PRELIMINARY PERFORMANCE PREDICTIONS 

FOR ELECTROMAGNETIC THROUGH-THE- EARTH 

MINE COMMUNICATIONS 

technical memorandum report to 

U. S. BUREAU OF MINES 

PITTSBURGH MINING AND SAFETY 

RESEARCH CENTER 

MARCH 1972 
PRELIMINARY PERFORMANCE PREDICTIONS 

FOR ELECTROMAGNETIC 

THRDUGH-THE-EARTH MINE COMMUNICATIONS 

A COLLECTION OF WORKING MEMORANDA 

TECHNICAL MEMORANDUM REPORT 

to 

U.S. BUREAU OF MINES 

PITTSBURGH MINING AND SAFETY RESEARCH CENTER 

PITTSBURGH, PENNSYLVANIA 15213 

Contract No. H0122026 

March 1972 

submitted by 

ARTHUR D. LITTLE. INC. 

CAMBRIDGE, MASSACHUSETTS 02140 

Arthur D Little Inc. 
TABLE OF CONTENTS 

List of Figures 

List of Tables 

I. INTRODUCTION 

11. WORKING MEMORANDUM #1 - SIGNAL POWER ESTIMATES 

FOR DOWNLINK THROUGH-THE-EARTH BASEBAND VOICE 

COMMUNICATIONS 

A. METHOD OF CALCULATION - qOWNLINK, 

HORIZONTAL WIRE ANTENNA 

B. NOISE IN MINES 

1. NOISE PRIMARILY DUE TO 6OHZ AND 

360HZ HARMONIC PEAKS 

2. NOISE PRIMARILY BROADBAND IMPULSIVE 

3. SIGNAL CURRENT AND POWER ESTIMATES 

111. WORKING MEMORANDUM #2 - SIGNAL POWER ESTIMATES 

FOR UPLINK THROUGH-THE-EARTH BASEBAND VOICE 

COMMUNICATIONS 

A. METHOD OF CALCULATION - UPLINK, LOOP ANTENNA 

B. NOISE ON THE SURFACE OVER MINES 

C. SIGNAL CURRENT AND POWER ESTIMATES 

IV. WORKING MEMORANDUM #3 - SPOT SIN PATIO ESTIMATES 

FOR UPLINK AND DOWNLINK THROUGH-THE-EARTH 

NARROWBAND CODE COMMUNICATIONS 

A. SIGNAL AND NOISE PLOTS 

B. SPOT SIGNAL-TO-NOISE RATIOS 

C. CONCLUDING REMARKS 

Page 

iii 

vi 

vii 

Arthur D Little,lnc 
LIST OF FIGURES 

Figure NO. Page 

Average Speech Spectra 1-2 

Downlink Signal, For Horizontal Wire 

Antenna - 1 Amp Current 

Horizontal Wire Antenna, Coupling 

Relationship 

Horizontal Component of Subsurface EM Noise 

Magnetic Field, Hy Gunn Quealy Mine, Rock 

Springs Wyoming (Westinghouse data) 1-7 

Allen Mine, Underground, 3000 Hz RW, 01-14; 

Antenna Sensitive Axis Pointed Vertical 1-8 

Allen Mine, Underground, 3000 Hz BW, 07-14; 

Antenna Axis Vertical; Electric Locomotive 

Pulling Out with Loaded Coal Cars 1-9 

Lincoln Mine, Underground, 3000 Hz BW, 03-23; 

in Control Room; Severe Trolley Impulsive Noise 

(Vertical) 1-1 1 

Allen Mine, Surface, 3000 Hz BW, 03-17; Antenna 

Sensitive Axis Pointed Vertical 1-12 

RMS Antenna Current for 12db S/N Ratio Over the 

Voice Band 500-3000 Hz for o =lo-2 mhoslmeter 1-14 

Shapes of Voice, Signal Attenuation, and Impulse Noise Spectra 1-16 

Loop Antenna, Coupling Relationship 2-2 

Uplink Signal for Loop Antenna, Unit Magnetic 

Moment 2-3 

Allen Mine, Surface, 03-17; Antenna Sensitive 

Axis Pointed Vertical 2-5 

EM Background Noise on Surface-Vertical Component, Hz Gunn Quealy Mine, Rock Springs, Wyoming 2-6 

Lincoln Mine, Surface, 3000 Hz BW, 07-22 2-7 

Artlnlr n l itti.- lnr 
LIST OF FIGURES (Continued) 

Figure No. Page 

2-6 Transmitter Magnetic Moment (RMS) Required for 

12db SIN Ratio Over Voice Bandwidth (500 Hz3kHz) for Vertical Field Components of Noise 

and Signal ( 0 = 10'2 nhos/meter) 

Relative Shapes of Signal Attenuation Factor 

and Voice Spectrum (from 1000 ft depth) 

Power Dissipated in Loop for 12db S/N Over 

Voice Bandwidth at Surface (Uplink Transmission) 

Downlink Signal for Horizontal Wire Antenna - 

1 Amp Current 

Uplink Signal for Loop Antenna - Magnetic 

Moment - 1460 Amp-~eter~ 

In-Mine Broadband Noise Densities for Downlink Communications 

Surface Broadband Noise Densities for Uplink 

Communications 

Lincoln Mine, Underground, 3000 Hz BW, 03-23; 

in Control Room. Severe Trolley Impulsive 

Noise (Vertical) 

Lincoln Mine, Underground, 750 Hz BW, 03-20; 

in Control Room, Severe Trolley IIUpulSive 

Noise (Vertical) 

Allen Mine, Underground, 750 Hz BW, 07-13; 

Antenna Axis Vertical; Electric Locomotive 

Pulling Out with Loaded Coal Cars 

Allen Mine, Underground, 750 Hz BW, 01-13; 

Antenna Sensitive Axis Pointed Vertical 

Horizontal Component of Subsurface EM Noise 

Magnetic Field, HyrGunn Quealy Mine, Rock 

Springs, Wyoming (Westinghouse data) 

Allen Mine, Surface, 750 Hz Bandwidth (BW), 

03-16 Antenna Sensitive Axis Pointed Vertical 
LIST OF FIGURES (Continued) 

Figure No. 

