Rayleigh graph paper with scales

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chosen so that Gaussian noise (e.g., thermal 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

receiver bandwidth.

Section III of this report describes the APD measurement

methods and results. Part B.l describes the measurement

instrumentation of an underground recording system, a data

transcribing system, and a data processing system. Part B.2

presents measurement techniques used for APD, rms , and average

measurements of noise in a coal mine. Part B.3 describes the

calibration procedure and estimate of accuracy in our measurements. Part C includes many APD's taken in a coal mine. An

analysis of these results also is included.

B. Noise Measurement Techniques

1 . Measurement Instrumentation

Section III, B of this report describes the system used

to measure the amplitude probability distribution statistics

of electromagnetic noise in coal mines. The system is an extension of one designed by Matheson [10]. See figures 42a,

b, and c.

75
COLLAPSIBLE

ANTENNA

LOOP

ANTENNA

f=10kHz-250kHz

FIELD

STRENGTH

METER A

BW=1 .4kHz

455kHz

MIXER

40kHz

FIELD

STRENGTH

METER

BW=3.3kHz

25kHz

OSC

455kH; MIXER 40kHz

f=150kHz-32MHz

CHANNEL 1

CHANNEL 2

CHANNEL 3 PORTABLE

TAPE

RECORDER

DIRECT

RECORDING

15 IPS

f=100Hz-50kHz

SCOPE

Figure 42a System for field recording.

76

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PORTABLE

TAPE

RECORDER

DIRECT

RECORDING

15IPS

CHANNEL 1

CHANNEL 2

CHANNEL 3

40kHz

25kHz

40kHz

LABORATORY

TAPE

RECORDER

DIRECT

RECORDING

15 IPS

CHANNEL 2

CHANNEL 4

CHANNEL 6

Figure 42b System for transcribing.

CHANNEL 2

CHANNEL 4

CHANNEL 6

LABORATORY

TAPE

RECORDER

DIRECT

RECORDING

15 IPS

40kHz

I 25kHz |

I

ATTENUATOR

40kHz

40kHz

SERVO

CONTROL

AMPLIFIER

G = 10

40kHz

DIGITAL

LEVEL

COUNTER

455kHz ATTENUATOR .455kHz TUNED

FREQUENCY

CONVERTER

Figure 42c System for ADP processing.

77 IB- U161~
a. Underground Recording System

The principal parameter measured is magnetic field

strength. Electrostatically shielded loop antennas are used

to intercept the magnetic field and to discriminate substantially against any electric - field component. For the frequency range between 10 kHz and 250 kHz, the loop antenna is

a collapsible, single-turn diamond configuration of area of

about 0.7 square meters. This loop has an inductance of

about a microhenry, and at low frequencies represents a very

low impedance source compared to the 50-ohm input impedance

of the field strength meter. Therefore, a balun with step-up

transformer is used to match the low impedance antenna to the

50-ohm input. For the frequency range between 150 kHz and

32 MHz, a single-turn, 38-cm diameter, circular loop antenna

is used with a balun. This loop antenna is also electrically

shielded and has an inductance of about a microhenry. The

magnitude of the impedance of the loop antenna varies from

one ohm at 150 kHz to 200 ohms at 32 MHz. A switch on each

balun allows use of several impedance -matching networks (four

for the low frequency case and eight in the high frequency

case), which consist of transformers and coupling capacitors

to give the desired match over the required frequency range.

The outputs of the baluns are fed into commercially available,

battery-powered, electromagnetic interference and field strength

meters (hereafter referred to as EIFS meters).
It has long been recognized that a mean square measure

is a very useful statistical measure. But although many EIFS

meters have detector functions such as peak, quasi -peak, and

average voltage, very few of them have the very important

function of rms voltage. The EIFS meters used for our

electromagnetic noise measurements are modified to measure

rms voltage simultaneously with average voltage [10] . The

characteristics of these modified EIFS meters used for our

noise measurements are listed in Table 1.

The particular system that we used actually measures rms

voltage, V , and average voltage, V . The system functions & ' rms & 6 avg

by automatically adjusting the receiver gain to keep a constant rms voltage at the output of the integrator following

the squared-voltage detector. The receiver has a logarithmic

gain control characteristic. One can examine the automatic

gain control (AGC) voltage and obtain directly the input rms

voltage in dB

In order to measure the APD , the AGC circuit is disabled.

Since the gain of the receiver is now constant, the magnitude

of the IF output is directly related to the bandlimited

input noise signal magnitude. To check the linearity and to

establish the dynamic range of the EIFS meter, a CW signal is

applied to the input of the EIFS meter. The relation between

the input and the IF output of the EIFS meter A is shown in

figure 43. The dynamic range of the EIFS meter used in our

noise measurements at 1.4-kHz bandwidth is found to be 65 dB

79
10 20 30 40 50 60 70

INPUT OF EIFS METER, dB RELATIVE TO ImV RMS

80

Figure 43 Linearity of Electromagnetic Interference and Field

Strength (EIFS) Meter.

80
Table 1. Characteristics of Electromagnetic Interference

and Field Strength (EIFS) Meters

(Manufacturer's Specifications)

EIFS Meter A B

Frequency Coverage 10 kHz - 250 kHz 150 kHz - 32 MHz

Sensitivity 0.01 yV 0.1 uV

cw 3-dB Signal Bandwidth* 1.4 kHz 3.3 kHz

Dynamic Range 6 7 dB 60 dB

Spurious Response

Rejection >50 dB >60 dB

Input Impedance 5 ohms with an 5 ohms with an

input VSWR less input VSWR less

than 1.2:1 than 41.2:1

*These values of the cw 3-dB signal bandwidth are based on

NBS measurements.
Since the bandwidth of the portable tape recorder is

limited to 50 kHz, the IF output from the EIFS meter is converted down from 455 kHz to 40 kHz using mixers. The circuit

diagram for the mixers used in our noise measurements is

shown in figure 44. A commercially available, battery-powered,

portable, analog magnetic tape recorder is used for recording.

