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Presented:

Date:

Location:

© SRK Consulting (UK) Ltd 2011. All rights reserved.

Talk 2 – Data Collection for Mine Design

Rockmass Characterisation from Combined

Borehole Geophysics and Core Logging

Philipp Mohr

Minex Central Asia Forum, Astana, Kazakhstan

31 March 2014

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Rockmass Characterisation – Why?

• Provides systematic description of physical properties of a

rockmass

• Forms the basis of determining rockmass strength by

downgrading of intact rock strength by taking into account rock

defects

• Rockmass strength forms the building block to geotechnical mine

design:

o Maximum achievable stable slope angles for open pits

o Underground mining method selection, stope dimensions and support

requirements

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Rockmass Properties – what to log?

Joint

Spacing Fracture Density

Aperture

• Intact rock strength (IRS) • Orientation of both cemented and open

fractures such as joints, bedding planes, foliation planes, or faults and fault zones

• Fracture density (RQD, fracture spacing, fracture frequency)

• Condition of Fracture Surface (shape, roughness, alteration and strength)

• Condition of Fracture Infill (strength, mineralogy and thickness)

• Groundwater and porewater pressure (hydrogeological tests)

• Field stress (over coring, lab tests)

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Approach to Rockmass Logging

• Most parameters required for rockmass

characterisation can be obtained from drill core

logging.

• However, orientation of fractures requires

orientated core.

• Inherent difficulties with core orientation makes the

case for core orientation alternatives

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Alternatives to Classic Core

Orientation –Televiewer Imaging • Optical Televiewer

• Acoustic Televiewer

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Optical & Acoustic Televiewer Imaging • Developed in the oil and gas industry and thus used to

be expensive

• Costs now competitive to core orientation tools

• Slim-line and mobile equipment available suitable for

mining applications

o Winch (60 kg / 500 m)

o Data Logger Unit

o Lap top with software

o Probe

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Optical Televiewer - how it works

• Based on using a downhole digital

camera and a prism to obtain a

continuous and oriented 360° image of

the borehole wall.

• Includes an orientation device to

measure accurate borehole deviation.

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Acoustic Televiewer - how it works • Generates an 360° image of the borehole wall by transmitting

ultrasound pulses from a rotating sensor.

• This is achieved by recording amplitude and travel time of the signals

reflected at the interface between drill fluid and borehole wall.

• Uses the in-built deviation device to orientate the televiewer image

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Acoustic - Travel Time and Amplitude • Travel time represents variations of the borehole radius (assuming that

the tool is well-centralised), can be also used as a borehole calliper.

• Amplitude represents the strength of the echo returning from the borehole

wall.

• Both can be used to pick structures from the image log.

• Picked sinus curves can then be automatically converted into dip and dip

direction using specialised software (WellCAD)

amplitude travel time optical digitised structure

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Optical Televiewer and Drill core Acoustic Televiewer

Picking structures off the image log

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Televiewer – what can it do? • A continuous detailed structure log

• Data are electronic and highly accurate

o Orientation of structures

o Aperture / thickness

o Depth

o Fracture cementation / type (semi quantitative and only from acoustic

probes)

» Open (air/fluid filled)

» Well cemented (infill similar to wall rock density)

» Weakly cemented (soft materials such as clays)

• Automated computation of RQD, spacing and frequency of fractures

for a defined geotechnical interval.

• Depth of groundwater

• Borehole calliper

• Orientation of principal horizontal rock stress

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Using televiewer image to measure

groundwater levels

air filled (using optical televiewer; no signal in acoustical log)

water filled (showing good signal in acoustic televiewer log, and poor signal in optical log)

Optical Televiewer

Acoustic Televiewer

Groundwater surface

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Rock Stress Direction from

Borehole Deformation • Determines direction of principal horizontal stresses from analysis of

stress induced break-outs and drill induced fractures of borehole

wall.

sH = max. stress direction (= direction of drill induced fractures) sh = min. stress direction (= direction of stress induced borehole break outs perpendicular to sH)

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Televiewer – Key Considerations • Cost competitive to core orientation but without its uncertainty.

• Equipment is compact and easy to mobilise (< 200 kg)

• Survey of a 500 m deep hole takes one day to complete including

processing of raw data.

• Data interpretation (structure picking) done in 1-2 days.

• Requires stable borehole walls – risk of loosing probe. Usually requires

checking hole by inserting a dummy probe first.

• Borehole should be thoroughly cleaned from any drilling mud before

commencing televiewer surveying.

• OTV works only in dry holes or in clean water filled holes.

• ATV works only in fluid filled boreholes, fluids can be muddy, but without

any trapped air.

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Combining geotechnical drill core

logging with borehole televiewer data • Making use of both methods creates highly accurate structure log.

• Allows confirming natural structures and excluding drill induced

fractures by comparing drill core against televiewer image.

• Joint condition parameters added from core logging (roughness,

infill, joint wall strength / alteration) to each orientated structure

allows for statistical analysis of joint properties .

• Results of statistical analysis allows to define characteristic shear

strength values for each structure set.

• Structure log can then be used to complete a geotechnical

summary log.