TIME AND AMPLITUDE STATISTICS 

FOR ELECTROMAGNETIC NOISE IN MINES 

Motohisa Kanda 

Electromagnetics Division 

Institute for Basic Standards 

National Bureau of Standards 

Boulder, Colorado 80302 

Prepared for: 

U. S. Bureau of Mines 

Pittsburgh Mining and Safety Research Center 

4800 Forbes Avenue 

Pittsburgh, Pennsylvania 15213 

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

NATIONAL BUREAU OF STANDARDS Rlchard W Roberts D~rector 
CONTENTS 

Page 

1.0 INTRODUCTION- - - - -- - - -- - - -- - - - - - - -- - - -- - - -- - - - - - - - - - - -- - - - - -- - - - - - - 1 

2.0 MEASURANDS FOR TIME AND AMPLITUDE STATISTICS FOR EM NOISE IN 

MINES------------------------------------------------------------- 2 

2 1 Allan Variance Analysis (AVA) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2 

2.2 Interpulse Spacing Distribution (1SD)----------------------- 3 

2.3 Pulse Duration Distribution (PDD)--------------------------- 4 

2 -4 Average Crossing Rate (ACR) -- --- ---- - --- - ----- -- - ---- --- --- - 4 

2.5 Amplitude Probability Distribution (APD)-------- ------------- 4 

3.0 NOISE MEASUREMENT TECHNIQUES - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4 

3 1 Measurement Instrumentation - -- - - - - - - - - - - -- - -- - - - - - -- - - - - - - - - - 4 

3.1.1 Underground Recording System-------------------------- 4 

3 1.2 Data Transcribing Sys tern- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 5 

iii 
LIST OF FIGURES 

Page 

Figure 1. Block diagram of underground recording system- - - - - - - - - - - - - - 11 

Figure 2. Block diagram of transcribing system----------------------- 12 

Figure 3. Block diagram of data processing system-------------------- 13 

Figure 4-a. AVA, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 11:25 a.m., Robena No. 4------ 14 

Figure 4-b. ISD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 11:25 a.m., Robena No. 4----- 15 

Figure 4-c. PDD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 11:25 a.m., Robena No. 4------ 16 

Figure 4-d. ACR, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 11:25 a.m., Robena No. 4------ 17 

Figure 4-e. APD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 11:25 a.m., Robena No. 4------ 18 

Figure 5-a. AVA, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p.m., Robena No. 4------- 19 

Figure 5-b. ISD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p.m., Robena No. 4------- 20 

Figure 5-c. PDD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p.m., Robena No. 4------- 21 

Figure 5-d. ACR, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p.m., Robena No. 4------- 22 

Figure 5 -e . APD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p.m., Robena No. 4------- 23 

Figure 6-a. AVA, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:35 p.m., Robena No. 4------- 24 

Figure 6-b. ISD, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:35 p.m., Robena No. 4------- 25 

Figure 6-c. PDD, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:35 p.m., Robena No. 4------- 26 

Figure 6-d. ACR, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2 :35 p.m., Robena No. 4----- - - 27 

Figure 6-e. APD, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:35 p.m., Robena No. 4------- 28 

Figure 7-a. AVA, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 24, 1973, 10:30 a.m., Grace Mine---------- 29 
LIST OF FIGURES (Continued) 

Page 

Figure 7-b. ISD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 24, 1973, 10:30 a.m., Grace Mine--------- 30 

Figure 7-c. PDD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 24, 1973, 10:30 a.m., Grace Mine--------- 31 

Figure 7-d. ACR, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 24, 1973, 10:30 a.m., Grace Mine--------- 32 

Figure 7-e. APD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 24, 1973, 10:30 a.m., Grace Mine--------- 33 

Figure 8-a. AVA, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 10, 1973, 1:10 p.m., McElroy Mine-------- 34 

Figure 8-b. ISD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 10, 1973, 1:10 p.m., McElroy M i n e - - - - - - - - 3 5 

Figure 8-c. PDD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 10, 1973, 1:10 p.m., McElroy Mine-------- 36 