The tape speed chosen on record and on playback is 15 inches

per second (ips) . At this speed the portable tape recorder

frequency response range is 100 Hz to 50 kHz at the ± 2 dB

points in the direct recording mode as shown in figure 45.

The input voltage range is adjusted to record the signal level

between 10 millivolts and 1 volt rms . The tape recorder gain

is adjusted for dB . The characteristics of this portable

tape recorder are listed in Table 2.

An external set of sealed, lead-acid batteries in an explosion -proof enclosure is used to drive the portable tape

recorder. The current is limited by a solid-state, currentlimiting circuit in series with a fuse. The power requirement

is approximately 13 watts at a nominal 17.5 volts. This

battery system allows about eight hours of recording.

b. Data Transcribing System

The cumulative peak-to-peak flutter of the portable tape

recorder is about 0.8 percent, whereas that of the laboratory

tape recorder is about 0.4 percent. The time displacement

error is perhaps more important , being microseconds for the
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Table 2. Characteristics of Portable Magnetic Tape Recorder

(Direct Record/Reproduce)

(Manufacturer's Specifications)

Tape Speed

Flutter

(cumulative peak-to-peak

flutter)

rms Signal to rms Noise

Crosstalk

Harmonic Distortion

Input Voltage Range

Input Impedance

Output Voltage

Output Impedance

Frequency Response at 15 ips

15 inches per second (ips)

0.8 percent

35 dB

35 dB below nominal signal

level

1.5 percent total harmonic

distortion

0.01 to 1 V rms

20 kft

1 V rms into 1-kfi load

100 n

100 Hz to 50 kHz at ± 2-dB

points

85
laboratory tape recorder and milliseconds for the portable tape

recorder. Therefore later, at our laboratory, the tapes are

transcribed through a laboratory tape recorder whose servo system can take out the flutter and wow introduced by the portable

tape recorder. The characteristics of this laboratory tape

recorder are listed in Table 3. To give a reference time base,

a stable 25-kHz signal is recorded on a separate track at the

time the mine recordings are made. At playback time (after

transcription) this signal is used to control the servo of

the laboratory tape recorder.

c. Data Processing System

The data processing system consists principally of the

laboratory analog magnetic tape recorder as a playback unit,

an amplifier, a tuned frequency converter, and a digital

level counter. The amplifier is used primarily for impedance

conversion between the output impedance of the laboratory tape

recorder and the input impedance of the tuned frequency meter.

The 40-kHz output of the laboratory tape recorder is converted

up to 455 kHz by the tuned frequency converter in order to

match the response band of the digital level counter.

The digital level counter provides a direct digital display of the percentage of the time each of 15 levels, 6 dB

apart, are exceeded. This instrument has five identical 18-dB

amplifiers in series. Each amplifier has a detector on its

86
Table 3.

Characteristics of Laboratory Analog Magnetic Tape Recorder

(Direct Record/Reproduce)

(Manufacturer's Specifications)

Tape Speed 15 inches per second (ips)

Flutter

(cumulative peak-to-peak 0.4 percent

flutter)

rms Signal to rms Noise 37 dB

Harmonic Distortion 1 percent

Input Voltage Range 0.3 to 3.0 V rms

Input Impedance 100 kQ

Output Voltage 1.0 V rms into 10-kfi load

Output Impedance < 100 fi Frequency Response at 15 ips 100 Hz to 75 kHz at ± 3-dB

points

87
output. Each detector is linear over an 18 -dB range, and its

output is discriminated to be in one of three levels, six dB

apart. Each level, whenever exceeded, drives a Schmitt trigger

which gates a clock on; the gated clock pulses are counted in

parallel in each of 15 (three times five) independent counters.

This gives a cumulative distribution. The lower-level

channels generally read nearly 100 percent. A 16th counter

reads the corresponding total clock pulses. Readout is on a

7 -digit display which has a maximum of 9 x 10 counts available for each channel. The clock rate can be adjusted from

1 kHz to 1 MHz in 1-2-5 steps.

The cw , 3-dB signal bandwidth of the whole system, including the recording, transcribing, and data processing systems,

is primarily determined by the data processing system. The

predetection bandwidth of the APD measurements for the frequency

range between 1.0 kHz and 250 kHz using EIFS meter A is 10 kHz,

whereas that for the frequency range between 250 kHz and 32 MHz

using EIFS meter B is 1.2 kHz. These predetection bandwidths

are indicated in each APD figure. The dynamic range of the

whole system is primarily limited by the magnetic tape recorder

to about 45 dB

The system used for recording, transcribing, and data

processing is shown in figures 42a, b, c. Figure 46 shows a collapsible loop antenna used in a mine for the frequency range

between 10 kHz and 250 kHz. The recording system which includes
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two EIFS meters, two mixers, a portable analog magnetic tape

recorder, an external set of sealed, lead-acid batteries, a portable oscilloscope, etc., used in a mine is shown in figure

47. Figure 48 shows the data processing system which consists

of a laboratory analog magnetic tape recorder, an amplifier, a tuned frequency converter and a digital level counter.

