the role of computers in the detection and research of ... · the role of computers in the...

RIEMER, K.L. The role of computers in the detection and research of seismic events on the West Wits Line. APCOM 87. Proceedings of the Twentieth International Symposium on the Application of Computers and

Mathematics in the Mineral Industries. Volume 1: Mining. Johannesburg, SAIMM, 1987. pp. 55-80.

The Role of Computers in the Detection and Research of Seismic Events on the West Wits Line

K.L. RIEMER

West Driefontein Gold Mine, Carletonville, South Africa

The West Wits Line first began yielding gold in the mid-1930s and is among South Africa's leading gold-producing areas. Gold mining operations are carried out amidst the occurrence of seismic events which cause both damage to underground excavations and injury and loss of life to production personnel. The feasibility of establishing a system to minotor the seismicity on all the Gold Fields mines was investigated during the early 1980s. This led to the establishment of a centralized seismic network in 1981.

The system was structured around three major objectives. It uses Perkin Elmer (now Concurrent Computer Corporation) 16 and 32 bit mini-computers. The hardware in use at both the central and remote sites is outlined, giving a good insight into the role of computers in the system. The system software reveals the fast flow of data through the network; it can be divided into three categories: (1) data logging and transmission - remote sites, (2) data capture and communications - central site, (3) applications and research - central site.

Several examples are given which show how the system is measuring up to its objectives. These case studies also indicate how the computers are effective in representing the seismic data collected. It is concluded that the use of computers enables the pin-pointing of seismic events on the West Wits Line within about 3 - 4 minutes of their oc<:urrence.

The choice of hardware is largely dependent on the objectives of system, and the Gold Fields Network has chosen mini-computers because of the speed of communication and the large amount of data and storage required for

research applications.

Introduction

The economic potential of Carletonville

area was realized with the discovery of

the gold bearing reefs of the Upper

Throughout its 55-year history the area

has been responsible for 15 gold mining

leases. 1 In addition to the high tonnages

Witwatersrand series by Dr Rudolf

Krahmann. This resulted in the

development of the West Wits Line (Figure

1) which was to add a new dimension to

South Africa's gold reserves. Mining

operations in the area star ted i n 1934

with t he Venterspost Gold Mine, and today

the West Wits Line is among t he top

producing regions of the country.

in the area, mi ning operat i ons are

extending to progressively greater depths,

with layouts in the region of 4 DOOm below

surface presently being planned.

One of the problems which has burdened

the gold mining industry since its early

days, and in which the West Wits Line is

no excepti on, is that of rockbursts and/or

seismic events. These dangerous releases

DETECTION AND RESEARCH OF SEISMIC EVENTS ON THE WEST WITS LINE 55

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of energy s how a ll the properties of

natural earthquakes and range in sca l e

from micro fractur i ng of t he rock to

violent r e l eases of ene rgy measuring 5,2

on the Richter Scale. The events can

disrupt mining operations by causing

frequent damage to underground workings as

well as inj uries and loss of life to

underg round pe rsonnel.

In confyonting

disciplines

seismology

of

have

these problems , the

Rock Mechanics and

received enormous

attention ove r the l as t twenty years. One

of the first t asks i n seismology was the

locat i on of the hypo centers of these

events ; th is need l ed to the estab.lishment

of a number of

individual mines

seismic networks on

from the late 19605

onwa rds . Although s ome of the early

systems us ed ana logue t echni ques. most of

the s ubsequent s ys t ems r esorted to digital

methods and the us e of computers to carry

out sei~mic r es ea rch.

Gold Fie l ds of South Africa has

extensive inte rests i n the Wes t Wits Line,

and it inves tigated the feasibility of

establishing a seismic network during the

late 19705 . The purpose of this paper is

to descri be the applicat i on of computers

and t heir rol e ill the detection and

research of s e ismic events using the Gold

Fields system on the Wes t Wi t s Line. The

development of the s e i s mic

outlined together with

object ives .

The compute r hardware

network is

its major

i n use is

described in some de tai l . and the role of

the compute r s i n t erms of the applications

software is then considered. Some

examples are put forward on how the system

is meeting i ts objectives, and finally

various conclusions on the system and on

the use of computers are outlined.