Figure 8-d. ACR, 1 MHz, vertical component, 1.0 kHz predetection 

bandwidth, April 10, 1973, 1:10 p.m., McElroy M i n e - - - - - - - - 3 7 

Figure 8-e. APD, 1 MHz, vertical component, 1.0 kHz predetection 

bandwidth, April 10, 1973, 1:10 p.m., McElroy Mine-------- 38 

Figure 9-a. AVA, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 11:40 a.m., Itmann No. 3------- 39 

Figure 9-b. ISD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 11:40 a.m., Itmann No. 3------- 4 0 

Figure 9-c. PDD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 11:40 a.m., Itmann No. 3------- 41 

Figure 9-d. ACR, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 11:40 a.m., Itmann No, 3------- 4 2 

Figure 9-e. APD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 11:40 a.m., Itmann No. 3------- 43 

Figure 10-a. AVA, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p.m., Itmann No. 3------- 44 

Figure 10-b. ISD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p.m., Itmann No. 3------- 4 5 

Figure 10-c. PDD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p.m., Itmann No. 3------- 46 

Figure 10-d. ACR, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p.m., Itmann No. 3------- 4 7 

Figure 10-e. APD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p.m., Itmann No. 3------- 4 8 

TIME AND AMPLITUDE STATISTICS FOR ELECTROMAGNETIC NOISE IN MINES 

Motohisa Kanda 

Abstract 

The time and amplitude statistics necessary to adequately 

describe electromagnetic (EM) noise in mines are illustrated 

through computer software techniques. They are 1) Allan Variance 

Analyses (AVA) , 2) Interpulse Spacing Distributions (ISD), 

3) Pulse Duration Distributions (PDD) , 4) Average Crossing 

Rates (ACR) , and 5) Amplitude Probability Distributions (APD) . These statistics are illustrated using data taken from a rather 

large store of raw analog data recorded in operational mines. 

The curves generated for the illustrations characterize 

the noise environment in the mines from which the corresponding 

data were taken, and should aid in the design of reliable communication systems for such mines. 

Key words: Allan variance analysis ; amplitude probability distribution; amplitude statistics; average crossing rate; electromagnetic interference; electromagnetic noise; impulsive noise; 

interpulse spacing distribution; magnetic field strength; manmade noise; pulse duration distribution; time statistics. 

1.0 Introduction 

The need for reliable communication systems in mines is a long-standing 

problem. For emergency use when all the power in a mine is cut off the residual electromagnetic (EM) noise is not a problem. However, if a communication 

system were designed for emergency use only, it would have two serious drawbacks. First, it would not be ready for use in an emergency; second, it 

would not be of any value during normal operations. Therefore, the Bureau 

of Mines decided to design communication systems that could be used for 

both emergency and normal operational conditions. 

During normal operation of a mine, the machinery used creates a wide 

range of many types of intense electromagnetic interference (EMI), and 

therefore ambient EM1 is a major limiting factor in the design of a 

communication system. 

There are several EM1 parameters that can be measured: magnetic field 

strength, H; electric field strength, E; conducted current, i, and voltage, 

v, between two conductors. Some measurements of each of these parameters 

were performed but only one parameter is discussed here, namely the magnetic 

field strength, H, the magnitude of the magnetic field vector [I]. Since 

there are a multitude of different sources that generate all the known types 

of noise, the resultant magnetic field strength noise vector is a function 

of its frequency, time, sensor orientation, and location. Small changes in 

these parameters can cause many tens of decibels variations in the measured 

field strength. 
3. Pulse Duration Distribution (PDD) , 

4. Average Crossing Rate (ACR) , and 

5. Amplitude Probability Distribution (APD) . These are discussed below in detail. 

2.1 Allan Variance Analysis (AVA) 

It is essential to know how much data to gather when dealing with statistical quantities. Therefore, in any measurement of a statistical phenomenon the minimum length of time over which the phenomenon is observed 

should be determined. Allan Variance Analysis (AVA) can be used to accomplish this determination. The basic idea to be discussed briefly below has 

been implemented often in the discussion of frequency stability [4,5,6], 

and is a special case (sample size two) of the more general Allan Variance 

discussed in the references. 
period, T. For a given maximum allowable deviation in y(t) the minimum 

averaging time can then be read off the graph. An example is given in 

section 4.0. A graph of the Allan Variance that clds;o,t decrgas,e,-wL5,hhh~ 

indicates that the record, y(t) , from which a2 12.~) was caw -- _3_I..._U*- 

ed ne& - 

to be longer. 'FOR R GWEH (e,) - (t,+%, 

The data are analyzed typically by computer via a program designed to 

compute the appropriate Allan variance. In the computer program log a 

versus log T is plotted on microfilm along with the associated confidence 

levels [7]. 