2 . Measurement Technique

The measurement technique is to record on magnetic tape a

time -varying analog signal whose amplitude (envelope) varies

proportionally to magnetic field strength, as seen through a specific receiver bandwidth. These signals are recorded for

about 20 minutes at each frequency and for each of three

antenna orientations. The tape is transcribed later through

a laboratory tape recorder whose servo system can take out the

flutter and wow introduced by the portable tape recorder. The

data processing system consists principally of the laboratory

tape recorder for a playback unit and a digital level counter

which provides a direct digital display of the percentage of

the time each of 15 levels, 6-dB apart, is exceeded.

The voltage levels applied to the recorder must be adjusted

(by controlling the gain of the field strength meters) so that

the curved portion of the APD falls within the dynamic range

of the magnetic tape recorder. To do this, we set the main

function switch to "noise" position and record rms and average

90
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values of noise in a coal mine on a fast -response basis. The

time constants for our measurements can be adjusted between

0.1 and 100 sec. depending on the circumstances. One can use

the shorter time constant for a Gaussian noise environment,

but longer time constants are required for an impulsive noise

environment. These "short term" rms and average values are

recorded and are used to set the gain of EIFS meters, but

these values are not reported here. We then set the main

function switch to peak position in order to disable the AGC .

By changing the gain of each EIFS meter, the IF output of each

EIFS meter is adjusted to about 100 mv peak to peak. A portable oscilloscope is used for monitoring. The necessity for

monitors is discussd earlier in this report.

3 . Calibration

The calibration of the entire measurement system, including the loop antennas, field strength meters, mixers, magnetic

tape recorders, impedance transforming amplifiers, and the

digital level counter, is performed by immersing the receiving

loop antennas in a known field, generated at the NBS field

strength calibration site. Thus all levels of field strength

are given in absolute units . This technique is called the

standard field method. It is used to calibrate loop antennas

at NBS from 30 Hz to 30 MHz. The ratio (calibration factor)

93
of the known field strength to the output of the unknown

system is calculated.

This standard field, a cw, quasi-static near-zone magnetic

field, is produced by a single-turn, unshielded, balanced

transmitting loop of known radius carrying a known current.

The magnitude of the field at the receiving loop, produced by

a single -turn circular transmitting loop, is given by the

following equation [11, 12, 13]:

H

2 T r

l

J 777272727372 2(d +r

1

+r

2

)

i \W7 I A (1)

where H is the magnetic field strength in rms amperes per

me t e r ,

r-. is the radius of transmitting loop in meters,

r_ is the radius of receiving loop in meters (if the

receiving loop is rectangular, use the radius of a circle having the same area) ,

d is the axial spacing in meters between the two coaxial

loops

I is the transmitting loop current in rms amperes, and

A is the free-space wavelength in meters.

The transmitting and receiving loops are positioned co

axially with respect to each other at a spacing of 1 to 2 meters

The spacing is determined by the desired magnitude of the calibrating field and the frequency. Equation (1) is valid for

94
determining the magnetic field strength only when r, , r

?

, and

d are small compared to A. The loop spacing should be at

least four times the radius of the larger of r and r

? for

equation (1) to be valid within one percent.

The calibration site should be in an area that is free

of sizeable metallic objects that might influence or distort the calibrating field. Normally, if the calibrating area

is cleared of metallic objects within two or three times the

loop spacing, d, there will be no appreciable effect. The

calibrating site should, therefore, be in an area relatively

free of overhead power lines, steel -reinforced walls, or

shielded enclosures. A non-metallic building with no overhead wiring makes a satisfactory calibration site.

The estimated limits of error for our 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 ± 3-dB quantization error limit.

This ± 3-dB quantization error would be improved to ± 1-dB

error by an improved calibration method, and future APD

noise data will be reported with improved accuracy.

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

and data processing, has a calibration uncertainty of

± 0.5 dB.

95
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, gain instability, etc.

4. The gain instability during measurements, gain

changes between measurements and calibration, and the

non-linearity of 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

C . Nois e Measurement Results: APD '

Many APD ' s of magnetic field noise were taken during

actual operation of the coal mine on December 5th and 7th,

1972. The loop antennas were placed about 300 meters from the

face area (location 5 in figure 5). Three orthogonal components

of magnetic field were measured at eight frequencies ranging

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

70 kHz, 130 kHz, 500 kHz, 2 MHz, 8 MHz, and 32 MHz. The length

of time for each measurement was 23 minutes.

The measured data are presented in 32 APD ' s . These APD '

are given as figures 49 through 80. The vertical axis gives

magnetic field strength, H, while the horizontal axis gives

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December 5, 1972. 11: 25 a.m. , Robena No. 4.

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Figure 64 APD , 32 MHz, horizontal component (E-W) , 1.2 kHz predetcction

bandwidth , December 5 , 1972, 5: 45 p .m . , Robena No . 4.

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Figure 66 APD , 30 kHz, horizontal component (N-S), 1 . kHz predetection

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Figure 69 APD , 500 kHz, horizontal component (N-S) , 1.2 kHz predetection

bandwidth, December 5 , 1972, 1:30 p.m., RobenaNo. 4.

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Figure 71 APD , 8 MHz, horizontal component (N-S), 1.2 kHz predetection

bandwidth, December 5, 1972, 5:20 p.m., RobenaNo. 4.

119

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Figure 72 APD , 32 MHz, horizontal component (N-S) , 1 . 2 kHz predetection

bandwidth, December 5 , 1972, 6:50 p.m., Robena No . 4.

