NBSIR 74-378

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, Pennslyvania 15213

NBSIR 74-378

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 B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

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 (ISD) 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 System 5 3.1.3 Data Processing System 5

3. 2 Calibration 6 4.0 NOISE MEASUREMENT RESULTS 6 5.0 CONCLUSIONS 8 6.0 RECOMMENDATIONS 8

7.0 ACKNOWLEDGMENTS 9

8.0 REFERENCES 10

iii
LIST OF FIGURES

Page

Figure 1. Block diagram o£ underground recording system 11

Figure 2. Block diagram o£ 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 , 19 72 , 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 , 19 72 , 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

iv
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 Mine 35

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 Mine 37

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 40

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 42

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 45

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 47

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

bandwidth, April 17 , 1973, 12:50 p.m., Itmann No. 3 48

v
LIST OF FIGURES (Continued)

Page

Figure 11-a. AVA, 1 MHz, vertical component, 1.2 kHz predetection

bandwidth, April 17 , 19 73 , 12:20 p.m., Itmann No. 3 49

Figure 11-b. ISD, 1 MHz, vertical component, 1.2 kHz predetection

bandwidth, April 17 , 1973 , 12:20 p.m., Itmann No. 3 - 50

Figure 11-c. FDD, 1 MHz, vertical component, 1.2 kHz predetection

bandwidth, April 17 , 1973 , 12:20 p.m., Itmann No. 3 51

Figure 11-d. ACR, 1 MHz, vertical component, 1.2 kHz predetection

bandwidth, April 17 , 1973 , 12:20 p.m., Itmann No. 3 52

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

bandwidth, April 17 , 1973 , 12:20 p.m., Itmann No. 3 53

vi
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 com- munication 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; man- made noise; pulse duration distribution; time statistics.

1 . 0 Introduct ion

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 EMI is a major limiting factor in the design of a communication system.

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

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

V, between two conductors. 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 [1]. 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.
The purpose o£ this report is to describe and illustrate five time and

amplitude statistics needed to describe the random variations of the EM noise

in mines. These statistics of EM noise are the statistical descriptors of the

noise process needed to design and evaluate a telecommunication systems that

will operate in a noisy environment [2,3],

In section 2.0, these time and amplitude statistics used for the analysis

of the EM noise measured in mines are described. Section 3.0 describes briefly

the measurement instrumentation of an underground recording system, a data

transcribing system, a data processing system, and their calibration procedures. Section 4.0 gives the results of the recorded data taken in the mines

analyzed by these time and amplitude statistics of EM noise taken in four different mines. The conclusions are given in section 5.0. A recommendation for

data processing into a statistical form is given in section 6.0.

2 . 0 Measurands for Time and Amplitude Statistics for EM Noise in Mines

EM noise generated in mines is generally a non-stationary, random process.

Therefore, the most meaningful measurands for EM noise generated in mines are statistical ones. We have used five time and amplitude statistics in order

to unravel the complexities included in the EM, man-made noise in mines. We

feel that the following five measurands compose a necessary and sufficient

set of statistics upon which intelligent communication system design decisions

can be made. They are:

1. Allan Variance Analysis (AVA)

,

2. Interpulse Spacing Distribution (ISD)

,

3. Pulse Duration Distribution (PDD)

,

4. Average Crossing Rate (ACR) , and

5. Amplitude Probability Distribution (APD)

These are discussed below in detail.

2.1 A llan 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 accom- plish 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.

2
A record of the phenomenon under consideration, yCt) , is divided into

a number of equal time segments of length x, and the average value of yCt)

,

yj^, of each segment is calculated by

Yk = ^ I yCt) dt,

^k

th where y-j^ is the k segment average starting at time t-^. Next, the sample

variance (sample size two), a^(2,T), of successive averages is calculated.

That is

aJ(2,T) = Y (y^-7^)^ = I-(>^k.r>^k^^ n=k

where

1 ^k = I

n=k\

is the average of the two successive segment averages y-j^ and Y-^+j^- The

Allan Variance, a; defined to be [4]Allan Variance, a^(2,T), for this special case (sample size two) is then

a^(2,T) = <aJ(2.T)> , where the brackets represent the average of o^(2,t) over all pairs of suc- cessive y^^ constructed from y(t).

The preceding calculation is repeated for various values of averaging

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 does not decrease with x indicates that the record, y(t), from which a^(2,x) was calculated needs

to be longer.

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 x is plotted on microfilm along with the associated confidence

levels [7].

2.2 Interpulse Spacing Distribution (ISP) [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

.

