Contract H0177098 

Arthur D. Little, Inc. 

BUREAU OF MINES * UNITED STATES DEPARTMENT OF THE INTERIOR 

Minerals Health and Safety Technology 
The views and conclusions contained in this document 

are those of the authors and should not be interpreted as necessarily representing the official 

policies of the Interior Department's Bureau of 

Mines or the United States Government. 

Arthur D Little, lnc. 
REPORT DOCUMENTATION PAGE 

8. Performing Organization Report No. 

C-81150 

1. Report No. 

- - 

9. Performing Organization Name and Address 

Arthur D. Little, Inc. 

25 Acorn Park 

Cambridge, MA 02140 

12. Sponsoring Organization Name and Address 

Pittsburgh Mining and Safety Research Center 

U.S. Bureau of Mines 

Pittsburgh, PA 

2. 

I 10. Project/Task/Work Unit No. 

3. Recipient's Accession No. 

4. Title and Subtitle 

Final Report on Development of a Through-the-Earth 

Monitor System 

I 15. Supplementary Notes I 

5. Report Date 

December 1980 

6. 

-- - - pp -- - - 

1 16. Abstract 

The object of this program was to design and build a self-contained system for remote monitoring of the atmosphere in mines. The system developed uses the mine pager phone line as 

the primary communication channel and also has the capability of using 208 Hz in the ELF 

band for wireless transmission through the earth. The depth for through-the-earth transmission is expected to be about 80 to 90 meters in high conductivity soil and is limited by 

the noise level in the ELF band-pass filters. The underground components of the TEM System 

are energized by mine power through a battery charger - storage battery power supply. In the 

event of mine power failure the battery capacity is more than sufficient for one week's 

loperation with interrogations every four hours. I 

I I 

ELF Data Transmission 

Through-the-Earth Communication 

17. Originator's Key Words 

Mine communications 

Mine monitoring 

18. Availability Statement 

Release unlimited 

1 

Phone line I I 

, 19. U.S. Security Classif. of the Report 

Unclassified 

1 I 

20. U.S. Security Classif. of This Page 

Unclassified 

21. No. of Pages 

29 

22. Price 
FOREWORD 

This report was prepared by Arthur D. Little, Inc., 

Cambridge, MA 02140 under USBM Contract Number H0177098. 

This contract was initiated under the Coal Mine 

Health and Safety Program. It was administered under 

the technical direction of the Pittsburgh Mining 

and Safety Center with Mr. Harry Dobroski as the 

Technical Project Officer. Mr. Doyne W. Teets was 

the Contract Administrator for the Bureau of Mines. 

This report is a summary of the work recently completed 

as a part of this contract during the period October 1977 

to December 1980. This report was submitted by the authors 

in December 1980. 

Arthur D Little, lnc 
TABLE OF CONTENTS 

Page 

List of Figures 

List of Tables 

INTRODUCTION 

PHASE I - ESTABLISH SYSTEM FUNCTIONAL REQUIREMENTS 

AND SELECT SENSORS 

Remote Unit Operating Modes 

System Operating Modes 

Sensor Evaluation and Selection 

PHASE I1 - DETAILED ENGINEERING DESIGN 

ELF Communication Link 

Transceiver Interface 

Monitor Unit 

PHASE 111 - SYSTEM FABRICATION AND PERFORMANCE 

EVALUATION FABRICATION 

Fabrication 

Performance Evaluation 

CONCLUSIONS 

INVENTIONS 

REFERENCES 

Arthur D Little, Inc. 
LIST OF FIGURES 

Figure 

Number 

1 TEM System 

2 TEMS Block Diagram 

3 Surface Unit Transceiver Interface 

4 Signal Timing and Communications Link 

5 Interface Logic 

6 TEMS Components 

7 TEMS Console 

Page 

12 

13 

15 

16 

18 

21 

22 

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LIST OF TABLES 

Table 

Number 

1 Measured ELF Receiver Performance 

2 Predicted Maximum Operating Depths 

Page 

24 

26 

Arthur D Little, Inc. 
INTRODUCTION 

The objective of this program was to obtain a self-contained monitor 

data transmission system that uses existing mine wiring as the primary means of data transmission, but which can also transmit directly 

through the earth. This objective was attained in a three-phase program: 