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99

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Figure 74 APD , 30 kHz, vertical component, 1.0 kHz predetection bandwidth,

December 7, 1972, 11: 20 a.m. , Robena No. 4.

122

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Figure 75 APD , 70 kHz, vertical component, 1.0 kHz predetection bandwidth

December 7, 1972, 11: 55 a.m. , Robena No. 4.

123

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Figure 76 APD , 130 kHz, vertical component , 1 . kHz predetection bandwidth

December 7, 1972, 1: 25 p.m. , Robena No . 4

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128

|g.76-133&~l
percentage of time the indicated level is exceeded. For the

recording time interval, one can see what percentage of time

a particular magnetic field strength was exceeded. In addition, one can easily infer the relative composition of the

noise, i.e., whether it is Gaussian, impulsive, or CW . The

Rayleigh distributed envelope of Gaussian noise has a slope

of -1/2. CW noise gives slopes greater (more positive) than

-1/2. Impulsive noise shows up as slopes of -4 or -5 or even

more negative [9]. The noise sources and transmission effects

should be considered in this respect. The impulsive noise

sources are typically trolley arcs and brush arcs. At the

lower frequencies, 10 kHz to 130 kHz, all wires, cables, and

rails serve as relatively low-loss transmission lines, and

noise generated in this part of the spectrum anywhere in the

mine is transmitted throughout the mine. At the higher frequencies, the transmission loss is higher, and unless there

is a local source of impulsive noise, the measured noise is

Gaussian except for a relatively small percentage of the time.

Above 2 MHz, for the location of these recordings (beside a trolley line but away from equipment except for occasional

passing trolleys), impulsive noise is present less than one

percent of the time. Harmonic power of the periodic powerline signal falls off above 10 kHz; this can be seen in the

spectral plots

129
The APD's are integrated to give rms and average values

of the field strength, according to the equations

H

avg

and

H

rms

J H dp(H)

o

I H

2 dp(H)

o

(2)

(3)

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 (i.e., the APD 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 an rms detector, the

readings will vary 10 to 20 dB over fractions of a second.

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 0.1 and 99 percent,

as well as rms and average values, are shown in figures 81

through 84 for three orthogonal field components measured on

December 5, and for the vertical component measured on

December 7, 1972. The predetection bandwidth for these APD

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measurements is normalized to be 1 kHz. Some fluctuations in

values occur because of different operating conditions during

different times of the day. Between 2:00 p.m. and 4:00 p.m.

the mining equipment operated less often, but trolleys

operated more often. Considerably lower level of EM noise

around 130 kHz was observed for all three orthogonal field

components on December 5, 1972. This peculiarity was not observed on December 7, 1972, nor was it observed on several

broad-band spectral plots taken in some other mines at other

times. Therefore we conclude that the considerably lower

level of EM noise around 130 kHz on December 5 was due to

some specific operation of this coal mine on this particular

day and is not considered to be a general nature of EM noise

in a coal mine. The 23 minute time period for each measurement is adequate for covering the variations due to the local

work cycle. Shorter periods display shifts of several

decibels in APD ' s ; longer periods (46 minutes) do not. The

emphasis in this program was to obtain noise measurements

during normal operation. We did not make measurements when

the mine was shut down. When shut down, the mine is quieter

by many tens of decibels.

135
IV. SPECIAL MEASUREMENTS

A. Surface Noise Measurements

Introduction

Surface noise measurements were made on December 5 and 7,

1972, near Blaker and Bailey Shafts of Robena No. 4 Coal Mine.

The Blaker Shaft data are probably typical of farming and

pasture countryside. The Bailey Shaft data are typical of

mine entrances near power substations and heavy electrical

machinery

These data are relevant to electromagnetic techniques for

locating entrapped miners and for special, through-the-earth,

communication systems. The upper frequency limit of interest

is probably no higher than 10 kHz for most practical uses.

This is because attenuation through the earth increases

rapidly above some "corner" frequency, often much lower than

10 kHz.

There are seven sections to part IV of this report. They

are brief because the measurement techniques, instrumentation,

and calibration are similar to those discussed in much more

detail in other sections of the report.

136
2 . Measurement Techniques

a. Spectra

The surface spectral measurements are made the same way

as the underground spectral measurements are made, but with

four differences: first, different equipment is used; second,

the frequency range is less, 100 Hz to 10 kHz; third, the

spectral resolution is correspondingly smaller; and fourth,

a laboratory tape recorder is used, thus eliminating the need

for transcribing data.

b. Amplitude Probability Distributions

Again, the measurement technique is similar to that used

in making underground measurements. The differences are the

same as the first and fourth listed above. The spot frequencies

covered are 10 kHz, 30 kHz, 70 kHz, and 130 kHz. These frequencies are selected for specific underground-surface measurement comparisons; the general information of this type is available elsewhere [14]

3 . Measurement In s trumentation

The instrumentation used is shown in figure 85. A

single laboratory tape recorder is used for both spectral

and amplitude probability distribution (APD) techniques. The

peripheral equipment is identical to that used in the underground system except active filters are used for the spectral

137
COLLAPSIBLE

LOOP

ANTENNA

BALUN

LOOP

ANTENNA

FIELD

STRENGTH

METER

IOkHz-250kHz

BALUN

455kHz

PREAMP

\oMIXER 40 kHz

LP FILTER

10 kHz

25 kHz

OSCILLATOR

EQUIPMENT

SIMILAR TO 3, O—

BUT WITH DIFFERENT

ANTENNA ORIENTATION

EQUIPMENT

SIMILAR TO 3,

BUT WITH DIFFERENT

ANTENNA ORIENTATION

CHANNEL I (FM)

