NBSIR 74-369

SURFACE MAGNETIC FIELD NOISE MEASUREMENTS

AT GENEVA MINE

J. W. Adams

W. D. Bensema

N. C. Tomoeda

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

June 1974

Prepared for:

U. S. Bureau of Mines

United States Department of the Interior

Pittsburg, Pennsylvania 15222

Working Fund Agreement HO 133005

NBSIR 74-369

SURFACE MAGNETIC FIELD NOISE MEASUREMENTS

AT GENEVA MINE

J. W. Adams

W. D. Bensema

N. C. Tomoeda

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

The views and conclusions contained

in this document should not be

interpreted as necessarily representing the

official policies or recommendations of the

Interior Department's Bureau of Mines of the

U. S. Government.

June 1974

Prepared for

U. S. Bureau of Mines

United States Department of the Interior

Pittsburg, Pennsylvania 15222

Working Fund Agreement HO 133005

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

"NATIONAL BUREAU OF STANDARDS Richard W Roberts Director
FOREWORD

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

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

contract was initiated under the Coal Mine Health and Safety

Research Program. It was administered under the technical

direction of the Pittsburgh Mining and Safety Research Center

with Mr. Howard Parkinson and Mr. Harry Dobrowski acting as the technical project officers.

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

contract during the period June 1973 to June 1974. This report

was submitted by the authors in September, 1974.
CONTENTS

Page

1.0 Introduction 1

2.0 Measurement System 2 3.0 Earth- Ionosphere Waveguide Effect on Propagated

Noise 3

4.0 Other Measured Data 4

5.0 Conclusions 5

6.0 Recommendations 5 7.0 Acknowledgments 5

8.0 References 6

9.0 Appendix 34

LIST OF FIGURES

Page

Figure 1. Field recording system 7 Figure 2a. Digitizing part of data processing system 8

Figure 2b. Fast-Fourier transform part of data

processing system 8

Figure 3. Spectrum of the horizontal, E-W component 9

Figure 4. Spectrum of the horizontal, N-S component 10

Figure 5. Spectrum of magnetic field strength vs. time-- 11

Figure 6. Spectrum of magnetic field strength vs. time-- 12

Figure 7. Map of surface over Geneva Coal Mine 13

Figure 8. Spectrum of surface EM noise at Lila Flats

over Geneva Mine 14

Figure 9. Spectrum of surface EM noise, north side of

Lila Point 15

Figure 10 . 10 kHz spectrum of surface EM noise at Lila

Canyon Overlook 16

iii
LIST OF FIGURES (Continued)

Page

Figure 11. 10 kHz spectrum o£ surface EM noise at Lila

Canyon Overlook 17

Figure 12. 10 kHz spectrum of surface EM noise at Lila

Canyon Overlook 18

Figure 13. 10 kHz spectrum of surface noise at Lila

Canyon Overlook 19

Figure 14. 10 kHz spectrum of surface noise at Lila

Canyon Overlook 20

Figure 15. 10 kHz spectrum of surface noise at Lila

Canyon Overlook 21

Figure 16. 10 kHz spectrum of surface EM noise at Lila

Canyon Overlook 22

Figure 17. 10 kHz spectrum of surface EM noise at Lila

Canyon Overlook 2 3

Figure 18. 10 kHz spectrum of surface EM noise at Lila

Canyon Overlook 24

Figure 19 . 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 25

Figure 20 . 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 26

Figure 21. 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 27

Figure 22. 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 28

Figure 23. 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 29

Figure 24. 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 30

Figure 25. 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 31

Figure 26. 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 32

Figure 27. 3 kHz spectrum of surface EM noise at Lila

Canyon Overlook 33

iv
SURFACE MAGNETIC FIELD NOISE MEASUREMENTS

AT GENEVA MINE

by

J.W. Adams, W.D. Bensema, N.C. Tomoeda

Measurements o£ surface magnetic field noise were

made at various locations over the Geneva Coal Mine

near Price, Utah, on June 12, 1973. The locations

selected were on the surface over emergency locator

beacons underground at depths between 350 meters

(1150 ft.) and 488 meters (1600 ft.). The surface

terrain where these measurements were made was

mountainous, and access was difficult. There were no

power lines within several miles, and the weather was

clear; therefore, the magnetic noise levels were about

as low as will normally occur.

Results of measurements of distant sferics indicate rather sharp cutoff frequencies below which broadband, impulsive noise is attenuated. The mechanism

of propagation for this noise above the daytime cutoff

frequency of 3500 Hz and the nighttime cutoff frequency

of 1700 Hz is deduced to be a waveguide formed by the

D or E layers of ionosphere as an upper plane and the

earth as a lower plane.

