NBSIR 74-390

ELECTROMAGNETIC NOISE IN ITMANN MINE

W. D. Bensema

M. Kanda

J. W. Adams

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

June 1974

Prepared for

U. S. Bureau of Mines

United States Department of the Interior

Pittsburgh, Pennsylvania 15222

Working Fund Agreement HO 133005

NBSIR 74-390

ELECTROMAGNETIC NOISE IN ITMANN MINE

W. D. Bensema

M. Kanda

J. W. Adams

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

The views and conclusions contained in

this document should not be interpreted

as necessarily representing the official

policies or recommendations of the Interior

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

Government

June 1974

Prepared for

U. S. Bureau of Mines

United States Department of the Interior

Pittsburgh, Pennsylvania 15222

Working Fund Agreement HO 133005

U S DEPARTMENT OF COMMERCE, Frederick B Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director
FOREWORD

This report was prepared by the National Bureau of Standards,

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

contract was initiated under the Coal Mine Health and Safety

Research Program. It was administered under the technical

direction of the Pittsburgh Mining and Safety Research Center

with Mr. Hov/ard Parkinson and Mr. Harry Dobroski acting as the

technical project officers.

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

this contract during the period January 1973 to June 1974.

This report was submitted by the authors in October 1974.

iii
CONTENTS

Page

1. INTRODUCTION 1

1.1 Background 2 1.2 Mine Description 3

2. ?1EASlJRE?'fENT INSTRWffiNTATION- -

- 5

3. SPECTRUM MEASUREMENT RESULTS 10

3.1 Introduction and Uncertainties 10

3.2 Antenna Sites 10

3.3 Electromagnetic (EM) Noise Spectrum Results 11

3.3.1 Interpretation 11

3.3.2 Cabin Creek Panel 11

3.3.3 Farley Panel 13

3.4 Miscellaneous Measurements 14

3.4.1 Electric Field 14

3.4.2 Measurements o£ Voltage Between Roof

Bolts 15

3.5 Intercomparison of Different Mines 16

3.5.1 Roof Bolt Voltage Intercom.parison 16

3.5.2 Summary of 1 kHz to 3 kHz Data 17

3.5.3 Intercomparison of 3 kHz to 200 kHz

Data 18

4. AMPLITUDE PROBABILITY DISTRIBUTION MEASUREMENTS 47

4.1 Introduction and Uncertainties 47

4.2 Results 48

4.2.1 Introduction 48

4.2.2 Measurement Results 49

4.2.3 RMS and Average Values 51

5. CONCLUSIONS 9 8

6. RECOMMENDATIONS 9 9

7. ACKNOWLEDGMENTS 100

8. REFERENCES 101

9. APPENDIX 102

iv
LIST OF FIGURES

Page

Figure 2-1. Block diagram o£ portable instrumentation- 7 Figure 2-2. System for field recording data to obtain

amplitude probability distributions 8

Figure 2-3. Block diagram of laboratory recording

system modified for field use 9

Figure 3-1. Map of Itmann Mine where measurements were

made, Cabin Creek panel 19

Figure 3-2. Spectrum with mine in operation, measurements made 10 meters from end of longwall

face. Cabin Creek panel. Spectral resolution of 78.1 Hz 20

Figure 3-3. Spectrum with mine in operation, measurements made 10 meters from end of longwall

face. Cabin Creek panel. Spectral resolution of 3.91 Hz 21

Figure 3-4. Spectrum with mine in operation, measurements made 5 meters from end of longwall

face, Cabin Creek panel. Spectral resolution of 78.1 Hz 22

Figure 3-5. Spectrum with mine in operation, measurements made 5 meters from end of longwall

face, Cabin Creek panel. Spectral resolution of 3.91 Hz 23

Figure 3-6. Spectrum with second shift in full operation, measurements made 5 meters from end

of longwall face. Cabin Creek panel.

Spectral resolution of 3.91 Hz- 24

Figure 3-7. Spectrum of quiet time between shifts,

measurements made 5 meters from end of

longwall face. Cabin Creek panel.

Spectral resolution of 78.1 Hz 25

Figure 3-8. Spectrum of quiet time between shifts,

measurements made 5 meters from end of

longwall face. Cabin Creek panel.

Spectral resolution of 3.91 Hz 26

V
LIST OF FIGURES (continued)

Spectrum with mine in operation, measurements made 10 meters from end of longwall

face, Farley panel. Spectral resolution

of 7 8.1 Hz

Spectrum with mine in operation, measurements made 10 meters from end of longwall

face, Farley panel. Spectral resolution

of 3 .91 Hz

Spectrum with- hydraulic pump operating,

measurements made 10 meters from end of

longwall face, Farley panel. Spectral

resolution of 78.1 Hz

Spectrum with hydraulic pump operating,

measurements made 10 meters from end of

longwall face, Farley panel. Spectral

resolution of 3.91 Hz

Spectrum with conveyor belt starting up,

measurem.ents made 10 meters from end of

longwall face, Farley panel. Spectral

resolution of 78.1 Hz

Spectrum with conveyor belt starting up,

measurements made 10 meters from end of

longwall face, Farley panel. Spectral

resolution of 3.91 Hz

Vertical component of magnetic field

spectrum about 3 meters from longwall

machine

Vertical component of magnetic field

spectrum at end of haulageway in Farley

panel

Vertical component of magnetic field

spectrum at end of haulageway in Farley

pane 1

Horizontal component of magnetic field

spectrum at end of haulageway in Farley

panel

vi
LIST OF FIGURES (continued)

Figure 3 - 19

Figure 3 - 20

Figure 3-21.

Figure 3 - 22

Figure 3-23.

Figure 3-24.

Figure 3 - 25

Figure 3-26.

Figure 3-27.

Figure 3-28.

Figure 3 -29

Figure 4-1.

Figure 4-2.

Page

Vertical component of magnetic field

spectrum at end of haulageway in Farley

panel 37

Vertical component of magnetic field

spectrum at end of haulageway in Farley

panel 38

Horizontal component of magnetic field

spectrum at end of haulageway in Farley

panel 39

Spectrum obtained with an active dipole,

0.75 meters from phone line. Spectral

resolution of 78.1 Hz 40

Voltage spectrum obtained between roof

bolts located 15.2 meters apart, Farley

panel. Spectral resolution of 78.1 Hz 41

Voltage spectrum obtained between roof

bolts located 15.2 meters apart, Farley

panel. Spectral resolution of 3.91 Hz 42

Summary of rms noise voltage measured

underground between roof -support -bolts

in four mines- 16

Comparison of logarithmic average of

magnetic field strengths of three mines

at six frequencies 43

Logarithmic average magnetic field strength

summary

Comparison of EM noise levels near operating machinery from four mines 45

Comparison of EM noise levels along

haulageways in four mines

APD , 10 kHz, vertical component. Cabin

Creek 53

APD, 30 kHz, vertical component, Cabin

Creek 54

vii
Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

4-3.

4-4.

4-5.

4-6 .

4-7.

4-8.

4-9 .