CHANNEL 2 (D I R)

CHANNEL 3 (FM)

CHANNEL 4(DIR)

CHANNEL 5 (FM)

CHANNEL 6 (DIR)

O— CHANNEL 7 (FM)

Figure 85 Surface recording system,

138

IB- H 1639 |
systems rather than passive filters. The size, weight, and

power limitations to be considered are only moderate, and mine

permissibility is not a requirement. The use of the laboratory

recorder with its excellent speed -control servo system allows

omission of one step, the data transcription. Also, the

small frequency range of the spectral measurements requires

only a four-to-one tape speed reduction for digitizing. The

record speed is 15 ips, with a 10-kHz bandwidth using FM mode;

reproduce speed is 3-3/4 ips, with a corresponding 2.5 kHz

bandwidth

The APD data must be recorded at the same speed, 15 ips;

direct record mode is used, and, for this recorder, the 75-kHz

bandwidth is quite adequate to record the 40 -kHz output of the

mixers. Reproduce speed is 15 ips into the same data processing

system used for the underground data processing.

4 . Location of Measurements

The location of measurements near Blaker Shaft is shown

in figure 86 . Although a power line (not shown) to a local farm

house passes within 20 meters of the measurement location, and

although a high tension line is about 88 meters away, there

is no heavy electrical machinery and no power substation within

about 229 meters. Thus the noise levels may be considered

typical for dairy farming countryside.

139
APPROXIMATE LOCATION

OF UNDERGROUND

MEASUREMENTS

LOCATION OF

SURFACE

MEASURE

MENTS

BLAKER SHAFT

ROBENA #4 COAL MINE

COMPRESSOR

AND FANS

Figure 86 Location of surface measurements

140

IB- 7 . -7»9
The original plan was to record surface data at a position directly above the position of the underground recordings,

but due to sustained heavy rains before and during the date of

these measurements, the overburden surface directly overhead

was inaccessible. Therefore, the measurements were made with

a horizontal offset of approximately 360 meters.

The measurements made near Bailey Shaft are about 30

meters from a power substation (with rectifiers for supplying

underground dc power), and about 50 meters from compressors

and other heavy electrical machinery. Thus, these data are indicative of a "worst case" environment, or at least what

normally must be expected within 50 meters of mine entrances

or other areas of heavy electrical power usage.

5 . Calibration

Calibration of each spectral system and each APD system

was performed as described in a previous section by immersing

each antenna in a standard magnetic field and following through

the entire procedure of record, reproduce, process, and display, thus calibrating each complete system.

The uncertainties in the spectral plots and APD plots are

the same as stated in appropriate previous sections.

6. Results of Spectral Noise Measurements

Three orthogonal components of surface H-field noise

spectral plots for Blaker Shaft are shown in figures 87

141
through 89. Similarly, three orthogonal components for

Bailey Shaft are shown in figures 90 through 92. The lower

curve in each figure is the system noise curve. The curves

have been corrected to display absolute values between 750

Hz and 10 kHz; values below this range have increased uncertainty; values above this range are not usable. The spectral

resolution is 7.81 Hz for figures 87 through 92.

With the exception of some 60-Hz harmonics the Blaker

noise varies between dB uA/m and -20 dB uA/m. Variations

are gradual, and not consistent, but sometimes there is a broad minimum between 1 kHz and 3 kHz and a slight peak near

4 kHz. The Bailey Shaft noise is much stronger; from 1 to

2 kHz it is about 30 dB yA/m; after that it falls off at

approximately 50 dB per decade.

7 . Amplitu de P robability Distribution Results

Only four APD measurements were made near Blaker Shaft and

none at Bailey Shaft. These were made at approximately the

same time, at the same frequencies, and with the same antenna

sensitive axes (vertical) as for measurements underground.

These APD's are shown in figures 93 through 96. The comparative results show surface noise above a working mine to be

about 25 dB less than underground noise within the mine,

although this generalization may be grossly in error near

sources of mine noise.

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USCOMM EJU.

Figure 93 APD, 10 kHz, surface, antenna sensitive axis

vertical, Robena No. 4, December 5, 1972,

11:45 a.m.

149
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Figure 94 APD, 30 kHz, surface, antenna sensitive axis

vertical, Robena No. 4, December 5, 1972,

2 :00 p.m.

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APD, 130 kHz, surface, antenna sensitive axis

vertical, Robena No. 4, December 5, 1972,

5 : 15 p.m.

99

ISCOMM - EM.

152

IB-H1629 |
At 10 kHz the surface noise varies between 26 dB )jA/m

(0.00011 of time) and -20 dB yA/m (99% of time).

A summary of the field strength excursions between 0.11

and 99% of the time as a function of frequency is shown in

figure 97 for the surface data,

B . Measurements of Voltage Between "Roof Bolts"

Measurements of RF voltage were made between three pairs

of roof bolts with intrapair spacings of 3.6 m, 8.5 m, and

13 m. Roof-bolt locations are shown in figure 5. Unshielded,

insulated copper wire is attached to the bolts using large

clips. The other end of the wire pair is connected directly

o to the input of an amplifier with 10 ohms input impedance.

The resulting voltage spectra are shown in figures 98, 99,

and 100, for spacing of 3.6, 8.5, and 13 meters, respectively.

