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a Part of the Center for Radiation Research. 

Located at Boulder, Colorado 80302. 

Part of the Center for Building Technology. 
Electromagnetic Noise 

in Robena No. 4 Coal Mine 

W.D. Bensema 

Motohisa Kanda 

John W. Adams 

E lectromagnetics Division 

Institute for Basic Standards 

National Bureau of Standards 

Boulder, Colorado 80302 

Sponsored by 

U.S. Bureau of Mines 

Pittsburgh Mining and Safety Research Center 

4800 Forbes Avenue 

Pittsburgh, Pennsyivania 15213 

-- - 

U.S. DEPARTMENT OF COMMERCE, Frederick 6. Dent, Secretary 

NATIONAL BUREAU OF STANDARDS. R~chard W Roberts. D~rector 

Issued April 1974 
For salt. Ir? thc. Sul,erintendrnt of L)ocurncnts, I!.S. (;overnrnent Printing Office, Washington, D.C. 20402 

(Order by SL) Catalogue No. C 13.46:654). $1 50 
CONTENTS 

Page 

A. Noise Measurement Techniques - - - - - - - - - - - - - - - - - - 5 

1. Description of Measurement Instrumentation------------------------------------ 5 

2. Description of Processing Instrumentation 11 

a. Laboratory Transcription Process -- - - - 11 

b. Laboratory Digitizing Process-------- 13 

c. Spectral Computat ion and Graphical output------------------------------- 14 

d. Description of Processing Software - - - 14 

B. Noise Measurement Results: Spectra------ ----- 2 3 

1. Mine Description and Antenna Sites - - - - - - - 2 3 

2. Electromagnetic Noise Spectrum Results - - - 2 5 

a. Introduction------------------------- 2 5 

b. Working Face Area (Locat ion 1) - - - - - - - 26 

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

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

e. Nip Point (Location 4)--------------- 5 7 f. Air-split---------------------------- 59 

g. Bailey Shaft (Location 6) - - - - - - - - - - - - 6 2 

h. Bailey Shaft (Location 7)------------ 71 

i. Summary Plot of Power Line Harmonics- 73 

111. AMPLITUDE PROBABILITY DISTRIBUTION MEASUREMENTS---- 73 

iii 
CONTENTS (continued) 

Page 

B. Noise Measurement Techniques------------------ 7 5 

1. Measurement Instrumentation-------------- 7 5 

a. Underground Recording System--------- 7 8 

b. Data Transcribing System------------- 8 2 

c. Data Processing System--------------- 86 

2. Measurement Technique-------------------- 90 

C. Noise Measurement Results: APD's------------- 96 

A. Surface Noise Measurements-------------------- 136 

2. Measurement Techniques------------------- 13 7 

a. Spectra--- ........................... 137 

b. Amplitude Probability Distributions- - 137 

3. Measurement Instrumentation-------------- 137 

4. Location of Measurements----------------- 139 

6. Results of Spectral Noise Measurements--- 141 

7. Amplitude Probability Distribution 

Results-,--------------------------------- 142 

C. Surface-Underground noise coherence tests------ 159 

D. Mine Phone Wire Measurements------------------ 162 

2. Measured Noise Current------------------- 166 

E. Trolley Wire Voltage Measurements------------- 166 
CONTENTS (continued) 

Page 

IX. APPENDIX--Decoding of Spectrum Captions- -- -- -- -- - - - 177 

LIST OF FIGURES 

Page 

Figure 1. 

Figure 2. 

Figure 3. 

Figure 4. 

Figure 5. 

Figure 6. 

Figure 7. 

Figure 8. 

Figure 9. 

Figure 10. 

Figure 11. 

Figure 12. 

Block diagram of portable instrumentation - 6 

Block diagram of laboratory transcription 

process----------------------------------- 12 

Block diagram of laboratory digitizing of 

data-------------------------------------- 12 

Block diagram of spectral computation, plotting, and printing-------------------- 15 

Map of antenna locations at Robena working section----------------------------------- 24 

Spectrum of continuous mining machine in 

full operation at face area (location 1) , antenna sensitive axis vertical - -- - -- -- -- - 2 7 

Spectrum of continuous mining machine in 

full operation at face area (location l), 

antenna sensitive axis, horizontal E-W- - - - 2 9 

Spectrum of continuous mining machine in 

full operation at face area (location l), 

antenna sensitive axis horizontal N-S-- --- 30 

Three-D plot of spectrum of continuous 

mining machine speeding up at face area-- - 32 

Expanded frequency scale of spectrum of 

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

sensitive axis vertical ------------------- 33 

Expanded frequency scale of spectrum of 

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

sensitive axis horizontal E-W------------- 34 

Expanded frequency scale of spectrum of 

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

sensitive axis horizontal N-S- --------- --- 3 5 
Figure 20. 

Figure 21. 

Figure 22. 

pg 

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

Expanded frequency scale in spectrum of 

13- ton electric locomotive pulling into 

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

Three-D plot of spectrum of noise from 

13-ton electric locomotive pulling into 

mine section, location 2, antenna sensitive axis vertical------------------------ 46 

Three-D plot of spectrum noise from 13- 

ton electric locomotive pulling into mine 

section, location 2, antenna sensitive 

axis horizontal E-W----------------------- 4 7 

viii 
LIST OF FIGURES (continued) 

Page 

Figure 23. 

Figure 24. 

Figure 25. 

Figure 26. 

Figure 27. 

Figure 28. 

Figure 29. 

Figure 30. 

Figure 31. 

Figure 32. 

Figure 33. 

Figure 34. 

Figure 35. 