Phase I - Establish System Functional Requirements and Select 

Sensors 

Phase I1 - Detailed Engineering Design 

Phase 111 - System Fabrication and Performance Evaluation 

The resulting Through-the-Earth Monitor System (TEMS) has a total 

capacity of 1,000 addresses and is under control of a central unit 

at which commands are dispatched and responses received. The TEMS, 

as delivered, has a single remote unit with sensors for fire, 

methane, and air flow. 

Arthur D Little, Inc 
PHASE I - ESTABLISH SYSTEM FUNCTIONAL REQUIREMENTS AND SELECT SENSORS 

The primary design goal for the Through-the-Earth Monitor System 

was to provide a means to monitor from a surface station carbon monoxide, methane, and air flow at various places in a mine. Existing 

mine wiring, when available, was to be used for data transmission 

with direct signal transmission through the earth to be used otherwise. The wireline transmission scheme developed for the Visual Display and Control System (VDCS), phase inversion modulation of a 

16 KHz carrier, was retained for the TEMS wireline mode. Other features of the VDCS were also retained, including the addressing and 

command formats and the data transmission protocol, so that both 

TEMS and VDCS remote units can be addressed by the TEMS control unit. 

Through-the-Earth communication is provided byan ELF data link. Unlike the VDCS remote units, the TEMS remote units are powered by 

a rechargeable battery which is kept charged by 115V ac or 270V dc 

mine power, when available. 

Remote Unit O~eratine Modes 

The following remote unit operating specifications were defined for 

the four combinations of wireline communication and mine power availability. It should be pointed out that different monitor units in 

the system can be in different modes at any given time. 

Mode 1 

Wireline communication and mine power are both available to the remote unit. This is the normal mode of operation. TEMS remote monitor 

units are fully powered-up continuously. They can be interrogated 

at any time, either manually or automatically (see below) and will 

send back their instrument readings immediately. 

Arthur D Little, Inc. 
Mode 2 

Wireline communication is available, but mine power is out at a given 

unit. That unit goes into a low power mode where only the front end 

of the communication circuits is active. When that unit is interrogated, it signals the surface that it has been powered-down, after 

which it goes into a four minute sensor warm-up cycle. After five 

minutes, the surface control unit will automatically re-interrogate 

it. After transmitting its instrument readings, it returns to the 

low power state until the next interrogation. 

Mode 3 

Wireline communication has been lost, but mine power is on at the 

remote unit. Communication is provided by the ELF channel which may 

require, because of the relatively short communication range of the 

ELF channel, that the control console and ELF antennas be moved to 

a surface location directly above the remote unit. Except for a signalling rate approximately twenty times slower, operation is the 

same as Mode 1. 

Mode 4 

Neither mine power nor wireline communication is available. ELF communication is used as in Mode 3, but the remote unit goes into a 

low power state between interrogations, as in Mode 2. 

System Ooeratinz Modes 

The TEM system was specificed to have three operating modes - Keyboard, Automatic, and Test. In the Keyboard mode, remote units are 

interrogated by manually entering addresses and commands on the console keyboard. Instrument readings are reported by NORYL/ERROR LEDs 

on the console and printed reports on the TTY. Monitored variables 

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outside a preselected range trigger an audible alarm. Supervisory 

verification of communications link integrity is provided and indicated as in the VDCS (1). - In the Automatic mode, TEMS monitor units 

are interrogated automatically every five minutes according to a 

polling list in the controller memory. In both Keyboard and Automatic 

modes, CURRENT, OLD, and ERROR lists are maintained as in the VDCS. 