The logarithmic average of the 360 Hz harmonics 3 through 8

for the 13-meter spacing is about -84.2 dB with respect to

1 V rms, i.e., 62 yV rms . The amplitudes in the spectra for

3.6- and 8.5-meter spacings are too low to allow reliable

scaling of voltages.

Figure 101 shows the voltage spectrum obtained from an 8.5-meter "dipole". Instead of being clipped onto the roof

bolts, the wires are merely left unconnected. The resulting

voltage is much higher. The logarithmic average of harmonics

153
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Figure 98

2 3

FREQUENCY, KHz

Voltage spectrum, 100 Hz to 3 kHz, Robena No. 4 mine, underground, roof bolts - 3.

-meter separation, 12:25 p.m., Dec. 7, 1972. Spectral reso- lution is 7.81 Hz.

155
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FREQUENCY, KHz Figure 99 Voltage spectrum, 1 J .,: to 3 k.iz, *oocm No. 4 •nine, underground, roof bolts- 8 . 5-meter separation, 12:20 p.m., Pec. 7, 1972, Spectral reso- lution is 7.81 Hz.

156
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FREQUENCY, kHz

Figure 100 Voltage spectrum, 100 Hz to 3 kHz, Robena No. 4 mine, underground, roof bolts -- 13-meter separation, 12:14 p.m., Dec. 7, 19 72. Spectral reso- lution is 7.81 Hz.

157

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E GENERATED NOISE

SYSTEM NOISE

iMVi ^

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FREQUENCY, KHz Figure 101 Voltage spectrum, 100 Hz to 3 kHz, Robena No. 4 mine, underground, dipole antenna, 8.5-meters

long, 12:24 p.m., Dec. 7, 1972. Spectral reso- lution is 7.81 Hz.

158
3 through 8 is 70.0 dB below 1 volt, i.e., 316 yV rms (14.2 dB

higher than the 13-meter roof bolt voltage)

Data taken simultaneously on a pair of roof bolts and on

a loop antenna show unlike responses. At one time, a particular

88-kHz signal would be picked up more strongly by the roof

bolts than by the loop. The same is true for impulses. To

illustrate this, figure 102 shows the signal received on a loop antenna at position 2 of figure 5. Figure 103 shows the

signal taken simultaneously on a pair of roof bolts separated

by 13 meters. A fairly strong impulse is shown on the bolts,

while this impulse does not appear above the loop system

noise. These differences in antenna pick-up characteristics

should be studied in more detail

C . Surface -Underground Noise Coherence Tests

A test was made of the coherence between noise at the surface and noise underground at Bailey shaft. Direct current

power lines are associated with the shaft. Very large mine

supply currents, both underground and on the surface, are

present near the receiving antennas. As a result, the measurement is probably more one of noise coherence along a 600

foot run of dc supply cable rather than of the coherence of

atmospheric or other noise directly between surface and

underground.

A system test was performed by injecting a single broadband noise source into the two channels used for the coherence

159
9$ *

Fieure 102 ^D plot of the voltage spectrum of magnetic field

strength obtained on a loop antenna, 74 kHz to

95 kHz, Robena No. 4 mine, underground,

12:14 p.m., Dec. 5, 1972, antenna sensitive axis

vertical, no impulse is detectable. Spectral

resolution is 1000 Hz.

160

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161
test. One channel required the use of 800 ft. (245 m) of

RG-58 cable. Test results show that the cables and taperecorder flutter introduced negligible coherence degradation

over the frequency band of interest.

Figure 104 shows the results of the coherence measurements

at Bailey Shaft. The coherence is above 0.85 in the frequency

range 270 Hz to 6.6 kHz. For this frequency range and for

these conditions, variations in noise measured on the surface

would be very similar to noise variations made underground.

The coherence as defined by Benignus [15] is:

G

2 (F)

Y

2 (F) = ^—

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xx ^

J yy

where G (F) is the cross power spectrum estimate, G (F) is

X y XX

the power spectrum estimate of time series x, G (F) is the

power spectrum estimate of time series y, and F is the frequency index

D . Mine Phone Wire Measurements

1 . Vol t age

Measurements on mine phone -wire voltage relative to the

rail conductor were made in the working section at location

numbered 2 in figure 5. Figures 105 and 106 show the spectrum

obtained December 5, 1972, during second-shift operation. The

162
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1.000*000 0.3086 9.997-001

FREQUENCY, KH;

Figure 104 Spectrum of the coherence of noise received on the

surface and underground, Robena No. 4 mine,

4:00 p.m., Dec. 7, 1972. Data spectra used to generate this spectrum were 300 Hz to 40 kHz with

a spectral resolution of 62.5 Hz.

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MINE GENERATED NOISE

FREQUENCY, KHz

Figure 106 Spectrum of the voltage measured on the mine phone wire relative to the rail, Robena No. 4 mine,

underground, 100 Hz to 3 kHz, 4:30 p.m., Dec. 5, 1972. Spectral resolution is 7.81 Hz.

165

a^
logarithmic average of harmonics 3 through 8 in figure 106

is 5 dB relative to 1 V rms, i.e., 1.8 V rms . An unusual

feature of the phone line voltage spectrum is the very high

amplitude at 60 Hz as shown on figure 106.

2 . Measured Noise Current

Figures 107 and 108 show the noise current in one mine

phone wire, measured at location 2 (figure 5) on December 7,

1972. The signals in the audio region do not appear to be

360 Hz harmonics. The current appears to be very low, and the

logarithmic average in the region of 360 Hz harmonics 3 through

8 is -92.8 dB relative to one ampere rms, i.e. 23 yA rms. The

current at 88 kHz peaks at about -80 dB , i.e., 100 yA rms.

