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6 June 2013 1
Rock Mass Characterisation using
Geophysical Logs
Peter Hatherly – Coalbed Geoscience
Terry Medhurst – PDR Engineers [email protected]
6th June 2013
Outline
1. Geophysical logging methods
2. Interpretation of geophysical logs
3. Geotechnical interpretation – the GSR
4. Modelling the GSR and geophysical logging
data
6 June 2013 2
1. Geophysical logging methods
Types of logs
1. Radiometric logs – natural gamma
– density (gamma-gamma)
– neutron (neutron-neutron)
– spectrometric logging
2. Sonic logs – conventional (first breaks)
– full waveform sonic (cement bond logs)
3. ‘Electric’ logs – resistivity
– conductivity
– dip meter
– self potential
– magnetic susceptibility
– induced polarisation
6 June 2013 3
Types of logs (cont)
4. Caliper logs – single arm
– multi arm
– full hole
5. Survey tools (magnetic and non-magnetic)
6. Imaging tools – acoustic scanner
– electrical imaging
7. Temperature
Geophysical logging in coal mining
The basic log suite:
– natural gamma
– density
– sonic
– caliper
– borehole survey
– (also neutron,
acoustic scanner,
resistivity,
spectrometric logs)
6 June 2013 4
1. Radiometric tools
• natural gamma radiation
– isotopes of 40K and 238U, 232Th & their daughters
• sources
– 60Co or 137Cs source for gamma rays
– PuBe or AmBe for neutrons
• detectors
– NaI Detector – gamma radiation
– 3He – neutron detection
• can be run inside rods and in and out of water (calibration issues)
Natural
gamma
Caliper
Detectors
Source
1.1 Natural gamma logging
• K, U & Th tend to be more common in shales
• total counts and spectrometric logging
• raw units in counts per second (cps). Calibration required to
convert to units of API (American Petroleum Institute)
• quartz sandstone = 10-30 API, shale 180-250 API, coal < 50
API.
• for clay content calculations:
• beware of kaolinitic siltstones as well as lithic & otherwise ‘hot’
sandstones (heavy minerals, felsic)
sandshale
shaleV
sand
)(clayevolumeShalVshale
6 June 2013 5
Natural radioactivity
• U – reasonably common, soluble – sedimentary & organic
concentrations
• Th – most commonly associated with monazite – heavy beach
mineral
• K - 7th most abundant element and occurs in many silicate
minerals including micas, feldspars and clays:
- Kaolinite: (Al2Si2O5(OH)4
- Montmorillonite/Smectite:
(Ca, Na, H)(Al, Mg, Fe, Zn)2(Si, Al)4O10(OH)2-xH2O
- Illite (K,H)Al2(Si,Al)4O10(OH)2-xH2O (hydrated muscovite)
natural
gamma log 150
170
190
210
230
250
270
290
0 150 300
API
Dep
th (
m)
6 June 2013 6
1.2 Density (gamma gamma) logging
• measures back scattered gamma radiation – Electron density true density
• raw unit in counts per sec (cps) - Calibration required to convert to kg/m3 (also units of t/m3 = g/cm3 )
• Some densities (t/m3)
fresh water 1 clay minerals 2.65- 2.75
salt 2.1- 2.4 basalt 2.7- 3.3
limestone 2.5- 2.85 gabbro 2.8- 3.5
granite 2.5- 2.8 sphalerite 3.5- 4
sandstone 2.5- 2.75 pyrite 4.9- 5.2
quartz 2.65 galena 7.4- 7.6
coal 1.3- 1.9
density,
gamma 150
170
190
210
230
250
270
290
0 1.5 3
g/cc
Dep
th (
m)
150
170
190
210
230
250
270
290
0 150 300
API
Dep
th (
m)
6 June 2013 7
Density
• Density of rocks need not be the same as the density of
the mineral constituents because of porosity.