3-1 1 

Page 

Lincoln Mine, Surface, 750 Hz BW, 07-21, 

Antenna Sensitive Axis Pointed Vertical 3-1 2 

EM Background Noise on Surface-Vertical Component, Hz*Gunn Quealy Mine, Rock Springs, 

Wyoming 3-13 

Wideband Power Density Spectrum for Florida 

Data (Feb., May, June, July 1968) Magnetic 

Field Horizontal Components 3-14 

Downlink (in mine) Signal-to-Noise Ratio for 

Horizontal Wire Antenna, 1 Amp Current (50 

Watts for 50 ohm Resistance), Overburden 

Conductivity, a = 10'~ mholmeter 3-16 

Downlink (in mine) Signal-to-Noise Ratio for 

Horizontal Wire Antenna, 1 Amp Current, 

o - 10-I mholmeter 

Uplink (surface) Signal-to-Noise Ratio for 

Loop Antenna, Overburden Conductivity 

o - lom2 mho/meter, Loop Moment 

NAI = 1460 Amp-meter 2 

Uplink (surface) Signal-to-Noise Ratio for 

Loop Antenna, Overburden Conductivity 

a - 10-I mholmeter, Loop Moment 

NAI - 1460 Amp-meter 2 

Arthrrr nl ittb lnr 
LIST OF TARI.ES 

Table No. Page 

1-1 RMS Antenna Current and Power Dissipated for 

Long-Wire 50-Ohm Antenna 1-15 

2-1 Required Loop Magnetic Moment, ~m~-~eter2 2-1 0 

2-2 Power Dissipation and RMS Voltage and Current 

in Uplink Circular Loop Antenna 2-1 1 

2-3 Power Dissipated and RMS Voltage and Current 

in Uplink Circular Loop Antenna 2-12 
I. INTRODUCTION 

This Technical Elemorandum Report is a collection of three working 

memoranda, prepared during the course of our work for the Bureau of Mines 

and bound together for convenient reference. These working memoranda 

present preliminary performance estimates, for baseband voice and narrowband through-the-earth electromagnetic communications systems of principal 

interest to the Bureau, for operational/emergency mine communications. 

The calculations were prepared to obtain early indications of the feasibility and governing parameters of such communications systems. They 

are hased on limited, but pertinent, coal mine electromagnetic noise data 

acquired to-date by Bureau of Mines contractors; and of theoretical signalattenuation characteristics for two transmitter antenna types of present 

interest to the Bureau. 

The memoranda examine the cases of baseband voice and narrowband 

communications for uplink and downlink transmissions, for frequencies up 

to 3kHz. Downlink transmissions are via a horizontal wire antenna, uplink transmissions via a vertical-axis loop antenna, for typical mine 

depths of 300, 600, and 1000 feet. Representative coal-mine overburden 

conductivities of lom2 mhoslmeter (moderate and common) and 10'1 

mhoslmeter (high) were used; for voice and narrowband, 10" for 

narrowband only. Examples of high, moderate, and low; surface and subsurface; harmonic and broadband-impulsive noise conditions were taken 

from NBS and Westinghouse (WGL) mine noise data, together with examples 

of high- and lowlevels of ELF atmospheric noise taken from M.I.T. Lincoln 

Laboratory data. 

These feasibility calculations are not intended to serve as definitive and complete treatments, but as a starting point: to establish 

first-order estimates of the magnitude and variability of transmitter 

power requirements under different noise, overburden conductivity, and 

mine depth conditions; to identify relationships, conditions, or frequencies that are likely to limit or enhance system performance; to 

reveal items requiring further investigation and data still required; 

and to suggest practical methods for optimizing system performance. 

These objectives were met by the calculations. Simple experiments to 

support these calculations can and should be carried out; together with 

more detailed investigations of specific modulation, coding, noisesuppression, voice-compression and signal-conditioning techniques, aimed 

at producing through-the-earth operational/emergency mine-communication 

systems that are not only effective, but practical and economically 

sound. 

vii Arthur D Little,Inc 
WORKING MEMORANDUM # 1 Case: 73912 Date: 27 January 1972 

11. SlGNAL POWER ESTIMATES FOR DOWNLINK THROUGH-THE-EARTH 

BASEBAND VOICE COMMUNICATIONS 

Preliminary calculations have been performed to determine the feasibility 

of communications systems using baseband voice signals, transmitted 

through the earth and received in the presence of various types and levels 

of electromagnetic noise in mines. Even without attempts to remove the 

major line components of the noise upon reception, or to give the optimum 

pre-emphasis and peak clipping to the initial transmitted voice spectrum, 

it appears that intelligible voice signals may be received for an appreciable fraction of the time in mines, using a horizontal wire antenna for 

the downlink. Of course, to design an effective operational cammunications system. techniques such as those just mentioned would have to be 

used, to improve signal-to-noise ratios by perhaps 12-20db. The implementation of these techniques poses no fundamental problems of great 

difficulty. 

A. METHOD OF CALCULATION - MIWNLINK, HORIZONTAL WIRE ANTENNA 

We have considered a situation in which voice is baseband transmitted (500Hz to 3kHz) through the earth by a horizontal wire antenna, 

terminated in a grounded electrode at each end. 

The voice spectrum over this bandwidth may be crudely represented 

by S /f as shown in Figure 1-1: where f is frequency in Hertz and So is 

an a?tPlitude gain factor with a value tt8at depends on the signal-to-noise 

ratio desired at the receiver. Hence the RMS current is 

If the series resistance of the antenna electrode/earth contact is 

assumed equal to 50 ohms (values between 5052 and 200 oare typical), 

the transmitter power required may be estimated as 

P = RI- 2 2 = 50 1- = So/12 watts 

A horizontal wire antenna produces a magnetic field 

Arthur DLittlelnc 
100 1000 10.000 

Frequency(Hz) 

Source: Handbook of Experimental Psychology, S.S. Stevens (ed.1. 

