Download - Large Open Pit

Geophysical Logging For The
Assessment of Blast Damage
Bingham Canyon Mine Utah
INTRODUCTION
This study took place at the Bingham Canyon Mine, Utah,
operated by Kennecott Utah Copper (KUC).
The goal of this study was to determine if the test borehole
geophysical methods are applicable for assessing changes
in rock mass properties from production blasts in an open-pit.
The boreholes were drilled under working mine conditions,
which required rapid drilling and logging to allow work to
continue. The drilling method, using a hammer rather than
coring, resulted in boreholes with a high degree of rugosity.
May 10, 2017
2
INTRODUCTION (continued)

Geomechanical stresses produced by blasting and
removal of lateral loading from an advancing mine face
are expected to disturb and weaken rocks by enlarging
existing fractures and creating new ones.

These changes might be associated with changes in
geophysical properties of the rocks as measured by
geophysical logging probes in boreholes adjacent to the
active mine face before and after blasting.
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INTRODUCTION (continued)

May 10, 2017
This analysis addresses changes in these logs that may
have occurred as a result of blasting and how those
changes might be related to rock mass properties in the
vicinity of the boreholes.
4
HOLE LAYOUT FOR THE SEDIMENTARY (Congor)
FORMATION TEST SITE

Air hammer rig
 Drilled to 100 ft.
 5.5 inch diameter
 Closest holes
were 60 ft. from
the blast
 10 ft. behind the
projected backbreak
May 10, 2017
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HOLE LAYOUT FOR THE INTRUSIVE (Monzonite)
FORMATION TEST SITE

Air hammer rig
 Drilled to 100 ft.
 5.5 inch diameter
 Closest holes
were 60 ft. from
the blast
 10 ft. behind the
projected backbreak
May 10, 2017
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BOREHOLE GEOPHYSICAL TOOLS

Mechanical caliper
 Acoustic televiewer (ATV)
 Optical televiewer (OTV)
 Full-waveform sonics (FWS)

May 10, 2017
The ATV and FWS tools can only be run in the fluid
filled portion of the boreholes, thus the boreholes
were filled with water as needed for acquisition of
ATV and FWS data.
7
MECHANICAL CALIPER - 9074 3-Arm Caliper

The caliper gives the borehole diameter as
indicated by the average deflection of three
mechanically linked arms attached to a
resistive sensor (linear potentiometer).

Changes in borehole diameter indicated by the
caliper log help with the interpretation of the
BHTV and FWS logs.
©Century Geophysical Corporation
May 10, 2017
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ACOUSTIC TELEVIEWER (ATV) – ALT ABI40

Oriented 360° scanned image of the borehole
wall
 1.2 MHz ultrasound pulses from a rotating
sensor
 2 mm depth resolution and 2.5 degree
horizontal
 Borehole fluid required
 Image not affected by clarity
 Drilling has major affects on image
 Tool centralization has major affects on image
May 10, 2017
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OPTICAL TELEVIEWER (OTV) – ALT OBI40








May 10, 2017
Oriented 360° color image of the borehole wall
DSP based digital CCD camera with color
resolution is 24 bit RGB values
2 mm depth resolution and 1 degree horizontal
Borehole fluid not required
Image is affected by clarity
Drilling has major affects on image
Tool centralization has major affects on image
Openings can be difficult to identify
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TELEVIEWER – NEAR IDEAL CONDITIONS
GNEISS
ATV
May 10, 2017
SANDSTONE
ATV
OTV
OTV
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TELEVIEWER PRE-BLAST & POST-BLAST
Cng-1
ATV Travel Time
May 10, 2017
ATV Amplitude
12
OTV Image
FULL-WAVEFORM SONIC (FWS) – MSI 2SAA-1000F
Transmitter (Tx) 15 kHz
 Three Receivers (Rx)
 Tx-Rx Spacings: 3,4,5 ft.
 P-wave Velocity
 S-wave Velocity
 Stoneley Wave Velocity
 Acoustic Porosity
 Amplitude
 Fracture Characterization

May 10, 2017
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SONIC DATA PROCESSING

Processed using the WellCAD software (©Advanced Logic
Technology) to filter and stack the waveform data to
remove extraneous noise beyond the source frequency
range of the probe, including low-frequency noise
generated by the movement of the probe in the borehole.

For a 5.5 inch diameter borehole in hard rock expect:
P mode at 10-15 kHz
S mode at 12-20 kHz
15 kHz selected as the ideal frequency for this study

May 10, 2017
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SONIC DATA PROCESSING (continued)

A frequency filter was used that consisted of removing
lower and higher frequencies from the data set using a
tapered filter centered on the 10-15 kHz range, with
tapering to cut-offs at 5 and 20 kHz.