The Gold Fields Network and system objectives

When the Go l d Fields Network was

establis hed . s everal objectives were laid

down for its operation. These have been

outlined by De Jongh and Klokow.~ For the

purpose of this s tud y , they are summarized

below:

Immediate objectives

It was decided that the system would

function as a management tool, including

the rapid location of seismic events.

This facilit y was vi tal j~ order to

implement rescue operations as soon as

possible whenever the need a rose .

Medium term objectivE's

Once the locations of seismic events had

been obtained they could be stored and

used to identify active geo logical

s tructures s uch as dykes and faults.

This would then assi s t in pl anning of mine

layouts and r emnant blocks. In addition,

it was hop ed t hat the information would

provide feedback on the use of various

support methods such as backfill and

stabilizing pillars .

Long term objectives

Once the patterns and the occurrence of

se1smicity had been observed on the mine,

it was hoped t o be able to study in more

detail the factors causing these events.

This would include detailed seismogram

analys is, stress migrations, laws of

attenuation and in general a more detailed

analYSis of the wave forms.

Motivation for a seismic network within

the Gold Fields Group started in about

1979 on the West Dr iefontei n mine. During

the next two years extensive work was

carried ou t by Or Rod Green (Bernard

Price Institute, University of the

Witwatersrand) and Miles Forsyth (Mining

Stress Systems) on ·the design of the

system and on the selection of suitable


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computer hardware.

During these early development stages it

was decided to include all the surrounding

Gold Fields Mines, and the early planning

attempted to evaluate individual mine

networks against one centralized system.

Ultimately, from the considerat i on of

s t aff r equirements alone, it was decided

to design a centrali zed sys tem involving

remote stations at each of t he mines which

would be linked to a central si te at the

West Driefontein Mine.

On t he computing hardware side, the

choice was not easy and had to be based on

the knowl edge of other existing networks.

Hewlett Packard These i nc luded

( Blyvoo~it zicht and Western Deep Levels)

and Perkin Elmer (Welkom). From the start

it was clear that multiple computers would

be involved: those at the remote sites

would be slave systems and would need to

log underground data and transmit th1S to

a more powerful computer at the central

site. Other computer hardware considered

a t the time included Prime and Vax, but

ultimately the dual bus architecture on

the Perkin Elmer hardware was considered

to be a major advantage. Sixteen bit

mini - computers were proposed for the

remote sites, with a 32 bit mini-computer

at the central site for data processing

and analysis.

One other system

was for logging at

proposed at the time

the remote sites by

means of a direct memory transfer into a

micro processor. The mini computers would then be used only to transfer the data to

the central si te. At the time this

proposa l was rejected as the assurance was

high that the 16 bit remote computers

could cope with the task.

In view of the priority to receive

seismic data as quickly as possible at the

central site, the communications link

envis aged had to operate at high speeds.

In order to achieve this, a UHF radio link

that would operate a t speed of ',Bk baud

was proposed.

The eventual layout of the system

adop ted is shown in Figure 2. Each remote

system is made up of an underground

network of geophones which generate

amplified and modulated signals to a

surf ace 16 bit compute r . The Perkin Elmer

1620 computer perfo rms an analogue to

digital conversion on t he data and logs

this to a system of files on a disc drive.

The computer i s also connec ted to a

ha rdware triggering device which i s

activated dUring the s t a rt and end of any

seismic events. Once the seismic data has

been logged to disc, t he 1620 computer

transmits the underground data to the

central site for further processing and

research work. The Cent ral mini-computer

software was designed t o support up to

eight mines. At present it is monitoring

five mines, namely Wes t Driefontein, Bast

Driefontein, Kloof, Doornfontein and

Deelkraal. (see Figure 1).

The Gold Fields seismic network has been

in operation for five

already recorded about

years

60 000

and has

events.

Seismi c data is transmitted to the central

si te within about 1-2 minutes. In

addition, messages and seismic results can

be transmitted back to remote sites at

will.

The features which tend to make the Gold

Fields network different from most of the

other existing networks are its size and

also its high speed radio communications.

During the initial five year period ,

problems have arisen on t he remo te sites

wi th both the computer hardware and

software. These have generally been in

the form of data losses during the logging

cycle which have tended to make some data


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unsuitable for locating.

Changes have been carried out to the

application$ software, but were not

successful in totally eliminating the

problem, It has become evident following

these changes that an upgrading of the

remote site computers may be necessary.

