GE 6477 DISCONTINUOUS ROCK 8. Fracture Detection Dr. Norbert H. Maerz Missouri University of Science and Technology (573) 341-6714 norbert@mst.edu 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. 1
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 2
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 Phase Detailed 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 3
Tertiary Phase Further core drilling, vertical, inclined, or horizontal Jointing information from drillhole television, impression packers, oriented core 1 integral-core sampling Downhole geophysical logging and tomography between holes to define rock mass quality and individual faults, etc. Downhole testing using a dilatometer to measure deformability Packer testing for hydraulic conductivity Installation of piezometers to study the groundwater regime and to 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 4
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) 5
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Extrapolating Satellite and Air Photography Convenience as a remote sensing tool Small scale only 2-D only 10
Under water lineaments False color infrared of vegetation patterns 11
Summary Data Sudbury - Lineament Analysis 12
8.2 Borehole logging 13
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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 15
Oriented core Reorienting Core Using a Goniometer http://www.accu-dril.com/products_core_goniometer76.htm 16
Bore hole optical imaging Bore hole acoustic imaging 17
Bore hole impression packer Electrical imaging FMS (Formation microscanner) image Electrodes on 4 individual electrode pads of a dipmeter 18
Terzaghi bore hole bias N Lsin d N (L/d=1) 90 1.0 60 0.86 45 0.71 30 0.5 0 0.0 Terzaghi bore hole bias Values of sin for a borehole inclined at 45 degrees. Great circle blind zone 19
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) 20
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 21
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 22
Surface methods: Seismic refraction Surface methods: Seismic reflection 23
Seismic reflection Seismic reflection 24
3-D section Microsiemics 25
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 26
Electrical methods - Relationship to hydraulic properties 27
8.3.5 Surface method: Ground penetrating radar Similar to reflection seismics in interpretation Different frequencies = different resolutions and penetration 28
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8.3.6 Bore hole methods Eliminate overburden, or part of travel path Look at local rock directly - important for shallow penetrating methods. 30
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 31
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 32
Acoustic waveform logging 33
Borehole temperature and flow 8.4 Other methods Movement instrumentation 34
Surface methods: Conventional monitoring Inclinometers Extensiometers Tilt meters Settlement gauges Pressure cells 35
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