The measurement systems used are similar to those

used earlier. The technique is to record broadband,

analog signals, digitize the data, and use a fastFourier transform to obtain spectral plots. This technique is novel in that it can measure simultaneously

all magnetic field energy within a limited portion of

the spectrum for a limited time, and, after processing,

reproduce the events occurring in that time interval

in great detail

.

Key words: Earth- ionosphere waveguide; electromagnetic

noise; EMI measurement technique; sferic interference.

1 . 0 Introduction

Magnetic field strength measurements were made on June 12,

1973, over the Geneva Coal Mine in the Book Cliff Mountain

Range east of Price, Utah. The locations selected were on

the surface over emergency locator beacons underground at
depths between 350 meters (1150 ft.) and 488 meters (1600 ft.)-

The surface terrain where these measurements were made was

mountainous, and access was difficult. There were no power

lines within several miles, and the weather was clear; therefore, the magnetic noise levels were about as low as will'

normally occur.

The primary purpose of the measurements was to determine

surface magnetic field noise levels so that performance of

emergency subsurface locator beacons of the U.S. Bureau of

Mines could be better predicted. These emergency locator

beacons are located many hundreds of feet underground and

when activated, generate magnetic fields in a pulsed-carrier

,

on-off mode for signaling to the surface, usually in emergency situations. They operate at frequencies below 3 kHz

where signal attenuation through the earth is relatively low;

however, the beacon signals are greatly attenuated by various

effects, and surface noise becomes a limiting factor.

2 . 0 Measurement System

The block diagram of the field recording measurement system is shown in figure 1. It consists of a balanced, shielded

loop antenna, balun, filter, and analog tape recorder. Later

in the laboratory, the analog signal is filtered, digitized,

fast Fourier transformed, and plotted on microfilm. See

figure 2 for the laboratory processing system. This gives an

output plot of one component of absolute magnetic field strength

versus frequency- -a spectral plot. The transform may be

repeated to allow three-dimensional plots, where time is the

additional variable.

This system is described in more detail in the Robena

Mine report [1]

.

2
3 . 0 Earth- Ionosphere Waveguide Effect

on Propagated Noise

During the time the measurements were being made, there

were no visible thunderstorms or clouds anywhere in sight,

and hence, the atmospheric noise was largely that propagated

from distant sources. During daylight, strong sferics were

present, primarily above 3500 Hz, as shown in figure 3. At

night, sferics came in above 1700 Hz, as shown in figure 4.

A three-dimensional view given in figure 5 shows more detail

of the daytime structure. A similar plot in figure 6 shows

the nighttime structure. Note the 2500 Hz and 1900 Hz subsurface coal mine beacon signals in figure 5. The 1900 Hz

beacon is almost obscured by the atmospheric noise at night

(see figure 6). Notice the sharp cutoff of noise at 1700 Hz

at night and the more gradual cutoff at 3500 Hz during the day.

Ionospheric effects on radio transmission have been widely

studied for years, but these measurements with this new system

show some fresh insights into earth-ionosphere waveguide

phenomena. A dramatic and sharp increase in attenuation of

propagated atmospheric noise at frequencies below the waveguide

cutoff frequency (as mentioned above) has been observed. About

ten dB of signal-to-noise ratio may be gained by operating at

a frequency below the waveguide cutoff frequency rather than

above the cutoff, as shown by the one example in figure 6.

The probable propagation mechanism is a parallel plate

waveguide formed by the D or E layers of the ionosphere and

the earth. The TE or TM modes are excited between the

parallel planes and have a cutoff frequency of

r nc T o ^c = 2i' ^ = 1,2,...,

where c is the velocity of light, and a is the spacing between

the plates [ 2]

.

3
, I£ a = 88 km, £^ = 1704 Hz, 3408 Hz, ... . If the D

layer is about 50 km above the earth, and if the E layer is

about 100 km high [3] , the cutoff frequencies calculated are

approximately correct. The height of maximum ionospheric

density may vary somewhat, and may not be the exact distance

needed for this model. This phenomenon should be further

investigated, as it relates directly to what frequencies that

should be used for the emergency locator beacons .

4.0 Other Measured Data

A map of the surface is shown in figure 7. Noise at

location Bl, 463 meters (1520 feet) over the 1900 Hz beacon, . is shown in figure 8. Noise at location CI, 442 meters

(1450 feet) over a 1700 Hz beacon, is shown in figure 9.

All the remaining figures are of noise at location Al

,

1150 feet over a 2500 Hz beacon.

Figures 10 through 18 show spectra of day, twilight, and

night noise to 10 kHz. Figure 13 shows a distant sferic.

