GE 6477 DISCONTINUOUS ROCK Instructional Objectives 8 ...web.mst.edu/~norbert/ge6477/Class...
Transcript of GE 6477 DISCONTINUOUS ROCK Instructional Objectives 8 ...web.mst.edu/~norbert/ge6477/Class...
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GE 6477 DISCONTINUOUS ROCK8. Fracture Detection
Dr. Norbert H. Maerz
Missouri University of Science and Technology
(573) 341-6714
Instructional Objectives
1. List the advantages and disadvantages of surface mapping of discontinuities.
2. List which characteristics of discontinuities can be obtained from oriented cores. Which cannot?
3. Compare optical, acoustic and electrical imaging of the inside of the borehole.
4. Explain the Terzaghi borehole bias and critique the methods of measuring fracture orientations in terms of that bias.
5. Discuss the effectiveness of the various surface geophysical methods in detecting discontinuities.
6. Determine what advantages there are in borehole tomography.
7. Give the advantages and disadvantages of conventional borehole logging.
Fracture Detection
• 7.1 Surface mapping
• 7.2 Borehole logging
• 7.3 Geophysical methods
• 7.4 Other methods
Site Investigation
1. Primary
2. Secondary
3. Tertiary
Primary (research phase)Research of topographic and geological maps and reports
Research of regional seismic and ground stress data
Walking reconnaissance of the site and of regional outcrops,exposures of the rocks in nearby tunnels and mines
Study of nearby water-well records
Air photo study using existing photography
Discussions with local residents and specialists
Preparation of base maps
Preliminary report on site conditions: plan next phase
Secondary PhaseDetailed logging of rock outcrop, statistics on jointing
Exploratory test pits and trenches: sampling
Index testing in situ and in the laboratory
Reconnaissance by helicopter or light airplane
Probe hole drilling using auger or air track
Seismic or electromagnetic geophysical traverses
Special air photography and supplementary interpretation
Preliminary, limited core drilling, logging, and testing
Definition of soil-rock interface topography
Rock mass classification
Progress report: plan next phase
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Tertiary PhaseFurther core drilling, vertical, inclined, or horizontal
Jointing information from drillhole television, impressionpackers, oriented core1 integral-core sampling
Downhole geophysical logging and tomography between holesto define rock mass quality and individual faults, etc.
Downhole testing using a dilatometer to measure deformabilityPacker testing for hydraulic conductivity
Installation of piezometers to study the groundwater regime andto monitor piezometric pressures
In situ stress determinations
Excavation and logging of exploratory trenches, adits, shafts
Large-scale in situ strength and deformability tests
Full evaluation of soil, rock, groundwater, and stress regimes
Final site investigation report
8.1 Surface mapping
Surface mapping features
• Orientation
• Spacing
• Persistence
• Roughness
• Wall Strength
• Aperture
• Filling
• Seepage
• Number of Sets
• Block Size
Surface mapping
• Advantages:• Most comprehensive
coverage: can see joints and pattern
• Least sampling bias
• Highest resolution
• Inexpensive for small natural exposures
• Disadvantages:• Natural outcrops rarely
in the best location
• Creating large exposures can be expensive
• Rock character can be different (weathered, more fractured)
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Extrapolating Satellite and Air Photography
• Convenience as a remote sensing tool
• Small scale only
• 2-D only
Under water lineamentsFalse color infrared of vegetation
patterns
Summary Data Sudbury - Lineament Analysis
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8.2 Borehole logging
Bore hole logging features
• Orientation
• Spacing
• Persistence
• Roughness
• Wall Strength
• Aperture
• Filling
• Seepage
• Number of Sets
• Block Size
Bore hole logging
• Advantages:• Drilling ubiquitous
• Pinpoint location
• Core can be preserved for later analysis
• Cost effective
• Natural sampling vehicle for lab tests
• Disadvantages:• Less precise data
• Borehole orientation bias
• Small scale structures
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Oriented coreReorienting Core Using a
Goniometer
http://www.accu-dril.com/products_core_goniometer76.htm
Bore hole optical imaging Bore hole acoustic imaging
Bore hole impression packer Electrical imaging
• FMS (Formation microscanner) image
• Electrodes on 4 individual electrode pads of a dipmeter
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Terzaghi bore hole bias
d
LN
sin
N (L/d=1)90 1.060 0.8645 0.7130 0.50 0.0
Terzaghi bore hole bias
• Values of sin for a borehole inclined at 45 degrees.
• Great circle “ blind zone”
Terzaghi bore hole bias
• Mutually orthogonal boreholes, to eliminate borehole bias
8.3 Geophysical methods
• 1) Large scale (surface sounding)
• 2) Intermediate scale (surface to borehole, borehole to borehole)
• 3) Small scale (rock adjacent to free surface)
8.3 Geophysical Methods
• 8.3.1 Seismic
• 8.3.2 Ultrasonic
• 8.3.3 Electrical
• 8.3.4 Electromagnetic
• 8.3.5 Radar
• 8.3.6 Bore hole methods
Geophysical methods
• Fractures are thin, thus are two dimensional anomalies
• Inverse relationship between depth of penetration and resolution
• Typical scenario: 1 equation, many unknowns
• Most methods highly dependent on deduction and interpretation
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Geophysical methods
• 1) Get and reduce measurement data.
• 2) Invert data to get local rock properties.
• 3) Deduce fracture properties from rock property data.
• 4) Interpret fracture geometries using a model.
8.3.1 Seismic Properties
• Velocity
• Attenuation
• Reflection
• Refraction
Surface methods: Seismic refraction
Surface methods: Seismic reflection
Seismic reflection Seismic reflection
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3-D section Microsiemics
8.3.2 Ultrasonic
• Low penetration, used generally to determine microscopic fissuring.
8.3.3 Surface methods: Electrical and Electromagnetic Methods
• Water filled fractures have higher electrical conductivity in low porosity rock
• Difficult to identify individual fractures, unless independent evidence is found
• Best for identifying fractured zones, especially effective if fractures are filled with conductive fluids
Electrical methods - Relationship to hydraulic properties
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8.3.5 Surface method: Ground penetrating radar
• Similar to reflection seismics in interpretation
• Different frequencies = different resolutions and penetration
8.3.6 Bore hole methods
• Eliminate overburden, or part of travel path
• Look at local rock directly - important for shallow penetrating methods.
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Bore hole method - Vertical seismic profiling
• Compromise between surface sounding and bore hole logs, by using a surface source, and down-hole geophones.
Bore hole methods: Tomography
• Seismic
• Radar
• Electric
• Electromagnetic
Bore hole methods -Conventional logging
• ADVANTAGES– Tried and proven
technology
– Consistent profile
– In situ properties
– Multiple independent measurements
• DISADVANTAGES– Properties unrelated to
fracture
– Rock disturbed by drilling
– Average fluid properties
– Directional bias
Acoustic waveform logging
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Borehole temperature and flow8.4 Other methods
• Movement instrumentation
Surface methods: Conventional monitoring
• Inclinometers
• Extensiometers
• Tilt meters
• Settlement gauges
• Pressure cells