In addition, the TEMS maintains an ALARM list of all monitor units 

which have experienced alarm conditions. There is also a Test mode 

which permits various types of system diagnostics in the field. These 

modes are described in detail in the VDCSITEMS Technical Manual (2). - 

Sensor Evaluation and Selection 

After reviewing the state-of-the-art of carbon monoxide, methane, 

and air velocity sensors, J-Tec Associates, Inc. of Cedar Rapids, 

Iowa was selected on the following basis: 

Single source of supply for all three sensor types; 

Units are available and meet all performance requirements; 

Units operate at 12V dc and consume little power; and 

Units have been demonstrated in the field. 

The VM-101 methane sensor detects methane concentrations in the 0 to 

4% range and creates a 0 to 5V dc output directly proportional to 

methane concentration. The VA-216 air draft sensor converts air speed 

in the range of 50to 1,500 feetlminute to a 0 to 5V dc output signal. 

Both of these units were designed and have been certified to be intrinsically safe. The CO-200 fire sensor detects carbon monoxide. Its 

output signal switches from a low (-0.5V) level to a high (-5~) level 

when the CO concentration exceeds 150 ppm. The CO-200 detector active 

element can become fouled, so a purge capability is provided. On 

receipt of a PURGE command the active element is heated slightly to 

drive off contaminants. During the 4%-minute purge cycle, a status 

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output is provided to warn users that the instrument readings are 

invalid. Detailed descriptions of these instruments are contained in 

the VDCS/TEMS Technical Manual (2). - 

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Figure 1 shows the interconnection of the components of this TEM system. 

Figure 2 is a block diagram of the TEM system which evolved in response to the functional requirements developed in Phase I. The primary design activities in Phase I1 related to the ELF communication 

link, the ELF link interface to the wireline link, and the remote 

monitor unit. The wireline transceiver design is the same as the corresponding design in the Visual Display and Control System (vDCS) (1). - 

ELF Communication Link 

The design process, including parameter tradeoffs, is described in 

detail in the TEMS Phase 11 progress report (3) - and is summarized 

here. The Sinha and Bhattacharya propagation model (4) - was used, 

assuming a homogeneous conducting earth. This model permits computation of the magnetic field at the earth's surface due to a vertical 

dipole at various frequencies and dipole depths. The underground 

transmitting antenna design was constrained by intrinsic safety re- 

-3 quirements which limit the antenna energy to 10 joules. This constraint restricts the number of ampere-turns so the antenna area must 

be made large to achieve reasonable transmitted signal levels. 

The final antenna design is a multiconductor cable with connectors 

at either end. This cable can be carried around a 60 foot coal pillar 

and, when the connectors are joined at the transceiver, becomes a 

large area multi-turn coil. 

The receiving antenna is a 300 turn, 12 inch diameter coil which gives 

an induced voltage greater than that contributed by the receiver electronics. System performance will therefore be limited by ambient noise 

rather than receiver design. 

Arthur D Little, Inc. 
TEM SYSTEM 

TTY 

(OPTIONAL) 

1 10 VAC + CONSOLE 

FIGURE. I 

POWER 

4 

A D 

, * 

RECEIVI )rlG 

ANTENNA < 

TRANSMITTING 

ANTENNA 

L J 

-------------- 

L 

MONITOR 

'U N IT 4 

1 

VELOCITY 

ABOVE GROUND 

-----------------_-___--- 

UNDLRGROUND 

PAGER 

PHONE 

LINE 

v 

r + 

TRANSMITTING 

ANTENNA 

I 7 

RECEIVING 

ANTENNA i # 
TEMS Block Diagram 

Figure 2 

CONSOLE r 

1 

T. 0. D. 

CLOCK 

CONTROL 

SBC 108 El -- -- 

l NTEL 

t 

DISPLAYS -' TRANSCEIVER 

INTER FACE 

-L PANEL 

LOGIC > 

~1 swlT!HEs 

A 

1 

WlRE 

LINE ELF 

XCVR. XCVR. 