Three-D plot of spectrum of noise from 13- 

ton electric locomotive pulling into mine 

section, location 2, antenna sensitive 

axis horizontal N-s----------------------- 4 8 

Spectrum at location 3, thirty meters from 

track, antenna sensitive axis vertical -- -- 51 

Spectrum at locati-on 3, thirty meters from 

track, antenna sensitive axis horizontal 

E-W--------------------------------------- 5 2 

Spectrum at location 3, thirty meters from 

track, antenna sensitive axis horizontal 

N-S--------------------------------------- 5 3 

Expanded frequency scale in spectrum at 

location 3, antenna sensitive axis vertical 54 

Expanded frequency scale in spectrum at 

location 3, antenna sensitive axis horizontal E-w-------------------------------- 5 5 

Expanded frequency scale in spectrum at 

location 3, antenna sensitive axis horizontal N-s-------------------------------- 56 

Spectrum at location 4, near electric 

motor for car-pull hydraulic pump, antenna 

sensitive axis horizontal E-W-- --- --- --- -- 5 8 

Expanded frequency scale in spectrum at air 

split, antenna sensitive axis vertical- --- 61 

Map of antenna locations near Bailey shaft 63 

Spectrum at location 6, thirty meters east 

of Bailey shaft, antenna sensitive axis 

vertical -------------- --------- ----------- 64 

Expanded frequency scale in spectrum at 

location 6, thirty meters east of Bailey 

shaft, antenna sensitive axis vertical ---- 6 5 
Figure 44. 

Figure 45. 

Figure 46. 

Figure 47. 

Figure 48. 

Figure 49. 

Figure 50. 

Frequency response of portable analog magnetic tape recorder----------------------------- 8 4 

Collapsible loop antenna as used in Robena 

Mine for frequencies between 10 kHz and 

250 k~~----------------------------------- 89 

APD recording system in operation in Robena 

Mine-------------------------------------- 91 

APD data processing system in operation--- 92 

APD, 10 kHz, vertical component----------- 9 7 

APD, 30 kHz, vertical component, Dec. 5, 1972-------------------------------------- 9 8 
LIST OF FIGURES (continued) 

Page 

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 , 

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-W- - - - 

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

2 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-S- - - - 

500 kHz, horizontal component N-S---- 

2 MHz, horizontal component N-S------ 

8 MHz, horizontal component N-S----- - 

32 MHz, horizontal component N-S- - - - - 

APD, 10 kHz, vertical component, Dec. 7, 1972-------------------------------------- 121 
Figure 83. 

Figure 84. 

Figure 85. 

Figure 86. 

Figure 87. 

Figure 88. 

Field strength excursions between 0.1% and 

99% of the time as a function of frequency, 

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

Field strength excursions between 0.1% and 

99% of the time as a function of frequency, 

vertical component, Dec. 7, 1973---------- 134 

Surface recording system------------------ 138 

Location of surface measurements---------- 140 

Spectrum of surface noise, 300 meters from 

Blaker shaft, antenna sensitive axis horizontal N-S- -- -- -- --- -- ---.- - --- - -- - -- - -- - -- 143 

Spectrum of surface noise, 300 meters from 

Blaker shaft, antenna sensitive axis horizontal E-W ---- -- -- ------ ---- ---- ----- --- -- 144 

xii 
LIST OF FIGURES (continued) 

Page 

Figure 89. 

Figure 90. 

Figure 91. 

Figure 92. 

Figure 93. 

Figure 94. 

Figure 95. 

Figure 96. 

Figure 97. 

Figure 98. 

Figure 99. 

Figure 100. 

Figure 101. 

Figure 102. 

Spectrum of surface noise, 300 meters from 

Blaker shaft, antenna sensitive axis 

vertical---------------------------------- 145 

Spectrum of surface noise, Bailey shaft, 

antenna sensitive axis horizontal N-S- - - - - 146 

Spectrum of surface noise, Bailey shaft, 

antenna sensitive axis vertical - - - - - - - - - - - 147 

Spectrum of surface noise, Bailey shaft, 

antenna sensitive axis horizontal E-W----- 148 

APD, near Blaker shaft, 10 kHz, surface 

vertical component, Dec. 5, 1973------- -- - 149 

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

APD near Blaker shaft, 70 kHz, surface 

vertical component, Dec. 5, 1973---------- 151 

APD near Blaker shaft, 130 kHz, surface 

vertical component, Dec. 5, 1973---------- 152 

Field strength excursions between 0 -1% and 

99% of the time as a function of frequency, 

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

Voltage spectrum of roof bolts, 3.6 meter 

spacing----------------------------------- 155 

Voltage spectrum of roof bolts, 8.5 meter 

spacing----------------------------------- 156 

Voltage spectrum of roof bolts, 13 meters 

spacing----------------------------------- 157 

Voltage spectrum of 8.5 meter dipole------ 158 

3D Spectrum at location 2, magnetic field 

strength obtained with loop antenna, no 

impulses, antenna sensitive axis vertical- 160 

xiii 
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 

Viaynesburg, 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 slr~plitude 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 transmiss ion line. Carrier 

trolley phone signals and harmonics are strong throughout 

the mine whenever the trolley phone is in operation. 

Additional informat ion 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 in~~rumentation ; 

spectral density; t i m e - d e p e n d e n t 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 desig~zed only 

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

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 transmiss ion 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 [I] 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 

measure three orthogonal components of magnetic field strength 
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 required, H and Hd. Results are given as the rms value of one component of magnetic field strength, H, versus frequency for discrete frequencies, or magnetic-field-strength spectrum density 

level, Hd, [I] 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 (APD1s). 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 -sys tem 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 

informat ion. 