Wiley, Chapter 26, Licklider, J.C.R. and Miller, G.A.. Figure 2, W. 1042 

FIGURE 1-1 AVERAGE SPEECH SPECTRA 

Arthur D Little. Inc. 
WORKING MEFlORANDllEl #I Case 73912 

where D is the depth of the mine, and is a dimensionless quantity. 

In Figure 1-2 plots are show11 of the shape of the attenuation as a 

function of frequency (1 AI Z/lrO) for the three depths D = 300, 600, 1000 

feet, using values for overburden conductivity of a = 10'~ and 10-I 

mhosfmeter. These plots were derived using the Westinghouse IA 1 curve 

in Figure 1-3. 

The RMS magnetic field at the receiver in the mine can then be 

written 

where IAI is a function of frequency. (The following calculations were done 

-2 only for the more favorable o = 10 mholmeter conductivity situation, commonly 

found over coal mines.) 

At D = 300 feet,we approximate 

At D = 600 feet,we approximate 

At D = 1000 feet,we approximate 

is of the form ae In the latter two cases, where 2T~ Q f 

the received magnetic field is 

Arthur D Little. Inc 
Horizontal Magnetic Field Signal Component Below Wire 

(db re 1 AMPIMeter) 

A d d 2 

00 m e N s do 

0 m 

0 a 0 0 0 0 0 
Arthur D Little. Inc 

Curves Apply With Receiver Located Directly 

Below Transmitter 

- 

- ii .d .- 

S - 

- 

- < 

I I I I I 

012345678 9 10 11 12 13 14 

Penetration into Medium (~16) in Skin Depths 

- 

Source: Westinghouse Georesearch Laboratory (WGL) 

FIGURE 1-3 HORIZONTAL WIRE ANTENNA, COUPLING RELATIONSHIP 
WORKING MICMOKANDUM #1 Cnse: 7301 2 

B. NOISE IN MINES 

Two different types of noise situations were considered: 

1. Noise Primarily Due to 6OHz and 360Hz Harmonic Peaks 

The noise magnetic field can be written 

N rms =iT 

where h are the noise peaks in A/M at all 60Hz and 360Hz harmonics lying i 

between 500 and 3000Hz. While there are 42 of these in all, in practice 95% 

or more of the noise power is often contributed by only 2 or 3 harmonics. 

Three cases were calculated: 

Gunn Quealy Mine, Rock Springs, Wyoming - Figure 1-4 measurements with mine shut down, horizontal component (Westinghouse data) 

N = 2.96 x AIM (low noise) rms 

Allen Mine, Colorado - W. D. Bensema (NBS*) measurements 

(Figure 1-5, Bensema NBS report) power line 6OHz and 360Hz 

harmonics primarily, vertical component 

Nrms - 7.7 x 10'~ AIM (low harmonic noise) 

(Figure 1-6, Bensema NBS report) electric locomotive pulling 

out - 60Hz and 360Hz harmonics primarily, vertical component 

N,, = 1.03 x AIM (high harmonic noise) 

2. Jioise Primarilv Broadband Im~ulsive 

The noise density plots of NBS and WGL data can be approximated. 

either over the whole bandwidth or individually over several segments of 

it, by relations of the fo~m 

*W. D. Bensema, Coal Mine ELF Electromagnetic Noise Measurements, NBS 

Report 10-739 (1972) - sponsored by U.S. Bureau of Mines. Working Fund 

Agreement H0111019. 
1-7 

Arthur D LittIelnc. 
Arthur D Little. Inc. 
Arthur D Little. Inc. 
WORKING MEMORANDUM #1 Case: 73912 

Then the noise magnetic field can be written 

One example studied was severe trolley impulsive noise, vertical 

component in the Lincoln Mine in Colorado, as shown in Figure 1-7 

(Bensema NBS report). 

A correction of -9db was applied to his data to take account of the 

7.8Hz bandwidth he used. 

Nrms = 3.71 x AIM (high impulsive noise) 

By way of comparison with the above noise values, it is worth noting 

that the harmonic noise in the same bandwidth on the surface at the Allen 

Mine shown in Figure 1-8 (Bensema NBS report) gives a value of 

Nrms = 5 x 10'4 AIM (harmonic noise on the surface). 

3. Signal Current and Power Estimates 

For intelligible voice comunications,a criterion of a signal-tonoise ratio equal to or greater than 12db is used. Computing this ratio 

for the signal-to-noise field quantities, without regard for specific 

field sensors, we get 

Hrms - 24 which represents a reasonable bound on the 

Nrms desired grade of service for voice communications. 

(In A3 telephony, a 6db audio SIN ratio produces just-usable-quality 

reception, and is reserved for operator-to-operator communications. 

Good commercial quality reception requires an audio SIN ratio on the 

order of 30db.) 

By using this criterion, and combining Eqs. (11) or (9) with (4) or 

(a), it is possible to determine the corresponding RMS current (or the 

power) required to achieve a 12db SIN ratio for voice communication through 
I! 0 0 2011 20 P.%tOOO 1.111000 tlRbl7l 0#:%:2¶ ¶ 

¶.9lOO¶ I00002 0.00.000 O.OO.PO0 20 40x0 40120 

0323250871 Rec, gain corr.' 0 Total conat c0rr.z-54.5 

1.000-002 0.2236 9.224-003 

I 

4 Voiceband Chosen 

Levels of Broadband Levels of Broadband 

Noise Taken from Noise Approximated by Linear 

I NBS Fig. 29 Below * Interpolotion from-75dB at 500 Hz 

250 Hz for Narrow to-98dB at 3kHz (after Corrections) 

Band Code Calcs. Plus 2 Harmonic Noise Peaks.(Circled) 

Frequency. Hertz 

Source: NBS Report Fig. 30 

FIGURE 1-7 LINCOLN MINE, UNDERGROUND. 3000 Hz BW. 03-23; IN CONTROL ROOM; 

SEVERE TROLLEY IMPULSIVE NOISE (Vertical) 
Arthur I3 Littklnc. 
WORKING MEMORANDUM #I Case: 73912 

the earth, to mines under a variety of circumstances. These results 

can only be rough estimates, given the simplified nature of the 

assumptions made in the above analysis. However, they are in an important sense "worst case" estimates, since we know that by tailoring 

the initial voice spectrum and shaping the received noise spectrum, or 

reducing its few major harmonic components, we can achieve large gains 

in the final signal-to-noise ratio. Note that the power calculations 

represent the average power delivered to a 50 ohm load. To deliver the 

required levels of average power - with linear amplifiers - requires 

approximately 10-15db more amplifier power handling capability, because 

of the dynamic range of normal speech. 