4-10

4-11

Figure 4-12

Figure 4-13

Figure 4-14

Figure 4-15

Figure 4-16

Figure 4-17

Figure 4-18

Figure 4-19

LIST OF FIGURES (continued)

Page

APD, 70 kHz, vertical component, Cabin

Creek 55

APD, 130 kHz, vertical component, Cabin

Creek 56

APD, 160 kHz, vertical component. Cabin

Creek 57

APD, 250 kHz, vertical component. Cabin

Creek 58

APD, 500 kHz, vertical component, Cabin

Creek 59

APD, 1 MHz, vertical component. Cabin Creek 60

APD, 2 MHz, vertical component, Cabin Creek 61

APD, 6 MHz, vertical component, Cabin Creek 62

APD, 14 MHz, vertical component. Cabin

Creek 63

APD, 32 MHz, vertical component. Cabin

Creek 64

APD, 10 kHz, vertical component, quiet

time, Cabin Creek 65

APD, 30 kHz, vertical component, quiet

time. Cabin Creek 66

APD, 130 kHz, vertical component, quiet

time, Cabin Creek 67

APD, 500 kHz, vertical component, quiet

time. Cabin Creek 68

APD, 1 ^4Hz, vertical component, quiet

time, Cabin Creek 69

APD, 2 MHz, vertical component, quiet

time. Cabin Creek '70

APD, 6 ]^4Hz, vertical component, quiet

time. Cabin Creek '^^

viii
LIST OF FIGURES (continued)

Page

Figure 4-20. APD , 10 kHz, roof bolts 72

Figure 4-21. APD, 10 kHz, vertical component, Farley

panel 7 3

Figure 4-22. APD, 30 kHz, vertical component, Farley

panel 74

Figure 4-23. APD, 70 kHz, vertical component, Farley

panel 75

Figure 4-24. APD, 130 kHz, vertical component, Farley

panel 76

Figure 4-25. APD, 250 kHz, vertical component, Farley

panel 77

Figure 4-26. APD, 500 kHz, vertical component, Farley

panel 78

Figure 4-27. APD, 1 MHz, vertical component, Farley

panel 79

Figure 4-28. APD, 2 MHz, vertical component, Farley

panel 80

Figure 4-29. APD, 10 kHz, vertical component, Farley

headpiece at the end of main belt haulage- 81

Figure 4-30. APD, 30 kHz, vertical component, Farley

headpiece 82

Figure 4-31. APD, 70 kHz, vertical component, Farley

headpiece 83

Figure 4-32. APD, 130 kHz, vertical component, Farley

headpiece 84

Figure 4-33. APD, 160 kHz, vertical component, Farley

headpiece 85

Figure 4-34. APD, 250 kHz, vertical component, Farley

headpiece 86

Figure 4-35. APD, 500 kHz, vertical component, Farley

headpiece

ix
Figure 4-36

Figure 4-37

Figure 4-38

Figure 4-39

Figure 4-40

Figure 4-41

Figure 4-42

Figure 4-43

Figure 4-44

Figure 4-45

LIST OF FIGURES (continued)

Page

APD , 1 MHz, vertical component, Farley

headpiece 88

APD, 2 I^lHz, vertical component, Farley

headpiece 89

APD, 6 MHz, vertical component, Farley

headpiece 90

APD, 14 MHz, vertical component, Farley

headpiece 91

APD, 32 MHz, vertical component, Farley

headpiece 92

APD, 10 kHz, roof bolts 93

Field strength excursions between 0.001%

and 99% o£ the time as a function of

frequency, vertical component, Cabin Creek 94

Field strength excursions between 0.001%

and 99% of the time as a function of frequency, vertical component, quiet time,

Cabin Creek 95

Field strength excursions between 0.001%

and 99% of the time as a function of frequency, vertical component, Farley panel,

April' 17 , 1973 96

Field strength excursions between 0.001%

and 99% of the time as a function of frequency, vertical component, Farley headpiece, April 19 , 1973 97

X

ELECTROMAGNETIC NOISE IN ITMANN MINE

Two different techniques were used to make

measurements of the absolute value of electromagnetic noise in an operating coal mine, Itmann No. 3

Mine, located near Mullens, West Virginia. The

electromagnetic environment created by 250-volt-dc

and 550- and 950-volt-ac machinery in two longwall

panels was measured and is reported. One technique

measures noise over the entire electromagnetic

spectrum of interest for brief time periods. It

is recorded using broadband analog magnetic tape,

and the noise data is later transformed to give

spectral plots . The other technique records noise

amplitudes at several discrete frequencies for a sufficient amount of time to provide amplitude

probability distributions.

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

distribution plots .

Key words: Amplitude probability distribution; coal

mine noise; digital data; electromagnetic communications; electromagnetic interference; electromagnetic

noise; Fast Fourier Transform; Gaussian distribution;

impulsive noise; magnetic field strength; measurement

instrumentation; spectral density; time-dependent

spectral density.

1. INTRODUCTION

This report gives data concerning electromagnetic noise

in a coal mine that uses dc rail haulage power and ac-powered

conveyor belt and longwall equipment. In this section, background information and a brief mine description are covered.

In Section 2, measurement instrumentation is discussed.

In Section 3, spectral plots of data are presented. In Section

4, amplitude probability distributions (APD) of magnet ic - field

noise are given.
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. With specific permission of the

Bureau of Mines, we will supply the additional data. A more

complete description of the measurement systems used is given

in the Robena Mine report [1] and the Grace Mine report [2].

1.1 Background

The need for reliable communication systems in mines is

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

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

problem. However, if a com.mun icat ion 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 operating conditions.

During operation, the machinery used in mines creates a wide range of many types of intense electromagnetic interference (EMI). This EMI is a major limiting factor in the

design of a communication system. The work reported here

gives the results of comprehensive measurements of this EMI

in critical communication locations where miners extract coal

as well as along haulageways.

There are several EMI parameters that can be measured:

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

One parameter was emphasized, magnetic field strength. There

are several reasons. First, electric field sensors are very

insensitive at lower frequencies, and hence probably will not

2
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

sense currents. Fourth, power line 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.

Although magnetic field strength measurements are emphasized here, even this one parameter is difficult to measure

meaningfully. The IEEE definition [3] of magnetic field

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

in this report. The resultant magnetic field strength noise

vector is a function of frequency, time, orientation, and

location. Small variations in these parameters can cause

several orders of magnitude difference in measured field

s trength

1 . 2 Mine Description

The results and data presented in this report are based

on measurements made on April 17 and 19, 1973, in the Itmann

No. 3 Coal Mine located near Mullens, southwest of Beckley,

West Virginia. The mine belongs to Consolidation Coal Company.

The coal in this mine occurs in a narrow seam, approximately

one meter thick, and is called "low-coal." The measurements

were made in the two sections of the mine using the longwall

mining technique. These sections were called Cabin Creek panel

and Farley panel and were separated by about 1500 meters at

the time of measurement. Overburden at Cabin Creek was

3
290 meters and at Farley was 440 meters. The mine has seven

conventional, room- and -pi 11 ar sections; measurements were

restricted to the longwall panels.

The mine uses 250 V dc trolley haulage to carry coal out

of the mine, and ac powered conveyor belt haulage from the

section to the trolley. All of the section longwall mining

equipment is ac powered, with the exception of a dc-powered

cable winch which was used occasionally to advance portions

of the longwall equipment. The face and associated longwall

equipment were 145 meters long. Each of the separate components

of the longwall mining equipment was made by a different

manufacturer, typically in England or Scotland. There were a total of six electric motors in the section ranging from 15 to

300 HP. The shear and face conveyor used 950 volts and the

stage loader and hydraulic pumps used 550 volts. The stepdown transformer supplying these voltages is kept 45 to 230

meters back from the face, and is supplied with 13,200 volts.

Vertical clearance for personnel passage in Cabin Creek

panel v/as limited to the 1.0 meter left by the extracted coal.

In Farley panel, clearance was increased to 1.2 meters by the

removal of some shale in the personnel passageways.

Acoustical noise at the longwall face may be high enough

to seriously degrade voice communication systems.

4
2. MEASURE^4ENT INSTRUMENTATION

Two measurement techniques were used. In the first

technique used a large portion of the spectrum is covered as

a "snapshot" at one relatively short period of time. In

three-dimensional form, several such "snapshots" can show how

drastically a signal varies, not only with frequency, but

also with time. In the second technique, variations over a 20 -minute time interval are measured at one frequency.

Usually, a set of twelve different frequencies was used.

Both techniques were used to measure two orthogonal components of magnetic field strength. This was done either by

using two systems simultaneously or by varying the orientation

of one system; both techniques were used in as many different

locations as possible.

With the exception of the electric dipole spectral plots

and the roof-bolt APD ' s , all measured noise is reported in

absolute quantities (instead of relative) to allow others to

make effective use of the data. For the magnetic field strength

measurements, the NBS field calibration site [4] was used with

each complete measurement system to assure correct calibration.