2.2 Interpulse Spacing Distribution (ISD) [8] 

The interpulse spacing distributions give the probability distribution 

for the spacing between successive pulses in the received noise process. 

These distributions are, of course, functions of the noise amplitude level. 

Seven distributions are given, each for a different noise amplitude level, 

covering the significant portion of the dynamic range of the noise process, 

approximately 45 dB. 
scribed in detail in the previous literature [9] . Figures 1, 2, and 3 give 

block diagrams of the underground recording system, the data transcribing 

system, and the data processing system. 

3.1.1 Underground Recording System 

Figure 1 gives the block diagram of the system. The principal parameter 

measured is magnetic field strength. Electrostatically-shielded loop antennas 

are used to intercept the radiated magnetic field and to substantially discriminate against any electric-field component. For the frequency range 

between 10 kHz and 250 kHz, the loop antenna is a collapsible, single-turn 

diamond configuration with an area of about 0.7 square meters. For the 

frequency range between 150 kHz and 32 MHz, a single-turn, 38-cm diameter 

loop antenna is used with a balun. A switch on each balun allows the use 

of several impedance-matching networks (four for the low frequency case and 

eight for 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). 

4 
In order to measure the time and amplitude statistics for the EM noise 

in mines, the AGC circuit of the EIFS meter is disabled. Since the gain of 

the receiver is now constant, the magnitude of the IF output is directly 

related to the band limited input noise signal magnitude. 

A commercially available, battery-powered, portable, analog magnetic 

tape recorder is used for recording. Since the bandwidth of this portable 

tape recorder is limited to 50 kHz, the IF output from the EIFS meter is 

converted down from 455 kHz to 40 kHz using mixers. The tape speed chosen on 

record and on playback is 15 inches per second (ips). At this speed the 

portable tape recorder frequency response range is 100 Hz to 50 kHz at the 

2 dB points in the direct recording mode, The input voltage range is 

adjusted to record the signal level between 10 millivolts and 1 volt rms 

with gain of 0 dB. 

An external set of sealed, lead-acid batteries in an explosion-proof 

enclosure is used to drive the portable tape recorder. The current is 

limited by a solid-state, 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 provides power for about 8 hours of recording. 

3.1.2 Data Transcribing System 

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

around 0.8 percent, whereas that of the laboratory tape recorder is around 

0.4 percent. The time displacement error is perhaps more important, being 

microseconds for the laboratory tape recorder and milliseconds for the 

portable tape recorder. Therefore later, in the laboratory, the tapes are 

transcribed through a laboratory tape recorder whose servo system can take 

out the flutter and wow introduced by the portable tape recorder. Figure 2 

gives the block diagram of the system. To give a reference time base, a 

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

recordings are made. At playback time (after transcription) this signal is 

used to control the servo of the laboratory tape recorder. 

3.1.3 Data Processing System 

This portion of the measurement system is different from any previously 

used. The data processing system consists principally of the laboratory 

analog magnetic tape recorder as a playback unit, an amplifier, an envelopedetector receiver, a differential amplifier, an active low-pass filter, and 

a digitizer. Figure 3 gives the block diagram of the system. The amplifier 

is used primarily for impedance conversion between the output impedance of 

the laboratory tape recorder and the input impedance of the envelope-detector 

receiver. The 40-kHz output of the laboratory tape recorder is fed into an 
Calibration procedures similar to those followed in developing the APD's 

reported earlier are used [9,14]. For average crossing rates and amplitude 

probability distributions, the number of times a selected level is exceeded 

is determined for each of seven levels. These seven levels are approximately 

2, 4, 10, 30, 100, 300, and 800 mV, and are calibrated using a cw signal each 

time the data are digitized. The estimated limits of error for EM noise 

measurements of time and amplitude statistics are 2 5 dB [9]. 