• Density is affected by pressure and temperature
• Density is also relevant to gravity measurements &
seismic wave propagation (sonic logging)
fma log
fma
ma
logitymatrixDensma
tyfluidDensif
1.3 Neutron (porosity) logging
• neutrons emitted from source loose energy through collisions
with particles of similar mass to neutrons (ie H)
• raw measurements in counts per second but calibrated to
provide apparent porosity
• H is present in the form of water – free (pore) water and bound
water in clays (also hydrocarbons in coal and hydroxides etc)
• coal and iron rich strata have high apparent porosities
• when free gas is present, (porosity crossover)
• for clay calculations:
NSh
DNshaleV
(assumes øDSh = 0)
D
6 June 2013 8
neutron,
density,
gamma
150
170
190
210
230
250
270
290
0 1.5 3
g/cc
Dep
th (
m)
150
170
190
210
230
250
270
290
0 150 300
API
Dep
th (
m)
150
170
190
210
230
250
270
290
0 50 100
Neutron Porosity %
De
pth
(m
)
2. Sonic logging
• usually provides P-wave velocity through rocks in borehole wall – raw measurement is transit time between P-
wave arrivals at receivers (originally measured in microsecs/foot)
– velocity in km/s, ft/s
– velocity (km/s) = 304.8/transit time (usecs/ft)
– also full waveform tool
• requires water coupling – no casing
– no S-waves when Vs < 1550 m/s
• P-wave velocities in rocks – coal < 2,500 m/s
– Sediments: 2,500 – 5,000 m/s
– Igneous: 4,000 – 7,000 m/s
6 June 2013 9
Borehole wave
propagation
• P-wave from Tx is
refracted as P- and S-
waves along borehole
wall which refract
again as P-waves to
Rx array
• P-wave picked as 1st
arrival at each Rx
• Stoneley wave travels
within borehole fluid
Full waveform signals
velocity,
neutron,
gamma,
density
150
170
190
210
230
250
270
290
0 1.5 3
g/cc
Dep
th (
m)
150
170
190
210
230
250
270
290
0 150 300
API
Dep
th (
m)
150
170
190
210
230
250
270
290
0 50 100
Neutron Porosity %
De
pth
(m
)
150
170
190
210
230
250
270
290
2000 4000
Velocity m/s
Dep
th (
m)
6 June 2013 10
Acoustic scanner
(televiewer)
• rotating head obtains
oriented acoustic image
(reflections off) of borehole
wall
– fracture mapping
• also full borehole caliper
– borehole breakout (stress)
112
112
VV
VV
oefficientCreflection
3. Resistivity logging
• raw measurement is voltage between electrodes – converted to resistivity (ohm-m)
• ionic conduction through pore water and electron flow in metallic minerals and clay
• requires fluid coupling
• no steel casing
• typical values coal: 200 –10,000 ohm-m
sediments: 20-1,000 ohm-m
minerals: < 1 ohm-m
• focussed (laterlogs) logs, use guard electrodes to force the current flow into the formation.
• micro-resistivity tools (dip meters) use electrodes on caliper pads
6 June 2013 11
Electrical properties
Resistivity ρ is defined by the resistance R of a volume
of material length l and cross sectional area A
ohm-metre
Conductivity, σ = 1/ρ siemen/metre
(From now on, R is used for resistivity, not ρ)
l
RA
Typical values of resistivity
6 June 2013 12
resistivity,
neutron,
gamma
150
170
190
210
230
250
270
290
0 150 300
API
Dep
th (
m)
150
170
190
210
230
250
270
290
0 200 400
Resistivity (ohm m)
De
pth
(m
)
150
170
190
210
230
250
270
290
0 50 100
Neutron Porosity %
De
pth
(m
)
150
170
190
210
230
250
270
290
80 100 120
Caliper (mm)
Dep
th (
m)
150
170
190
210
230
250
270
290
15 20 25
Temperature C
De
pth
(m
)
150
170
190
210
230
250
270
290
0 150 300
API
Dep
th (
m)
150
170
190
210
230
250
270
290
0 1.5 3
g/cc
Dep
th (
m)
temperature
caliper,
gamma,
density
6 June 2013 13
2. Interpretation of geophysical logs
“Pretty, but does it mean anything?”
LAS (Log ASCII Standard) format
Header
Data
6 June 2013 14
Import LAS format files to Excel
Basic Excel chart
CODE
0
0.5
1
1.5
2
2.5
3
0 100 200 300 400 500 600
6 June 2013 15
Image files
• No standard formats for image files – i.e. files with multiple
measurements at each depth level
– Full waveform sonics
– Scanners
– Dip meters
• Use of proprietary binary formats
An approach to quantitative interpretation
• Determine porosity using density log
• Determine clay content using:
– Natural gamma log
– Neutron and density logs
– Resistivity and density logs
• Calculate velocity using porosity and clay content and
check with calculated velocity – provides a check on
porosity and clay determinations
• Calculate Geophysical Strata Rating (GSR)
Assume a clastic rock model where:
porosity + clay + quartz = 1
1 quartzshale VV
6 June 2013 16
Equations
porosity
sandstoneshale
sandstoneshaleV
NSh
DNshaleV
shalequartz VV 1
fma
ma
)(446.094.677.5 7.16 eVvelocity shale
Eberhart-Phillips et al, Geophysics (1989)
water
shaleshaleRR
RV81.0
1 2
clay
quartz
(density log)
(natural gamma log)
(neutron & density logs)
(resistivity & density
logs)
Demonstration
6 June 2013 17
Some practical considerations, pitfalls
and additional matters
Sampling interval
90
100
110
120
130
140
150
160
170
558 558.1 558.2 558.3 558.4 558.5 558.6 558.7 558.8 558.9 559
Depth (m)
Natu
ral gam
ma (
AP
I).