A complication to making these measurements is the mine

environment, which is generally humid, dusty, poorly lighted,

and potentially explosive. In Itmann Mine, the restriction

caused by the one meter clearance in the passageways caused

a significant reduction in the total number of locations where

measurements could be made. Battery-operated, dust-protected,

permissible gear was used for most of our portable measuring

equipment.

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

and hence two different magnetic field strength parameters are

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

component of magnetic field strength, H, versus frequency for

5
discrete frequencies, or as one component of magnetic-f ieldstrength spectrum density level [3] , H^, versus frequency for

broadband noise in the spectral plots. In the amplitude

probability distributions, results are given as the rms value

of one component of magnetic field strength versus percent of

time this value is exceeded. The APD gives the distribution

of the actual instantaneous values only as far as the

measurement -sys tem detector bandwidth will allow the detector

to follow the time variations of the actual magnetic field.

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

the results are applicable for a communication receiver whose

bandwidth is similar to the measurement -system detector

bandwidth

Three measurement systems were used to make measurements

underground. The three systems are described in the block

diagrams shown in figures 2-1, 2-2, and 2-3. For a detailed

description of these systems, see two previous reports [1,2].

There are some differences in the systems used in Itmann Mine

from the systems used in Robena #4. The systems used in

Itmann are the same ones used in Grace.

The first system measures data for spectral plots and

is fully permissible and portable. The second system measures data for statistical descriptions of time variations,

most commonly amplitude probability distribution; it is also

permissible. The third system is not permissible but is

portable (with considerable effort) ; it records data for both

spectral plots and statistical presentations.

6
SIGNAL MONITORS

LOUD

SPEAKER

SYSTEM

#1

LOOP

ANTENNA BROADBAND

AMPLIFIER

OSCILLOSCOPE

IMPEDANCE

TRANSFORMER

10 METERS

OF CABLE

lOO-kHz

LOW PASS

FILTER

SYSTEM

#2

SYSTEM

#3

200WZ CRYSTAL

OSCILLATOR

PORTABLE

TAPE RECORDER

(30 ips, record)

TRACK I (FM)

TRACK 2 (DIR)

TRACK 3 (FM)

TRACK 4 (DIR)

TRACK 5 (FM)

TRACK 6 (DIR)

SHORT

TRACK 7 (FM)

MICROPHONE EDGE TRACK

(VOICE)

Figure 2-1 Block diagram of portable instrumentation. FM

tracks are used to record from 40 Hz to 100 kHz

direct tracks are used from 3 kHz to 320 kHz.

Systems 2 and 3 are identical to system 1. When

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

filters are eliminated, and the amplifier bandwidth is increased from 100 kHz to 300 kHz. The

microphone is used for occasional vocal comments

by the operator.

7

11-74 -29151
COLLAPSIBLE

ANTENNA

LOOP

ANTENNA

f =1 0kHz -250kH z

455kHz

FIELD

STRENGTH

METER A

BW = 1 .4kHz

MIXER

40kHz

25kHz

OSC

FIELD

STRENGTH o METER

BW = 3 .3kHz

455kH2^ MIXER

4 0 kHz.

f =1 50kHz-32MHz

CHANNEL 1

CHANNEL 2

CHANNEL 3

PORTABLE

TAPE

RECORDER

DIRECT

RECORDING

15 IPS

f =100Hz-50kHz

SCOPE

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

probability distributions.

8

IB-74 -I9lb]
COLLAPSIBLE

LOOP

ANTENNA

FIELD

STRENGTH

METER

IOkHz-250kHz|

455kHz MIXER 40 kHz

10 kHz

HIGH PASS

FILTER

BALUN

LOOP

ANTENNA

(SENSITIVE AXIS VERTICAL)

COLLAPSIBLE

LOOP

ANTENNA

FIELD

STRENGTH

METER

l50kHz-32MHz

455kHz

LOOP

ANTENNA

(SENSITIVE AXIS VERTICAL)

-O25 kHz

OSCILLATOR

EQUIPMENT

SIMILAR TO 3, O—

BUT WITH HORIZONTAL

ANTENNA ORIENTATION

MIXER 40 kHz

20 kHz

LOW PASS

FILTER

RECORDER

CHANNEL I (FM)

CHANNEL 2 (DIR)

CHANNEL 3 (DIR)

CHANNEL 4 (DIR)

CHANNEL 5 (DIR)

CHANNEL 6 (DIR)

CHANNEL 7 (FM)

30ips

Figure 2-3 Block diagram of laboratory recording system modified

for field use.

9
3. SPECTRUM MEASURFJfENT RESULTS

3.1 Introduction and Uncertainties

In this section of the report, spectrum plots are

presented and discussed. Most of these plots present magnetic

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

different locations and results can be used to characterize

electromagnetic noise levels generated by most fixed and

mobile equipment used in this mine. Some plots give data

to 3 kHz, 20 kHz, and 2 30 kHz.

The spectra to 100 kHz, to 3 kHz, and to 20 kHz have

uncertainties of ± 1 dB over the following portions of the

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

kHz or 10 kHz to 100 kHz as stated or shown. The 3 kHz spectra

have the ± 1 dB uncertainty from 100 Hz to 3 kHz. Between 40 Hz

and 100 Hz the uncertainty is ± 6 dB. The 20 kHz spectra

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

The spectra shown to 2 30 kHz have an uncertainty of ± 2 dB

from 3 kHz to 180 kHz.

3 . 2 Antenna Sites

Figure 3-1 is a map of Cabin Creek panel as it was on

April 17, 1973, the first day measurements were made. The

shear would operate back and forth across the face, as the

mining operation advanced. The recording equipment was set up

in the dinner man hole, and the antennas were deployed in the

locations labeled A and B.

On the second day measurements were made in Farley panel,

which has similar equipment but was arranged in the mirror

image of Cabin Creek panel. The layout can be determined by

looking at figure 3-1 through the back side of the drawing.

No separate map is given for Farley panel.

10
3 . 3 Electromagnetic Noise Spectrum Results

3.3.1 Interpretation

When reading values from the spectra in this report,

keep the following points in mind:

1. Field strength values above the upper roll -off

frequency and below the lower roll -off frequency are

not calibrated and are therefore not shown on the spectra.

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

3. The broadband noise between spectral peaks is as

seen by a receiver having the same bandwidth as the Fast

Fourier Transform (FFT) spectral resolution bandwidth

used to compute the spectrum. The correct units for the

background noise between peaks are microamperes per meter

per square root x hertz [(yA/m)//x Hz], where x is the

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

1-to-lOO-kHz graphs) . An easy way to obtain the spectral density per (one)

root hertz for broadband noise is to subtract the required

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

(yA/m) //Hz , For spectra with a resolution bandwidth of 78.1

Hz, subtract 10 log-|^Q (78.1) or 18.93 dB.

The Appendix gives the code key used in determining the

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

each spectrum. The resolution bandwidth is also given on the

ordinate of the plots .

3.3.2 Cabin Creek Panel

Figure 3-2 upper curve, shows the magnetic field noise

spectrum from 1 kHz to 100 kHz received at the antenna location identified as A (in figure 3-1). The antenna sensitive

11
axis was vertical. Location A is near the dinner man hole,

10 meters from the end of the face, and is a busy area in the

section. The lower curve in this, and in following figures,

is the receiving system noise. It is included to indicate

frequency ranges in which system noise may predominate. The

lower curve is obtained by replacing the loop antenna with a dummy antenna. This figure shows data from 1 kHz to 100 kHz

from one system and from 10 kHz to 100 kHz from another system

that had lower system noise. An obvious feature in figure 3-2

is a signal at 17.8 kHz. This is from the U.S. Navy transmitter NAA located in Cutler, Maine. With the exception of one

shallow location in a hardrock mine, this is the only underground mine location where reception of a distant VLF station

occurred. This strong signal from NAA in a coal mine under

this large overburden (290 meters) is very unusual. The minegenerated noise shown on figure 3-2 extends to 100 kHz at the

-20 dB relative to 1 yA/m level. Between 60 and 75 kHz, there

is a dip in amplitude of 8 dB or more. Figure 3-3 shows the

expanded spectrum at location A, ten meters from the end of

the longwall face, with the mine operating. Figure 3-3 shows

a noise spectrum dominated by powerline harmonics of 60 Hz.

