geophysical logging in planning & minimizing costs in shafts and slopes presentation to illinois...
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Geophysical Logging in Planning & Minimizing Costs in Shafts and SlopesPresentation to Illinois Mining Institute
Marion, Illinois • August 26, 2015
Authors:John E. Feddock, P.E., Senior PrincipalMark S. Smith, GLS Business Unit ManagerPhil Waters, Senior Supervisor - GLS
Abstract•This presentation deals with the excavation of shafts and slopes for underground mines. Through properly located boreholes and the use of geophysical logging it is possible to identify and investigate ground control and the prediction of strata strength characteristics prior to excavation and avoid costly delays due to geologic and hydrologic changes that impede construction and increase costs.
•Through geophysical logging it is possible to predict compressive strength and tensile strength of strata, identify faults and fracture patterns, fissile shales in addition to water inflow and outflow zones and conductivity for controlling water during excavation.
What is Geophysical Logging ?
Geophysical Logging Services
Geophysical Logging Probes
0033 Density Probes 9051 Casing Collar Locator
9055 Deviation / Neutron Probes
9096 Gyro Deviation Probe
9041 Temperature-Fluid Conductvity-Res. Probes
Downhole Cameras – three models for various applications
9035 Compensated Density Probe
9511 Induction Probe
9136 Density Temperature Deviation Probe
9721 EM Flowmeter
9804 Acoustic Televiewer Probe
0026 Three-arm Caliper/Deviation/Temperature
9321 Full-wave Sonic Probe
Discrete Interval WaterSampler
Core Photos
Standard Working Depth to 4,000 Feet
Deeper upon request
The High-Resolution Density Probe
Model No. 9030
Contains a very short spacing Americium source
Records natural gamma, caliper, guard resistivity (in fluid) and open hole density
Recording at 50 times per foot
Purpose
Provides for the identification of sedimentary strata by an estimate of its density and response to the nuclear source. The log can be interpreted to provide a general description of the type of strata.
Gamma-Density Logwith Caliper and Resistivity Traces and Interpreted Geology
The Full Wave Sonic Tool
Model No. 9320
Single transmitter and dual receiver
Records 10 times per foot
Sonic velocities are proportional to the compressional strength of
the rock where the state of stress (σ) in a direction is
proportional to the particle velocity (V) in that direction.
Elastic-wave (acoustic) methods Propagation paths for P, S, mud, and tube waves. (After Lobo, 1987)
Propagation paths for P, S, mud, and tube waves. (After Lobo, 1987)
Elastic-wave (acoustic) methods
S-w
ave
Mud w
ave
Receiver
1.0 foot
Receiver
Tube wave
Tube wave
3.0 feet
Transmitter Tube wave
P-w
ave
Sonic Log Traces
Trace of Acoustic Wave Train
Focused Resistivity
Century Model No. 0033 combination probe with natural gamma, density, guard resistivity and caliper.
Uses a 4-inch current electrode centered within two guard electrodes
Total length of 55 inches
Focused Resistivity Probe Schematic
55 inches
Guard
Electrode
ElectrodeGuard
ElectrodeCurrent
Tensile Strength versus Guard Resistivity
y = 58.772x - 474.26
R2 = 0.8775
0
200
400
600
800
1,000
1,200
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Resistivity (Ohm-ft)
Te
ns
ile
Str
en
gth
(P
SI)
Acoustic Televiewer
Model No. 9804
Images the borehole sidewall using high-resolution sound waves (similar to sonogram)
Picture displayed in both amplitude and travel time
Digitizes 256 measurements around the boreholes
Sample interval (0.005 meters/0.02 feet)
Oriented directionally with magnetometers and inclinometers
Acoustic Televiewer
Acoustic Televiewer : The circle produced by the intersection of a dipping plane and borehole is “unrolled” and appears as sine curve
Televiewer Log – Travel Time Televiewer Log - Amplitude
45º Fracture dipping North 10º East
Televiewer Log Trace
Interpretation of a Highly Fractured zone over 8 feet
From the log we can Plot Stereographs and Rose diagrams for wedge and plane failure analysis
Down Hole Video Camera Use
Geological Logging Systems’ latest camera systems have:
• Internal rotating and external fixed lighting
• Ring lighting (not shown) that gives an unobstructed downward view. Pan-tilt-zoom model for large shaft –mine void use.