System hardware

In the Gold Fields seismic network,

computers play a significant role in terms

of both speed and efficiency of handling

data. In order to understand this role it

is first necessary to outline the hardware

used in more detail . Because the remote

MINING: ROCK MECHANICS

,

1620 machi nes may be replaced with more

power f ul )2 105 the remote systems will be

described in terms of the l atter. Some of

the more important features of t he Perkin

Elmer 32 bit hardware are f irs t outlined

and then the system is described in terms

of t he remote s ites, the central site and

the operating sys tems.

The Perkin ELmer 3200 family of

computers are hardware inte rrupt driven

machines . They have four different

hardware i nterrupt Levels and can handle

up to 1023 devices on these four levels .

In addition, the computers have eight sets

of sixteen 32 bit regis t ers of which four

are speC i a lly l aid as ide to support the

four interrupt levels. Si gnificant in

this ha r dware is the dual bus architecture

which inc ludes a high speed direct memory

access COMA) bus as well as a slower speed

multiplexor bus. The hardware also has

high i nput / output band widths of 8 Hb / sec

on the 3230 and processing power of 1.0

MIPS .

One of t he latest developments in the

Pe r kin Elmer 32 bit hardware has been in

the field of parallel processing. These

32 bit sys tems can support multiple

processing units under the same operating

system. This gi ves the hardware more

power by increasing the processing

capabilities as well as their I /O

performance . These developments are

important when considering the large

amount of scientific deve lopment work that

must continually be performed on the

hardware a t the centra l site. Upgrades to

multi processing systems are only possible

from the model 3230 and upwards .

Remote slfes

The hardware configuration of t he 3210

mini-computer recently installed at the

Deelkraal Gold Mine is shown in Figure J.

This computer is configured with I MB

of memory and has a 16+16 MS disc drive

and analogue to digital converter

connected to the DMA bus. The 32 analogue

channels are wired singl e- endedly and the

conversion is 12 bits at 32 KHz. The di sc

drive is a fixed removable system and

s tores the sys t em software and the data files for logging.

The slower mult ip l exor bus houses a

s ingle synchronous adap t er wh ich i s

connected via a v-J 5/RS232 modem and

interface to the UHF radio l i nk with the

central site. Also connected to this bus

is a contact closure detector for hardware

triggering of the s eismic events, a two

line mul tiplexor board , a 8 KB loader

storage unit (LSU) and a rea l time c lock .

The two line multiplexor supports a system

console, and the 8 KB LSU provides a

facility for loading the operating system

"from t he disc drive . The data transfer

rate on the DMA bus i s up to B MB/ sec,

while on the slower mul ti plexor bus it i s

approxima t e Ly 400 K8/sec. The process ing

power of the )2 10 computer is 0,6 MIPS.

Central site

The computer

site comprises

computer with

hardware at the central

a Perkin Elmer 3230 mini-

3 MB of memory . The dual

bus arc hitecture of this cgmpute r and the

configuration of i t s peripheral s are shown

i n Figure 4 . This centra l mi ni-computer

is responsible fo r t he data capture from

t he remote sites as well as development of

the applications and research s oftware.

The DMA channel supports all t he on-line

storage devices which includes 4 disc

drives and

dr ive . The

sof tware are

drive. The

compatibility

a dual density magne tic t ape

operating system and s ys tem

normall y housed on a 300 MB

other drives are used for

with the remote sites and

for data storage.

The s lower multiplexor bus supports the

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remainder of the devices on the system.

This inc ludes the eight synchronous ports

to the radios via two quad synchronous

adapters and a line control module . The

standard graphics and

terminal s are connected via

async hronous

and 8 line

mul tip lexor while a 2 line multiplexor

s upports a Pe r kin IHmer 550 sys tem

console . Prin t ing is s uppor ted via a 600

line /min cent ronics pr i nter, and an 8 KB

LSU provides a fac ility for loading the

operating sys tem from any of t he on-line

storage devices.

One of the non standard features on the

system i s a Tektronix 401Li graphics

t e rmi na l with a para l le l interface and

un i ve rsa l l og i c interface (ULl). Although

the ULI is a standard Perkin RImer

product. t he parallel interface and driver

rout i nes for the ULI were deve loped by the

CSIR t n Pretoria . This device, yhi ch is

capable of operating at s peeds of about

30 000 characte r / sec, i s used i n order to

s peed up the process of locating se ismic

events .