Figure 3-4 was measured at location B, five meters from

the end of the longwall face, with the mine in operation.

The noise spectrum at location B, with the antenna axis vertical, has about the same shape as the spectrum at location A,

but is 4 to 10 dB higher. The spectra recorded at B, as at

location A, shows a rise in noise level from about 80 kHz to

100 kHz. Figure 3-5 shows the expanded spectrum at location

B. The spectrum taken with the antenna sensitive axis horizontal gives 4 to 10 dB less noise than with the sensitive

axis vertical. Figure 3-6 shows the expanded spectrum at location B later, during the second shift, with the mine in full

operation. Figure 3-6 is included because of the high level

(108 dB relative to 1 yA/m) of 60 Hz noise.

12
Figure 3-7 shows the noise spectrum measured at Cabin

Creek panel during the quiet time between shifts at location

B. This spectrum is about 10 to 20 dB quieter than when the

mine is in full operation. Figure 3-8 shows the expanded

spectrum measured at location B between shifts. The 60 Hz

noise here is 46 dB lower than when the mine was in full

operation during the second shift.

3.3.3 Farley Panel

Figure 3-9 shows the spectrum measured ten meters from

the end of the longwall face in Farley panel. Farley panel

was the mirror image of Cabin Creek. The measurement location for figure 3-9 corresponds to location A on figure 3-1.

While the noise in Farley between 10 kHz and 20 kHz is sometimes as much as 10 dB higher than in Cabin Creek, generally,

the noise in Farley may be considered lower than the noise in

Cabin Creek panel. Neither the signal from NAA nor the increase in noise between 80 and 100 kHz show up in Farley.

Figure 3-10 shows the expanded spectrum at location A, Farley

panel, with the mine in operation. In Farley, the 60 Hz noise

is 24 dB lower, the 360 Hz noise is the same amplitude, and

most other harmonics are lower by as much as 15 dB , than at

Cabin Creek.

Figure 3-11 shows the spectrum measured at Farley panel,

ten meters from the end of the longwall miner, during the quiet

time at lunch hour. Only the hydraulic pump, associated with

the 134 roof jacks, was running. This pump was left running

much of the time. The spectrum is essentially the same as shown in figure 3-9 when the mine was in operation. Figure

3-12 shows the expanded spectrum at Farley during lunch hour.

The noise at 60 Hz is 4 dB lower during lunch hour. With some

minor variation, the remainder of the expanded spectrum is

about the same during lunch hour as during mine operation.

13
Figures 3-13 and 3-14 show the spectrum recorded in

Farley panel, ten meters from the end of the longwall face.

The ac-powered conveyor belt was just starting up. Evident

on figure 3-13 is powerline related noise out to 100 kHz,

occurring with broad peaks about every 20 kHz. This noise

signature is probably associated with an ac motor starting,

and is probably of correspondingly short duration. Figure

3-14 shows 7 dB higher 60 Hz noise than with the mine in

operation (figure 3-10). Induction motors typically require

about 2.5 times (8 dB) more current to start than to run.

Probably the increased 60 Hz noise comes from the increased

starting current being drawn by the starting of the conveyorbelt motor.

Data taken with the third system in Farley Panel are

shown in the following figures. Figure 3-15 shows a spectrum

from 0.8 to 20 kHz taken about three meters from an operating

shear on the longwall machine. Figure 3-16 shows typical

noise at the end of the rail haulage, about 200 meters from

the longwall face. Figures 3-17 and 3-18 show two components

of noise generated by an unidentified piece of machinery.

Figure 3-19 shows an expanded (to 20 kHz) spectra of figure

3-17. There were strong cw signals at frequencies higher than

our systems were normally operated; however, two illustrations

are shown at frequencies higher than the system's calibration

is valid. One signal shown in figures 3-20 and 3-21 is

at 202.5 kHz; it may be a control signal specific to this

mine. It did generate higher harmonics that were detected

with other equipment.

3.4 Miscellaneous Measurements

3.4.1 Electric Field

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

on the second day while in Farley panel. This dipole has not

14
been calibrated, nor have any system nonlinear i ties in gain

vs. frequency been removed by a correction curve. Therefore,

all field strength information is only qualitative. Figure

3-22 shows unprocessed data results obtained with the dipole

ends pointed parallel with the drift and 0.75 meters from the

phone line. Measurements taken with the dipole pointed across

(perpendicular to) the drift gave fields that were 10 to 20 dB

stronger than those shown in figure 3-22. Fields were 20 to

30 dB weaker when the dipole was 4 meters from the phone line.

This indicates the noise was coming from the phone line. The

electric field data shown in figure 3-22 was taken simultaneously with the magnetic field data in two previous figures,

3-13 and 3-14. The mine conveyor belt was just starting up.

There is some similarity in the general shape of the two 1 kHz

to 100 kHz spectra shown in figures 3-13 and 3-22.

Because of the relative nature of the electric field

measurements, the principal conclusions that can be drawn

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

Farley panel; (2) it is primarily powerline related, similar

to the magnetic field noise; (3) the electric noise field is

maximum for horizontal dipole orientation across (perpendicular to) the drift; and (4) the source of the electric

field noise appears to be the phone line.

3.4.2 Measurements of Voltage Between Roof Bolts

A single roof -support -bolt measurement was performed at

Farley panel on the second day. The separation between the two

bolts was 15.2 meters, and the voltage was measured using nonshielded copper wire clipped to the bolts. Figures 3-23

and 3-24 show the resulting spectra. No receiver system noise

curves are available for these spectra. It is not possible to

say that the voltage measured between bolts was induced by any

15
single mechanism. It may be any combination o£ electric field

and magnetic field acting on the copper wires connected to the

bolts, as well as by any potential produced by current flow

between the bolts

3.5 Intercomparison of Different Mines

3.5.1 Roof Bolt Voltage Intercomparison

Figure 3-25 shows a summary table of the rms noise voltage

measured between roof bolts of various spacings , in four mines.

Logarithmic

Frequency Roof Bolt Voltage Average

Mine (Hz) dB Relative to 1 Vrms Voltage

Itmann 1080 -46

(coal) 2160 . -60 -53

McE Iroy 10 80 - -62

(coal

)

2160 -6 7 -64.5

Grace 10 80 -68

(hardrock) 2160 -68 -68

Robena 1080 -80

(coal) 2160 . -81.5

Figure 3-25. Summary of the rms noise voltage measured

underground between roof -support -bolt s in four mines.

The logarithmic average is the arithmetic average of the dB

values. Two frequencies within the voice baseband were chosen

for comparison: 1080 Hz, the third harmonic of 360 Hz, and

2160 Hz, the sixth harmonic of 360 Hz. The results show that

the logarithmic averages for these two frequencies give the

highest voltage (-53 dB with respect to 1 V rms) for Itmann,

and the lowest value for Robena (-81.5 dB with respect to

1 V rms) . Possibly a factor contributing to the low roof

bolt voltage in Robena, is the fact that the bolts were on a

16
line perpendicular to the haulageway. In Itmann, the bolts

were on a line along the haulageway. In McElroy and Grace

this distinction could not be made.

3.5.2 Summary of 1 kHz to 5 kHz Data

Figure 3-26 is a summary of magnetic field strength at

powerline harmonic frequencies observed within Itmann (acpowered longwall), Robena (all 600 v dc) and the Grace (dieselpowered haulage) mines. Plotted are the dB averages of six

powerline harmonics. This logarithmic average masks the

highest single value. Average fields at five locations in

Itmann are plotted as a function of distance from the nearest

current -carrying cable or wire. The six frequencies chosen

are between 1020 Hz and 2940 Hz and are either the third

through eighth harmonic of 360 Hz, or the' highest adjacent

powerline harmonic.