3
2.3 Pulse Duration Distribution (PDD) [8]

The pulse duration distributions give the probability distribution for

the pulse widths and are given in terms of the percentage of pulses which

exceed various widths in seconds. As in section 2.2, distributions are given for each of the seven levels.

^ 2.4 Average Crossing Rate (ACR) [8]

The average crossing rates present the average number of times the noise

amplitude crosses various levels and is given as positive crossings per second

versus noise amplitude level.

2 . 5 Amplitude Probability Distribution (APD)

The amplitude probability distributions are presented as a fraction of

the time the envelope exceeds various levels. The APD is the most common statistical measurand required for analysis of the performance of communication

systems. The details of the measurement of APD's and the resulting plots of

data taken are given in previous literature [8,9,10,11,12].

3 . 0 Noise Measurement Techniques

3 . 1 Measurement Instrumentation

This section of the report describes the system used to measure the time

and amplitude statistics of EM noise in mines. Parts of this system are described 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 -f ield 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 EI FS 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 13 watts 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

5
envelope -detector receiver to extract the baseband noise. Since the detector

output is coupled with a fixed, dc bias voltage (- 6V) , the differential amplifier is used to take out this dc offset. The baseband noise is then fed into

an active, low -pass filter to cut off the higher frequency components above

3 kHz before digitizing. For digitizing, 10 k samples per second with 4096

samples per record are used to produce 2500 records. This corresponds to

about 17 minutes of real-time data.

A large digital computer is used to compute the time and amplitude statistics of the EM noise data recorded in the mines. The basic software was developed by L.R. Espeland and A.D. Spaulding of the Office of Telecommunications, Institute for Telecommunications Service [13], and was modified by the

author. It is outside the scope of this report to describe the software in

any detail, but the program and corresponding information will be available

from the author upon request.

The 3-dB cw signal bandwidth of the whole measurement system, including

the recording, transcribing, and data processing systems, is primarily

determined by the data processing system. The predetection bandwidth of the

measurements for the frequency range between 10 kHz and 250 kHz is 1.0 kHz,

whereas the bandwidth for the frequency range between 250 kHz and 32 MHz

is 1.2 kHz. The appropriate predetection bandwidths are indicated in each

figure. The dynamic range of the whole measurement system is primarily

limited by the magnetic tape recorder to about 45 dB

3 . 2 Cal ibrat ion

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 ± 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

6
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%

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

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

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

Allan Variance, a^(2,T), tends to decrease as the averaging period, t, is

increased. However, there are cases when the Allan Variance, a^(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

7
the time, the pulse widths exceed 200 milliseconds at the EM noise level o£

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

Interpulse 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 illustrated 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 provide 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 hard- ware was developed to present the data in the required statistics presented

here without the serious limitations due to this software approach.

8
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.

9
8.0 References

[I] The Institute o£ Electrical and Electronics Engineers, Inc., "IEEE

Standard Dictionary of Electrical and Electronics Terms," IEEE Std. 100,

1972 .

[2] Crichlow, W.Q., et al . , "Amplitude Probability Distributions for Atmospheric Radio Noise, NBS Monograph 23, Nov. 1960.

[3] Thompson, W.J., III, "Bibliography of Ground Vehicle Communications and

Control," AKWIC Index, Report No. DOT-TSC-UMTA-71 -3, July 1971.

[4] Allan, D.W. , "Statistics of Atomic Frequency Standards," Proc. IEEE,

Vol. 54, No. 2, pp. 221-230, Feb. 1966.

[5] Barnes, J. A., et al . , "Characterization of Frequency Stability," IEEE

Trans. Instrum. ^ Meas . , Vol. IM-20, No. 2, pp. 105-120, May 1971.

[6] Shoaf, J. H. , Halford, D. , Risely, A.S., "Frequency Stability Specification and Measurement: High Frequency and Microwave Signals," NBS

Technical Note 632, Jan. 1973.

[7] Lesage, P. and Audoin, C. , "Characterization of Frequency Stability:

Uncertainty Due to the Finite Number of Measurements," IEEE Trans.

Instrum. § Meas., Vol. IM-22, No. 2, pp. 157-161, June 1973.

[8] Disney, R.T. and Spaulding, A.D., "Amplitude and Time Statistics of

Atmospheric and Man-made Radio Noise," ESSA Technical Report ERL 150-

ITS 98, Feb. 19 70.

r9] Bensema, W.D., Kanda, M. , Adams, J.W. , "Electromagnetic Noise in Robena

No. 4 Coal Mine," NBS Technical Note 654, April 1974.

[10] Adams, J.W., Bensema, W.D. , Kanda, M. , "Electromagnetic Noise in Grace

Hardrock Mine," NBS Technical Note 657, June 1974.

[II] 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 19 69.