Stacking consists of adding and averaging waveforms
from adjacent stations, a standard procedure in seismic
wave analysis.

A stack of three (3) waveforms was used in the display,
and a stack of five (5) was used for the semblance
processing.
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SONIC DATA PROCESSING (continued)
RAW
May 10, 2017
FILTERED FILTERED
STACKED
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RAW
STACKED STACKED
FILTERED
WHAT DO SONIC WAVEFORMS SAMPLE?

SAMPLE VOLUME IS A WAVELENGTH: L = V/f = 1 or 2
ft.

WAVES PROPAGATE AROUND VOIDS OR FRACTURES

Vp AND Vs ONLY SAMPLE MICROFRACTURES

WAVES ARE SCATTERED BY DISCONTINUITIES

VELOCITY CHANGES – MICROFRACTURES

COHERENCE CHANGES - MACROFRACTURES
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WAVEFORM PROPAGATION
In this study we
concentrated on
observed changes to:
Compressional-Wave
May 10, 2017
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
Shear-Wave

Fluid (Tube)-Wave
BOREHOLE WAVES PROPAGATE AS MODES

BOREHOLE ACTS AS AN ACOUSTIC RESONATOR the borehole geometry selectively propagates certain
constructively interfering frequency packages (denoted as
modes)
 GEOMETRY SELECTS FOR CONSTRUCTIVE
INTERFERENCE
 MULTIPLE NARROW-BAND MODES CAN INTERFERE
 BEST WAVEFORMS WITH SINGLE SET OF MODES –
P, S, and TUBE
 NO DISTINCT FIRST BREAKS, THEREFORE
 SEMBLANCE PROCESSING (CROSS-CORRELATION)
REQUIRED
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MICROSEISMIC PROPAGATION
Some key properties and relationships




May 10, 2017
ROCK PROPERTIES
E YOUNG’S MODULUS
ν POISSON’S RATIO
SEISMIC WAVE PROPAGATION
Vp2 = M/ρ M = (1-ν)E/(1-ν-2ν2)
Vs2 = μ/ρ μ = E/(2(1+ν))
STATIC VERSUS DYNAMIC MODULI
Vp AND Vs APPLY TO SUB WAVELENGTH SCALE
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ANALYSIS METHODOLOGY
May 10, 2017

Compared the televiewer data to identify any
changes that might have occurred between the
pre-blast and post-blast runs.

Generalized inspection of the sonic waveform
plots for the boreholes with sufficient data to
identify intervals of coherent wave propagation,
and to identify any possible changes in the
extent of those coherent intervals in the postblast data.
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ANALYSIS METHODOLOGY (continued)
May 10, 2017

Qualitative inspection of the sonic waveform plots
and semblance analysis.

Micro-seismic velocity measurement from the fullwaveform sonic data

Sample waveform analysis

Wave mode amplitude study - using a window of
400 to 600 us for P and 800 to 1100 us for S and
1100 to 2000 us for Tube.
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SEMBLANCE ANALYSIS – Int-1 EXAMPLE
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WAVE MODE AMPLITUDE - Cng-1 EXAMPLE
Gain was constant (16) for pre to post runs.
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WAVE MODE AMPLITUDE - Int-2 EXAMPLE
Note complicating factor, gains changed by operator from pre to post runs.
Gain = 4
May 10, 2017
Gain = 16
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ANGLO AMERICAN BLASTING TRIAL- FWS

FWS data from six cored holes, before and after
blasting runs
 The tool appears to be a Weatherford 4-reciver tool
with Rx spacings of 60 cm, 80 cm, 100 cm, and 120cm
 We were provided FWS data from only the 120 cm
receiver for before and after blast runs
 Semblance processing was not possible without data
from all receivers
 We did do a simple wave mode amplitude study using
an assumed window of 350 to 550 us for P and 550 to
800 us for S; we assumed Gain constant for all runs.
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WAVE MODE AMPLITUDE – ANGLO DATA
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SUMMARY AND CONCLUSIONS

Bingham – Air Hammer - The set of televiewer and sonic
waveform data obtained in the Bingham boreholes before
and after blasting show little, if any, consistent differences
that can be attributed to changes in rock properties
produced by blast effects.

Anglo – Core - However, the wave mode amplitude for P
and S from the FWS logs obtained from the Anglo test
appear to be a reasonably good indication of the extent of
wave disruption by "defects" in the borehole wall, and
therefore a good way to compare the condition of the rock
mass before and after.
May 10, 2017
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SUMMARY AND CONCLUSIONS (continued)
May 10, 2017

The analysis of the Bingham and Anglo FWS data
allowed us to state that a significant result of this
assessment is the importance of borehole conditions
when using centralized borehole geophysical tools
such as FWS and BHTV.