The logarithmic average of noise from 1 kHz to 3 kHz in

Cabin Creek panel of Itmann was 40 dB relative to 1 yA/m with

the mine in operation. The noise was 7 dB less between shifts.

The noise in Farley panel was 28 dB (as measured with the

third system, first day) and 29 dB (as measured with the first

system, second day) relative to 1 yA/m during operation, and

was also 29 dB during lunch hour. The noise in Farley was

5 dB higher during the time the conveyor belt was starting up

than while running, and 8 dB higher measured 70 meters back

from the face, near the end of the dc-powered rail haulage.

This last finding is consistent with the observation that, in

general, dc machinery is noisier than ac machinery.

Figure 3-27 is a summary of magnetic field strength over

the same frequency region as measured in the McElroy Coal Mine

[7] (ac-powered continuous miner and head loader, dc-powered

shuttle cars and trolley)

17
A comparison of the magnetic noise from 1 kHz to 3 kHz

in the four mines shows

1. Robena is noisiest, McElroy is next, followed byGrace and Itmann, which are about equal.

a. The highest logarithmic average of section noise

measured one meter from a cable in McElroy is 15 dB

relative to 1 yA/m (at the power center) . The noise

is 16 dB higher at the "air split" on the haulageway

in Robena, and 21 dB higher at the car pull in

operation in Robena than the 15 dB value in McElroy.

b. The highest logarithmic average of haulageway

noise measured 2 meters from a cable in McElroy is

7 dB stronger than the noise measured 2 meters from

a load-haul -dump in Grace. The Grace noise is equal

to the noise measured in Cabin Creek panel in Itmann.

c. For a given constant distance from the currentcarrying cable in each of the four mines measured,

the maximum logarithmic average noise between 1 kHz

and 3 kHz varies over a rather small range. For

example, at the 2 meter distance from the cable (or

noise source) , the highest noise was 48 dB (in McElroy)

,

and the lowest was 40 dB (in Grace and Itmann)

The range of values is only 8 dB. Part of the

reason for this low spread may be due to the parameter

chosen for measurement, the logarithmic average.

This average masks the peak value, which is dominant

in causing interference to a broadband system.

3.5.3 Intercomparison of 3 kHz to 200 kHz Data

Comparative spectra from 3 kHz to 200 kHz from four mines

are shown in figures 3-28 and 3-29. The Robena curve is only

calibrated to 100 kHz.

18
roof caved area (gob)

/ / / /

ooooooo o o^^^

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main

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belt haulage

trolley

tracks

main belt headpiece

approximately 1000 meters

distant .

Figure 3-1 Map of Itmann Coal Mine, Cabin Creek panel long-wall

mining section. Farley panel is a minor image of

this section.

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46
4. AMPLITUDE PROBABILITY DISTRIBUTION MEASUREMENTS

4 . 1 Introduction and Uncertainties

Statistical representations are required since the variations of field strength are, in general, random. The amplitude

probability distribution (APD) of the received noise envelope

is one of the most useful statistical descriptions of the

noise process for the design and evaluation of a telecommunications system operating in a noisy environment [5,6,8].

By plotting the cumulative APD on Rayleigh graph paper,

one can show clearly the fraction of time that noise envelope

exceeds various levels. Rayleigh graph paper is chosen with

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

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

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

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

noise and the receiver bandwidth.

With the exception of the roof -support bolt measurements,

all APD measurements are reported in absolute quantities.

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

to the overall accuracy of our measurements are listed below:

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

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

One-decibel step attenuators are used to achieve the

± one decibel.

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

data processing, has a calibration uncertainty of

± 0.5 dB [4]

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

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

dropout, cross-talk, etc.

47
4. The gain instability during measurements, gain

changes between measurements and calibration, and the

non-linearity o£ electromagnetic interference and field

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

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

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

uncert ainty

6. Connector losses and BNC cable losses, particularly

at higher frequencies above 100 kHz, contribute ± 2.0 dB

uncertainty.

Some additional uncertainty beyond the stated measurement

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

was taken to provide at least one meter separation from

metallic objects wherever possible. However, coal, rock, or

earth was sometimes immediately adjacent to a loop antenna.

In all observed cases, this had no effect at frequencies up

to 1 MHz. Above 1 MHz, earth and other reflections did in

som.e cases cause ± 1 dB variations, even with a shielded,

balanced loop antenna. An estimate is that an additional

± 5 dB uncertainty might be advisable. However, due to the

complexity of the shielded loop in the mine environment, this

uncertainty cannot be rigorously bounded without substantial

additional analysis.

4.2 Results

4.2.1 Introduction

APD measurements were made on April 17- and 19, 1973,

during operation in the Itmann #3 Coal Mine located near

Mullens, southwest of Beckley, West Virginia. Descriptions of

this mine are given in section 1.2. APD measurements were

48
made at three locations. The first set o£ APD measurements

was made on April 17, 1973, at Cabin Creek panel. Here, APD

measurements were made both during the operation of the mine

and during a period when the mining equipment was turned off.

In addition, APD measurements between roof -support bolts were

performed. The second set of APD measurements was made on

April 17 at Farley panel, and the third set of measurements

was made on April 19 at the headpiece for the main conveyor

belt for Farley panel beside the mainline rail haulage. In

these sets of APD measurements, only the vertical components

of magnetic field were measured. In addition, APD measurements between roof-support bolts were performed at the Farley

headpiece on April 19, 1973.

4.2.2 Measurement Results

Figures 4-1 through 4-12 show the APD ' s of the vertical

component of the magnetic field noise measured during operation of the mine at Cabin Creek panel, location A, figure 3-1,

on April 17, 1973. APD ' s measured during a period when mining

equipment was not operating are shown in figures 4-13 through

4-19. In these "quiet" measurements (with levels that are

sometimes higher than during normal operation), the vertical

components of magnetic noise were measured at frequencies

ranging from 10 kHz to 6 MHz. Above 100 kHz, the magnetic

field strength levels are not substantially different during

quiet time compared to normal operation. The harmonics of

88 kHz trolley phones extend to 8 MHz, and there are other

higher frequency carriers, e.g., 202.5 kHz.

Figure 4-20 shows the roof -support bolt measurement made

at Cabin Creek panel, location A in figure 3-1, on April 17,

1973. The separation between the two roof-support bolts was

1.5 meters, and the APD ' s were measured using non-shielded

copper wire connected between the roof-support bolts and

49
the electromagnetic intensity and field strength (EIFS) meter.

The dc resistance measured with a dc ohmmeter connected to

these roof-support bolts was 30kQ, . The RF impedance measured

at 10 kHz using a CW generator, RF voltmeter, and RF current

meter was 8 kfi . It is not easy to analyze what was measured

through non-shielded copper wires. It is a combination of

electric field coupled through a dipole with a lossy surrounding medium (i.e., coal) and of magnetic field through a lossy

loop antenna, and of voltage induced by current flowing through

the medium between the roof bolts. Therefore, relative voltage is the parameter given. The rms value is arbitrarily

assigned the value 0 dB.

Figures 4-21 through 4-28 show APD measurements of the

vertical component of magnetic field noise measured about 70

meters (230 ft.) from the longwall face in Farley panel on

April 17, 1973. Figures 4-29 through 4-40 show APD ' s of the

vertical component of magnetic field noise measured at the

headpiece for Farley main conveyor belt on April 19, 1973.

Generally, the levels at the headpiece are about 20 dB higher

than the levels 70 meters from the face. The length of time

for each APD measurement is nominally 20 minutes; for the

relatively long work cycle in a longwall mine, 20 minutes is

insufficient time to provide statistical validity. Also,

along main haulageways, the time required may be much longer

than 20 minutes

Figure 4-41 shows the roof-support bolt measurements

made at the Farley headpiece on April 19, 1973. The separation between the two roof-support bolts was one meter, the dc

resistance between these roof bolts was 30 kf^, whereas the RF

impedance at 10 kHz was 5.5 kfi. Again, the units are relative

voltage, with the rms value arbitrarily assigned the value

0 dB.