The 32JO computer is connec t ed to a 20

KVA uniterruptable payer supply yith a

batt ery back-up facili ty for about 48

hours . In addition to this, there i s a 20

minute

faci lity

a live .

battery back-up auto res tart

ior keeping t he memory content

Operlltlng systems

Both the 32 10 and the 3230 computers use

the standard Perkin Elmer operating system

05/32. Thi s is a real t ime multi-tasking

opera t i ng sys tem developed s pecifically

for the 3200 series of computers which

s upports any of the s tandard languages,

e.g Fortran. Cabal, Pascal. Basic, etc.

In the Fo rtran environment i ts supports a

universal

features.

compiler with optimizing

Task execution and the system

utilities can be controlled t hrough the

use of Perki n El mer' s poye r fu l command

substi tution system . In addition. the

sys tem is easily ope rated and controlled

t hrough a central sys tem co nsole .

This gives the operator a rea l time task

interface with control and display

f unctions for all tas ks in the sys t em.

The sys t em can also be moni t ored t hrough

the use of a facilit y s howing CPU usage,

curren t tasks etc.

OS/32 also provides a base onto yhich

higher l evel sys tems software can be

built. At present both sites use the

multi- termina l monitor (MTM ) . This is a

time- shari ng facility which is us eful for

contro l ling and scheduling development

work on the systems. The MTH Facility can

s uppo r t up to 255 system accounts and 64

users operating from either i nteractive

t ermina l s or from a batch environment .

16 bit systems

The hardware configuration of the 16 bi t

compute rs. whi ch are current ly being used

on f our of the remote s ites. although

essentia l l y the same, does have a few

significant differences. The 16 bit

systems also have a dual b~s architecture

but with much slower transfer rates of

2MB/ sec and 150 KB /sec on the DMA and

mul tiplexor buses , respective l y . The 16

bit machines a lso use t~o ana logue to

digital converters yith 16 channels per

board differentially yired and driven at

16 KH z by a small externa l clock .

The major difference betyeen the tyO

computers is in the memory systems . The

1Mb in t he 32 10 i s directly addressable

whil e the 128 Kb on t he 1620s is a paging

memory sys tem. This means t hat if the

operating system on the 16 bit is greater

than 32 Kb then these machines are

effectively reduced to 6Li Kb capacity, and

t he extended memory options are not open

to t he user. Disc storage on the 16 bit

DETECTION AND RESEARCH OF SEISMIC EVENTS ON THE WEST WITS UNE 63

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hardware is on a 10 MS fixed/removable

s ys tem. One other fea ture which l imits

the capability of the 16 bit systems is

the fact that the operating system on

these computers (05 / 16) is no longer

supported by Perkin Elmer . This came

a bout as a ma jor policy decision in the

ear ly 19805 and has signif i cant ly affected

t he adaptabil ity of t hese computers to

t hei r task

network .

i n t he existing seismic

Applications software and the role of computers

The real use a nd application of the

computer equipment in the seismic network

can best be seen by examini ng the software

conf iguration a t both the remote and

central sites. The essential func tions of

the systems a re both t he logging and fast

transfer of data; the s oftware is best

divided into three categories:

( I ) Loggillg a nd. communica tions - Remote

sites .

(2) Data capture and communications-

Cent ral site .

(3) Applications and r esearch - Central

site .

Once again t he softwar e at the remote

s i tes wi l l be described using t he newly

acquired 3210 computer.

Data logging and communications: remote site

The main task of the remote s ite

computers is to perform an analogue to

digita l conversion on the 32 underground

da ta channels and the n log the data to

files on the disc. Whenever a seismic

event occurs, the computers must capture

as much of the informat ion as possible and

t he n transfer this data at high speed (48

K baud) to the central site.

to the

computers

basic

must

logging

al so

In addition

function the

monitor the

communications line for messages and data

being passed from the central s i te .