10
COLLAPSIBLE

ANTENNA

LOOP

ANTENNA

f = 1 0l<H2-250kHz

455kHz

FIELD

STRENGTH

METER A BW = 1 .4kHz

MIXER

40kHz

FIELD

STRENGTH

METER

BW=3 .3kHz

25kHz

osc

455kHz

MIXER 40kHz

f=150kHz-3 2MHz

CHANNEL 1

CHANNEL 2

CHANNEL 3 PORTABLE

TAPE

RECORDER

DIRECT

RECORDING

15 IPS

f =1 00Hz-50kHz

SCOPE

Figure 1. Block diagram of underground recording system.

11
PORTABLE

TAPE

RECORDER

DIRECT

RECORDING

1 SIPS

40kHz

LABORATORY

TAPE

RECORDER

DIRECT

RECORDING

15 IPS

CHANNEL 1 CHANNEL 2

CHANNEL 2 25kHz — CHANNEL 4

CHANNEL 3

40kHz

CHANNEL 6

Figure Z. Block diagram of transcribing system.

12
<

ro <

CO

CQbO • H

CO

CO

(L) OOU

a,os

bO

nj

.1-1 Xuo

bJO

•1-1

13
033114

lAU MIN. = 3.000 T/TAU = 1.000

L

MEAN VALUE - 1.5920+001

10 Q

1

I

u

I

I —

0.1 I 10 100 1,000

AVERAGING TIME t 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.

14
Lineor by - y log.J-lnp) 20,000

6,000

2,000

JD00I.00I.0I .1 .5 I 5 10 20 30 40 50 60 70 80 85 90 95

PERCENT OF TIME ORDINATE IS EXCEEDED

9a

re 4-bo ISD, 10 kHz, vertical component, 1.0 kHz predetection

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

15
Lineor by - y log|Q(-lnp) 20.000

6,000

2.000

PERCENT OF TIME ORDINATE IS EXCEEDED

Figure 4-c. PPD, 10 kHz, vertical component, 1 . 0 kHz predetection

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

16
10 20 30 40 50 60 MAGNETIC 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.

17
Linear by - j log|Q(-lnp)

Percent of Time Ordinate is Exceeded

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

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

18
034114

TAU MIN. = 3.000 T/TAU = 1.0000

MEAN VALUE = 3.5882+001

10'

1II

III

]

B

m II II

1 i: III —

I

r

0.1 I 10 100 1,000

AVERAGING TIME r IN SECONDS

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

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

21
-40 -30 -20 -10 0 10 20 MAGNETIC FiaO STRENGTH, H (dB RELATIVE TO I MICROAMPERE PER METER)

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

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

22
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.

23
035114

TAU MIN. = 3.000 T/TAU = 1.0000

MEAN VALUE = 5.5 330+001

—I r mil

B

4 E

t

B

r

0.1 I 10 100 1,000

AVERAGING TIME r IN SECONDS

Figure 6-a. AVA, 2 MHz, vertical component, 1,2 kHz predetection

bandwidth, December 5, 1972, 2:3 5 p.m., Robena No. 4.

24
Linear by -7 logj-lnp)

20,000

6,000

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.

25
ure 6-c. PDD, 2 MHz, vertical component, 1.2 kHz predetection

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

26
oil I I I I I I 1

-60 -50 -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, 2:35 p.m., Robena No. 4.

27
Linear by - j log|Q(-lnp)

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.

28
038114

TAU MIN. = 3.000 T/TAU = 1.0000

MEAN VALUE = 3.4767+001

10' m IIIM

_U —

IL

I

E -E

-J

0.1 I 10 ICQ 1,000

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.

29

I.

i
4

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

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

30
Linear by - y logi^l-lnp) 20,000

6,000

2,000

600

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

PERCENT OF TIME ORDINATE IS EXCEEDED

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

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

31
-40 -30 =20 -10 0 10 20 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.

32
Linear by - y log .(-In p)

Percent of Time Ordinate is Exceeded

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

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

33
051120

TAU MIN. = 3.000 T/TAU = 1.0000

MEAN VALUE = 2.3393+001

10

1 II nil

II nil

Z

H

r B

-I

0.1 I 10 ioo 1,000

AVERAGING TIME r IN SECONDS

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

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

34
i Figure 8-b. ISD, 1 MHz, vertical component, 1.2 kHz predetection

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

35
Lineor by -j log (-In p) 20,000

6,000

2,000

600

200

S 60

1

-bd

IDdU u

33 df

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

PERCENT OF TIME ORDINATE IS EXCEEDED

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

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

36
I0,000p

-50 -40 -30 -20 -10 0 MAGNETIC RELD STRENGTH, H (dB RELATIVE TO I MICROAMPERE PER METER)