Figures 19 through 27 show expanded spectra of day,

twilight, and night noise. These spectra are valid from

100 Hz to 3 kHz.

Data in figures 8 through 27 is absolute and has an uncertainty of ± 1 dB [1]. This uncertainty only applies over the

following frequency ranges: figures 8 and 9, 300 Hz to

2600 Hz; figures 10 through 18, 560 Hz to 10 kHz; figures 19

through 27, 100 Hz to 3 kHz. See section 9.0, Appendix, for

the code key to use in determining the meaning of the numbers

in the header block at the top of each spectrum. The resolution bandwidth is given on the ordinate of the plots.

4
5 . 0 Conclusions

The surface noise at a remote site, away from powerlines,

will not be free of powerline harmonics; their amplitudes will

be reduced.

The earth- ionosphere may provide a waveguide to propagate distant noise, particularly above 3500 Hz during the day

and above 1700 Hz at night. These frequencies are valid only

during the period covered by these measurements, as ionospheric

phenomena are quite time, geographically, and seasonally

dependent

.

6 . 0 Recommendations

These limited results indicate that emergency locator

beacon frequencies should be selected below 1700 Hz and between harmonics of the 60 Hz powerline frequency.

Additional measurements should be made over a diurnal

cycle and during each of the four seasons. Higher gain baluns

and/or amplifiers should be used to lower system noise.

7 . 0 Acknowledgments

Ed Niesen assisted with the field work, Winston Scott

assisted with the data processing, and Sharon Foote and

Janet Becker performed typing services.

David Sterns at the University of Colorado assisted with

digitizing data.

Carl Fisher and Ruben Mayes of Westinghouse Georesearch

Laboratory made arrangements with Mr. Watson of Geneva Mine

of U.S. Steel Co. Fisher and Mayes assisted in specifying

locations, frequencies, and overburden with respect to

emergency locator beacons

.

Frank Cowley and Lorne Matheson of the National Oceanic

and Atmospheric Administration assisted with computer software

and data processing.

5
8.0 References

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

Noise in Robena No. 4 Mine," NBS Technical Note 654,

April 19 74.

Ramo and Whinnery, "Fields and Waves in Modern Radio"

[John Wiley § Sons, Inc., New York, New York, Second

Edition, p. 328, p. 335, etc., 1959).

Terman, "Electronic and Radio Engineering" (McGrawHill Book Co., Inc., New York, New York, Fourth Edition,

p. 825 , 826 , 19 55)

.

6
LOOP ANTENNA,

SENSITIVE AXIS VERTICAL

SIMILAR TO

BUT WITH ANTENNA

SENSITIVE AXIS

HORIZONTAL, E-W

SIMILAR TO 3

BUT WITH ANTENNA

SENSITIVE AXIS

HORIZONTAL, N-S

25 KH^

Figure 1. Field Recording System

/

7
CHANNEL

3

ANALOG

TAPE

5

RECORDER

7

> _

LOW

PASS

FILTER

DIGITIZER

ONE

CHANNEL

AT A TIME

Figure 2a. Digitizing part of data processing system

Figure 2b. Fast-Fourier transform part of data processing system

8
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01 BAliVl^y 9P '^H U1SN3Q 'dPiyM Hi3N3}liS-Q131i-01i3N3VW S

— LA

a:: n~> <

o

CO

0)3

•H

9
10
Figure 5 Spectrum of magnetic field strength vs. time. Antenna

placed on surface of ground above Geneva Coal Mine,

daytime. "Lost-Miner Beacon" pulses showing at 2500 Hz

are from a transmitter beacon straight dovm 351 meters

(1150 feet) . Pulses at 1900 Hz are from a transmitter

0.8 kilometers (1/2 mile) away under 463 meters (1520 feet)

of overburden.

11
Spectrum of magnetic field strength vs. time. Antenna

placed nn surface of ground above Geneva Coal Mine,

nighttime. " L ost- IVl iner Beacon" pulses at 2 500 Hz can

still be seen but those at 1900 Hz are nartially obscured

by the atmospheric pulses (lightning static) propagating

in from over the horizon. This plot shows that by placing

transmitter frequency below 1600 Hz, atmospheric

interference can be reauced at least 10 dB, a factor

of 10 to I reduction in power.

12
Geneva Coal Mine Entrance

Figure 7 Map of surface over Geneva Coal Mine.