4.0 Noise Measurement Results 

Data for time and amplitude statistical analysis of the EM noise were 

taken during actual operation of the following mines: 1) Robena No. 4 Coal 

Mine, Waynesburg, Pennsylvania; 2) Grace Hardrock Mine, Morgantown, Pennsylvania; 

3) McElroy Coal Mine, Moundsville, West Virginia; and 4) Itmann No. 3 Coal 

Mine, Mullens, West Virginia. The detailed descriptions of these mines are 

given in the previous literature [9,10,11,12]. Although usually three 

orthogonal components of the magnetic field were measured at ten to twelve 
frequencies in the range from 10 kHz to 32 MHz, time and amplitude statistical 

analysis for the vertical component of the magnetic field at 10 kHz, 70 kHz, 

and 1 MHz only are reported as illustrative examples in this report. 

Five figures compromise a set where each of the five time and amplitude 

statistics is displayed ; namely, Allan Variance Analysis (AVA) , Interpulse 

Spacing Distribution (ISD) , Pulse Duration Distribution (PDD) , Average 

Crossing Rate (ACR), and Amplitude Probability Distribution (APD). Each 

of these sets of curves shown in figures 4 through 11 displays different 

measurements made at the four mines listed above. More comprehensive results 

of amplitude statistics alone are given in the previous literature [9,10,11,12] . The length of time for each measurement is 23 minutes, and 17 minutes 

of actual real time data is processed to obtain the time and amplitude 

statistics for the total noise in the mines. Each result of Allan Variance 

Analysis shows the validity for the choice of a seventeen minute measurement 

time. This seventeen minute time period for each measurement is adequate 

for covering the variations due to the local work cycle, at least for short 

(3-4 minute) cycles common in working sections of room-and-pillar mines. For 

example, figure 6-a shows the result of the Allan Variance Analysis (AVA) of 

EM noise at 2 MHz measured at the Robena No. 4 Coal Mine. The figure indicates that the 0.3 second averaged data have a standard deviation of 18% 

(Q 1.5 dB) from 17 minute averaged data, whereas the 5 minute averaged data 

have only a standard deviation of 5% (s 0.4 dB) from 17 minute averaged data. 

Generally, when the noise generation mechanism is relatively stationary , the 

Allan Variance, a2 (2 ,r) , tends to decrease as the averaging period, r , is 

increased. However, there are cases when the Allan Variance, a2 (2 ,T) , does 

increase with an increase of the averaging period, T, as shown in figure 10-a 

(taken at 70 kHz at the Itmann No. 3 Coal Mine). It is believed that the work 

cycle for this longwall mine is much longer than several minutes, and 17 

minutes of real time data is not sufficiently long enough to give statistically 

meaningful results for an entire work cycle. 

For Average Crossing Rates (ACR) and Amplitude Probability Distributions 

(APD) , the seven levels which are exceeded are approximately 2, 4, 10, 30, 

100, 300, and 800 mV. For Interpulse Spacing Distributions (ISD), and Pulse 

Spacing Distributions (PSD) , these are the levels at which a particular time 

interval is exceeded. These seven levels are calibrated using a cw signal 

each time the data was digitized. The time intervals that are exceeded are 

0.2, 0.6, 2, 6, 20, 60, 200, 600, 2000, 6000, and 20,000 milliseconds. The 

interpretations of these other four time statistics are rather obvious. For 

example, in figure 4 -b , the Interpulse Spacing Distribution (ISD) indicates 

that 55 percent of the time the interpulse spacing exceeds 2000 milliseconds 

at the EM noise level of 46 dB above one microampere per meter. Similarly, in 

figure 4-c, the Pulse Duration Distribution (PDD) indicates that 12 percent of 
the time, the pulse widths exceed 200 milliseconds at the EM noise level of 

2 dB above one microampere per meter. It should be noted that although the 

Inte rpulse Spacing Distributions (ISD) and the Pulse Duration Distributions 

(PDD) are complementary to each other, one cannot be obtained from the other. 