11. SPECTRUM MEASUREMENTS 

A. Noise Measurement Techniques 

1. Description of Measurement Instrumentat ion 

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 
IioRT A TRACK 7 ( FM) 

MICROPHONE EDGE TRACK 

(VOICE) 

Figure 1 Block diagram of pcrtable 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. 
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 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- 
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 port ion 

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 re - 

chargeable sealed lead-acid batteries. The batteries, a solid- 
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 
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 P r o c e s s i n g I n s t r u m e n t a t i o n 

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 reduct ion 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 
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. 

3-244 ips AT A TIME 

(REPRODUCE) 

-TRACK 3 (FM) 

- 

LOW 

Fl 

PASS 

LTER 

- 

-TRACK 5 (FM) 

DIGITIZER 

( I6,OOO 

SAMPLES/SEC) 

AND DIGITAL 

TAPE 

TRANSPORT 

TRACK 2 (DIR 

OSCl LLOSCOPE 

SIGNAL MONITOR 
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 

I n s t r w n e n t a t i o n 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 -cont rol servo (including 

the motor) is controlled by the crystal oscillator signal 

originally recorded at 200 kHz in the field. Most of the 
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 
DIGIT1 ZED DATA MICROFILM DATA SPECTRA 

(TAPE) PLOTTER - (MICROFILM) 

MEASUREMENT COMPUTER 

PARAMETERS 

( CARDS) FFT COMPUTER 

PROGRAM "PEAK FINDER" 

(CARDS) PRINTOUT 

DATA SPECTRA 

(MICROFILM) READER 

PRINTER 

DATA SPECTRA 

(PAPER PRINTS) 

Figure 4 Block diagram of spectral computat ion, plotting, 

and printing. 
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. 
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 represent - 

ing 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 correct ion curve algorithm, will be discussed under antenna 

calibration. 
ygy jg 

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 1-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 
qpq

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 
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. 
bration 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 
current. The transmitting standard loop antenna was used as 

the wire with known current. Therefore, the uncertainty of 

k 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 k 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 t 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 
ANTENNA LOCATION = /\ 

DRAWING NOT TO SCALE 

SIDECUT XOO 

WORKING FACE 

SIDECUT X5 I 

Map of working section 3-main, 10-right, 2-room, Figure in Robena No. 4 coal mine as it was on Oecembcr 

5, 1972 wllc~l measurements were being made. 
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 (pA/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 1-to-100-kHz 

graphs). The correct units for the background noise 

between peaks are microamperes per meter per square root x 

hertz [(pA/m)/m], where x is the spectral resolution 

of the FFT (x equals 125 Hz for the 1-to-100-kHz graphs). 

2 5 
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 
MICROAMPERE ONE TO RELATIVE dB H, STRENGTH, F;ELD MAGNETIC RMS 

OR FREQUENCIES; DISCRETE FOR METER, PER 

TO RELATIVE dB H, LEVEL, DENSITY SPECTRUM RMS MAGNETIC-FIELD-STRENGTH 

BAND NOISE BROAD FOR ,/EL, MICROAMPERE-PER-METER PER ONE 
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 
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RMS MAGNETIC FIELD STRENGTH, H, dB RELATIVE TO ONE MICROAMPERE 

PER METER, FOR DISCRETE FREQUENCIES; OR 

RMS MAGNETIC-FIELD-STRENGTH SPECTRUM DENSITY LEVEL, Hd, dB RELATIVE TO 

ONE MICROAMPERE-PER-METER PER fiz, FOR BROAD BAND NOISE 
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 be- 
FREQUENCY , kHz 

Figure 9 '~II.:,? - i; s;'-ctrum of lilagilztic Cicld strcrigtil obtni~!ecl oi; 3 loop antenna, 73 kllz to 95 kilz, Ltobctla 

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RMS MAGNETIC-FIELD-STRENGTH SPECTRUM DENSITY LEVEL, Hdl dB RELAT

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ing, 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 Sth, 25 dB 

for the 6th, 12 dB for the 7th, and 9 dB for the 8th. The logarithmic average (average dB value) of these six harmonics is 

26 dB pA/m. These values are obtained from a computer printout from the program "Peak Finder". 
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59 dB pA/m. The logarithmic average strength of harmonics 

3 through 8 at the face (vertical moment) was 26 dB 1.1A/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 
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RMS MAGNETIC-FIELD-STRENGTH SPECTRUM DENSITY LEVEL, Hd, dB RELA

ONE MICROAMPERE-PER-METER PER , FOR BROAD BAND NOI
MICROAMPERE ONE TO RELATIVE dB HI STRENGTH, FIELD MAGNETIC RMS 

OR FREQUENCIES; DISCRETE FOR METER, PER 

TO RELATIVE dB Hdl LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

BAND NOISE BROAD FOR $!%t PER MICROAMPERE-PER-METER ONE 
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OR FREQUENCIES; DISCRETE FOR METER, PER 

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Figure 17 spectrum 

a loop 

mine, usitive 

from tr

ton loc

Spectra
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RMS MAGNETIC-FIELD-STRENGTH SPECTRUM DENSITY LEVEL, Hdl dB RELATIV

ONE MICROAMPERE- PER-METER PER ,/mZ , FOR BROAD BAND NOISE 
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. T h i r t e e n - t o n locomotive pulling miners into section. Spectral resolution is 1 kHz. Relative amplitude is shown. 
Figure 22 Thre

tain

Xo. 

sens

haul

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Figure 23 Three-D spe

tained on a 

No. 4 mine, 

sensitive a

trolley wir

Thirtccn-ton 

tion. Spec

amplitude i
1 these plots were shown in the six preceding spectra (antenna 

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

95 kHz is selected fo~ 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 abo-tie 1 uA/m. The impulse can then be inferred to be about 45 dB pA/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. 