Three computer programs have been

established in order to achieve these

goals .

logging

These are LOG3216, a Fortran

routine; XFER3216, a Fortran

buffer packing routine ; and COMM32 16, an

Assembler line communications routine ,

All three programs s hare a common b lock of

memory from which they access information

s uch as input / output buffers, (iIe flags,

etc . The programs a re all interrupt

driven and use real time trap handling

routines t o execute the appropriate s ub-

routines when required. The t as k t raps

enabled in these programs include t ask

calls, tas k message, I/O proceeds and

power res torations. The basic s tructure

of the system together with the paramete rs

passed between the programs is shown in

Figure 5,

The logging program issues r eads to t he

analogue conve r te r and has a read

outstanding on the contac t closure device

in order to moui.tor the hardwa re trigger

unit. The program then goes into a trap

wait state and under normal conditions

keeps taking I/O proceed traps from the

a nalog converter. Loggin~ proceeds

direc t l y to two buffers in memor y that are

each the same size as one physical

cylinder on the disc. These two buffers

are alternated , and when one is fu l l the

program writes it away

disc. If the stat us

c losure device does

to a

of

not

cylinder on

the contact

change , the

l agging s witches between t he f i rst two

cylinders of the data file using them as a

history period. If the contact closu re

device goes high , then logging proceeds to

t he rest of t he cylinders on disc.

The logging routine has task

communications with the buffe r packing

routine. Parameters are passed every time

a seismic event has occurred and logging


r , I

is comple ted . If no free data files are

found on the disc then t he buffer packing

i s notified and loggi ng is paused .

Logging i s res tarted once the trans f e r

r out ine has freed a da t a file. The on l y

other parameters received by the loggi ng

program are t hose to start and end a t i me

event. A time event i s a message s ent

back from t he central si t e whe re t he firs t

fou r byt es conSis t of t he ASCII l etters

'time'. Once the buffer packi ng sees this

message . i t passes a par amete r to LOG321 6

t o commence event logging , and 1.5 seconds

l a t e r passes it a second pa rameter t o

s top. The event is then handled as usua l

a nd the data is transmi tted back to t he

central s i te. The time event i s part of a

process initiated at the central si t e to

determine whe ther any r emo t e s ite is in a

normal wor ki ng s tate. The l oggi ng prog ram

logs dat a from all the channels of the

analogue conve rters rega rd less of whether

o r not t hat channel is i n good worki ng

order.

The buffer pac king rou tine when loaded

goes into a t rap wait state and waits fo r

calls from ei ther the logging or the

communicat ions routine. The program only

packs channe l data which 1s good f o r

locat i ng purposes through the use of 4

trigger group words in the task common

block. The routine packs the data into a

5 12 byte buffe r i n task common from the

re l evant da t a file on disc , and once it

has been inte rrupted by the logging. a

s eries of call s and inte r rupts are carried

out with t he communications routine. The

first and last buffe r s are alt.'ays

different and contain the 4 trigger group

words and t he end

respectively . Task

r eceived and issued

bf the file marker ,

calls are al so

for data messages

coming i n from the centra l site . With t he

exception of t he time event , this program

will write all mes sages to the s ystem

console and a pri nt ing device. The

routine can also r eceive task messages

about i t s s tatus from the system consol e.

and in contrast to the logg ing progr am it

assigns the data files only when required.

The l i ne communicati ons routine al so

starts in a trap wait state with a read

outstanding on the communications l i ne .

Whenever event data has t o be wri t ten to

the l i ne the program rece ives a call from

the buffer packing rout i ne and hal ts the

read on this l ine and begins t he

transmi ssion s equence. Task call s a r e

received and sent back to the buffer

packing , as shot.'n in Figure 5. Tas k calls

are al s o handled with t he buffer packi ng

in orde r to process t he i nformation being

sent from the central site. This

communicat ions routine has no access to

the da t a fi les on disc , and it us es the

data buf f ers that are placed in the tas k

common area by the buf fe r packing . In

view of the control required on the line

itsel f . the program is not suited t o

Fortran and i s the only routine wri tten in

Assembl er l anguage.

The commun i cations on the system uses a

specia l l y developed bisynchronous protocol

specifica lly suited to the network. This

was writ ten by John Fr e eman (DGA) to

bypass the RJE protocol initi ally

insta l l ed on the system.

The exi s ting 16 bit s ys tems use simila r

applications software although the pr ogram

struc t ur e is slightly different and all

the softwar e is writt en in Assembler code.

Three programs, name ly a logging routine

(LOG1 6 ). a combined buf fer packing and

line communications rout ine (XHIT16) , and

a message handling routine (MESG16 ) , are

used to achieve similar results.

66 MINING: ROCK MECHANICS

Data capture and communications: central site

The chief function of the mini-computer

at the central site is to process the data

sent to it by each of the remote sites.