In figure 4-d, the Average Crossing Rate (ACR) characteristic indicates that 

the average positive crossings at an EM noise level of 20 dB above one microampere per meter are 300 crossings per second. Further, in figure 4-3, the 

Amplitude Probability Distribution (APD) indicates that 20 percent of the 

time, the EM noise exceeds 28 dB above one microampere per meter. These five 

time and amplitude statistics are essential to adequately describe EM noise 

in mines. 

5.0 Conclusions 

The time and amplitude statistics illustrated in this report are those 

necessary to adequately describe the time-dependent, EM noise in mines. 

They are 1) Allan Variance Analyses (AVA), 2) Interpulse Spacing Distributions 

(ISD) , 3) Pulse Duration Distributions (PDD) , 4) Average Crossing Rates (ACR) , 

and 5) Amplitude Probability Distributions (APD) . These statistics are illus - 

trated using a rather large store of raw analog data recorded in operational 

mines through computer software techniques . The parameter assessed through 

this statistical approach is the magnetic field strength. The absolute values 

of the magnetic field strength analyzed are obtained with calibrated equipments. The frequency ranges of the magnetic field strengths cover from 

10 kHz to 32 MHz. The length of time necessary to provide statistical 

validity was determined from the Allan Variance Analysis to be about 17 minutes 

for most of the mines measured. The curves generated for the illustrations 

characterize the noise environment in the mines from which the corresponding 

data were taken, and should aid in the design of reliable communication systems 

for such mines. 

6.0 Recommendat ions 

There are shortcomings in using these particular computer software techniques to obtain the statistics presented. First, cost is very high due to 

the number of man-hours needed to digitize and process the data. Second, 

if the length of time required to pr-ovide statistical validity were longer 

than 17 minutes, other practical limits would be encountered in use of the 

computer. 

A significant reduction in both time and cost would be achieved if hardware was developed to present the data in the r.equired statistics presented 

here without the serious limitations due to this software approach. 
7.0 Acknowledgments 

The research presented in this report was performed with the direction 

of J.W. Adams . The association with him was pleasant and inspiring, and 

his continuous encouragement and contributions are gratefully acknowledged. 

Those also making significant contributions to this project were F. C. 

Cowley, T.B. Gray, and M.C. Candelaria of National Oceanic and Atmospheric 

Administration, and D.M. Sterns of the University of Colorado for data processing; A.D. Spaulding of the Institute for Telecommunication Sciences and 

W.W. Scott, Jr. of the National Bureau of Standards for much assistance in 

preparing this manuscript. 

S.C. Foote and J.R. Becker provided typing assistance, while 

N. C. Tomoeda and J. S. Spencer provided drafting assistance. 
[ll] Kanda, M., Adams, J.W., Bensema, W.D., "Electromagnetic Noise in McElroy 

Coal Mine," NBS Technical Note 658, June 1974. 

[12] Bensema, W. D. , Kanda, M. , Adams, J.W. , "Electromagnetic Noise in Itmann 

No. 3 Coal Mine," NBS Technical Note 659, June 1974. 

[13] Espeland, L.R. and Spaulding, A.D., private communication. 

[14] Taggart, H.E. and Workman, J. L., "Calibration Principles and Procedures 

for Field Strength Meters (30 Hz to 1 GHz) ," NBS Technical Note 370, 

March 1969. 
COLLAPSI3LE 

ANTENNA 

Figure 1. Block diagram of underground recording system. 

11 

f =10kHz-250kHz 

FIELD 

STRENGTH 

METER A1 

LOOP 

ANTENNA 

455kHz =- 

BW=1 .4kHz 

25kHz 

OSC CHANNEL 2 

r 

CHANNEL 1 . ' 

MIXER 

, 

40kHz * 

f =I50 kHz -3 2MHz RECORDER 

DIRECT 

RECORDING 

f=100Hz-50kHz 

SCOPE , 

FIELD 

STRENGTH 

METER 

BW=3.3 kHz 

CHANNEL 3 

455kHzb 

PORTABLE 

t 

MIXER i 

40kHz 

) 
Figure 2. Block diagram of transcribing system. 