50
4.2.3 RMS and Average Values

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

of the field strength, according to the equations

00 H

avg

- / H dp(H)

o and

00 H

rms

-/ dp(H)

where H represents the magnetic field strength of the noise,

and p is the probability that the measured field strength

exceeds the value H. These quantities are also dependent on

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

possibly other parameters. Finite series are actually used

for the numerical integration. The rms and average values so

arrived at are identified on each graph and are time averages

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

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

rms value is directly relatable to noise power. With these

wide variations of field strength with time, the most suitable

presentations ^re statistical.

Excursions of field strength between 0.001 and 99 percent,

as well as rms and average values, are summarized in figures

4-42 through 4-45. The predetection bandwidth for these APD

measurements either is 1 kHz or is normalized to 1 kHz. Figure 4-42, vertical component. Cabin Creek, is the summary

figure for curves shown in figures 4-1 through 4-12. Figure

4-43, vertical component, quiet time, Cabin Creek, is the summary

figure for curves shown in figures 4-13 through 4-19. Figure

4-44, vertical component, Farley Panel, is the summary figure

for curves shown in figures 4-21 through 4-28. Figure 4-45,

51
vertical component, Farley Headpiece, is the summary figure

for curves shown in figures 4-27 through 4-40.

Magnetic field strength generally decreases monotonically

with increasing frequency at 20 dB per decade. A significant

exception occurred during "quiet" time. Possibly due to the

heavy use of 88 kHz trolley phones (with their attendant

harmonics), during this period, the levels from 100 kHz to

6 MHz were frequency independent, and higher than during

normal operation at the higher frequencies.

52
Lineor by --j- log|(,{-lnp)

_{Q I II II i 1 1 III I—I II mill—I—I M I I 1 I I I—I—I—I—iiii I II i

1 11 M I I—I—I—I 1—I 1

1)001.001.01 .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95 98 99 Perctnt of Tim* Ordinat* it Excetded

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

April 17, 1973, 11:40 a.m.. Cabin Creek, Itmann if3.

53
Lineor by --^ logi^l-lnp)

CO

Of -

m

E«

0*a.E

o

o

CD

o>C

(/)

L

u

n

c

o>o2

.0001.001.01 5 K) 20 30 40 50 60 70 80 85 90 95

Percent of Time Ordinate is Exceeded

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

April 17, 1973, 12:20 p.m.. Cabin Creek, Itmann #3.

54
-70

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

Percent of Time Ordinate is Exceeded

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

April 17, 1973, 12:50 p.m.. Cabin Creek, Itmann #3.

55
Linear by --j- log (-Inp)

if)

ir

4-«E

a

Q.E

o

o

CD

c

a>wo

ii.u

Olco

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

Percent of Time Ordinate is Exceeded

98 99

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

April 17, 1973, 1:30 p.m.. Cabin Creek, Itmann #3.

56
Linear by -f log|g(-lnp)

30| II Ill I I III I I I I I I I lli M M I I I I I

\ \ ]

1

-60

-70 ' I M I I I I I I I I M I I I I I 1 1 1 1

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

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

April 17, 1973, 2:20 p.m.. Cabin Creek, Itmann #3.

57
Llneor by --^ log|g(-lnp)

Percent of Time Ordinate is Exceeded

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

April 17, 1973, 3:00 p.m., Cabin Creek, Itmann #3.

58
-70

iOOOI.OOI.OI .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95

Percent of Time Ordinate is Exceeded

98 99

Figure 4-7 APD, 500 kHz, vertical component, 1,2 kHz predetection bandwidth,

April 17, 1973, 11:40 a.m.. Cabin Creek, Itmann #3.

59
Linear by --^ log J-lnp)

-80' I ' I ' ' I I I ' "" 11 " I I I i I I I I—\ \—\ 1

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

Percent of Time Ordinate is Exceeded

Figure 4-8 APD, 1 MHz, vertical component, 1.2 kHz predetection bandwidth,

April 17, 1973, 12:20 p.m.. Cabin Creek, Itmann //3.

60
Linear by --j- logu,(-lnp)

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

Percent of Time Ordinate is Exceeded

Figure 4-9 APD, 2 MHz, vertical component, 1.2 kHz predetection bandwidth,

April 17, 1973, 12:50 p.m.. Cabin Creek, Itmann #3.

61
Figure 4-10 APD, 6 MHz, vertical component, 1.2 kHz predetection bandwidth,

April 17, 1973, 1:30 p.m.. Cabin Creek, Itmann #3.

62
coS

-80

-90 ' II '

' ^'iii^'i 11^ ' ' I I I I M I I

I—\ 1

\ 1

1)001.001.01 .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95 98 99

Percent of Time Ordinate is Exceeded

Figure 4-11 APD, 14 MHz, vertical component, 1.2 kHz predetection bandwidth,

April 17, 1973, 2:20 p.m.. Cabin Creek, Itmann #3.

63
-90

-100

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

Percent of Time Ordinate is Exceeded

Figure 4-12 APD, 32 MHz, vertical component, 1.2 kHz predetectlon bandwidth,

April 17, 1973, 3:00 p.m.. Cabin Creek, Itmann /if3.

64
Linear by -

-j- \oq^J,-\r\p)

-70 '

'

' " ' ' '

'' ^

' '

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

Percent of Time Ordinate is Exceeded

Figure 4-13 APD, 10 kHz, vertical component, 1.0 kHz predetection bandwidth,

April 17, 1973, 3:40 p.m.. Quiet Time Measurement, Cabin Creek,

Itmann #3.

65
-70

-80 I ' I 'II' " I ' \\\\\\\ \

\

I I I M I I I

\ 1

\ 1

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

Figure 4-14 APD, 30 kHz, vertical component, 1.0 kHz predetection bandwidth,

April 17, 1973, 3:50 p.m.. Quiet Time Measurement, Cabin Creek,

Itmann if 3,

66
Lineor by --j- log,Q(-lnp)

JW)0I.00I.01 .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95 98 99

Percent of Time Ordinate is Exceeded

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

April 17, 1973, 4:05 p.m., Quiet Time Measurement, Cabin Creek,

Itmann #3.

67
40

Lintor by --^ log^{-lnp)

-50

-60

JOOOI.OOI.OI .1 .5 I 5 K) 20 30 40 50 60 70 80 85 90 95 98 99

Percent of Time Ordinate it Exceeded

Figure 4-16 APD, 500 kHz, vertical component, 1.2 kHz predetection bandwidth,

April 17, 1973, 3:40 p.m., Quiet Time Measurement, Cabin Creek,

Itmann #3.

68
-40

-50

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

Percent of Time Ordinate is Exceeded

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

April 17, 1973, 3:50 p.m.. Quiet Time Measurement, Cabin Creek,

Itmann )?3.

69
-40

-50

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

Figure 4-18 APD, 2 MHz, vertical con^jonent, 1.2 kHz predetectlon bandwidth,

April 17, 1973, 4:00 p.m., Quiet Time Measurement, Cabin Creek,

Itmann #3.

70
Linear by -j log (-In p)

-40

-50l II ' I II I'l' I ' ' ' ' '

I \\\\\\\\\ II M I I I I

\ 1\ 1

j0001.OOI.OI .1 .5 J5 10 20 30 40 50 60 70 80 85 90 95 98 99 Percent of Time Ordinate is Exceeded

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

April 17, 1973, 4:05 p.m.. Quiet Time Measurement, Cabin Creek,

Itmann #3.

71
72
Lineor by -y log J-lnp)

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

Percent of Time Ordinate is Exceeded

Figure 4-21 APD, 10 kHz, vertical component, 1.0 kHz predetection bandwidth,

April 17, 1973, 12:30 p.m., Farley Panel, Itmann #3.