The results, in terms of the RMS antenna current required, are 

shown in Figure 1-9 and Table 1-1 for the mine noise cases discussed 

above at the three depths of 300, 600, and 1000 feet. Vertical components of the noise fields have been compared with the horizontal 

signal field components produced by a horizontal wire antenna, because 

NBS published primarily vertical component graphs of in-mine noise. 

Though the degree of variability between vertical and horizontal components must still be ascertained, it will probably fall well within 

the wide variation experienced in the vertical field alone. The 

highest noise level (Lincoln Mine, severe trolley impulsive noise) 

requires on the order of 100 kilowatts for intelligible voice communication at a depth of 1000 feet, and 23 kilowatts even at 600 feet. for 

a 50 long-wire antenna impedance. These power levels may mean that 

voice communication to someone on a moving trolley will not be possible 

through the earth. 

In addition, both the trolley impulsive noise and voice spectra fall 

off with increasing frequency, as shown in Figure 1-10, as opposed to 

just the voice spectrum as in typical voice communications systems; so 

that a S/N ratio greater than 12db, and correspondingly higher transmitter powers may be required to ensure intelligibility under such noise 

conditions. Signal pre-emphasis before transmission, and noise and 

signal-spectrum shaping upon reception, can probably be used to advantage 

here. However, the dominance of this noise type and level is relatively 

infrequent at any given point in a mine, because harmonic interference 

is the most prevalent. Hence, by using techniques, such as harmonic 

rejection filters on reception and signal pre-emphasis and peak clipping 

on transmission, it appears that intelligible through-the-earth voice 

communications to mines may be possible at reasonable power levels for 

the great majority of the time. Further investigation is clearly warranted, including the effects produced by specific pick-up sensors. 

Arthur D Littlelnc 
Arthur D Little. lnc 
WORKING MEMORANDUM 81 Case: 73912 

TABLE 1-1 

RMS ANTENNA CURRENT AND AVERAGE POWER DISSIPATED 

FOR LONG-WIRE 50-OHM ANTENNA 

(for o = lo-' mholmeter) 

Depth of Mine, Feet 

Gunn Quealy Mine .074 

(low noise) 

Allen Mine .19 

(low noise) 

Allen Mine 2.6 

(high harmonic noise) 

Lincoln Mine 9.1 

(high impulsive noise) 

These transmitter powers produce a signal-to-noise ratio of 12db over 

the voice band upon reception under the specified noise conditions. A 

power handling capability of 10-15db more than the average power dissipated is required if linear amplifiers are used for voice transmission (see text, p. 1-13). 

Arthur D Little Inc 
Arthur D Little lnc 
WRKING MEMORANDUM # 2 Case: 73912 Date: 3 February 1972 

111. SIGNAL POWER ESTIMATES FOR UPLINK THROUGH-THE-EARTH 

BASEBAND VOICE COMMUNICATIONS 

Some companion calculations to those described in Working Memorandum 

#I of 27 January 1972, on downlink through-the-earth communications have 

been performed on the uplink voice channel, using a vertical loop transmitter. 

A. METHOD OF CALCULATION - UPLINK, LOOP ANTENNA 

The magnetic field at the surface is given by 

INA IG I (1) 

IHZ 1 , vertical component, where IGI , as derived 

by Westinghouse, is plotted in Figure 2-1. Using Figure 2-1 curves of 

I G I / 2n d versus frequency for three mine depths, with earth conductivities of 10'2 and 10-I mhos/meter, were plotted in Figure 2-2. 

The value of the RMS magnetic moment = NAIrms), required for 

a signal-to-noise ratio of 12db between the vertical components of 

signal and noise magnetic fields at the surface, under a variety of surface noise conditions, has been calculated for these three depths. 

The same l/f voice spectrum, and the methods of signal-to-noise computation described in the previous memorandum, have been used. A current 

source for the loop has been assumed, so that the spectrum level of 

NAI - l/f. As for the horizontal-wire antenna downlink case, calculations were performed only for the more favorable 0-lom2 mhos/rneter conductivity situation, commonly found over coal mines. 

At D = 300 feet, we approximate 

At D = 600 feet we approximate 

At D = 1000 feet, we approximate 

Arthur D Little Inc 
FIGURE 2-1 LOOP ANTENNA, COUPLING RELATIONSHIP 

Arthur D Little.lnc 
Arthur D Little, Inc. 
WORKING MEMDRANDUM 62 Case: 73912 

Since 1s of the ci e -6f , the received field can be 

27~~~ 

written in the form 

as in Equation (8) of Working Memorandum #l. 

B. NOISE ON THE SURFACE - OVER MINES -- 

Surface Noise Conditions: 

(a) Allen Mine (~igure 2-3, Bensema NBS report) 60 Hz and 360Hz 

harmonics primrily.vertica1 component. 

Nrms = 5 x 10'~ A/M (high noise) 

(b) Gunn Quealv Mina Figure 2-4, 60 Hz harmonics and broadband 

background noise, vertical component (Hestinghouse data) 

NKms = 2.34 x lom4 AIM (medium noise) 

(c) Lincoln Mine (Figure 2-5, Bensema NHS report) 60 Hz harmonics 

primarily, vertical component 

Nms - 1.33 x 10'~ AIM (low noise) 

Broadband noise levels were neglected in the noise power calculations, whenever the spectrum level as seen by a 1tIz bandwidth filter was 

(over the individual 500 or 1OOOHz wide segments used to sum noise 

contributions) more than 40db below the highest harmonic peak lving 

between 500 and 3000Hz. In cases (a) and (c), this criterion is satisfied over the whole of the voice bandwidth, whereas in case (b) the 

background noise does not drop more than 30dh below the 660Hz peak, and 

hence has been inc1uded.h the noise computation. 