E . Trolley Wire Voltage Measurements

Figures 109 and 110 show the spectrum of the voltage

measured on the 600 volt dc trolley wire relative to the rail

in the working section at 4:59 p.m. on December 5, 1972. The

logarithmic average of harmonics 3 through 8 of 360 Hz is +10.1

dB relative to 1 volt rms, i.e., 3.2 volts rms. The amplitude

of the 88-kHz mine phone FM signal is 29 dB above a volt, i.e.,

28.2 volts rms. Variations in harmonic voltages are seen by

comparing figures 109 and 111, taken two days apart. In figure

111, logarithmic average of harmonics 3 through 8 is -10.2 dB,

i.e. 0.31 V rms. The 88-kHz mine phone carrier was measured

to be 100 volts rms.

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FREQUENCY, Mb

Figure 108 Spectrum of the current on one phone wire obtained

with a current probe, Robena No. 4 mine, underground, 100 Hz to 3 kHz, 11:42 a.m., Dec. 7, 1972.

Spectral resolution is 7.81 Hz.

168

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FREQUENCY, KHi

Figure 110 Spectrum of the voltage measured on the 600 V dc

trolley wire relative to the rail in the working

section, 100 Hz to 3 kHz, Robena No. 4 mine,

underground, 100 Hz to 3 kHz, 4:59 p.m., Dec. 5,

1972. Spectral resolution is 7.81 Hz.

170

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171
V. CONCLUSIONS

1. The amplitude of magnetic field noise as a function

of frequency is shown in the spectral plots of section II, B.

In most cases, the noise energy in this mine occurs in the

spectrum below 10 kHz and commonly is 50 to 100 dB above 1

microampere per meter in this noisy portion of the spectrum

when measured near (within about 3 meters) equipment or power

cables

2. Above 20 kHz, the magnetic noise is generally within

-20 to +30 dB with respect to one microampere per meter,

except at trolley-phone carrier frequencies. It may be considerably lower in some locations.

3. The 88 or 100 kHz trolley phones, when voice modulated, cover ± 10 kHz. In some cases, they create harmonics

up to 8 MHz.

4. Moving 30 meters away from machinery and power lines

causes a significant drop in noise, up to 40 dB.

5. The APD's show a 40 to 50 dB variation of noise with

time, with impulsive noise present up to 30 percent of the time

at frequencies below 1 MHz, while the impulsive noise is

usually present less than one percent of the time at frequencies

above 1 MHz.

6. The APD's also show that there is a general decrease

in noise as frequencies increase, ranging in this mine from

+20 dB uA/m at 10 kHz to -40 dB yA/m at 30 MHz. These are

time -averaged rms values.

172
7. Magnetic field noise measured on the surface above

a mine depends on proximity to surface noise sources, but is,

in general, less than underground noise, at least in the

winter. It varied from +10 dB yA/m at 10 kHz to -30 dB yA/m

at 130 kHz at one location about 300 meters from Blaker Shaft.

8. Surface noise may be correlated to underground noise

over a portion of the spectrum dominated by power line harmonics; at Robena Mine this is below 10 kHz. At higher frequencies, there is no correlation.

9. Trolley wire-to-rail noise voltages have spectral

contents similar to H-field results; peak amplitudes are at

360 Hz (approximately 100 volts rms) and other power line

harmonics. Transients sometimes exceed these peak values.

10. Roof-bolt voltages also have spectral contents

similar to H-field spectra; they may be more sensitive to impulsive noise in some cases than loop antennas are, but the

reverse was also observed. Further investigation is needed.

VI . RE COMMENDATIONS

Electromagnetic noise is a dominant factor in range

limited communication systems. In mine communication systems,

range -limiting factors are high attenuation, low size-weight

considerations, cost considerations, power -permissibility

limitations, and others. In addition, the in-mine environment is much noisier than most other environments, and has

173
more different noise sources of different types than most

other environments. Therefore, the character and magnitude

of this electromagnetic noise must be known in order to

be overcome.

Our principal recommendation is to use the data and tech

niques discussed in this report to study the character and

magnitude of electromagnetic noise in mine environments. It

should be recognized that there is no one interpretation or

solution

In the distant future, improved equipment design may reduce the noise.

VII. 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, and Frank Cowley.

Those making valuable but less time-consuming contributions

are Gerry Reeve, Bob Matheson, Don Spaulding, John Chukoski,

Lome Matheson, and Dave Lewis.

Winston Scott and Don Halford provided much assistance in

proofreading, while Sharon Foote and Janet Becker typed tirelessly through many versions. Jocelyn Spencer provided

drafting assistance.

174
Finally, none of this would have been possible without

the excellent cooperation of Bob Goddard, Bill Zeller,

and others at Robena No. 4 Mine of United States Steel Corporation .

175
VI11 - REFERENCES

[1] The Institute of Electrical Electrical and *n,q Electronic m

Engineers, Inc.,

IEEE Dictionarv o£ Electrirai ectllcal and an ^ Electronic n

Terms,

Std. 100, 1972.

[2] Electric Motor-Driven Mine E quipraent and Accessories

Schedule 2G, Federal Register, Vol. 23> Xo . 54, March 10

1968.

[3] Corbin, C.S., Servo f rom Tape, Honeyvell Technical Talk,

Denver (MS-211), July 25, 1971.

[4] Burnett, E.D fori -i * - p t n , .u., Lorliss, E.L.R., and Berendt, R.D., Magnetic recordingng nfot arnnctir acoustic A~+ data on audiofrequency tape,

XBS Technical Note 718, April 1972.