73
Lineor by --^ log J- In p)

30

20

10

-10

-20

-30

-40

1

1 1

1

1

1

1

1

1 N

1

I 1

-i

1

i M n

r

L ere lo

.1

1

i !

1

1

j

1

i 1

\

1

t

i

1 j

A 1

1

i

1

1 RMS

1

1

1 4h AVG 1

1

1

i

f

1

i

- - t

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

Percent of Time Ordinate is Exceeded

Figure 4-22 APD, 30 kHz, vertical component, 1.0 kHz predetection bandwidth,

April 17, 1973, 1:15 p.m., Farley Panel, Itmann #3.

74
Linear by - y log,Q(-lnp) 20| M il

I 11 1

I

I I 1 1

1 I

I I I I

I I i iiiii Ill—rn—

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

Percent of Time Ordinate is Exceeded

Figure 4-23 APD, 70 kHz, vertical component, 1.0 kHz predetection bandwidth,

April 17, 1973, 2:00 p.m., Farley Panel, Itmann #3.

75
20

Lineor by --^ loQigl-lnp)

S

Q.

ata

E

ook.u1

o

a>

k. m

o

X

o>c

01u

*— U)

t> iZu

c

D>O

-10

-20

-30

-40

-50

-60

-70

-80

,0001 .00) .01 .1 .5 I

I -|- -

!

1

-|

4

-r

-i

h —

——

t —

1

i -+•

\— -41

1

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-

h

--L

- -

\

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4

fr

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h[—

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j —-T F

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-[ G( d N oi s e

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-i -[

\

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--f

i -i1 4

r- 4—

|-

r4

4-

4

4

- 4

4

-|

4

4

-j- 4L

-|

4

.._

4

1 —

5 10 20 30 40 50 60 70 80 85 90 95 Percent of Time Ordinafe is Exceeded

98 99

Figure 4-24 APD, 130 kHz, vertical component, 1.0 kHz predetection bandvd.dth,

April 17, 1973, 2:45 p.m., Farley Panel, Itmann #3.

76
Lineor by --j- lognj(-lnp)

Percent of Time Ordinate is Exceeded

Figure 4-25 APD, 250 kHz, vertical component, 1.2 kHz predetection bandwidth,

April 17, 1973, 12:30 p.m., Farley Panel, Itmann #3.

77
Lirnor by -

-J- log,j,(-lnp)

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

Percent of Time Ordinate is Exceeded

Figure 4-26 APD, 500 kHz, vertical component, 1.2 kHz predetection bandwidth,

April 17, 1973, 1:15 p.m., Farley Panel, Itmann #3.

78
Linear by --j- log|J-lnp)

-20

2 -30

-40

-50

-60

-70

-90

JOOOl.OOI.OI .1 .5 I

1 --f

1

1

1 —

-t "1fs— f- \ 1—1 --i - i

4-

i —

i

1 r

4r i f

•T

1 i-

- L- -H -4

a -1 II lie V7eiic'rate d ^Joi

—

1

: — 4 —

1

1

I —

—i

1 r

1 -

1 OhACi

1

1 —1 r

1

1

i -4 1 — -

I 4

1 — ! -

i

5 10 20 30 40 50 60 70 80 85 90 95

Percent of Time Ordinate is Exceeded

98 99

Figure 4-27 APD. 1 MHz. vertical conrponent, 1.2 kHz predetection bandwidth,

April 17, 1973, 2:00 p.m., Farley Panel, Itmann ff3.

79
Linear by --^ log|Q(-lnp)

-20

-30

-50

-60

-70

-80

-90

-100

.0001.001.01 .1 .5 I

-T

-| -L --|

--1

-I

-I i

-4 1

1

j

1

1 -| 1

i 1

-i 1

j| 1 ne 6e d i

1

1

1 lie 1 c

4-

i i +

1

4t —

i

IS

! K i

i

>

!

I 1

-i- -4

I ii

1 1

-| I

i

1 +—

+

J.

]- -

t

--

.._

5 10 20 30 40 50 60 70 80 85 90 95

Percent of Time Ordinate is Exceeded

98 99

Figure 4-28 APD, 2 MHz, vertical component, 1.2 kHz predetection bandwidth,

April 17, 1973, 2:45 p.m., Farley Panel, Itmann #3.

80
Linear by log|Q(-lnp)

1

1

1 1

, J..

Tf

jl 4 -14 —

\-

1

4

r

4

jl

— 4-4

i

1

j

1 1— —

-4

itt-^

-4

fj

- 4i—

f- ^-

4-4

rrln

1

1

1 t— '

1 m

LMine Generated Noise

1

1 wi 1

1

-t 4

-T

I -jl

4n

1 --t

f- —j-L ^-

44— ^ RMS

— —

-i

i 1 4 4

t— M V

1

--t- 1—

r4^ -

i

1 f4—

i ft

--t- 4

4

r

j_ j

1 ^

-4

1

i

1 T

4

\ —

4

n Noi 1

-

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

Percent of Time Ordinate is Exceeded

4-29 APD, 10 kHz, vertical component, 1.0 kHz predetection bandwidth,

April 19, 1973, 10:20 a.nu^ Farley Headpiece, Itnann #3.

81
Lineor by - lo9,o("l"P)

a:

9>a.

0)a

E

oo

m

T3

CPC

0)

L

o

cO

.0001.001.01 .1 .5 I 5 K) 20 30 40 50 60 70 80 85 90 95

Percent of Time Ordinate is Exceeded

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

April 19, 1973, 10:50 a.m., Farley Headpiece, Itmann #3.

82
Linear by -

-j- log,Q(-lnp)

J00OI.OOI.OI .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95 98 99

Percent of Time Ordinate is Exceeded

Figure 4-31 APD, 70 kHz, vertical component, 1.0 kHz predetectlon bandwidth,

April 19, 1973, 11:25 a.m., Farley Headpiece, Itmann #3.

83
Linear by --^ log^^t-lnp)

Percent of Time Ordinate is Exceeded

Figure 4-32 APD, 130 kHz, vertical component, 1.0 kHz predetectlon bandwidth,

April IP, 1973, 12:00 p.m., Farley Headpiece, Itmann #3.

84
Linear by - y \oq^^{-\np)

-50

-60' " 1 1

' II' "' 'I '

' ' ' '

'— I I I M I I I\ \—\ 1

.0001,001,01 ,1 ,5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99

Percent of Time Ordinate is Exceeded

Figure 4-33 APD, 160 kHz, vertical component, 1.0 kHz predetectlon bandwidth,

April 19, 1973, 12:30 p.m., Farley Headpiece, Itmann #3.

85
Lin«ar by -f log,Q(-lnp)

-70 ' " " I'll " '

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

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

Percent of Time Ordinate is Exceeded

Figure 4-34 APD, 250 kHz, vertical component, 1.0 kHz predetectlon 1 April 19, 1973, 1:00 p.m., Farlev Headpiece, Itmann #3.

86
Linear by loQujC-lnp)

-60

-70

jOOOI.OOI.OI .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95

Percent of Time Ordinate is Exceeded

99

Figure 4-35 APD, 500kHz, vertical component, 1.2 kHz predetection bandwidth,

April 19, 1973, 10:20 a.m., Farley Headpiece, Itmann #3.

87
Linear by --^ log|p(-lnp)

30|

!i i i m ill I i iiiiiiii I n 1 1 11 1 1 1 I I 1 1 1 I I I—n—

20

& -50 o

-60

-TO' II ' Illllll I I I M I I I I I I I I— II I ^ 1 ^ >

JOOOI.OOI.OI .1 .5 I 5 K) 20 30 40 50 60 70 80 85 90 95 98 99 Percent of Time Ordinate is Exceeded

Figure 4-36 APD, 1 MHz, vertical component, 1.2 kHz predetection bandwidth,

April 19, 1973, 10:50 a.m., Farley Headpiece, Itmann //3.