12 
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AVERAGING TIME r IN SECONDS 

Figure 4-a. AVA, 10 kHz, vertical component, 1. 0 kHz predetection 

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

%DB N-L) .xhF+~rr~. %DB-+PCZ-AQ 

-5% :PL+- a,b D% )I 1 I 

r; I, 1, 1ILV L, 4~ b8- 

Figur& 4-be ISD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 11:25 a. m., Robena No. 4. 
Figure 4-c. PPD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 11:25 a. m., Robena No. 4. 
-. . 0 10 20 30 40 50 60 

MGNETIC FIELD STRENGTH, H (dB RELATIVE TO I MICROAMPERE PER METER) 

Figure 4-d. ACR, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 11:25 a. m., Robena No. 4. 
Percent of Time Ordinate is Exceeded 

Lineor by - IOCJ,,(-III p) 

80 

70 

h 

v> 

z 

a 

L 60 a 4- 

a 

E 

L 

a 

Q 

z 50 

a 

E 

0 

2 -' 40 E 

4 

0 

t 

a 

> '= 30 

- 0 

2' 

fD 

u 

u 

= 20 - 

f 

CT 

C 

z 

lo 

E! 

a - - 

LL 

0 

. - 

C 

a 

c 0 CT 

0 

Figure 4-e. APD, 10 kHz, vertical component, 1. 0 kHz predetection 

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

z 

-10 

-20 

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

-- 

-- 

-- 

--. 

-2 -- 

- 
034114 

TAU MIN. = 3.000 T/TAU = 1.0000 

MEAN VALUE = 3.5882+001 

I 10 100 

AVERAGING TIME T IN SECONDS 

Figure 5-a. AVA, 70 kHz, vertical component, 1. 0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p. m. , Robena No. 4. 
Figure 5-b. ISD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p.m., Robena No. 4. 
Linear by - log,,(-~n p) 

1)~1.001.01 1 .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 

PERCENT OF TIME ORRINATE IS EXCEEDED 

Figure 5-c. PDD, 70 kHz, vertical component, 1.0 kHz predetectlon 

bandwidth, December 5, 1972, 2:45 p.m., ~obena'No. 4. 
Figure 5-d. ACR, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p.m., Robena No. 4. 
Percent of Time Ordinate is Exceeded 

Figure 5-e. APD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, December 5, 1972, 2:45 p. m., Robena No. 4. 
Figure 6-a. AVA, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:3 5 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 6-b. ISD, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:3 5 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 6-c. PDD, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:3 5 p. m., Robena, No. 4. 
-60 -30 -40 -30 -20 - 10 0 

MAGNETIC FIELD STRENGTH, H (dB RELATIVE TO I MICROAMPERE PER METER) 

Figure 6-d. ACR, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:3 5 p. m., Robena No. 4. 
Percent of Time Ordinate is Exceeded 

Figure 6-e. APD, 2 MHz, vertical component, 1.2 kHz predetection 

bandwidth, December 5, 1972, 2:35 p.m., Robena No. 4. 
038114 

TAU MIN. = 3.000 T/TAU = 1.0000 

MEAN VALUE = 3.4767+001 

I 10 100 

AVERAGING TIME T IN SECONDS 

Figure 7-a. AVA, 1 MHz, vertical component, 1. 2 kHz predetection 

bandwidth, April 24, 1973, 10:30 a . m . , Grace Mine. 
PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 7-b. ISD 1 MHz, vertical component, 1.2 kHz predetectlon 

bandwidth, April 24, 1973, 10:30 a.m., Grace Mine. 
Figure 7-c. PDD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 24, 1973 1 10:3 0 a. m. , Grace Mine. 
MAGNETIC FIELD STRENGTH, H (dB RELATIVE TO I MICROAMPERE PER METER) 

Figure 7-d. ACR, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 24, 1973, 10:30 a. m. , Grace Mine. 
Percent of Time Ord i note is Exceeded 

Figure 7-e. APD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 24, 1973, 10:30 a. m. , Grace Mine. 
I 10 100 

AVERAGING TIME T IN SECONDS 

Figure 8-a. AVA, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 10, 1973, 1:10 p. m., McElroy Mine. 
Linear by - log,,( -1" 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 8-b. ISD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 10, 1973, 1:10 p. me, McElroy Mine. 
Figure 8-c. PDD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 10, 1973, 1:10 p.m., McElroy Mine. 