l~ote that for each 3-D graph, the information shown during the 1 

(interval of time, 0 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 
below 10 kHz. The 88-kHz trolley phone shows up clearly at 

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

23 dB uA/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. 
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BAND NOISE BROAD FOR JEZit, MICROAMPERE-PER-METER PER ONE 
MICROAMPERE ONE TO dB RELATIVE HI STRENGTH, FIELD RMS MAGNETIC 

OR FREQUENCIES; DISCRETE FOR METER, PER 

TO RELATIVE dB Hdt LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

BAND NOISE BROAD FOR ,/E%zt PER MICROAMPERE-PER-METER ONE 
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OR FREQUENCIES; DISCRETE FOR METER, PER 

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1 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 

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

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

1 operator. The antenna sensitive axis is horizontal E-8. 

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. I 

(The maximum spectrum value is at 78 dB pA/m at 1000 Hz, drops 1 

to 47 dB pA/m at 10 kHz, and is down to 25 dB pA/m at 30 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 I 

field strengths are measured at multiples of power line fre- 

,quencies in cuts containing primary mine dc power cables. 
8s 

MICROAMPERE TO ONE RELATIVE dB HI STRENGTH, FIELD RMS MAGNETIC 

OR FREQUENCIES; DISCRETE FOR METER, PER 

TO RELATIVE dB Hd, LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

NOISE BAND BROAD FOR fit, PER MICROAMPERE-PER-METER ONE 
Other spectra taken at antenna location #4 showed 

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

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

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

26.0 dB pA/m at the face and 59.4 dB pA/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 
MICROAMPERE ONE TO RELATIVE dB H, STRENGTH, FIELD MAGNETIC RMS 

OR FREQUENCIES; DISCRETE FOR METER, PER 

TO RELATIVE dB Hd, LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

NOISE BAND BROAD FOR , PER MICROAMPERE-PER-METER ONE 
wrr3Plm 

'd r. r. yi 

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r P. 

tr@ zn r.0 r N 

vlne or+: 4 roo 3 tmu @WP. IRMS MAGNETIC FIELD STRENGTH, H, dB RELATIVE TO ONE MICROAMPE

PER METER, FOR DISCRETE FREQUENCIES; OR 

RMS MAGNETIC-FIELD-STRENGTH SPECTRUM DENSITY LEVEL, Hd, dB - RELAT

ONE MICROAMPERE-PER-METER PER ,/mi , FOR BROAD BAND NOIS
py 

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 uA/m. The harmonic amplitudes remain fairly constant (between 58 and 70 dB pA/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 

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

noise becomes predominant) is the 25th (9 kHz) with an ampli- 
ANTENNA LOCATION 

PP

M

I *WATER PUMP AN

PUMP 

ROOM 
MICROAMPERE ONE TO RELATIVE dB H, STRENGTH, FIELD RMS MAGNETIC 

OR FREQUENCIES; DISCRETE FOR METER, PER 

TO RELATIVE dB Hdl LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

BAND NOISE BROAD FOR , ,JEKL PER MICROAt*!PERE-PER-METER ONE 

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1.95-00 5.6640 1 0.00+000 O.OO+OOO 20 41008 41008 

8 $ 12 9 72Galn corr., rcc.= 14 tot conrt: 65.9 

C=34 RG= 20( 1466) DG= 0 FG= -1 AGE 52 

t* 000+009 a 0.3086 1 .769+ 008. 

SYSTEM NOISE 

10 

0 

- 10 

FREQUENCY, \Hz 

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 meast of the 

bottom of Bailey shaft. Spectral resolution is 

7.81 Hz. 
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 
C. O , 

LcrnETVI- 
FREQUENCY, tiHz 

~i~~~~ 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. 
MICROAMPERE ONE TO RELATIVE dB HI STRENGTH, FIELD RMS MAGNETIC 

OR FREQUENCIES; DISCRETE FOR METER, PER 
c,mnaruvl 

c~nlnlr. m 

ul P-r3 r(D 

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dB pA/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 strengtlir 

are unusually high. The logarithmic average of harmonics 3 

through 8 is 98.2 dB pA/m. Subtracting 26.0 dB pA/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. 
r3m3wm 3 COT-. 

oOL3-13 on mm -.m Or+ Ole" 31 2.8. 

c c 

3p13 -.rc, >as l;l w oaorto 

MICROAMPERE ONE TO RELATIVE dB , H STRENGTH, FIELD MAGNETIC RMS 

OR FREQUENCIES; DISCRETE FOR METER, PER 

TO RELATIVE dB Hd, LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

BAND NOISE BROAD ,FOR PER,/^^ MICROAMPEL-PER-METER ME 

-J,t 

0. 

00 Y 00 

n - 

P, 0 

- 
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 measure - 

ments 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. 

I I I. AMPLITUDE PROBAB ILITY DISTRIBUTION MEASUREMENTS 

A. Introduction 

The amplitude probability distribution (APD) of the re - 

ceived 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, 91. 

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 
Figure 41 Average magnet

Mine in the fre

produced by pow

360 Hz. The tw

2 meters repres

1080 and 2Sb0 H

bandwidth for m

110- 

0 f NEAR MAIN MINE SU

I- 100- 

0 

2 

I- 4 90- W 

i 

i 

FACE MINING MACHIN

-4 

P 

9 

i i 

10 - 

Oi 

I I I I 

2 3 4 5 

DISTANCE TO NEAR
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 I11 of this report describes the APD measurement 

methods and results. Part B.1 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 measure - 

ments. Part C includes many APD1s taken in a coal mine. An 

analysis of these results also is included. 

