Talk 2 Data Collection for Mine Design Rockmass ...
Transcript of Talk 2 Data Collection for Mine Design Rockmass ...
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Presented:
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© 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.