1 cm vs 5 cm vs 10 cm
6 June 2013 18
0
50
100
150
200
250
558 558.5 559 559.5 560 560.5 561 561.5 562
Depth (m)
Gam
ma r
ay r
esponse (
AP
I) .
0.01m samples
0.05m samples
0.1m samples
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
4.8
5
558 558.5 559 559.5 560 560.5 561 561.5 562
Depth (m)
Velo
city (
km
/s)
0.01m samples
0.05m samples
0.1m samples
ss st coarsening
upwards
st ss st ss st coarsening
upwards
4 m
Natural
gamma
Acoustic
scanner
Core
Sonic
Low gamma shales
gamma
100
120
140
160
180
200
220
240
260
0 50 100 150 200 250
API
depth
(m
)
density & sonic
100
120
140
160
180
200
220
240
260
1.00 2.00 3.00 4.00 5.00
units
depth
(m
)
neutron porosity
100
120
140
160
180
200
220
240
260
0.00 0.10 0.20 0.30 0.40 0.50
fraction
depth
(m
)
resistivity
100
120
140
160
180
200
220
240
260
0 100 200 300 400
ohm-m
depth
(m
)
Bald Hill
Claystone –
kaolinitic and
iron rich
6 June 2013 19
Caving S1942
514
524
534
544
554
564
574
584
594
90 100 110 120
caliper
depth
(m
)
S1942
514
524
534
544
554
564
574
584
594
0 50 100 150 200 250
resistivity
depth
(m
)
S1942
514
524
534
544
554
564
574
584
594
1.00 2.00 3.00 4.00 5.00
density & velocity
depth
(m
)
80 m
Clay occurrence
Dispersed clay
Laminated clay
Structural clay
Katahara, K.W., 1995. Gamma ray log response
in shaly sands. Log Analyst 36(4)
6 June 2013 20
Clay vs porosity cross-plots
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.05 0.1 0.15 0.2 0.25 0.3
porosity
cla
y
dispersed
laminated
structural
390 m - 550 m
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.05 0.1 0.15 0.2 0.25 0.3
DEPO
Vshale
gam
ma
Narrabeen Group - laminated porosity & clay
390
410
430
450
470
490
510
530
550
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.05 0.1 0.15 0.2 0.25 0.3
porosity
cla
y
dispersed
laminated
structural
6 June 2013 21
non-coal, 90m - 217 m
0
0.2
0.4
0.6
0.8
1
1.2
0 0.05 0.1 0.15 0.2 0.25 0.3
DEPO
Vshale
gam
ma
porosity & clay
90
110
130
150
170
190
210
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)
Non-coal 90-217 m
Moranbah Coal Measures - structural
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.05 0.1 0.15 0.2 0.25 0.3
porosity
cla
y
dispersed
laminated
structural
4. Geotechnical interpretation – the GSR Velocity
100
110
120
130
140
150
160
170
180
190
200
1.00 2.00 3.00 4.00 5.00
km/s
depth
(m
)
GSR
100
110
120
130
140
150
160
170
180
190
200
0 20 40 60 80
units
depth
(m
)
Porosity & Clay
100
110
120
130
140
150
160
170
180
190
200
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)+
6 June 2013 22
Some background
• Sonic velocity / UCS relationships have been available and
utilised since early work (1980’s) of McNally, Davies, Ward
and others.
• It has been recognised that care is required in applying these
relationships from site to site, particularly in low strength
rocks.
• Through a series of ACARP projects since 2002, we have
looked more carefully at the relationships between
geophysical log responses, rock type and rock strength. This
has resulted in the development of a rating scheme we have
called the Geophysical Strata Rating (GSR).