B. Noise Measurement Techniques 

1. Measurement Instrumentation 

Section 111, 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 [lo]. See figures 42a, 

b, and c. 
Figure 42a System for field recording. 

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 [lo] . 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 

volt age, 'rms ' and average voltage, V . The system functions 

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. 
INPUT OF EIFS METER, dB RELATIVE TO IrnV RMS 

Figure 43 Linearity of Electromagnetic Interference and Field 

Strength (EIFS) Meter. 
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 pV 0.1 pV 

cw 3-dB Signal Bandwidth* 1.4 kHz 3.3 kHz 

- Dynamic Range 67 dB 60 dB 

Spurious Response 

Reject ion 

Input Impedance 50 ohms with an 50 ohms with an 

input VSWR less input VSWR less 

than 1.2: 1 than 41.2:l 

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

NBS measurements . 
recorder. The current is limited by a solid

state, current

limiting circuit in series with a fuse. The power requirement 

is approximately 1 3 w a t t s 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 


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 

15 inches per second (ips) 

0.8 percent 

35 dB below nominal signal 

level 

1.5 percent total harmonic 

distortion 

Input Voltage Range 0.01 to 1 Vrms 

Input Impedance 20 kR 

Output Voltage 1 V rms into 1-kR load 

Output Impedance 100 R 

Frequency Response at 15 ips 100 Hz to 50 kHz at 2 2-dB 

points 
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 
Table 3. 

Characteristics of Laboratory Analog Magnetic Tape Recorder 

(Direct Record/Reproduce) 

(Manufacturer's Specifications) 

Tape Speed 

Flutter 

(cumulative peak -to -peak 

flutter) 

rms Signal to rms Noise 

Harmonic Distortion 

Input Voltage Range 

Input Impedance 

Output Volt age 

Output Impedance 

Frequency Response at 15 ips 

15 inches per second (ips) 

0.4 percent 

1 percent 

0.3 to 3.0 V rms 

100 kR 

1.0 V rms into 10-kR load 

100 Hz to 75 kHz at + 3-dB 

points 
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 

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 

Figure 48 APD data pro
voltages. These two readings give true rms and average 

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 term1' 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) 
g )

d is the axial spacing in meters between the two coaxial 

loops, 

I is the transmitting loop current in rms amperes, and 

X is the free-space wavelength in meters. 

The transmitting and receiving loops are positioned coaxially 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 
determining the magnetic field strength only when r 1, r2' and 

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

least four times the radius of the larger of r and r2 for 1 

equation (I) 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 5 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. 
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 APDfs. These APDfs 

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

magnetic field strength, H, while the horizontal axis gives 
Linear by - 10q,~(-In pl 

Figure 49 APD , 10 kHz, vertical component, 1.0 kHz predetection bandwidth, 

December 5, 1972. 11: 25 a.m. , Robena No. 4. 
Figure 50 APD . 30 kHz, vertical component, 1.0 kHz predetection bandwidth. 

December 5, 1972, 2: 00 p .m . , Robena No. 4. 
.0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 51 APD , 70 kHz, vertical component, 1.0 kHz predetection bandwidth, 

December 5, 1972, 2: 45 p.m., Robena No. 4. 
Fig ;ure 52 APD, 130 kHz, vertical component, 1.0 kHz predetection bandwidth, 

December 5, 1972, 5: 45 p.m., Robena No. 4. 
.0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 53 APD , 500 kHz, vertical component, 1.2 kHz predetection bandwidth., 

December 5, 1972, 12: 45 p . m . Robena No. 4. 
.00011)0( -01 .I .5 I 5 I0 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 54 APD . 2 MHz, vertical component, 1.2 kHz predetection bandwidth, 

December 5, 1972, 2: 35 p.m., Robena No. 4. 
Linear by - $ log,,(-In p) 

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

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 55 APD, 8 MHz, vertical component, 1.2 kHz predetection bandwidth, 

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

P- MAGNETIC FIELD STRENGT

01! 
PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 57 APD , 10 kHz, horizontal component (E-W) , 1.0 kHz predetection 

bandwidth, December 5, 1972, 12: 45 p .me, Robena No. 4. 
.0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 5 8 APD , 30 kHz, horizontal component (E-W) , 1.0 kHz predetection 

bandwidth, December 5, 1972, 2: 30 p .m., Robena No. 4. 
hlAGNETIC FIELD STRENGlXf-I, 11 (dB RELATIVE TO 1 MICROAMPERE PER 
.0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 60 APD , 130 kHz, horizontal component (E-W) , 1.0 kHz predetection 

bandwidth, December 5, 1972, 6: 15 p .ma, Robena No. 4. 
Figure 61 APD , 500 kHz, horizontal component (E-W) , 1.2 kHz predetection 

bandwidth, December 5, 1972, 11: 25 a.m. , Robena No. 4. 
PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 62 APD, 2 MHz, horizontal component (E-W) , 1.2 kHz predetection 

bandwidth, December 5, 1972, 2: 00 p.m., Robena No. 4. 
.0001.001 .O1 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

ure 63 APD , 8 MHz, horizontal component (E-W) , 1.2 kHz predetection 

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

I-' 

N 

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

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 65 APD, 10 kHz, horizontal component (N-S) , 1.0 kHz predetection 

bandwidth, December 5, 1972, 1: 20 p .m., Robena No. 4. 
.0001.001 .01 S .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 66 APD , 30 kHz, horizontal component (N-S) , 1.0 kHz predetection 

bandwidth, December 5, 1972, 3: 05 p .m., Robena No. 4. 
.0001.001 .01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 9 8 99 