Location

Code

Overburden Beacon Frequency

(Hz)

Al 350 2500.0

Bl 463 1900.0

CI 442 1700.0

Dl 503 922.5

13
C_3

U_l CIS CO

U-l

— CO

CJ5 era

CO u-l

Ui-J

12 0 0 2048 20 2.15*000 4.88*000 09/25/75 I"':01:56 9 37 1.95-005 -4. 46» 00 1 0. 00 * 000 0.00* 000 20 43008 45008 8 77 S 12 73GQin corn., rec.= -40 tol const. = 12.4

C=23 RG=1 0000 (-40dB) D&

20 I. 0 00 + 002

C F&=

0. 308S

0 A&= 52

7. 023+CCl

1900 Hz BEACON

2,500

FREQUENCY, Hz

Figure 8 spectrum of surface EM noise at Lila Flats over Geneva Iline, Location Bl on Figure 7. 1900 beacon coming through

463 meters (1520 ft.) of overburden is barely visible. Antenna sensitive axis was vertical. Daytime. June 12, 1973.

14
12 0 0 2048 20 2.15«000 4.88*000 09/25/73 17:04:39 13 57

0 625 1,250 1,875 2,50 0

FREQUENCY, Hz

YigUVB 9 Spectrum of surface EM noise, north side of Llla Point, over Geneva Mine, Location CI on Figure 7. 1700 Hz beacon

throtir;'- 442 meters (1450 ft.) of overburden is not visible. Antenna sensitive axis was vertical. Daytime. June 12, 1973.

15
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31
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33
9 . 0 Appendix

Decoding of Spectrum Captions

Spectrum captions are generally organized into the following format:

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

where

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

MP

2 where MP = 12

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

NZS = Restart spectral average after output, example, 0

(restarted)

NDA = Data segment advance increment, example, 2048

NPO = Number of spectra averaged between output calls , example, 2 0

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

DF = Resolution bandwidth, spectral estimate spacing in

hertz, example, 62.5

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

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

Frame= Frame set number, example, 10

Serial = Film frame serial number, example, 42.

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

DTA = Detrending filter parameter a, example, 0.00195

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

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

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

NSA = Number of periodograms averaged, example, 20

NRP = Number of data points processed since spectrum

initialization, example, 43008

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

34
Third line: RUN, SESSION, MONTH, DAY, YEAR Gain corr. , rec. = tot. constr. =, where

Run and Session = the title o£ the portrayed frame identifying

the digitizing session and run number,

example, 21 8 3

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

example, 8 25 73

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

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

example , 46 .

4

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

C = correction curve used with data, example, 25

RG = receiver gain and accompanying correction in dB added to

the data, example, 200 (-6 dB)

DG = digitizer gain, example, 0

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

example, 0

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

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

Top of Scale = largest scale marking for computer drawn

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

Standard Error = standard error of curve, example, 0.3162

Spectral Peak = largest spectral peak observed, example,

4.108+003 (4.108 x lO"^)

35
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUl^LICATION OR REPORT NO. NBSIR 74-369

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE 5. Publication Date 6/74

6. Performing Organization Code

276.55

7. AUTHOR(S)

J. W. Adams, W. D. Bensema, N. C. Tomoeda

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Labs

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2768412

11. Contract/Grant No. H0133005

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

U. S. Bureau of Mines

Pittsburgh Mining and Safety Research Center

4800 Forbes Avenue

Pittsburgh, Pennsylvania

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.) Measurements of surface magnetic field noise were made at various locations over

the Geneva Coal Mine near Price, Utah, on June 12, 1973. The locations selected were

on the surface over emergency locator beacons underground at depths between 350 meters

(1150 ft.) and 488 meters (1600 ft.). The surface terrain where these measurements

were made was mountainous, and access was difficult. There were no power lines within

several miles, and the weather was clear; therefore, the magnetic noise levels were

about as low as will normally occur. Results of measurements of distant sf erics indicate rather sharp cutoff frequencies

below which broadband, impulsive noise is attenuated. The mechanism of propagation for

this noise above the daytime cutoff frequency of 3500 Hz and the nighttime cutoff

frequency of 1700 Hz is deduced to be a waveguide formed by the D or E layers of ionosphere as an upper plane and the earth as a lower plane.

The measurement systems used are similar to those used earlier. The technique is

to record broadband, analog signals, digitize the data, and use a fast- Fourier transform to obtain spectral plots. This technique is novel in that it can measure simultaneously all magnetic field energy within a limited portion of the spectrum for

a limited time, and, after processing, reproduce the events occurring in that time

interval in great detail.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Ear th-ionosphere waveguide ; electromagnetic noise; EMI

measurement technique; sferic interference.

18. AVAILABILITY Unlimited

i For Official Distribution. Do Not Release to NTIS

I

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

I

^ Order From National Technical Information Service (NTIS)

Springfield, Virginia 221'31

19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

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20. SECURITY CLASS

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