In addition, the central site uses a small

program (TALK16) to transmit messages and

seismic information back to the remote

sites. All the software apart from the

communications program has been written

using Fortran 77; the program structure

and flow of information is shown in Figure 6.

Three programs are responsible for the

flow of data coming in from the remote

sites. namely a line communications

routine (COMM32), a file handling routine

(LOG32), and a data manipulating routine

(GFSAOOO). The first two programs share a

common block of memory, and all three

programs are resident in memory for 24

hour data handling. The programs are all

interrupt driven in a similar way to the

remote site software.

COMM32 and LOG32 work interactively

during event transmission. COMM32 reads

the data off the line into memory buffers,

and when complete calls .LOG32 to write the

buffer away to a disc file. These files

are the same size as the communications

buffers and are built up buffer by buffer

until the data is complete, Once the mine

seismic file is full, a task call is sent

to GFSAOOO telling it that an event is

ready for processing. GFSAOOO uses the 4

trigger group words received in the first

buffer to decode which channels are being

sent and writes the channel and other

information concerning the event to two

files, a seismic data file (seisgram.dat)

and a located information file

(locatedx.dat). The located files store

data for 28 000 events each and therefore

form a series of files which are backed up

to tape when complete. These files have

a header record for each file and 256

halfwords of data for every event. The

seismic data file has a 64 byte record

length and stores a sample (halfword) for

every channel per record. The file has a

header record keeping a count of the first and last events in the file, and a header

record for each event followed by the data

for that event. The seisgram file is

temporary in nature, and is designed to

store the seismic data for a full day's

events, This is done to keep access times

to a minimum since it is the file from

which the seismograms are graphically

drawn during the locating process.

Once the day's locating has been done,

the signal data for all the located events

is rewritten to a larger file of similar

format through an archive process. These

files can grow to 680 000 records after

which time they are renamed and backed up

onto a single tape for research purposes

(see Figure 6),

Once GFSAOOO has finished the data

processing a message is logged to the

system console giving details of the event

as known, and the event is then ready for

locating and further user applications.

The time lapse between the trigger unit

going high at the remote site and the

message from GFSAOOO is about 2-3 minutes,

during which time the computers have

handled about 160 K bytes of data.

Applications and research: central site The data logging and communications

software at the central and remote sites

show that for a centralized system the

computers are efficient at transferring

fairly large amounts of data. Another

important advantage of using computers is

seen in their speed and ease of accessing

this data for various applications.

Seismic locations on some of the earlier


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SAMPl.ING RATE 15 638.0 EVX EV' EV, ,VT ERROR (M)

12000 -9000 - ''''' -20 17380 -3339 - - 2487 97 1470

21312 ~5725 -2941 " 9477 20672 -7774 -2181 80 1220 20293 -7333 -2269 94 330 20331 -7930 -2345 79 100 20332 -1927 -2355 76 31 20332 -7926 -2357 76 " 7n332 -793' -2358 76 " EVENT 22617 10 ARRIVALS

• • • • • • • • • • • CHANNEL ARRIVAL THEOR DlfF VELOCITY 015T

2P 276 276 0 5.39 1180 '5 39T 399 -, 3.67 11 BD 7P 234 236 -, 5.96 ,43 75 319 332 -15 3.68 943

10P '20 173 7 5.50 573 105 331 133 , 3.'02 573 'le "6 150 6 5.47 438 '15 '" 196 _9 3.95 ,,. '" 113 119 -6 6 .90 '50 "5 153 146 7 3.38 256

EVX EV, CV, EV7 ERROR (N)

20332. -7926 - 23 52 75 " ON EAST DRIEFONTEIN CHANNEL " ENERGY = 0.36674 ENTER MAGNI rUDE FOR THIS EVENT (0,367)

FIGURE 7. Graphical representation of channel data for location purposes, and details of location calculation

networks,3 were carried out using a string

analogue technique. Traces of the actual

seismic waves were obtained on mechanical

recorders. Some of the earlier systems

actually stored the data on tape and

reproduced the signals on photographic

paper so that events were never able to be

located immediately. The string analogue

a scale model of the was basically

geophone array. The time differences

between the primary and secondary waves

were then scaled off on a length of string

and the method performed a kind of manual

least squares, to find the best fit for

all the travel times .