PERCENT Of TIME ORDINATE IS EXCEEDED 

Figure 6 7 APD , 70 kHz, horizontal component (N-S) , 1.0 kHz predetection 

bandwidth, December 5, 1972, 5: 20 p .ma, Robena No. 4. 
-- + ...7 *-*- - . . .+. .. .. .- .. . - - .--. - *-. -- --- ~---* -?* - , , 

 1- - I 

-- ~- +-- .--- - +- - - . - - - - ..---..A *+ - .-. --. I 

I 

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

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 6 8 APD , 130 kHz, horizontal component (N-S) , 1.0 kHz predetection 

bandwidth, December 5, 1972, 6: 50 p .m., Robena No. 4. 
Figure 69 APD , 500 kHz, horizontal component (N-S) , 1.2 kHz predetection 

bandwidth, December 5, 1972, 1: 30 p .m., Robena No. 4. 
D001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 70 APD , 2 MHz, horizontal component (N-S) , 1.2 kHz predetection 

bandwidth, December 5, 1972, 3: 05 p.m., Robena No. 4. 
.0001.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

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

bandwidth, December 5, 1972, 5: 20 p.m., Robena No. 4. 
9~01.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 72 APD , 32 MHz, horizontal component (N-S) , 1.2 kHz predetection 

bandwidth, December 5, 1972, 6: 50 p.m., Robena NO. 4. 
.0001.001 .01 .I .5 I 5 16 20 30 40 50 60 70 80 85 90 9 5 9 8 9 9 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 73 APD , 10 kHz, vertical component, 1.0 kHz predetection bandwridth, 

December 7, 1972, 10: 45 a. rn . , Robena No. 4. 
AlAGNETIC FIELD STRENGT
.0001.001.01 .I .5 1 5 lU 20 30 40 50 60 70 80 85 90 95 9 8 9 9 

PERCENT OF TIME ORDINATE IS EXCEEDED 

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

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

................ 

-. 

, , 

. ~ 

-- .... ....... ..--- .- .- ... 

... .. ... .-.. 

... * .-., 

...... ... 

- -- .............. 

. . - - . - -- ..-./+-~. * .. ---.: ... i 

I i 

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

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 76 APD , 130 kHz, vertical component, 1.0 kHz predetection bandwidth, 

December 7, 1972, 1: 25 p .m. , Robena No. 4. 
.0001.001 .01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 9 8 9 9 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 7 7 APD , 500 kHz, vertical component, 1.2 kHz predetection bandwidtl 

December 7, 1972, 10: 45 a. m. , Robena No. 4. 
.0001.001.01 .I 5 1 5 10 20 30 iZ 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 78 APD , 2 MHz, vertical component, 1.2 kEz predetection bandwidth, 

December 7, 1972, 11: 20 a, m . , Robena No. 4. 
~01.001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Ire 79 APD , 8 MHz, vertical component, 1.2 kHz predetection bandwidth, 

December 7. 1972, 11: 55 a.m.. Robena No. 4. 
.0001.001 .01 .I 5 1 5 10 20 30 40 50 60 70 a0 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 80 APD , 32 MHz, vertical component, 1.2 kHz predetection bandwidth, 

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

60 I I 1 I IlttI 

$ 

Figure 82 Field strengt

of the time a

component E-W

-20 

-40 -60 

-80 

- - 

I I 1 1 11111 

1 

.I1! 

OkHz 

100 
40 I 1 1 11 111 I I 1 I 11 111 I r 1 11 111 I 11 11111~ 

20 - X MAXIMUM (0.1%) 

RMS 

- 

o AVERAGE 

0 - 

-20 - 

-40 - - 

-60 - - 

-80' I I I I I1111 I I I1 I111 I Illllll 1 I I 1 1111, 

lOkHz 100 1 MHz 10 10 

FREQUENCY 

Figure 83 Field strength excursions between 0.1% and 99% of 

the time as a function of frequency, horizontal 

component N-S, December 5, 1973. 
20 

1 OkHz 100 

Figure 84 Field strength excu

of the time as a fun

component, December 
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 APD1s; longer periods (46 minutes) do not. The 

emphasis in this program was to obtain noise measurements 

during normal operat ion. We did not make measurements when 

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

by many tens of decibels. 
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. 
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 I n s t r u m e n t a t i o n 

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 
EQUIPMENT 

SIMILAR TO 3, CHANNEL 7 (FM) 

BUT WlTH DIFFERENT 

ANTENNA ORIENTATION 

Figure 85 Surface recording sys tern. 
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. Locat ion 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. 
POWER 

LINE 

BLAKER SHAFT 

ROBENA # 4 COAL MINE 

COMPRESSOR 

AND FANS 

Figure 86 Location of surface measurements. 

140 
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 
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. 
3S10N OKVB OVOllB 104 'H ll3d ll313W d3d-313dWV01131W IN0 

01 3AllV131 BP "H '13~31 AlISN3O Wnll3MS H13N3~1S-0131j-3113N3VW SWll 

llO 'S313~3nD314 113DSIO 101 'll313W M3d 

3ll3dWVOll31W 3N0 01 3hIlV13M BP 'H '~13N311S OBI4 3113N3VW SWll 
MICROAMPERE ONE TO RELATIVE dB H, STRENGTH, FIELD MAGNETIC RMS 

OR DISCRETE FREQUENCIES; FOR PER METER, 

TO RELATIVE dB Hd, LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

BAND NOISE BROAD FOR m, PER MICROAMPERE-PER-METER ONE 

-& gf "ye y, 

m=* 0 0 -u 0 - 44: 4:: 