C. SIGNAL CURRENT AND POWER ESTIMATES 

A plot of the loop magnetic moment required for voice transmission 

with a 12db SIN ratio as a function of depth is shown in Figure 2-6. 

Shapes of the signal attenuation factor and voice spectrum, as functions 

of frequency, are shown in Figure 2-7. The magnetic moment results are 

presented in Table 2-1. In Tables 2-2 and 2-3 the power dissipated in 

Arthur D Little,lnc 
Arthur D Little. Inc. 
FIGURE 2-4 EM BACKGROUND NOISE ON SURFACE-VERTICAL COMPONENT, Hz n 

GUNN QUEALY MINE. ROCK SPRINGS, WYOMING 

Arthur D Little.lnc 
Arthur D Little, lnc. 
- 

- lo6 A-~2 - 

- 

- 105 A-~2 

- - 

Allen Mine 

- lo4 A.M' (Bensema, Fig. 8) - 

Gunn Quealy Mine 

(Westinghouse Surface Data) 

- - 

Lincoln Mine 

(Bensema, Fig. 25) 

103 A.M~ I I I I I I I 0 

300 400 500 600 700 800 900 1000 

Depth (Feet) 

FIGURE 2-6 TRANSMITTER MAGNETIC MOMENT (RMSI REQUIRED FOR 12db SIN RATIO 

OVER VOICE BANDWIDTH (500 Hz -3kHz) FOR VERTICAL FIELD COMPONENTS 

OF NOISE AND SIGNAL (o 10.' mhoslmeter) 
FIGURE 2-7 RELATIVE SHAPES OF SIGNAL ATTENUATION FACTOR AND 

VOICE SPECTRUM (from 1000 Ft. Depth) 

-40 

-50 

T - -60 

6 

e; 

(0 

.c 

V) 

b r -70 

- a, 

i 

n 

u 

a, .$ .so 

- m 

; 

-90 

-1oc- 

- 

Shape of Original 

l/f Voice Spectrum 

, 

Shape of Signal 

- Attenuation Factor, 

from 1000 Ft. 

la = 10'2 mhos/m.l 

- 

- Shape of Received Voice 

Spectrum from 1000 Ft. 

I I I I I I I II I I I I 11 ll 

500 1000 2000 3000 10.000 

Frequency (Hz) 
WORKING MENORANDUM Case: 73912 

TABLE 2-1 

REQUIRED LOOP MAGNETIC MOMENT, AMP-METER' 

-2 for a = 10 rnho/meter* 

Depth of Mine (Feet) 

- 300 - 600 1000 

Surface Noise at: 

Allen Mine 10,500 96,700 547,000 

(harmonics only) 

Gunn Quealy Mine 4,910 44.300 269,000 

(harmonics and 

broadband) 

Lincoln Mine 2,790 25,200 153,000 

(harmonics only) 

*For a signal-to-noise ratio of 12db over the voice band upon reception. 

Arthur D Little lnc 
WORKING MEMORANDUM 112 Case: 73912 

TABLE 2-2 

AVERAGE POWER DISSIPATION AND RMS VOLTAGE AND CURRENT 

IN UPLINK CIRCULAR LOOP ANTENNA 

for a = mhofmeter* 

w: 100 feet periphery, 20 turns of No. 8 gauge wire (-1285 in. diameter) 

Resistance R = 1.3 ohms; 

Induatance L = 27 millihenries; 

NA = 1460 meter2 

Surface Noise Condition Depth of Mine (Feet) 

300 - 600 1000 

I (A) V (V) P(Watts) I&) ~gi P (W)I (A) V (V) p (W) Tm6 rms ---- rms -- rms 

*For a 12db signal-to-noise ratio across the voice band upon reception. 

Arthur D Littleinc 

390 82,000 200,000 

180 38,000 44,000 

100 22,000 14,000 

Allen Mine 7.2 1,500 6 7 

(high) 

Gunn Quealy Mine 3.4 690 14.7 

(medium) 

Lincoln Mine 1.9 400 4.7 

(low) 

65 13,000 5500 

30 6,300 1200 

17 3,500 390 
WORKING MEMORANDUM 112 Case: 73912 

TABLE 2-3 

AVERAGE POWER DISSIPATED AND RMS VOLTAGE AND CURRENT 

IN UPLINK CIRCULAR LOOP ANTENNA 

for o = loe2 mholmeter* 

Loop: 100 feet periphery, 10 turns of No. 2 gauge wire (.2576 in. diameter) 

Resistance R = 0.12 ohm: 

~~,j~~~~~L = 6.1 millihenries; 

Surface Noise Condition 

Allen Mine 14 

(high) 

Gmn Quealy Mine 6.7 

(medium) 

Lincoln Mine 3.8 

(low) 

Depth of Mine (Feet) 

300 - 600 

v 

rms 

P Inns vnns P 

(v) (Watts) I (A) (v) (w) 

- 1000 

I rms 

v 

rms 

P 

(A) (V) (W) 

*For a 12db signal-to-noise ratio a c r o s s t h e v o i c e band upon reception. 

Arthur D Little Inc 
WORKING MEEIORANDUH #2 Case: 73912 

the loop antenna, and the KMS current and voltage under these transmission conditions, are presented for two alternative loop configurations 

of reasonable size. The RMS voltage has been calculated for the case 

where the loop signal current spectrum level varies as l/f, so that 

where L is the inductance of the loop, and w i' "'f are the initial and final 

edges of the voice bandwidth (i.e.,500 and 3000Hz). These data are also 

presented in Figure 2-8. Throughout, a value of 10'2 mhosfmeter has been 

assumed for the overhurden conductivity. 

