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