,'V 0 

0 0~3, . c. - .- pu"- B 

fg, ; 

;:; 

0 0 0 

g:-, .. - It -75 :. :: -. . O ., - ,510, 2 gz: .! I, 1' 

2 U ..B 

0' 2 2 N 

;g: f 

'N" ", 

No ' 

0 
MICROAMPERE ONE TO RELATIVE dB H, STRENGTH, FIELD RMS MAGNETIC 

OR DISCRETE FREQUENCIES; FOR METER, PER 

TO RELATIVE dB Hd, LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

NOISE BAND FOR BROAD m, PER MICROAMPERE-PER-METER ONE 
MICROAMPERE ONE TO RELATIVE dB H, STRENGTH, FIELD RMS MAGNETIC 

OR FREQUENCIES; DISCRETE FPR METER, PER 

TO dB RELATIVE Hdr LEVEL, DENSITY SPECTRUM RMS MAGNETIC-FIELD-STRENGTH 

BAND NOISE BROAD ,FOR mrz PER MICROAMPERE-PER-METER ONE 
LP-l 

MICROAMPERE ONE TO RELATIVE dB ti, STRENGTH, IELD f RMS MAGNETIC 

OR DISCRETE FREQUENCIES; FOR METER, PER 

TO RELATIVE dB Hd, LEVEL, DENSITY SPECTRUM RMS MAGNETIC-FIELD-STRENGTH 

BAND NOISE FOR BROAD m, MICROAMPERE-PER-METER PER ME 
MICROAMPERE ONE TO RELATIVE dB STRENGTH, H, FIELD MAGNETIC RMS 

OR FREQUENCIES; DISCRETE FOR METER, PER 

TO RELATIVE dB Hd, LEVEL, DENSITY SPECTRUM MAGNETIC-FIELD-STRENGTH RMS 

NOISE BAND BROAD FOR m, PER MICROAMPERE-PER-METER ONE 
Linear by - log,,(-In p) 

11001 .001.01 .I .5 1 5 10 20 30 40 50 60 70 80 85 90 95 9 8 99 

PERCENT OF TIME ORDINATE IS EXCEEDEO uscoYM - m 

Figure93 APD, 10 kHz, surface, antenna sensitive axis 

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

11:45 a.m. 
POO1.001.01 .I 5 1 5 I0 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED ~S~OYY . m 

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

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

2:00 p.m. 
'Ti 

Pa 

09 MAGNETIC FIELD STRENGTH, H (dB RELATIVE TO 1 MICROAMPERE P
Fig 

-60 

-70 

iI 111111 1 IIlli!irl I l i l l l I l I l I I I I 1 ' illllli 11 11 1 I I I 1 I I I ,.I 

fi~O1.001.01 1 5 1 5 N) 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED -.MI. 

;ure 96 APD, 130 kHz, surface, antenna sensitive axis 

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

5:15 p.m. 
At 10 kHz the surface noise varies between 26 dB pA/m 

(0.0001% of time) and -20 dB pA/m (99% of time). 

A summary of the field strength excursions between 0.1% 

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 

to the input of an amplifier with lo8 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 pV 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' 
ti, SIKCN~JIII, rlkLu MH~N~ILL 

:JIICI?OAML'EKE ONE TO RELATIVE dB 

RMS METER, PER 
I. 95001 5.074001 0.00*000 0.00*000 20 49008 4f000 

10 94 12 7 72Ga1n corr., rec.= 0 tot const.=-31.0 

C=35 RG= 100 ( OdBl DG= 0 FG=-31 AG= 0 ,. 000-006 0.3086 2.80 1 -00f. 

- - - - - 

0 I 2 3 4 

FREQUENCY, kHz 

Figure 98 Voltage spectrum, 100 Hz to 3 kHz, Robena No. 4 

mine, underground, roof bolts-3.6-meter separation, 12 :25 p.m., Dec. 7, 1972. Spectral resolution is 7.81 Hz. 
FREQUENCY, kHz 

Figure 99 \lo1 tag" spcctrui~i, 1 v' ..: ; i.,.~, .,oi,cnii XO. 4 

qine, underground, roof bolts-8.5-meter separation, 12:20 p.m., !kc. 7, 1972, Spectral reso- 

!ution is 7.81 Ilz. 

w S 2 2 -130.. 

2 O0 

W =-a 

me 

", 5 w -140.- 

- ZZ 

W 

a 5 d id -150.. r 

2 CL 

0 

7 m 

'0 

m 

5: & 

CE 

-160 

0 I 2 3 4 
RMS VOLTAGE, dB RELATIVE TO ONE VOLT, FOR DISCRETE FREQUENCIES; O

RMS VOLTAGE SPECTRUM DENSITY LEVEL, 

dB RELATIVE TO ONE VOLT PER JUIHz, FOR BROAD BAND NOISE 
12 0 0 2048 20 l.S4*000 7.81*000 11/09/75 14:S6:48 S 7 

1.95-001 S.B7*OOI 0. OO*OOO 0. OO*OOO 20 45008 45000 

9 94 12 772Galn corr., rec.= 0 tot const.=-31.0 

C=35 RG= 100C OdB) DG= 0 FG=-31 AG= 0 

MINE GENERATED NOISE . 

FRE QU EEY , kHz 

Figure 101 Voltage spectrum, 100 tIz to 3 kHz, Roberla No. 4 

mine , underground, dipole antenna, 8.5-meters 

long, 12:24 p.m., Dec. 7, 1972. Spectral resolution is 7.81 Ilz. 
3 through 8 is 70.0 dB below 1 volt, i.e., 316 pV 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. ,4 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 
~i~~~~ 102 3D 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. 
Q+o Cho 

0 OdO 

w *kk 0 

00 wad 

Z *m 3 C, N m 0 cd - ha .d 

AcmdQ) 

ao-( ks F' 

d .duo cl 0 -cd.d.d 

ts,aLnw ac, 
quency index. 