Figure 2-8 shows that the power required varies, under the noise conditions encountered, from easily realizable levels of up to around 100 

watts for mine depths in the range of 300 to 600 feet, to the very high 

values of several hundred kilowatts for the deepest mines (1000 feet) 

under the worst noise conditions. 

Two transmitter loop configurations were chosen with a view to 

keeping the diameter of the conductor 'sbundle'' roughly constant. The 

weight increased appreciably from 100 lbs in loop (a), to 150 lbs in 

loop (b). It is clear that sizable reductions in power can be achieved 

by careful design of the loop antenna, but a more thorough investigation 

is needed with proper weighting given to factors such as weight, volume, 

area, and cost. These gains may, however, not be sufficient to achieve 

satisfactory voice transmission from the deepest mines under the worst 

surface-noise conditions, because of the practical limitations imposed 

on a subsurface system compared with a surface installation. But as in 

the downlink case, significant gains may he possible by selectively 

rejecting the worst harmonic contributions by a series of notch filters, 

and the use of pre-emphasis and peak-clipping techniques on transmission. 

These possibilities should be explored further, including the effects of 

specific pick-up sensors, as in the downlink case. 

Arthur D Little,Inc. 
Loop Periphery 100 Ft. 

- Loop (a1 : 20 Turns, #8 Wire 

-'- - Loop (bl : 10 Turns. #2 Wire 

FIGURE 2-8 AVERAGE POWER DISSIPATED IN LOOP FOR 12db SIN 

OVER VOICE BANDWIDTH AT SURFACE (Uplink Transmission) 

Arthur D ~ittte. Inc 
WORKING MEMORANDUM #3 Case: 73912 Date: 29 February 1972 

IV. SPOT SIN RATIO ESTIMATES bWR UPLINK AN1) DOWNLINK THROUGH-THE-EARTH 

NARROWUANI) COIIE COFIMUNICATIONS 

Estimates of the spot signal-to-noise ratio (S/N) have heen made as 

a function of frequency for a variety of in-mine and surface noise conditions likely to be encountered by narrowband code communications systems 

between mines and the surface. Both uplink and downlink through-the-earth 

channels were treated. The spot SIN ratio is defined as 

s = Signal Power (g) (~oise Power Density ) 

spot 

where the signal (S) is expressed in units of amp/meter, and 

the broadband noise density (N) in units of amplmeter per JtIz. 

These calculations have been carried out with the limited available noise 

data, to provide preliminary estimates of performance trends and limitations to be expected for low-frequency narrowband code comunications 

systems, and to help identify related data gaps or limitations. Narrowband systems are of interest to the Bureau of Mines because of the 

possibility of obtaining code communication under circumstances where 

voiceband communication may be ineffective (e.g., in high noise and/or 

high signal-attenuation conditions). 

A. SIGNAL AND NOISE PLOTS 

Figures 3-1 and 3-2 show, respectively, the signal field strengths 

at the receiver for a downlink horizontal-wire antenna with a current of 

1 ampere (50 watts for the 50-ohm antenna of Working Memorandum#l), and 

an uplink loop an enna with a current of 1 ampere and a magnetic moment 

of 1460 amp-meter' (1.3 watts for the 20-turn loop of Working Memorandum#2). The signals (i.e., the vertical and horizontal magnetic-field 

components, directly above the loop or below the wire antenna, respectively) are plotted at three mine depths (300 600, and 1000 feet) for 

representative overburden conductivities (lo-' and 10-I mhoslmeter). 

Figures 3-3 and 3-4 are plots of the broadband noise density both in the 

mine and on the surface respectively, for a variety of conditions. 

These data* are taken from W.D. Bensemass NBS report., WGL measurements, 

and some atmospheric noise measurements on the surface in Florida (unrelated to any mining activity) reported by 3. E. Evans in M.I.T. Lincoln 

Laboratory Technical Note 1969-18, March 1969. Figures 3-5, 3-6, 3-7, 

3-8. 3-9, 3-10, 3-11, 3-12, and 3-13 present the original data plots. 

In a majority of the at-mine cases, vertical noise components are shown. 

since most measurements have been taken of these. Some in-mine measurements of broadband horizontal noise components (Figures 17 and 18 of 

Bensema's NBS report) give results not very different from the low 

vertical broadband noise component plotted from Figure 14 of Bensema's 

work, in Figure 3-8 of this memorandum. It is reasonable to suppose 

that broadband horizontal noise is likely to be no higher than the 

worst broadband vertical noise. 

*Adjusted to a 1 Hz bandwidth. 

Arthur D Little Inc 
3-2 

Arthur D Little Im 
o = 1 0-2 mhoslmeter 

--- 1 o = 10' mhoslmeter 

-21 0 I I I 11111 I I I I I Ill I I I I Ill1 

10 100 1000 3000 10.000 

Frequency (Hz) 

FIGURE 3-2 UPLINK SIGNAL FOR LOOP ANTENNA - MAGNETIC MOMENT = 1460 AMP-METER * 
3-4 

Arthur D Little,lnc 
FIGURE 3-4 SURFACE BROADBAND NOISE DENSITIES FOR UPLINK COMMUNICATIONS 

-80 

-100 

e '=- u 

w 

E" -120 

. 

E 

a - 

?! 

5 -140 

>- 

w .- * 

C 

i5 

. -160 

z" 

0 

C 

2 

2 

-180- 

-200 

-220 

- 

- 

- 

- 

- Allen Mine (Fig. 7, Bensema) Vertical Component 

- - Lincoln Mine (Fig. 24 Bensema) Vertical Component 

--e Gunn Quealy Mine (WGLI Vertical Component 

- -+- Low Atmospheric Noise (Florida, Winter) Evans, 1969: Horizontal Component 

High Atmospheric Noise (Flordia, Summer) 

- 

1 

I I I1 1111 I I I1 1111 I I I1 1111 

10 100 1000 10.000 

Frequency(Hz) 
Arthur D Little,lnc 
Arthur D Little lnc. 
Arthur D Little,Inc. 
Arthur D Little,lnc 
FIGURE 3-9 HORIZONTAL COMPONENT OF SUBSURFACE EM NOISE MAGNETIC FIELD, Hy, 

GUNN QUEALY MINE, ROCK SPRINGS WYOMING (Westinghouse Data) 

3-10 

Arthur D Little, Inc 
.. . 