3 6 
AVERAGE POSITIVE CROSSINGS PER SECOND 
Percent of Time Ordinate is Exceeded 

Figure 8-e. APD, 1 MHz, vertical component, 1.0 kHz predetection 

bandwidth, April 10, 1973, 1:10 p.m., McElroy Mine. 
TAU MIN. = 3.000 T/TAU= 1.0000 I - 

MEAN VALUE = 1.7540+001 

Figure 9-a. AVA, 10 kHz, vertical component, 1. 0 kHz predetection 

bandwidth, April 17, 1973, 11 :40 a. m. , Itmann No. 3. 
PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 9-b. ISD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 1 1 :40 a. m. , Itmann No. 3. 
Figure 9-c. PDD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 11 :40 a. m. , Itmann No. 3. 
MGNETIC FIELD STRENGTH, H (dB RELATIVE TO I MICROAMPERE PER METER) 

Figure 9-d. ACR, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 11:40 a. m., Itmann No. 3. 
D001.00( .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 9-e. APD, 10 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 11:40 a. m. , Itmann No. 3. 
I 10 100 

AVERAGING TIME r IN SECONDS 

Figure 10-a. AVA, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p. m. , Itmann No. 3. 
Linear by - log,,(-ln p) 

1)OOI .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 

Figure 10-b. ISD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p. m. , Itmann No. 3. 
6001.001 131 .I 5 1 5 10 20 30 40 50 60 70 60 85 90 95 98 99 

PERCENT OF TIME ORDINATE IS EXCEEDED 

Figure 10-c. PDD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p.m., Itmann No. 3. 
AVERAGE POSITIVE CROSSINGS PER SECOND 

- - 

0 

P 

- - 0 0 

1C 

is 

Z 

I I I 11 1111 I I 1111111 1 I 1 11 1111 I I I 11 1

0 

V, + 

w m 

Z 

n 

+ 

-= I 

N, 

I 

n 

e 

m 

R: 

r 

3 

s =- 

a 

- 

s 

2 

P 

% O- m 

8 ,- I 1 1 1 11111 I I I Illlll I I I111111 I I I 11 1
0 

Q) .- 

LL 

0 

. - 

C 

0, -50 C 

0, 

0 

2 

-60 

-70mll 'llnmmmn iIIliIIIiI1 I I mmmlllll~ I I I I 11 111 I llllllllll 1 I 1111111 I I 1 I 

0001.001 Dl .I 5 1 5 I0 20 30 40 50 60 70 80 85 90 95 98 99 

Percent of Time Ordinate is Exceeded 

Figure 10-e. APD, 70 kHz, vertical component, 1.0 kHz predetection 

bandwidth, April 17, 1973, 12:50 p. m. , Itmann No. 3. 
TAU MIN. = 3.000 T/TAU = 1.0000 

MEAN VALUE = 2.7118+001 

AVERAGING TIME T IN SECONDS 

Figure 11- a. AVA, 1 MHz, vertical component, 1. 2 kHz predetection 

bandwidth, April 17, 1973, 12:20 p. m. , Itmann No. 3. 
Figure 11 -b. ISD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 17, 1973, 12:20 p.m., Itmann No. 3. 
Figure 11-c. PDD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 17, 1973, 12:20 p. m. , Itmann No. 3. 

Linear by - ~og,~(-in p) 

40 

Percent of Time Ordinate is Exceeded 

Figure 11 -e. APD, 1 MHz, vertical component, 1.2 kHz predetection 

bandwidth, April 17, 1973, 12:20 p. m. , Itmann No. 3. 
The curves generated for the illustrations characterize the noise environment in the mines from which the corresponding data were taken, and 

should aid in the design of reliable communication systems for such mines. 

17. KEY WORDS (Alphabetical order, separated by semicolons) 

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