SURFACE MAGNETIC FIELD NOISE MEASUREMENTS 

AT GENEVA MINE 

J. W. Adams 

W. 0. 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 

~ittsbur~; Pennsylvania 15222 

Working Fund Agreement HO 133005 

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

NATIONAL BUREAU OF STANDARDS Rtchard W Roberts D~re~tor 
This report was prepared by the National Bureau of Standards, 

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

contract was initiated under the Coal Mine Health and Safety 

Research Program. It was administered under the technical 

direction of the Pittsburgh Mining and Research Center 

with Flr. Howard Parkinson and Mr. Harry 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 

Introduction- -- - - -- ---- - - -- - - ----- -- --- - ---- - -- - -- - - 1 

Measurement System-- - --- -- -- -- ------ -- ----- --- - - -- -- 2 

Earth-Ionosphere Waveguide Effect on Propagated 

Noise-- -- - - -- ---- ----- --- ------ ------ ---- ----------- 3 

Other Measured Data ----- - --------- ------------------ 4 

Conclusions - - -- - - -- - -- - -- -- - -- - -- - - -- - - - - - - - - - -- - - -- 5 

Recommendat ions - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 5 

Acknowledgments -- - - -- - - -- - - - - - - - -- - - - - - - - - - - - - - - - - - - 5 

Ref erences - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

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 of 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- - - -- - - -- - - -- - - --, - - - - - - - - - - - - - - 2 0 

Figure 15. 10 kHz spectrum of surface noise at Lila 

Canyon Overlook-- -- ---- ---- - ------ -- --- - ------ 2 1 

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

Canyon Overlook- - - -- - - -- - - - - - - - - -- - - - - - - -- - - - - 2 2 

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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2 5 

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

Canyon Overlook- - - -- - - - - - -- - - - - - -- - - -- - -- - - - - - 2 6 

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

Canyon Overlook- - - - - -- - - -- - - - - - - - - - - - - - - - - - - - - 2 7 

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

Canyon Overlook- -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2 8 

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- -- - - -- - - -- - - -- - --- - ---- - - - -- - - 3 1 

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- - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - 3 3 
SURFACE MAGNET1 C FIELD NOISE MEASUREMENTS 

AT GENEVA MINE 

by 

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

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 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 ; EM1 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 Sys tem 

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 [I]. 
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 p~o_bab_~e_p_r-opa~ation .- 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 

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

the plates 121. 
If a = 88 km, fc = 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 B1, 463 meters (1520 feet) over the 1900 Hz beacon, 

is shown in figure 8. Noise at location C1, 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 [I]. 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. 
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 Recommendat ions 

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. 
8.0 References 

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

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

April 1974. 

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

(John Wiley 6 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, 1955). 
I OSCILLOSCOPE I 

LOOP ANTENNA, I 

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25 kHz 

AN ALU ti 

TAPE RECORDER 

CHANNEL 

Figure 1. Field Recording System 

FILTER 3 . t 
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CHANNEL 

AT A TIME 

4 

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ANALOG 

TAPE 5, RECORDER 

Figure 2a. Digitizing part of data processing system 

C 

74 

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MICROFILM 

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placed on surface of ground above Geneva Coal Mine, 

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

are from a transmitter beacon straight down 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. X 
Figure 6 

Soectrum of magnetic field strength vs. time. Antenna 

 laced nn surface of ground above Geneva Coal Mine, 

in. "Lost-Miner Beacqn" pulses at 2500 Hz can 

still be seen but those at 1900 Hz are rartially obscured 

by the atmos~heric pulses (I ightning static) prqnagating 

in fr~m over the horizon. This plot shrlws that by placing 

transmitter frequency bel~w 1600 Hz, atmospheric 

interference can be reauced at least 10 dB, a factnr 

qf 10 tq I reducti~n in nower. 
Figure 7 Map of surface over Geneva Coal Mine. 

Location Overburden Beacon Frequency 

Code (Hz) 
Rlrl

RhlS C

O
0 625 1,250 1,875 2,500 

FREQUENCY 

Figure 9 spectrum of surface CII noise, north side of Lila Point, over 

c'eneva Mine, Location C1 on Figure 7. 1700 llz bencoq 

thro~,.,'. 442 meters (1450 ft.) of overhurden is not viqible. 

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9.0 Appendix 

Decoding of Spectrum Captions 

Spectrum captions are generally organized into the following format : L 

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

where 

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

2MP where MP = 12 

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

NZS = Restart spectral average after output, example, 0 

(restarted) 

NDA = Data segment advance increment, example, 2048 

NPO = Number of spectra averaged between output calls, 

example, 20 

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

DF = Resolution bandwidth, spectral estimate spacing in 

hertz, example, 62.5 

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

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

Frame= Frame set number, example, 10 

Serial = Film frame serial number, example, 42. 

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

DTA = Detrending filter parameter a, example, 0.00195 

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

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

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

NSA = Number of periodograms averaged, example, 20 

NRP = Number of data points processed since spectrum 

initialization, example, 43008 

NPP = Number of data points processed since data initialization, example, 43008. 
Third line : RUN, SESSION, MONTH, DAY, YEAR Gain corr. , rec. = 

tot. constr. =, where 

Run and Session = the title of the portrayed frame identifying 

the digitizing session and run number, 

example, 21 83 

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

example, 8 25 73 

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

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

example, 46 .4 

Fourth line: C =, PG =, 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 

4 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 lo3) 
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 fhe first letter of the first key word unless a proper 

name; separated by semicolons) Ear th-ionosphere waveguide ; electromagnetic noise ; EM1 

measurement technique; sf eric interference. 

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