. O 2010 22 1.02.JO: I.S.005 12/32/-: ::-50::5 3 

? 91-OCS .I &!.DO: O.OOrOC0 O.JO.DEO 2: li¶Z: 4:32: 

03!6210771 Rec. gain c0rr.z -12 Total const cor-. =--&. 3 

1. 000-003 0.2236 :.0?6-264 

-1401 I 

0 I25 250 375 5W 625 750 815 loo0 

Source: NBS Report. Fig. 7 Frequency, Hertz 

FIGURE 3-10 ALLEN MINE, SURFACE, 750 Hz BANDWIDTH (BWI, 03-16 

ANTENNA SENSITIVE AXIS POINTED VERTICAL 
Arthur D Littlelnc 
FIGURE 3-12 EM BACKGROUND NOISE ON SURFACE-VERTICAL COMPONENT. Hz. 

GUNN OUEALY MINE, ROCK SPRINGS, WYOMING 
Source: Lincoln Laboratory - Evans,TN - 1969-18.26 March 1969 Fig. 4-3 

-80 

h 

. -90 

. 

z 

U 

w 

r -100- 

FIGURE 3-13 WIDEBAND POWER DENSITY SPECTRUM FOR FLORIDA 

DATA (Feb., May. June, July 1968) MAGNETIC FIELD 

HORIZONTAL COMPONENTS 

July A NS (7-24 1500) 

NS (6-26 2033) 

- 

x NS (6-25 1833) June {' o '" EW 

- 

V NS (6-27 1620) 

NS (5-7 1800) 

May {+ EW 

Arthur D Little Inc. 

3 

m 

D 

.- 5 -110- "? 

C 

E 

5 -120- Representative High - 

o Atmospheric Noise 

(Florida summer) 

V) 

-130- 

2 

L 

-140- 

-150- 

epresentative Low 

-160- Atmospheric Noise 

(Florida. Winter) 
WORKING MEYORANDUN #3 Case: 73912 

It is apparent from Figures 3-3 and 3-4 that in several instances 

the broadband noise density has not been plotted down to the minimum 

frequency - at - which --- noise measurements - - were made. Fpr ox plotting purposes, cut-off frequencies were chosen because of two factors which can 

significantly distort very-low-frequency broadband noise estimates made 

from the NBS and WGL plots. These can be illustrated by reference to 

Bensema's work. First, in the presence of a high-level harmonic (particularly 6OHz and 180Hz), the characteristics of the analysis filter used 

are such that the plotted level of the noise "floor" between harmonics 

may be raised significantly above its actual value. Secondly, as is 

apparent from the system test with the antenna disconnected (Figure 15, 

Bensema), tape recorder noise rises steeply below 100Hz, and may severely 

contaminate low-frequency broadband noise measurements under some conditions. Broadband noise levels between harmonic peaks at very low 

frequencies (e.g., below 250Hz or so), presented by NBS and WGL, may 

therefore represent upper limits, that are possibly lOdb or more above 

the actual broadband noise floor. More-careful measurements, or 

reprocessing of existing noise recordings, are required before these lowfrequency uncertainties can be removed. 

B. SPOT SIGNAL-TO-NOISE RATIOS 

The spot signal-to-noise ratios for the transmitter antennas and 

noise conditions just described are plotted in Figures 3-14 and 3-15 for 

the downlink and in Figures 3-16 and 3-17 for the uplink channels. In 

each case, two values of overburden conductivity have been used which 

represent moderate and high values (10'2 and 10'1 mhos/meter) for coal 

mine regions. 

Figures 3-15 and 3-17 reveal that, for both the uplink and downlink channels, the optimum frequency for narrowband code communication 

systems for the deepest mines under conditions of high earth conductivity 

(10'1 mhoslmeter) lies considerably below ,lkHz. At a depth of 1,000 feet, 

a frequency on the order of 1OOHz seems optimum. At a depth of 

only 300 feet there is little variation in the spot signal-to-noise ratio 

over frequencies in the range between 100 and 500Hz. The downlink channel 

calculation at moderate earth conductivity (10'~ rnhos/meter), shown in 

Figure 3-14 indicates that a frequency near lOOOHz would be better than 

one in the 100-500Hz range. However, in all except the highest noise 

cases for deeper mines (i.e., Lincoln Mine noise at 600 and 1,000 feet). 

good spot signal-to-noise ratios of lOdb or more are predicted down to a 

frequency of 200Hz. Note that this refers to a horizontal-wire antenna 

with a current of 1 ampere. A current of 4 amperes would provide a lOdb 

spot signal-to-noise ratio at 200Hz even at 1,000 feet, under even 

Lincoln Mine high-noise conditions. The uplink channel calculation, 

shown in Figure 3-1 6, at moderate earth conductivity (1 om2 mholmeter) 

demonstrates less sensitivity to frequency than the downlink case, and 

frequencies in the range of 100 to 1000H~ appear about equally 

satisfactory. 

3-15 

Arthur D Little,lnc 
For Noise Conditions 

- Allen Mine (Fig. 14, Bensema) Vertical 

- - Allen Mine (Fig. 22, Bensema) Vertical 

-+- Lincoln Mine (Fig. 30, Bensema, Fig. 29 below 250 Hz) Vertical - Gunn Quealy Mine (WGL Data) Horizontal 

FIGURE 3-14 DOWNLINK (IN MINE) SIGNAL TO NOISE RATIO FOR HORIZONTAL WIRE ANTENNA, 

1 AMP. CURRENT (50 Watts for 50 ohm Resistance),OVERBURDEN CONDUCTIVITY 

C = 10-2 mholmeter 

- 70 

m 

3 

% .- 

o 

. 

z 

C 

rn 

i7, 

0 

n fn 

30- 

10 

0.- 

-10 

90- 

- 

D=300' _-+a D=600' 

D=600' 

50- D=1000' 

0- 

/ 

0' 

/0 ,/- 

// . / 

// Y' /+ D=600' 

/+ D=30OS '/ M. 