D. Mine Phone Wire Measurements 

1. Voltage -- 

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 
0 4 8 12 

FREQUENCY, kHz 

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 spectr-urn were 300 Hz to 40 kHz with a spectral resolution of 62.5 Hz. 
OR FEECiliEtiCIES; DISCRETE ONE 'JOLT, FOR TO RELATIVE dB VOLTAGE, KMS 

, LEVEL DENSITY VOLTAGE SPECTRUM RMS 

NOISE BAKD FOR BROAD , fit PER VOlT OIJE TO RELATIVE dB I I 0 CbO>--JcnPW - I I I 

OOOOOOO&OO 
I2 0 0 2048 20 1.34+000 7.81+000 11/19/73 15:00:07 2 2 * 

1.95-00 1.63*001 0. OO*OOO 0. OO*OOO 20 45008 43008 

1 9g-12 572Goin corr., rec.' 6 tot con,t.= 5.5 

C=14RG= 50( 6dB) DG= OFG= -1 AG= 0 

I I MlNE GENERATED NOISE 

FREQUENCY, kHz 

Figure 106 Spcctl-urn of the voltage measured on the mine phone 

wire relative to the I-ail, Robena No. 4 mine, 

underground, 100 I-Iz to 3 kllz, 4:30 p.m., Dec. 5, 

1972. Spectral ~.csolution is 7.81 112. 
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. 
3SION ON10 (IVOE0 104 ' Z- E3d 3EldWV 3NO 01 3AllVlB 0P 

' l3A31 AlISN3Q HflEl33dS lN3EEn3 SWll 

110 "313~3n83114 31M3SlO 804 ' 383dYV 3140 01 3h11V13ll 0P '1~31111n3 SWll 
I-'. 

UJ z 

(D (D w. 

n- I 

. (D 

C 

WJ 0 " GoID rt 

RMS CURRENT, 

dB RELATIV

RMS VOLTAG

dB REL
0 .d r-- 

o,QE 

OC 

(d 0- U 

u> a, 4.d .G 

ouo 

> mz - . a; m S=N CCCO 

u ucm 

OOU c, GI- a0 l mQmu4 
causes a significant drop in noise, up to 40 dB. 

5. The APD1s 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 pA/m at 30 MHz. These are 

time-averaged rms values. 
7. Magnetic field noise measured on the surface above 1 

( 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 pA/m at 10 kHz to -30 dB pA/m 

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

! 8. Surface noise may be correlated to underground noisel 

I 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, RECOMMENDATIONS - 

Electromagnetic noise is a dominant factor in range- 

( limited communication systems. In mine communication systems, ( 

I range -limiting factors are high attenuation, low size -weight 1 

considerat ions, cost considerat ions, power -permissibility 

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

Lorne Matheson, and Dave Lewis. 

Winston Scott and Don Halford provided much assistance in 

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

drafting assistance. 
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. 
1967, pp. 70-73. 

Greene, F .M., NBS field-strength standards and measurements (30 Hz to 1000 MHz), Proc. IEEE, Vol. 55, No. 6, 

June 1967, pp. 970-981. 

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

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

Thompson, W. I., 111, Bibliography of ground vehicle communications and control. AKWIC index, Report No. DOTTSC-UMTA-71-3, July 1971. 
[A] Shaver, tlarry V., V. Elaine Hatfield; George H. Hugh, 

!Jan -made radio noise parameter identification task, 

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

1 101 JIatheson, R. J . , Instrumentat ion problems encountered 

making man-made electromagnetic noise measurements for 

predicting communication system performance, IEEE Trans. 

on EFf compatibility, Vol. ENC-12, No. 4,( November 1970, 

pp, 151-158. 

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

meters at the National Rure-au of Standards, NBS Circular 

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

of loop antennas at VLF, NBS ,J, Res., - 64 ,, N.o. 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) . 

[IS] 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." UXOMM . ERI 

IX. APPENDIX 

Decoding of Spectrum Captions 

Spectrum captions are generally organized into the following format: 

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

where 

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

zMP where MP = 11 

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 .I68 

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 
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 lo-') 

Standard Error = standard error of curve, example, 0.3162 

Spectral Peak = largest spectral peak observed, example, 

7.747-006. (7.747 x 
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, 0 

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

digit, example, -1 

AG = absolute gain correction added to data, example, 0 
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. 

17. KEY WOKLIS (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 strength1 

r L-- ' For Off~clal 13istribution. 110 Not Release to NTIS 

instrumentation; spectral density; time-dependent spectral density. 

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Cryogenic Data Center Current Awareness Service 

(Publications and Reports of Interest in Cryogenics). 

A literature survey issued weekly. Annual subscription: Domestic, $20.00; foreign, $25.00. 

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. 

Superconducting Devices and Materials. A literature 

survey issued quarterly. Annual subscription: $20.00. 

Send subscription orders and remittances for the preceding bibliographic services to the U.S. Department 

of Commerce, National Technical Information Service, Springfield, Va. 22151. 

Electromagnetic Metrology Current Awareness Service 

(Abstracts of Selected Articles on Measurement 

Techniques and Standards of Electromagnetic Quantities from D-C to Millimeter-Wave Frequencies). 

Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription 

order and remittance to the Electromagnetic Metrology Information Center, Electromagnetics Division, 

National Bureau of Standards, Boulder, Colo. 80302.