Overview
1. Sonic velocity and UCS
2. The measurement and meaning of sonic velocity
3. Geophysical Strata Rating (GSR) for clastic rocks and
coal
6 June 2013 23
Transit time vs UCS
Oyler, Mark and Molinda, NIOSH 2010
Recognised limitations to UCS
relationships
• site specific
• function of rock
type
• depth dependent
Lawrence 1999
6 June 2013 24
Roadway roof hazard map - Crinum Mine (Payne, 2008)
Steps involved in analysis:
1. Identify roof units on the basis of sonic, gamma logs
2. Choose representative sonic transit times
3. Convert to UCS
4. Prepare roof sections & plans – ½ m, 2 m and 12 m
Average UCS for 2m roof (Payne, 2008)
6 June 2013 25
Rippability
Caterpillar Performance Handbook,
edition 38
What is seismic velocity measuring?
• Velocity is responding to bulk conditions in-situ
– defined by the elastic constants (it is related to the bulk
modulus and shear modulus)
– a function of fracturing and defects
– a function of composition and pressure
6 June 2013 26
3/4
KVp
sV
P-waves
(compressional
waves)
S-waves (shear
waves)
Seismic wave velocities
Dynamic vs static modulus
0
10
20
30
40
50
0 5 10 15 20 25
Static modulus (GPa)
Dynam
ic m
odulu
s (
GP
a)
Oaky Ck
Callide
(greater by a factor of 2)
6 June 2013 27
Velocity vs fracture
0
25
50
75
100
2.5 3.5 4.5 5.5
P-wave velocity (km/s)
RQ
D
0
5
10
15
20
Cra
cks/
m
RQD
Fracture
frequency
Sjogren et al 1979
Porosity
Clay
Quartz
2 .5
3 .0 3 .5
4 .0
4 .5
Velocity vs composition
epeshalep epVV
7.16446.073.194.677.5
5 MPa
confinement
Eberhart-Phillips et al, Geophysics (1989)
6 June 2013 28
Velocity vs pressure
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4
0 100 200 300 400 500 600 700
Depth (m)
Velo
city (
km
/s)
25 Mpa/km
mixed
15 Mpa/km
shale = 50%
porosity = 10%
)(446.094.677.5 7.16 eVV shalep
What is seismic velocity measuring?
• Velocity is responding to bulk conditions in-situ
– defined by the elastic constants (it is related to the bulk
modulus and shear modulus)
– a function of fracturing and defects
– a function of composition and pressure
• Velocity is related to UCS as much as modulus is
related to UCS
strain
str
ess
stiff rock is strong
soft rock is weak
6 June 2013 29
Geophysical Strata Rating (GSR)
EMPIRICAL SCHEME 0-100
• Sonic (P-wave) velocity to estimate in-situ rock mass
properties.
• Consideration of pressure and compositional factors:
– Porosity
– Clay content
• Defect scores according to variability in previous scores
(‘fracture’) and variability in clay content (‘bedding’).
Medhurst and Hatherly since 2002
For GSR calculations
Geophysical logging data:
• Sonic logs
• Density logs (for porosity)
• Natural gamma logs (for clay content) but can also use
neutron logs and resistivity logs
Implemented using macros written for Excel
6 June 2013 30
Geophysical Strata Rating
1. Strength score 0 to 55 plus
2. Cohesion score 10 to 25 plus
3. Porosity score -15 to 0 plus
4. Moisture score -10 to 0 plus
5. Defect score
– fracture score 0 to 10 plus
– bedding score 0 to 10
For velocity range 2.5 to 5 km/s…
upper limit 100, lower limit 5
Initial
(intact)
GSR
0
10
20
30
2 2.5 3 3.5 4 4.5 5
Velocity (km/s)
Co
he
sio
n s
co
re
1. Strength score
2. Cohesion
score
Qtz > 0.67
Qtz < 0.57
0
20
40
60
2 2.5 3 3.5 4 4.5 5
Velocity (km/s)
Str
en
gth
sco
re
(correct velocity for
pressure)
6 June 2013 31
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Porosity
Vsh
ale
3. Porosity score
4. Moisture
score
-15
0
0
0.2
0.4
0.6
0.8
1
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Porosity
Vsh
ale
0
-10
Only applied if Vp < 4 km/s.
Linearly scales to zero while 4
< Vp < 5
5. Defect score
• Fracture score: 0 to 10
Fractures cause variations in initial GSR so use
variability (derivative) in GSRi to define fracture score
(10 = no variability)
• Bedding score: 0 to 10.