Since these early days several digital

algorithms have been developed for the

location of seismic events. 4 These

methods normally use the time difference

of arrivals to set up a system of linear

equations in four unkh3wns. A method of

least squares regression is usually used


to minimize the difference between the

arrivals, and because compute~s are so

suited to this numerical type of analysis

locations can usually be obtained within

minutes.

The GFSA system uses an algorithm set up

by Dr S. Spottiswood at Blyvooruitzicht

Gold Mine, and the parallel interface used

on the graphics terminal helps to speed up

this process even further. Windows

depicting 1 000 samples of 10 channels at

a time are drawn on the screen, and the

user selects the primary and secondary

wave arrival times by means of a graphics

screen cursor, Several options are

available to the user during the selection

of these arrivals and once he is

satisfied, a location can be obtained in

seconds. See Figure 7. Once the located information has been

stored in the relevant file, the computers

are efficient at accessing and analyzing , the seismic data. This is shown by the

daily information which is presented to

management and which is used for simple

type analysis. Printouts of the located

information can be obtained within minutes

and simple statistics can easily be

obtained from user written programs.

All this information is shown in

Figures 8 and 9.

These examples indicate that computers

with fast processing speeds are usually

well suited to the location and research

of seismic events. On other systems where

micro-seismic data is being processed and

approximately 100 events are triggered

every hour, computers are unrivalled in

data handling.

Case studies In its five years of operation the Gold

fields Seismic Network has seen the

inclusion of five mines. Priority has

always been given to operation of the

10

system as a management tool, This has

meant that maintenance work to ensure the

rapid location

always taken

development

Unfortunately.

of seismic events has

priority over

and

the

resear ch

system

work.

large amount of

maintenance wo~k, involving both the

underground technical equipment an the

computers themselves. has meant that the

system has not developed toward the

analysis of the accumulated data as it

might have done. Nevertheless, during

this time period several case histories

can be highlighted which show how the development of the system can be measured

against its oYiginal objectives.

On 27 December 1983 a 4.2 magnitude

seismic event shook the Carletonville

area. The intensity at the West

Driefontein offices was particularly high,

and management anticipated it to be

somewhere on the mine. The seismic system

was able to calculate the location and

magnitude of the event within minutes.

The hypocentey was located close to an

area of mining on West Driefontein, and

rescue operations were put into effect

immediately. thus eliminating any wasted

time in trying to establish the area of

concern, and allowing panic reduction in

distant, undamaged areas. As it turned

out. this event caused underground damage

to the surrounding mining excavations.

Five workers were fatally injured and a

total of 89 workers were hospitalized.

Since this incident, several others have

also occurred involving events between 2

and 4 on the Richtey Scale which confirm

that the system has met the required

immediate objective.

The medium term objectives of the

network were involved with seismic

improved aspects of mine planning

including mining layouts and support


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GESA SEISMIC NETWORK

DISTRIBUTION Cf EVENTS >1. ~ DURING AUGUST 19B4

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26.17.2% FIGURE 9. Graphical representation of s,eismlc information

methods. Two examp les have been used to

show ho\o1 the sys tern is meeting some of its

roles ellv~saged in the medium tern .

Figure 10 shows the locality plan of a

shaft pillar complex on West Driefontein

t hat is presently being ext racted. In

J uly 1986 mi ning conditions in the area

began to deteriorate, and the Rock Mec hanics departme nt was asked to

investigate the methods of mining in the

area. The seismic data far the area was

included in a report . Prior to t he

deterioration the area had been mined i n

both easterLy and westerly directiDns from

the 14-1 ) Carbon Leader raise (see Figure

10). Min ing in a westerly di rection was

72

progressing to~ards a geological fault

structure and was stopped purely by

accident in about January 1986 owing to

labour prob J. ems . Mining was subsequent l y

continued in an easterly direction onl y

from February to J ul y 1986 . After this

period mini ng was again anticipa t ed on t he

westerly pane l s. and the Rock Mechanics

department was asked t o investigate the

situation .

By comparing a Simp le plot of the

seismicity during the periods 1 June 1985

)1 January 1986 (both panels) and

1 February 1986 - 4 July 1986 (east side

only. see Figures 11 & 12), the Rock

Mechanics report was ab le t o show that the


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FIGURE 14. Plan showing scismicity in the area of proposed 30 Level foolwall drive fo r period 1.1.83 - 1.3.84


>-

77

westerly mining ~as the main cause of

seismicity ,n the area, rhe

recommendation put forward by the report

was carried out and the westerly panels in

the area ~ere abandoned. Since this

decision no furher problems were

experienced in the area.

