GE 6477 DISCONTINUOUS ROCK Instructional Objectives 8 ...web.mst.edu/~norbert/ge6477/Class Notes/08_detect.pdf · Detailed logging of rock outcrop, statistics on jointing Exploratory

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GE 6477 DISCONTINUOUS ROCK8. Fracture Detection

Dr. Norbert H. Maerz

Missouri University of Science and Technology

(573) 341-6714

[email protected]

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