A A 

Fl t 

# 

A 

'SBC 8O/2Oe 

C PU 

co 4 ?' 

r V 1 

CH4 

-1 - 

WlRE 

Ll NE 

XC VR 

ELF. 

XC VR - 

r 

AIR 

VEL. 

4 

CI 

C 

, 

TRANSCEIVER 

INTERFACE 

t 

1 

* 

MONITOR UNIT 

SENSORS 

MINE POWER B ATT. CHGR. 
The ELF carrier frequency of 208 Hz was chosen following an analysis 

of measured ambient noise data from several mines which showed a 

broad minimum at 180 and 240 Hz. The receiver bandwidth was chosen 

as 12 Hz to accommodate the desired signalling rate and the receiver 

filter was designed to give at least 50 dB rejection at 180 and 

240 Hz. Phase inversion modulation is used as in the wireline communication link. The detector is a phase-locked loop, the loss-of-lock 

signal indicating a binary zero. 

Transceiver Interface 

Figure 3 shows the transceiver interface design for the surface control unit. The wireline or ELF channel is selected by a switch on 

the control console. The UART and its various buffers and control 

circuits are shared by the two channels, only the clock rate and 

the data paths being changed by the switch. The switch also sends 

a signal, WIRE, to the controll.er which sets the time out period 

for the selected channel. Figure 4 illustrates the signal timing 

on the communication link for the wireline channel. Signals and 

timing on the ULF channel are identical except for a twenty-fold 

reduction in bit rate. 

The communication cycle begins when the system controller sets up 

two data bytes, TRA and TRB, in its output ports C4 and C8. These 

bytes contain the 10 bit remote unit address and a two bit command 

code. Transmission is initiated by the signal XMIT which causes the 

transmitter to send a string of 64 bits as a training sequence for 

the underground receiver phase-locked loop. The two data bytes are 

then transmitted and the transceiver switches to the receive mode 

to await a reply from the underground unit. 

When the UART has assembled the first received data byte it sends 

a DR (Data ~eady) signal to the controller. The controller interrupts 

whatever it is doing, collects the data byte, and issues a DRR (Data 

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- 

OR 

TO INTER 

INTEL I 

SBC 108 L" TRANSCEIVER 

I 
Signal Timing and Communications Link 

Figure 4 
Ready Reset) to the transceiver to acknowledge receipt of the data. 

After all six data bytes, the address, command code, and four bytes 

of monitor data have been received, the controller checks the received 

address and command for equality with those transmitted, then processes 

the monitor data for display. The second received byte also contains 

bits indicating the loss of mine power and the state of the CO unit 

purging circuit. The four monitor data bytes contain measured levels 

of carbon monoxide, methane, air flow, and battery voltage. 

The underground unit transceiver interface is similar in principle 

but is complicated by the necessity for the unit to recognize when 

it is being addressed, either via the ELF or wireline channel, and 

when that occurs to turn on the wake-up power to the remaining communication circuits and control electronics and, in the event that 

mine power is off at that unit, to turn on instrument power. Figure 5 

shows the interface logic. A separate UART monitors the ELF receiver 

output continuously. When it recognizes the ELF training sequence 

a switch is operated to connect the data UART to the ELF transceiver. 

Monitor Unit 

Referring to Figure 2, the monitor unit is seen to consist of the 

transceivers and interface described above, sensors for carbon monoxide, 

methane, and air flow, and a batterylcharger unit. In normal wireline 

operation with mine power on, the monitor unit replies with a set 

of four sensor and battery voltage readings each time it receives 

a POST command. The CO sensor, however, has a special purge state 

into which it can be placed by a PURGE command from the surface. While 

in this state, which lasts for about four minutes, the semiconductor CO 

sensor element is heated to drive off impurities which may effect 

its readings. During the purge state, CO readings will be incorrect 

so a PURGING flag in the second echo byte is set to warn the controller 

to discard readings from units interrogated while in their purge cycle. 