Changes in the quality of tool centralization between
runs from different times can introduce differences in
the data both subtle and large; borehole rugosity is a
critical complicating factor to any rock mass blast
study. The presentation of the tool to the borehole wall
must be the same for pre and post runs.
29
SUMMARY AND CONCLUSIONS (continued)

Failure occurs by deformation concentrated along certain
favorably orientated planes where local dilatation along
grain boundaries allows slippage to occur. Incipient
failure of wall rocks near a mine face would likely be
concentrated along such planes.

Orientation might otherwise be influenced by the
prevailing stress field, which is tension aligned
perpendicular to the mine face.

Dilatation would therefore be along vertical planes, and
unlikely to be intersected by vertical boreholes
May 10, 2017
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SUMMARY AND CONCLUSIONS (continued)

May 10, 2017
Specific to the Bingham Test - Blast effects could
have been concentrated on the most easily disturbed
intervals, which would be least likely to produce
coherent sonic wave propagation, and to be so densely
fractured as to be difficult to image effectively with
acoustic logs.
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SUMMARY AND CONCLUSIONS

The set of televiewer and sonic waveform data obtained in
the Bingham boreholes before and after blasting show
little, if any, consistent differences that can be attributed to
changes in rock properties produced by blast effects.
 Bingham – Air Hammer
 However, the wave mode amplitude for P and S from the
FWS logs obtained from Anglo appear to be a reasonably
good indication of the extent of wave disruption by
"defects" in the borehole wall, and therefore a good way to
compare the condition of the rock mass before and after
 Anglo - Core
May 10, 2017
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SUMMARY AND CONCLUSIONS (continued)

Failure occurs by deformation concentrated along certain
favorably orientated planes where local dilatation along
grain boundaries allows slippage to occur. Incipient
failure of wall rocks near a mine face would likely be
concentrated along such planes.

Orientation might otherwise be influenced by the
prevailing stress field, which is tension aligned
perpendicular to the mine face.

Dilatation would therefore be along vertical planes, and
unlikely to be intersected by vertical boreholes
May 10, 2017
33
SUMMARY AND CONCLUSIONS (continued)
May 10, 2017

Blast effects could have been concentrated on the
most easily disturbed intervals, which would be least
likely to produce coherent sonic wave propagation,
and to be so densely fractured as to be difficult to
image effectively with acoustic logs.

A significant result of this assessment is the
importance of borehole conditions on the use of
these geophysical measurements for this
application.
34
SUMMARY AND CONCLUSIONS (continued)
The classes of fractures that we were concerned with in
this study were:

MICRO-FRACTURES – AROUND GRAIN
BOUNDARIES

MACRO-FRACTURES – MECHANICAL
DISCONTINUITIES IN THE ROCK

DILATATION FRACTURES – MACROSCOPIC
DISCONTINUITIES TO ACCOMMODATE SHEAR AT
INCIPIENT FAILURE
May 10, 2017
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SUMMARY AND CONCLUSIONS (continued)
POSSIBLE CHANGES IN ROCK MASS PERCOLATION

Dilation may not affect vicinity of the borehole
 Dilation can affect large-scale permeability
 Hydraulic stimulation studies – Major changes in
flow can occur without any detectable change in the
other geophysical log data (BHTV, FWS, caliper)
 Before and after slug testing
 Flowmeter testing can indicate the depth of flow
 Flow modeling codes can be used to estimate
hydraulic aperture changes
May 10, 2017
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RECOMMENDATIONS

In future studies the set of test holes should include a
number of inclined holes (30 to 45 degrees from vertical).

The wave mode amplitude for P and S appears to be a
reasonably good indication of the extent of wave
disruption by "defects" in the borehole wall, and therefore
a good way to compare the condition of the rock mass
before and after.

However, care must be taken to ensure that tool
parameters are maintained constant and unchanged for
pre-blast and post-blast logging runs, specifically Gain.
May 10, 2017
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RECOMMENDATIONS (continued)

Increased permeability can be measured with highresolution flowmeter logging during injection of water
into boreholes to sustain a water level above static.

When flow logs are not obtained, simple slug testing
(monitoring the rate of water level decline as a function
of depth to water level) could determine if significant
changes in permeability have occurred in the rock mass.
May 10, 2017
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RECOMMENDATIONS (continued)
HEAT PULSE FLOWMETER – MSI HFP-2293
Increased permeability can be measured with
high-resolution flowmeter logging during
injection of water into boreholes to sustain a
water level above static.

Detects small outflows to network
 Affected by entire network and not just local
 Response to dilation away from hole
 Outflow induced by adding water for other
probes
 Flowmeter identifies exact depth of outflow
May 10, 2017
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