• Very low lux sensitivity that allows horizontal scan viewing into large openings such as mine voids, tunnels, etc.
• Rugged stainless steel construction, hermetically sealed with internal nitrogen charge.
• Explosion proof with up to a 2500-psi pressure rating,
• Viewing distances from fractions of inches out to 70 feet or more depending upon lighting and void conditions.
• A 5,000-foot depth capacity as mounted in a four wheel drive Ford F-350 with off-road capabilities
• Various models for use in boreholes from 3 inches in diameter up to large shafts and 6 inch boreholes into mine voids.
Down hole and Side Views from Camera
Immersed Side View of Borehole Wall
Immersed Side View of Borehole Wall Showing Water Inflow “Cloud”
Down hole Views from Camera
Down Hole View
Down Hole View with ring light
Down hole and Side Views from Camera
Down hole view of Borehole and Pilot Hole
Down hole view of Item Lost in Pilot Hole
Close-up view of Item Lost in Pilot Hole
Shaft and Mine Void Inspection Camera
Imersed Side View of Borehole Wall
Old Mine Void thru 6 inch borehole
Wide angle in Photo above.
Zoomed in on Pillar 60 feet away, on right.
Now that we have all of this data what do we do with it?
?
Knowledge of fractures and bedding planes allows prediction of possible unstable rock conditions in the shaft walls or the slope roof.
Prediction of rock strengths are an indicator of weak rock strata and provides information for rock bolting or secondary support.
In sedimentary rock, porosity is inversely proportional to the modulus of elasticity and the wave velocity, giving an indicator for water bearing strata.
Ground water information gives the ability to predict potential water inflow and the design of water rings in shafts or effective grouting plans.
In sedimentary rocks sonic wave velocities can be treated as if they are isotropic, but in metamorphic and igneous rocks tectonic stress can have a large effect on sonic wave velocities.
Information Available
Different Rock Types can create Difficult Mining Conditions
Interbedded Sandstone and Shale (Stack rock) can result in very difficult mining conditions especially during construction of shafts and slopes. Weak or incompetent strata is difficult to predict from only core logs. When it occurs weak or incompetent strata allows for overbreak during blasting and/or rock falls during excavation.
A Slope with an overbreak problem involved a Geology Assessment and Interpretation of Rock Stability
Slope Logging Activities
• Three bore holes were located near the base of the slope but the two holes near the top of the slope were located plus or minus 1600 feet away from the center line of the slope.
• Core for three of the holes were logged 50 feet above the coal seam, while the rest of the hole relied on drillers logs.
• Although two of the five holes had geophysical logs for sonic, density, gamma ray, and caliper performed by Geological Logging Systems (GLS), subsidiary of Cardno, only one had agreement with geologists log.
• Using the geologists and sonic logs, an RMR rating was subjectively estimated for seven zones along the line of the slope .
• Zones of weak roof were identified and a recommended bolt length was estimated so that excavation of the slope could continue safely.
The outcome was favorable but delays occurred when weak strata interfered with the normal excavation cycle and forced an assessment of possible future adverse geology.
The sonic, density and gamma logs provided the opportunity to predict the weak rock areas and identify the necessary reinforcement such that the excavation cycle could proceed with minimal interference.
Recommendations• Drill core holes on or near the center line of shafts, and at the bottom or radius of
the slope, slightly lower than the midpoint, and between the slope collar and the upper quarter point. For example if a slope will have an inclined length of 1700 feet, core holes should be located ~400 feet, ~1000 and ~1700 feet from the collar.
• Geophysical logging of each hole should include:• density, gamma ray, and caliper density, and caliper logs for geologic
interpretation, • acoustic televiewer, sonic, and resistivity logs for fracture analysis and
rock strength evaluation, and • temperature log to identify water zones.
• The geophysical information can be used to predict: • unstable or weak strata that will require blasting precautions and/or roof
support, • water bearing strata that may require enhanced water control by
grouting, or installation of water rings and dewatering holes.
Resource/Reserve Evaluations Geotechnical Evaluations (Subsidence) Geophysical Logging Hazard Prediction Field Exploration Water Resources Management Probable Hydrologic Consequences Evaluations Mine design and Plant Evaluations
THANK YOU
Mining and Mineral Services
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