Boundaries between sands and silts/clays reflected in
the gamma log. Use derivative of clay content to
provide bedding score (10 = no variability)
6 June 2013 32
Velocity
100
110
120
130
140
150
160
170
180
190
200
1.00 2.00 3.00 4.00 5.00
km/s
depth
(m
)
GSR
100
110
120
130
140
150
160
170
180
190
200
0 20 40 60 80
units
depth
(m
)
Porosity & Clay
100
110
120
130
140
150
160
170
180
190
200
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)
+
Geophysical Strata Rating
porosity & clay
90
100
110
120
130
140
150
160
170
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)
porosity & clay
100
110
120
130
140
150
160
170
180
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)
porosity & clay
116
126
136
146
156
166
176
186
196
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)
porosity & clay
138
148
158
168
178
188
198
208
218
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)
porosity & clay
164
174
184
194
204
214
224
234
244
0.00 0.25 0.50 0.75 1.00
fraction
depth
(m
)
GSR
90
100
110
120
130
140
150
160
170
0 20 40 60 80
units
depth
(m
)
GSR
100
110
120
130
140
150
160
170
180
0 20 40 60 80
units
depth
(m
)
GSR
116
126
136
146
156
166
176
186
196
0 20 40 60 80
units
depth
(m
)
GSR
138
148
158
168
178
188
198
208
218
0 20 40 60 80
units
depth
(m
)
GSR
164
174
184
194
204
214
224
234
244
0 20 40 60 80
units
depth
(m
)
GSR reflects rock
quality in a
geological context
6 June 2013 33
53
35
53
36
53
42
53
68
53
49
54
23
57
02
70
51
637800 638000 638200 638400 638600 638800 639000 639200 639400 639600 6398000
2
4
6
8
10
12
5
10
15
20
25
30
35
40
45
50
55
60
65
UCS section
GSR section modelled using Golden Surfer
GSR for coal & carbonaceous materials
• Require different treatment because porosity calculations assume that discrete rock grains are present.
• Velocity is abnormally low compared to other rocks but there is still some inherent strength.
• Generally bright coals are weaker and have lower ash than dull coals.
• Analysis requires recognition of these rock types and then applying appropriately modified GSR formula.
density & velocity
520
525
530
535
540
1.00 2.00 3.00 4.00 5.00
units
depth
(m
)
GSR
520
525
530
535
540
0 20 40 60 80
units
depth
(m
)
6 June 2013 34
GSR vs CMRR
• There are differences between GSR and CMRR, particularly in weaker mudstones
• CMRR limited by sensitivity of PLT data in weak rocks (CMRR < 30)
• Differences can also occur depending on how units are determined
• For an approximate conversion for GSR > 25
CMRR ≈ 0.5GSR + 20
Q-value vs velocity
Barton, 2002
6 June 2013 35
5. Modelling the GSR and geophysical
logging data
Geological modelling of a stratified deposit
• Establish positions for lithological boundaries within boreholes – Core, chips, geophysical logs
• Interpolate boundaries between boreholes (create wireframes)
• Take chosen geological parameter (ore grade, coal quality) from borehole and apply geostatistics to interpolate parameter values between boreholes and within boundaries
• Requires sufficient boreholes and test results. Not usually the case with geotechnical work.
• Geophysical logging data can be modelled in this same way
6 June 2013 36
Natural gamma logs
Density logs
6 June 2013 37
Control surfaces
Clay content model
6 June 2013 38
GSR model
Channel example 1. Clay content faults
6 June 2013 39
Channel example 2. GSR
6 June 2013 40
Coal seam silling. GSR
Seam splitting. GSR
6 June 2013 41
Seam splitting model without control
surfaces for splits
Grid spacings 25 m x 0.1 m
Average GSR 0-5m into roof
6 June 2013 42
Average GSR 0-5m into roof no
control surfaces
Average GSR 0-5m into roof
With lithological control No lithological control
6 June 2013 43
Demonstration – interactive 3D GSR and
clay content models
Conclusions 1 - modelling
• Calibrated geophysical logs provide a rich source of
information on physical properties of the strata.
– coal & other lithological boundaries, clay content, GSR
• Geophysical logging data can be modelled in the same way
that geologists model seams and their quality
• To properly construct the models, the lithological boundaries
need to be honoured
6 June 2013 44
Conclusions 2 - applications
• Geotechnical applications for GSR & geophysical models
include:
– 3D models of strata characteristics
– Hazard plans for roof horizons
– Understanding of caving behaviour
– Basis for understanding highwall behaviour
– Blast design
– Floor failure
– A starting model for 3D numerical modelling
– Etc.
Conclusions 3 – developments
• ACARP C20025. Investigations for open pit geomechanics
using geophysical logs. Report completed in May 2103
• ACARP C20032. Dynamic response of longwall systems and
their relationship to caving behaviour. Medhurst, LVA (Hoyer)
& Hatherly. Completion within next few months.
• New ACARP project C22008. Investigation into roadway roof
support design using Geophysical Strata Rating. Medhurst &
Hatherly. 18 months.