'. I /+' 

D=600' .' / /-- ,+ -- 

D=1000' // t-- /t' +A D=1000' 

- / ,t- - - 

I / 

/ +' / +' / ,+-- - +- 

Signal Power -tO 

ISIN) =( -- 

)+-- 

0° 

Spot Noise Power Denssty +- -tO 

I I I 1,111, t- 4- 

I I I, 111, I I 11 11, 

10 100 1000 10.000 

Frequency (Hz) 

For NAI = 1460 ~mpmeter~ 

(N= 20 Turns, I = 1 Amp, P =1.3 Watts 

Same Loop as Used in WM2. 

Voice Band Calculations, Table 2-2) 

- 

For Noise Conditions -~=1000' 

Allen Mine (Fig. 7 Bensema) (Vertical) 

-- Lincoln Mine (Fig. 24 Bensema) (Vertical) 

4 Gunn Quealy Mine (WGL Data) (Vertical) 

- + - Representative Low Atmospheric Noise (Florida, Winter) 

-Representative High Atmospheric Noise (Florida, Summer) Evans, 1969(Horizontal) 

Signal Power 

Noise PowerDensity 

-50 I I I IIIII I I I I 1111 I I I I I III 

10 100 1000 10,000 

Frequency (Hz) 

FIGURE 3-16 UPLINK (SURFACE) SIGNAL-TO-NOISE RATIO FOR LOOP ANTENNA.OVERBUR0EN 

CONDUCTIVITY a - 10-2 rnholmeter.LOOP MOMENT NAI = 1460 ~mp-meter2 
FlOURE 3-17 UPLINK (SURFACE) SIGNAL TO NOISE RATIO FOR LOOP ANTENNA,OVERBURDEN CONDUCTIVITY 

U= 10 -1 mholmeter, LOOP MOMENT NAI = 1460 Amp-meter2 

I 

, I. / -+ /+ For NAI = 1460 Amp.-meter 2 

D=300' - 0 

0+ (N=20 Turns, I= 1 Amp., 

- I P=1.3 Watts: Same Loop as used in - - \ WM2 Voice Band Calculations, Table 2-21 

D=300' ---, -- -.. D=W \ -- ,+-+ 

- /--_ 

+-- 

D=1000';f-- 5 

D=600' *-0-0 

D=300' 

-- - 

D=1000' , D=600' 

- 

- 

- 

For Noise Conditions 

-Allen Mine (Fig. 7 Bensema) (Vertical) 

- - Lincoln Mine (Fig. 24, Bensema) (Vertical) 

\ 

\ D=1000' 

'b 

\ D=600' 

-Gunn Quealy Mine (WGL) (Vert~cal) - 

-+- Representative Low Atmospheric Noise (Horizontal) (Florida Winter) Evans, 1969 (Horizontal, Vepresentative High Atmospheric Noise (Florida Summer) 

(SIN) Signal Power 

spot =(Noise Pwer Density 

- 

D= 1000' 

1 t I IIIII I I I I Ill1 I I I Ill* 
WORKING MEMORANDUM #3 Case: 73912 

The spot signal-to-noise ratios plotted for Evans' noise measurements in Figures 3-16 and 3-17 are probably representative of the range 

of expected performance on the surface when broadband man-made noise is 

not dominant; the Florida Winter curve is characteristic of the cases 

when thunderstorms are distant, and the Florida Summer curve is characteristic of times of intense local thunderstorm activity. For communication 

via the vertical signal component, the S/N estimates are probably 12 to 20db 

too pessimistic, because the horizontal noise components (which are 

larger than the vertical) were used in the calculations. On the other 

hand, for EM location systems that depend on detecting and locating a 

null in the horizontal component of signal magnetic field, the SIN curves 

are probably more than 30db too ovtimistic, because horizontal signal 

null depths can be more than 50db the vertical component signal 

strength directly above a loop signal source. 

C. CONCLUDING REMARKS 

A generalized tentative conclusion is that frequencies on the order 

of 100 to 500Hz appear attractive for the design of narrowband code 

through-the-earth communications systems to cover a wide variety of mine 

depths,conductivities, and noise conditions. This holds for both uplink 

and downlink channels, and particularly under conditions of high-signal 

attenuation and high noise. The limited noise data available indicate 

that even for deep (1,000 ft) mines with high noise and conductivity, 

more-than-adequate signal-to-noise levels should be attainable for 

narrowband code systems with practical signal sources operating at 100 

to 500Hz. However, for moderate conductivity mines, the data indicate 

that frequencies up to about. 1-to-2kHz may provide improved performance. 

If high conductivity, de?~ mine conditions assume a high priority, 

more accurate measurement of the broadband-noise power spectral density 

and/or processing of existing noise recordings at very low frequencies 

will be required to obtain more exact estimates of the noise 

densities; so that optimum frequency bands for narrowband code communications systems can be identified and selected with a higher degree of 

confidence. Since broadband man-made and atmospheric noise contributions are also nonstationary and non-Gaussian, it is important to obtain 

and utilize noise amplitude and time statistics, so that full advantage 

can be taken of existing methods for optimizing digital signaling in 

such noise environments. 

This discussion has not touched on ways in which it may be possible 

to take advantage of certain broadband noise characteristics to improve 

the performance of narrowband code comrmnications systems. For example, 

when broadband noise is highly impulsive in nature (which can be determined by 0bSe~ation and by measuring amplitude and time-interval exceedance ~robabilities), then it may be practical to use wideband-limiting 

Arthlrr n I ittlo inr 
WORKING MEMORANDUM #3 Case: 73912 

or other impulsive-noise-discrimination techniques to good advantage. 

Some of these techniques may not be usable in the presence of strong 

harmonic interference, but clearly they merit further investigation 

concerning their applicability and possible application. 

Arthur D Little,lnc. 
CAMBRIDGE, 

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NEW YORK 

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WASHINGTON 

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