Arthur D Little, Inc. 

When mine power is on, the gel cell battery is kept charged by the 

charger circuit. When mine power goes off, the remote unit is powered 

down so that only the transceiver circuits essential to signal reception are operating. A MINE POWER OFF flag in the second echo byte 

signals this condition to the controller on the surface. When the 

powered-down monitor unit receives a POST command, full power is 

restored to the unit. However, it can not report data immediately 

because the CO sensor goes into a four minute purge cycle when it is 

powered-up. Under these circumstances the control console indicates 

that the unit has gone into a WARM-UP state. After five minutes has 

elapsed the system controller automatically re-posts the unit at 

which time the correct sensor and battery voltage readings are sent 

to the surface. Following the data transmission the remote unit 

reverts to its low power state. 

The monitor unit was designed to operate for one week with interrogations every four hours. 

Arthur D Little, Inc. 
PHASE 111 - SYSTEM FABRICATION AND PERFORMANCE EVALUATION FABRICATION 

Fabr icat ion 

Figure 6 shows most of the TEM system components. At the left is 

the monitor unit and its receiving antenna. The control console containing all surface electronics is in the center and on the right 

are the battery and charger unit and one of the three sensors. Not 

shown are the receiving antenna for the control unit, the two transmitting antennas and the other two sensors. Figure 7 is a closeup 

of the control console. Operating controls and displays duplicate 

to a large extent those found on the VDCS console and described in 

the VDCS final report (1). - TEMS controls and displays are also described in detail in the TEMS Operator's Manual (5). - Displays not 

found in the VDCS are: 

Mine power - On/Off, 

Battery voltage - Adequate/Low, 

Carbon monoxide level - Safe/Alarm, 

Methane level - Safe/Alarm, 

Air flow - Safe/Alarm, 

Purge, 

Warm-up, 

Monitor/List, and 

Time of day. 

The first seven displays listed report on the state of the remote 

unit as described above. The LIST LED indicates that, in the Automatic mode, the STATION STATUS display is cycling through items on 

the CURRENT, OLD, ERROR, or ALARM list as specified by the setting 

of the LIST SELECT switch. The MONITOR LED indicates that, in the 

Automatic mode, the STATION STATUS display is cycling through the 

list of all monitor units which have been posted since the last System Reset and which are not on the ALARM list by virtue of a measured 

value of battery voltage, CO level, CH4 level, or air flow being outside a specified range. LIST/MONITOR display mode is selected by 

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Arthur D Little Inc. 
TEMS Console 

Figure 7 

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the switch of the same name. The Time of Day Clock indicates 24 hour 

time and is set by fast and slow buttons at the rear of the console. 

When an alarm condition is detected an audible alarm sounds. This 

alarm can only be turned off by the ALARM RESET or SYSTEM RESET buttons. 

Performance Evaluation 

The wireline receivers will respond to signals as small as 18 millivolts which permits operation with transmission losses as high as 

54 dB. The completed system was tested in the wireline mode for performance in the presence of noise by injecting white noise, bandlimited to 20 KHz, onto the wireline and recording the noise level 

at which errors began to occur in data reception. These values, normalized to the receiver bandwidth of 500 Hz, were about 5 millivolts 

for the surface and underground receivers. Since the noise level 

in the noisiest mine is expected to be as high as 35 millivolts in 

a 500 Hz band, the wireline receivers would have to be operated at 

reduced gain in such a mine. This is not a real problem because the 

receiver sensitivity is larger than necessary to meet system design 

goals based on 40 dB wireline loss. 

Measured performance of the ELF receivers is summarized in Table 1, 

from which it can be seen that a signal strength of at least 28 microvolts at the antenna terminals is required for reliable operation. 