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

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

37
(f)

a>E

Eoo

30

20

10

Lineor by - y log|Q(-lnp)

2 -10

>

•;i -20

<i>

GO

T3X -30

c

05 _

0)

40

« -50 o

-60

-70

1

% s

, Mine Generated Norse

1

DM s

Svstem Noise

— -

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

Percent of Time Ordinate is Exceeded

99

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

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

38
054120

TAU MIN. = 3.000 T/TAU= 1.0000

MEAN VALUE = 1.7540+001

—

(

3 E I c

E ^

E

E

B

1

0.1 I 10 100 1,000

AVERAGING TIME r IN SECONDS

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

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

39
Lineor by - y log (-In p)

20 dB

lOdB

2dB

6dB

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

PERCENT OF TIME ORDINATE IS EXCEEDED

98 99

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

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

40
Lineor by - y \oq^^{-\r\p) 20,000

6,000

2,000

200

0.6

1

1

—-1

!

j

i

1

1

1 4-- —1

1

1 -1-

-r

1

1 --j

r

"""1+XL —

1

!

j

1

1

)d -(

2dB

)d B

^

V]dnUL

1)001.001.01 .1 .5 1 5 10 20 30 40 50 60 70 80 85 90 95 98 99 PERCENT OF TIME ORDINATE IS EXCEEDED

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

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

41
MAGNETIC 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.

42
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.

43
05 8120 - ,

TAU MIN. = 3.000 T/TAU = 1.0000

MEAN VALUE = 2.4862+001

10

b

... -

t

::

ii.

r.

0.1 I 10 100 1,000

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.

44
Linear by - y log|Q(-lnp)

6,000

2,000

600

200

0.6

-\t

\fVrn

n

II 4dR -RHR II

|[-

II

2 JD Od:

1-M

• [-

[-

- [- —

—

-22(1B

13

1)001.001.01 .1 .5 I 5 ID 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.

45
Figure 10-c. PDD, 70 kHz, vertical component, 1.0 kHz predetection

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

46
MAGNETIC FIELD STRENGTH, H (dB RELATIVE TO I MICROAMPERE PER METER)

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

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

47
Linear by - j log,Q(-lnp)

CO

tr

i_

(i> a>E

a.

<Da>Q.

E

o

o

0)>

k. m

C

0>

10«

ilu

0)c

CPo

.0001.001.01 .1 .5 1 5 K) 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.

48
056120

TAU MIN. = 3.000 T/TAU = 1.0000

MEAN VALUE = 2.7118+001

1.0 111

1 r

6 =

c

r.

Ci

I III

0.1 I 10 100 1,000

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.

49
Lineor by log,p(-lnp)

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

• PERCENT OF TIME ORDINATE IS EXCEEDED

98 99

ure 11-b. ISD, 1 MHz, vertical component, 1.2 kHz predetection

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

50
Linear by -j logigC-lnp)

20,000

6,000

2,000

600

200

0.6

1

i

1

1

1 I

!

i 4-.

-

1 _;

i- 10 \1

B

t

]

1)001.001.01 .1 .5 I 5 K) 20 30 40 50 60 70 80 85 90 95 98 99 PERCENT OF TIME ORDINATE IS EXCEEDED

ure 11-c. PDD, 1 MHz, vertical component, 1.2 kHz predetection

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

51
-40 -30 -20 -10 0 10 20 MAGNETIC FIELD STRENGTH, H (dB RELATIVE TO I MICROAMPERE PER METER)

Figure 11-d. ACR, 1 MHz, vertical component, 1.2 kHz predetection

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

52
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.

53
FORM HBS.114A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Acc«t»ion

BIBLIOGRAPHIC DATA NBSIR 74-378

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Time and Amplitude Statistics for Electromagnetic Noise

in Mines

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S) 8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

DEPARTMENT OF COMMERCE

Washington, D.C. 20234

10. Project/Task/ Work Unit No. 272ft41 7

11. Contract/Grant No,

12. Sponsoring Organization Name and Address

Same as Item 9.

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.) 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) Interpuls

Spacing Distributions (ISD) , 3) Pulse Duration Distributions (PDD)

4) Average Crossing Rates (ACR) , and 5) Amplitude Probability Distribution

(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

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

Allan variance analysis; amplitude probability distribution; amplitude

statistics ;, average crossing . rate electromagnetic interference; electro

^ft^l^iLm^TENHW?' field strength; man

ulsive noi ! •rengi man-made noise; pulse duration da-isiia^^gi^ftbipn; t|ime statisri

UNLIMITED.

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TO NTIS.

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