A second example is highlighted through

the siting of a footwall haulage to be

connected between two shaft systems on the

West Driefontien Gold Mine (see Figure

13). The existing 30 level haulage

connection also shown on the plan was

damaged in 1984 due to a large seismic

event which was located on the system of

dykes intersecting the haulage. Once again seismic data was used in determining

the best position for this haulage.

Figure 13 shows t he layout of the mining

with the two proposed positions for the

haulage, Figure 14 represents a plot of

the seismicity in the area for the period

1/1/83 - 1/3/84, and Figure 15 a section

of all the events within a 200 m radius of

the line marked x-x on Figure 14.

It can be seen from figure 15 that most

of the seismicity in the area was limited

to within about 100 m of the reef plane.

Also important in the location of this

haulage was a 100 m thick shale horizon

located about 100 m in the footwall of the

reef. The first option for this haulage

(position 1) was found to intersect the

weaker shale horizon. In addition it ~as

intersected by a complicated system of

dykes and it would also have fallen under

the final mining remant in the area.

Consequently the second option (position

2) was chosen as it was situated much

deeper in the footwall and was considered

to be well out of the range of any

possible seismic damage.

As far as the long term objectives are

concerned, the system has not yet met any

78

of its requirements. In recent months, a

large amount of time has been spent on the

development of a data base specifically

for seismic research. This has involved

the writing of a digitizing program that

will be able to store data for any

Variables considered relevant to seismic

research. This will include i nfonnation

such as geological features, sampling

values, induced mining stresses, accident

statistics and support types. Each of these variables will be stored in separate

files on a separate disc volume. It is

hoped that this base of information will

help the system meet more of its medium

term objectives and provide a foundation

on which to base the long term research

projects.

Conclusion The Gold Fields Seismic Network was

established in 1980 in order to monitor

the seismicity occurring in association

with the gold mining operations of the

West Wits Line. The system is an example

of how computers have been used to assist

with a problem that causes the industry

frequent setbacks. Data is transmitted

from a number of remote sites to a central

processing station at speeds of 48 K baud.

The system uses Perkin "Elmer mini-

computers to achieve these goals. rhe

speed and efficiency of the computers in

handling the necessary data mean that

seismic events can be located within the

space of about 3 4 minutes. Further

analysis on this data can be carried out,

and several examples have been outlined to

show that computers are ideally suited to

this area of study.

Although the present system uses mini

computers in this particular application,

it is by no means true that they are the

only type of hardware suited to the task.

It is possible to set up seismic systems


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ES1 6~

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79

using smalle r micro computers. Most of the

technical breakthroughs which have taken

place in the micro-computer indus try over

the las t 10 years were not avai lable in

the 19705 when mos t o[ t he exist i ng

netwo rks wer e designed. The type of

hardware chosen for any seismic

appl i cat ion wil l depend largely on the

object i ves of that system. The GoLd

Pie l ds Network was des i gned as a

centralized computer system because of its

siz e, the speed of data transfers

req uired, the large amount of data

handling and the mi nimum staff

req ui rement .

References

1 .JANISCH,P.R. Turning geo logy to

80

accoun t . Economic risk and return on

t he Wes t Wits Line. Geocongress'86.

Extended Abstrac ts, Geological

Society of South Africa. 1986. pp. 229-

232.

2. DE. JONGU ,C.L. and KLOKOW,J.W. The use of

a seismic network as a management

tool. 5th International Congress on

Rock Hechanics. Melbourne Australia,

1983 , Section D pp 101-107

3 .COOK, N.G .W.

rockburs t s .

SymposiUJ1J

University

Sei smic l ocation of

Proceedings of the fifth

on

of

Rock Mechanics.

Minnesota, May 1962 .

Oxford,Pergmon, 1963. pp. ~93 -516.

/1. ECCLES ,C.D.and RYDfffi,J . A. Seismic

location algo rithms: a comparative

evaluation . Rockbursts and Seismicity

in Hines . Johannesburg,Sout h African

Institute of

19B~. pp 89-92

Mini ng & Metallurgy.

MINING: ROCK MECHA NICS

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