From the Phase II report on this contract, that signal strength is 

predicted to be: 

V = 2rfp N Ar Q HZ Volts 

o r 

where f = Frequency = 208 Hz 

Nr = No. of turns in receiving antenna = 300 

A = Area of receiving antenna = .07 sq. meters r 

Q = 3.6 

H = Vertical component at surface due to buried dipole Z 

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Surface Unit 

Receiver self-noise (input shorted) 

Minimum signal for operation 

Maximum signal for operation 

Receiver gain to BNC = 45 dB 

Dynamic range = 40 dB 

Mine Monitor Unit 

Receiver self-noise (input shorted) 

Minimum signal for operation 

Maximum signal for operation 

Receiver gain to BNC = 41 dB 

Dynamic range = 49 dB 

Table 1 

Measured at Referred 

Test BNC to Input 

Measured ELF Receiver Performance 

24 

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and H = z 

NT I A7 Ampereslmeter 

2T D~ 

where N7= No. of turns in transmitting antenna = 12 

I = Maximum value of transmitting antenna current permitted by intrinsic safety = 0.25 amperes 

AT= Area of transmitting antenna = 334 sq. meters 

D = Depth of mine monitor unit in meters 

G = Attenuation factor from propagation model, a function 

of frequency and overburden conductivity 

When the known system parameters are used in the above equations, 

the received signal strength is predicted to be: 

v=- 19*8G Volts 

D~ 

Table 2 shows the variation in predicted received signal strength 

as a function of mine unit depth and overburden conductivity. The 

table shows that to achieve 28 microvolts of signal the maximum operational depths will be 87 and 78 meters with conductivities of 0.05 

and 0.5 Mhoslmeter, respectively. 

Current drain for the completed monitor unit was measured to be 0.011 

amperes in the quiescent state, or about 1.8 ampere-hours for a week. 

Posting the unit every four hours, including the warm-uplpurge cycle, 

consumes another 1.2 A-H for a total of 3.0 A-H for a weeks' operation. Since the battery is rated at 6 A-H the performance exceeds 

the specification. 

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Depth 

(meters) 

Table 2 

S igna 1 

-- G (microvolts) 

Predicted Maximum O~eratine De~ths 

Arthur D Little, Inc 
CONCLUSIONS 

The Through-the-Earth Monitor System has the capability of making 

remote measurements at many locations in a mine and transmitting 

data to the surface either over pager phone lines or directly through 

the earth. In their present form the wireline transceiver circuits 

have more than enough sensitivity to operate with the specified maximum line transmission loss of 40 dB. In noisier mine environments 

they can be operated at reduced gain with little, if any, sacrifice 

in communication range. 

The present ELF transceiver design is predicted to permit communication to depths of 87 meters through overburden of conductivity 

0.05 Mhos/m and to depths of 78 meters through overburden of conductivity 0.5 Mhos/m. The performance is presently limited by a high 

value of receiver noise, a consequence of the choice of a low power 

active filter as the receiver tuning element. This performance could 

be improved at the cost of size or battery life by using a passive 

filter, lower-noise amplifiers, or both. 

Arthur D Little, Inc 
INVENTIONS 

There were no inventions made in conjunction with this contract. 

Arthur D Little, Inc 
REFERENCES 

1. "Final Report on Development of a Visual Display and Control 

System,"'Arthur D. Little, Inc., USBM Contract 50177020, October 

1980. 

2. "Technical Manual for Through-the-Earth Monitor System and 

Visual Display and Control System," Arthur D. Little, Inc., 

USBM Contracts 50177020 and H0177098, December 1980. 

3. "Development of a Through-the-Earth Monitoring Transceiver System," Arthur D. Little, Inc., USBM Contract H0177098, Phase 

I1 Report. 

4. "Vertical Magnetic Dipole Inside a Homogeneous Earth," Sinha 

and Bhattacharya, Radio Science, Vol. 1 (New Series), No. 3, 

March 1966. 

5. "Operators Manual for the Through-the-Earth Monitor System," 

Arthur D. Little, Inc., USBM Contract H0177098, December 1980. 

Arthur D Little, Inc.