88
-80' I' ^' ' " I " '

I I I I I I M I I lllllllll II I I I I I—I 1

\ 1

J0OOI.OOI.OI .1 .5 I 5 K) 20 30 40 50 60 70 60 85 90 95 98 99 Percent of Time Ordinate is Exceeded

Figure 4-37 APD, 2 MHz, vertical component, 1,2 kHz predetectlon bandwidth,

April 19, 1973, 11:25 a.m., Farley Headpiece, Itmann #3.

89
Lintor by --^ log^(-lnp)

o

-70

-80' II H I ' I I I I M M M I I I li n ill l I I 1

I M I I I \ 1 \ 1

AOOIjOOIjOI .l.5l5l0 20 M40S0 60TOaOI5 90 95 96 99

Percent of Time Ordinate is Exceeded

Figure 4-38 APD, 6 MHz, vertical component, 1.2 kHz predetectlon bandwidth,

April 19, 1973, 12:00 p.m., Farley Headpiece, Itmann #3.

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

Percent of Time Ordinate is Exceeded

Figure 4-39 APD, 14 MHz, vertical component, 1.2 kHz predetection bandvridth,

April 19, 1973, 12:30 p.m., Farley Headpiece, Itmann #3.

91
Linear by - j log,Q(-lnp)

-90

-100 ' ' " I ' I "I ' l"' I I I I M I I I I I I I I I II I

\ 1\ 1

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

Figure 4-40 APD, 32 MHz, vertical component, 1.2 kHz predetection bandwidth,

April 19, 1973, 1:00 p.m., Farley Headpiece, Itmann #3.

92
Lineor by loguj(-lnp)

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

Percent of Time Ordinate is Exceeded

Figure 4-41 APD, 10 kHz, roof bolt measurement, 1.0 kHz predetection bandwidth,

April 19, 1973, 1:30 p.m., Farley Headpiece, Itmann #3.

93
8 X •>

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E o E o 2 > - S tr < 5

X • O <

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c

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0)

<3

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53 9

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( SIAId -idlduj jad ajedtuDOJOiuLi i 04 3A!;d|8j gp ) H

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94
s

8

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X • O <

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c

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( SIAId jsd ajaduuoojojuu | dA\iD\Qj gp )

95
5

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tr < i

X • O <

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cr

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( SiAld -Jdldtu J8d dJddujDOJOjUJ | o; aAiioisj gp )

96
8

55

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( SIAJd jad ajsdujDOJOjUJ i oi aAiiojaj gp

97

)
5. CONCLUSIONS

In Itmann, we found that the haulageways usually had

higher EM noise levels than exist near the face area. This

was different from other mines; usually the face equipment

is the prime source of noise. At higher frequencies, above

100 kHz, Itmann was noisier than other mines. So called

"quiet" periods have low noise levels only at low frequencies,

and only at considerable distances from rail haulageways.

The 88 kHz trolley phones in Itmann No. 3 may not have

worse harmonic output than exists in other mines, but relatively high levels of harmonic signals were observed to 8 MHz

in this mine on a spectrum analyzer. This was not true in

one other mine, Grace Mine, but this measurement was not made

in most mines visited.

There were also carriers present on an intermittent basis

at frequencies other than the 88 kHz of the trolley phone.

A strong one was at 202 kHz. In general dc machinery is

noisier than ac machinery.

98
6. RECO^IMENDATIONS

In Itmann, in addition to the lower than usual powerline -harmonic noise and the average impulsive noise, there are

more intermittent cw carriers and harmonics of these carriers

than in any other mine visited. Therefore, longer intervals

of interference may be present at more discrete frequencies,

and design of communication equipment should take this into

account.

99
7. ACKNOWLEDCLIENTS

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, Anne Rumfelt, Nancy Tomoeda, Winston Scott,

Frank Cowley, and David Stearns. Those making valuable but

less time-consuming contributions were Gerry Reeve, Bob

Matheson, Don Spaulding, John Chukoski , Lorne Matheson,

Dave Lewis , and Sharon Foote

Winston Scott provided much assistance in proofreading,

while Sharon Foote and Janet Becker typed tirelessly through

many versions. Jocelyn Spencer and Barbara Bolton provided

drafting assistance.

Finally, none of this would have been possible without

the excellent cooperation of John Burr of Lee Engineering

Company and of Bill Starling, Wes McDonald, John Henry Johnson,

Hamp Clark, Thurman Tilley, and Jack Holt of Consolidation

Coal Company.

100
8. REFERENCES

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

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

April 1974.

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

Noise in Grace Hardrock Mine, NBSIR 74-388, June 1974.

The Institute o£ Electrical and Electronic Engineers, Inc.,

IEEE Dictionary of Electrical and Electronic Terms,

Std. 100, 1972.

Taggart , H.E. and Workman, J.L., Calibration Principles

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

,

NBS Tech. Note 370, March 1969.

Crichlow, W.O., et al . , Amplitude -probability distributions

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

Thompson, W.I., III, Bibliography of ground vehicle communications and control. AKWIC index. Report No. DOTTSC-UMTA-71-3, July 1971.

Kanda, Motohisa, Adams, J.W. , Bensema, W.D., Electromagnetic Noise in McElroy Coal Mine, NBSIR 74-389.

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

Part I: Estimates for Business, Residential, and Rural

Areas, OT Report 74-38, June 1974.

101
9. 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 = 12

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

NZS = Restart spectral average after output, example, 0

(restarted)

NDA = Data segment advance increment, example, 2048

NPO = Number of spectra averaged between output calls,

example, 20

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

DF = Resolution bandwidth, spectral estimate spacing in

hertz, example, 62.5

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

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

Frame= Frame set number, example, 10

Serial = Film frame serial number, example, 42.

Second line: DTA DA(1) DAC2) DA(3) NSA NRP NPP , where

DTA = Detrending filter parameter a, example, 0.00195

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

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

DA(3) = Detrending filter average, K=3, example, 0

NSA = Number of periodograms averaged, example, 20

NRP = Number of data points processed since spectrum

initialization, example, 43008

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

102
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, 21-83

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

example , 8 25 73

Gain corr. rec. = receiver gain correction, example, 0

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

example, 46.4

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

C = correction curve used with data, example, 25

RG = receiver gain and accompanying correction in dB added to

the data, example, 200 (-6 dB)

DG = digitizer gain, example, 0

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

example, 0

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

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

Top of Scale = largest scale marking for computer drawn

graph, example, 1.000+004 (1.0 x 10^)

Standard Error = standard error of curve, example, 0.3162

Spectral Peak = largest spectral peak observed, example,

4.108+003 (4.108 x 10^)

103
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 74-390

4. TITLE AND SUBTITLE

Electromagnetic Noise in Itmann Mine

2. Gov't Accession

No.

7. AUTHOR(S)

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

3. Recipient's Accession No.

5. Publication Date June 1974

6. Performing Organization Code

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Labs

DEPARTMENT OF COMMERCE

Washington, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

U. S. Bureau of Mines

Pittsburgh Mining & Safety Research Center

4800 Forbes Avenue

Pittsburgh, Pennsylvania 15213

13. Type of Report & Period Covered

Rppnrf tn Spnn.gm

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual 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, Itmann No. 3 Mine, located near Mullens, West Virginia. The electromagnetic environment created by 250-volt-dc and

550- and 950-volt-ac machinery in two longwall panels was measured and is reported. One

technique measures noise over the entire electromagnetic spectrum of interest for brief

time periods. It is recorded using broadband analog magnetic tape, and the noise data is

later transformed to give spectral plots. The other technique records noise amplitudes

at several discrete frequencies for a sufficient amount of time to provide amplitude

probability distributions.

The specific, measured results are given in a number of spectral plots and amplituc^e

probability distribution plots.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Amplitude probability distribution; coal mine noise, digital

electromagnetic communications; electromagnetic interference; electromagnetic noise;

Fast Fourier Transform; Gaussian distribution; impulsive noise; magnetic field strength;

measurement instrumentation; spectral density; time-dependent spectral density.

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