High-resolution, short-range, in-mine geophysical techniques for the delineation of South African orebodies

The CSIR Research and Innovation Conference

Natural Resources and the Environment

Michael van Schoor

Senior Research Geophysicist

28 February 2006

Slide 2 © CSIR 2006 www.csir.co.za

Agenda

• Introduction to geophysicsWhat does a geophysicist do?(An analogy)

• Gold and platinum mining in South AfricaAn overview – why, where, how.

• Geological problemsPotholes (platinum)Reef slopes and terraces (gold)

• Geophysical techniquesGround penetrating radar (GPR)Borehole radar Electrical resistance tomography

(ERT)

• Case studiesWaterval Mine (GPR)Mponeng Gold Mine (Borehole

Radar)Western Platinum Mine (ERT)

• Conclusion

• Future research

• Acknowledgements

Introduction to geophysics

An analogy…

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What does a geophysicist do?

• Like any applied science it starts with a problem…

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What does a geophysicist do?

• that requires an accurate assessment…A quantitative assessment of the problem is required to help define the most

appropriate course of action

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What does a geophysicist do?

• “A geophysicist does to the earth what a radiologist does to the human body”

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What does a geophysicist do?

• Some of the techniques we use are adapted versions of the medical equivalent – e.g. electrical resistance tomography (ERT) is based on medical impedance tomography

Gold and platinum mining in South Africa

An overview

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Gold and platinum mining in South AfricaEconomic impact

• South Africa produces approximately 75% of world’s platinum and 14.5% of world’s gold

• The bulk of the above Pt and Au is extracted from only two geological occurrences, the Bushveld Complex (Pt) and the Witwatersrand Basin (Au)

• Significant reserves and resources remain to be extracted

• Orebodies are relatively easy to mine because of their planar geometry

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Gold and platinum mining in South AfricaTypical mining layout

S haft

H aulage

Raises

Reef plan e

To-be- min ed a rea

C rosscuts

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Gold and platinum mining in South Africa Bushveld Complex (Pt)

R UST E NBU RG

AngloplatsImpala

LonplatsOthers

N

PO LO K WAN E (P IET ER SBU RG )

Messina Lebow a

Twickenham

W ES TER NLIM B

Two R ive rs (JV)

Potgieters rus t

40 km

R ustenburg

K roondal Lonpla ts

Am andelbu lt

N ortham

Un ion

Bafokeng-R asimone

Im pa la

20

Marikana

M arula

PRE TO RIA

0

M od ikw a

N O R TH ER NLIM B

EASTE R NLIM B

C rocodileR iver

P andora (JV)

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Gold and platinum mining in South AfricaWitwatersrand Basin (Au)

Geological problems

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Geological problems

• On a regional scale, the planar reefs are remarkably continuous and fairly straightforward to map ahead of mining

• On a mine-scale, disruptive geological features often distort or displace the economic horizon, thereby complicating mining:

Geological faultsRollsTerracesDykesPotholes Iron-rich ultramafic pegmatite (IRUP) bodies

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Geological problemsBushveld Complex - potholes

?

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Geological problemsBushveld Complex - potholes

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Geological problemsWitwatersrand Basin – slopes and terraces

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Geological problems Rock engineering / safety issues

Access tunnel / working area

Parting plane / marker horizon

0 m

~ 6 m

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Geological problemsImpact

• Distorts / displaces the economic horizon

• Uneven grade distribution

• Poor ground conditions

• Compromises safety

• Hampers production

Geophysical techniques

Ground penetrating radar (GPR)

Borehole Radar

Electrical resistance tomography (ERT)

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Ground penetrating radar (GPR)Concept

G PRsystem

Ta rg et ho rizo n

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Ground penetrating radar (GPR)Typical output

P os ition (m )T

ime

(ns)

0.0 4.0 8.0 12.0 16.00

25

50

75

Direct arrival

Hyperbolicreflector

Layeredreflector

R inging

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Ground penetrating radar (GPR)Commercial equipment

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Borehole radarConcept

TargetHorizonBorehole

Receive Antenna

Transmit Antenna

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Borehole radarCSIR’s Aardwolf system

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Electrical resistance tomographyConcept

0 100 200 m

R e s is tiv itya n o m a ly

Ra

ise

S t r ike -p a ra lle ld e ve lo pm e n t

Ra

ise

E le c tro de sD is ru pt io n

in re e f p lan e

Unmined targetblock

Shaft

Haulage

Raises

Reef plane

To-be-mined area

Crosscuts

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Electrical resistance tomographyIn-mine ERT system

Case studies

GPR – Waterval Platinum Mine

Borehole Radar – Mponeng Gold Mine

ERT – Western Platinum Mine

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Ground penetrating radar (GPR)Waterval Platinum Mine

2 m

A ccess tunne lo r w o rk ing a rea

Curved jo ints

LeaderSeam

?

0 m

??

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Ground penetrating radar (GPR)Waterval Platinum Mine

C U RV E DJO IN T S

L E A D E RS E A M

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Borehole radarMponeng Gold Mine

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Borehole radarMponeng Gold Mine

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Borehole radarMponeng Gold Mine

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Electrical resistance tomographyWestern Platinum Mine

ACCESS AROU ND SURVEY AREA DISRUPTED REEF

RA

ISE

19

E6

SPD TUNNEL

RA

ISE

19E

5

ELECTRODE LOCATIONS

MINED OUT AREA

N

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Electrical resistance tomographyWestern Platinum Mine

A

B

17

18

22

20

19

21

23

24

16

14

13

15

5

4

21

3

8

9

7

10

6

11

12

K N O W NIR U P

O C C U R R E N C E

K N O W NP O T H O LE

O C C U R R E N C E S

5.7

3 .0log a

~ 35 m

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Integrated geophysical (“toolbox”) approach

GPR

Borehole Radar

ERT

Maximum Range

~ 8 m (500 MHz)

~ 50 m (100 MHz)

100-200 m

Resolution

Few centimetres

Few tens of centimetres

Few metres

Applications

Mapping continuity and topography of marker horizons and parting planes in roof Identification of hazardous structures e.g. joints (i.e., mainly for rock engineering purposes)

Mapping reef continuity and topography at exploration phase Following-up ERT anomalies to obtain info of third dimension (depth)

Mapping of potholes, IRUPs in unmined blocks Mapping of grade variations

Conclusions

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Conclusions

• High-resolution geophysical techniques can be used to predict reef topography ahead of mining

• Choice of techniques allows mine to use an integrated geophysical assessment approach

• Routine application of geophysics can result in significant savings Reducing the amount of conventional exploration drilling Selective extraction of higher grade areas

• In-mine geophysics can make a valuable contribution to safety Identifying hazardous roof conditions Optimum placement of roof bolts

Future research

Magnetic resonance sounding (MRS)

Induced polarisation (IP)

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Future research

• The use of the magnetic resonance sounding (MRS) technique to detect hazardous water and gas occurrences ahead of mining developments.

• Supplementing existing in-mine ERT technique with the induced polarisation (IP) technique to improve target discrimination and resolution.

Acknowledgements

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Acknowledgements

• PlatMine Collaborative Research Programme• AngloGold Ashanti Ltd.• Mponeng Gold Mine

• Co-authors and colleagues: Petro du Pisani Declan Vogt Mpho Nkwana Stepens Letlotla Johann Haarhoff Reinhard Bilgeri Karlo Walker Liam Candy