[5] Welch, P. D ., The use of fast pourier transform fQr the

estimation of power spectra. A method based on time

averages over short, modified, periodograms , IEEE Trans,

on Audio and Electroacous tics , Vol. AU-15, Xo. 2, June

196", pp. 70-73.

[6] Greene, P.M., XBS freld-strength standards and measurements (30 Hz to 1000 MHz), Proc. IEEE, Vol. 35, Xo. 6,

June 1967, pp. 970-981.

[7] Crichlow, K.Q., et al., Amplitude -probability distributions

for atmospheric radio noise, XBS Monograph 23 (1960b).

[8] Thompson, ».!., m> Bibliography of ground vehicle communications and control control. AKWIC innr -; j index, Report Xo. DOTTSC-UMTA-71-3, July 1971.

176
[9] Shaver, Harry V., V. Elaine Hatfield, George H. Hugh,

Man-made radio noise parameter identification task,

Final, Report - -Naval Electronics Systems Command, Standard Research Report, SRI Project 1022-2, May, 1972.

[10] Matheson, R.J., Instrumentation problems encountered

making man-made electromagnetic noise measurements for

predicting communication system performance, IEEE Trans.

on EM. compatibility , Vol. EMC-12, No. 4, November 1970,

pp. 151-155.

[11] Greene., F.M., Calibration of commercial radio field strength

meters at the National Bureau of Standards, NBS Circular

517, 1951. , . . .

:

[12] Jean, A.G. , Taggart , H.E.,,and Wait, J.R., Calibration

of loop antennas at VLF , NBS J. Res., 64_, No. 3, 1961.

[13] Greene, F.M., The near-zone magnetic field of a small

circular-loop antenna, NBS J. Res., 71C , No. 4, OctDec. 1967.

[14] C.C.I.R. Report 322, World distribution and characteristics

of atmospheric radio noise, Documents of the Xth Plenary

Assembly, I.T.U. , (1964) .

[15] Benignus , V.A. , Estimation of the coherence spectrum and

its confidence interval using the Fast Fourier Transform,

IEEE Trans, on Audio and Electroacoustics , Vol. AU-17,

No. 2, June 1969, pp. 145-150.

Errata - NBS Technical Note 654

Figures 87 through 92, substitute "surface noise" for "mine

generated noise."
Errata (Continued)

Insert above Section C, p. 96

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 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

I S( OMM • HO
IX. 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

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

MP

2 where MP = 11

NDT = Detrending option, example, (dc removed)

NZS = Restart spectral average after output, example,

(restarted)

NDA = Data segment advance increment, example, 2048

NPO = Number of spectra averaged between output calls,

e x amp 1 e , 2

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

177
DA(2) = Detrending filter average, K=2 , example,

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

NSA = Number of periodograms averaged, example, 2 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, DAY, MONTH, YEAR Gain corr. , rec. =

tot. constr. =, where

Run and Session = the title of the portrayed frame identifying

the digitizing session and run number,

example , 33 5 5

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

e x amp 1 e , 5 12 7 3

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

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

example , -30.5.

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

Top of Scale = largest scale marking for computer drawn

graph, example, 1.000-005 (1.0 x 10

_5

) Standard Error = standard error of curve, example, 0.3162

Spectral Peak = largest spectral peak observed, example,

7.747-006. (7.747 x 10~ 6

)

178
The preceding coding was used extensively in the Robena

report, but the format has recently been changed, and a few

examples of the new format will be found in this report. In

the new format, the old fourth line is now the new fifth line

and the new fourth line has the following format:

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

C = correction curve used with data, example, 34

RG = receiver gain and accompanying correction in dB added to

the data, example, 10 (20 dB)

DG = digitizer gain, example,

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

digit, example, -1

AG = absolute gain correction added to data, example,

179
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLK ATION OR REPORT NO. NBS Tech Note 654

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITL1

Electromagnetic Noise in Robena No. 4 Coal Mine

5. Publication Date April 1974

6. Performing Organization Code

7. AUTHORlS)

W. D. Bensema, Motohisa Kanda, J. W. Adams

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAMF AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10. Project/Task/Work Unit No. 2728412

11. Contract/Grant No. H0133005

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

7/72-12/73

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary ol 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 coal mine, Robena No. 4, located near Waynesburg,

Pennsylvania. One technique measures noise over the entire electromagnetic spectrum of

interest for brief time periods. With present instrumentation, the spectrum can be

covered from 40 Hz to 400 kHz. It is recorded using broad-band analog magnetic tape,

and the noise data are later transformed to give spectral plots. The other technique

records noise envelopes 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. The general results are that at frequencies below 10 kH

power line noise within the mine is severe. Impulsive noise is severe near arcing

trolleys, and at lower frequencies near any transmission line. Carrier trolley phone

signals and harmonics are strong throughout the mine whenever the trolley phone is in

operation.

Additional information beyond that included in this report may be obtained from the

authors, who are with the Electromagnetics Division of the National Bureau of Standards.

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; emergency communications;

Fast Fourier Transform; Gaussian distribution; impulsive noise; magnetic field strength

measurement instrumentation; spectral density; time-dependent spectral density.

18. AVAILABILITY [X Unlimited

_ For Official Distribution. Do Not Release to NT1S

Order brum Sup. of Doc, U.S. Government Printing Office Washington, D.< . 20402, SI) Cat. No. ( M. 4 6:654

Order From National Technical Information Service (NTIS)

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