using passive and active seismic data to better understand
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Using passive and active seismic data to better understand the stress conditions in a Canadian mine
April 2016
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Copyright © Institute of Mine Seismology 2016
From Hasegawa et al. 1989
Sources of Mine Seismicity
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Seismic Wave Propagation
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Relative Velocity Changes
Aim is to see how changes in velocities (directly related to changes in stress) can be correlated with mining.
Once we know this we can use future observations of velocity (stress) changes to help the mine with understanding the state of the rockmass.
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Wave Propagation
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Seismic Events
4502 events in 3 years (10 trigger or more)
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Seismic Source
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Moment Tensor
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Moment Tensor
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Moment Tensor
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Moment Tensor Decomposition
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Moment Tensor Decomposition
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Seismic Events
4502 events in 3 years (10 trigger or more)
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Moment Tensors
3504 events with MT
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Moment Tensors – slip events
1583 events with large DC component
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Hemlo Mine B-Zone Stress Conditions
After Coulson, 2009
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Principle Stress Trend (°)
Plunge (°)
Magnitude (MPa/ metre)
σ1 358 10 0.0437
σ2 093 28 0.0299
σ3 250 60 0.0214
After Coulson, 2009
B-Zone Stress Conditions
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Moment Tensors – Away from Mining
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Inferred Stress Directions
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C-zone
This is the adjacent ore-body so stress conditions were extrapolated from the known B-zone stress conditions.But the C-zone ore-body has been rotated on an adjacent fold limb (Muir 1997).
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Moment Tensor - C-zone
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Stress Inversion C-zone
Region A Region B Region C
Model 1 σ1
Seismic Proxy(P-axis)
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Stress Inversion C-zone
Region A Region B Region C
Model 1 σ2
Seismic Proxy(B-axis)
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Stress Inversion C-zone
Region A Region B Region C
Model 1 σ3
Seismic Proxy(T-axis)
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Stress Inversion C-zone
Region A Region B Region C
Model 2 σ1
Seismic Proxy(P-axis)
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Stress Inversion C-zone
Region A Region B Region C
Model 2 σ2
Seismic Proxy(B-axis)
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Stress Inversion C-zone
Region A Region B Region C
Model 2 σ3
Seismic Proxy(T-axis)
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Seismicity vs Modelling
We have compared modelling parameters with seismic information● maximum shear stress, orientation of principal axes vs● source locations and mechanisms
This is somewhat simplistic ● regions of maximum shear stress provided by elastic model
may yield before the analysed seismicity is recorded. ● seismic events can be associated with specific weaknesses,
which may locate outside the regions of maximum shear stress.
● P , B and T axes of source mechanisms represent proxies of maximum principal stresses provided the strength of the rock mass is isotropic and homogeneous. These axes can deviate from the principal stresses significantly for preferred planes of weakness.
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Modelling Seismicity
Original idea was from M.G.D. Salamon (1993) and it was elaborated on by A.M. Linkov (2005, 2013).
We implemented a variant of this (Malovichko, Basson, 2014).
It forecasts seismic events associated with disturbances of the stress field by mining.
Requires information about in situ stress and failure criteria for the rock mass, joint sets and specific 3D surfaces (e.g. faults).
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The modelled seismicity can be compared with the observed data in a rigorous way using two fundamental characteristics: ● amount of co seismic deformation (seismic potency)● geometry of co seismic deformation (source mechanisms)
Calculate two parameters for each cell namely:● ratio of modelled and observed cumulative potencies,● minimum rotation angle between the average source
mechanisms of modelled and observed seismic events.
Modelling Seismicity
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Modelling Seismicity
Observed
East South
Model 1
Jul 2013
Nov 2013
Mar 2014
Jul 2014
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Modelling Seismicity
Observed
East South
Model 2
Jul 2013
Nov 2013
Mar 2014
Jul 2014
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Modelling Seismicity
Observed
East South
Model 1
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Modelling Seismicity
Observed
East South
Model 2
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Modelling Seismicity
Cumulative Potency
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Modelling Seismicity
Comparison with model 1
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Modelling Seismicity
Comparison with model 2
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Modelling Seismicity
Model RMSD (log(Pobs
/Pmod
)) Median misorientation angle[deg]
1 0.98 79
2 0.96 53
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Active Seismic Source
Part of the Ultra-Deep Mining Network projects
Hosted by Barrick Williams Mine, Hemlo
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Seismic Wave Velocities
In rock seismic wave velocities depend on:● Young's modulus, Poisson's ratio and density of intact rock in 3D● Fractures (density, orientation and wet/dry)● Stresses (amplitude and orientation)
Only fractures and stresses change in response to mining
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Seismic Wave Velocities
Passive seismic tomography can yield seismic velocities, but poor resolution (few %) due to ● Unknown source locations● Non-repeating signals
We can get much higher resolution (0.01%) with a controlled repeating seismic source
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Seismic Source
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Active Seismic Source
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Radiation Patterns
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Radiation Patterns with Attenuation
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Experimental Design
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Active Source Signal
Signal recorded by reference sensor 0.5 m away
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Active Source Signal
Signals as the slug bounces
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Active Source Signal
Signal recorded by sensor 158 m away
S-wave arrival on the sensor
Reference pulse
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Resolution
For a signal of dominant frequency f0 and signal-to-noise ratio SNR,
the smallest measurable time shift determined by cross-correlation is
δt ≥ 1/(2πf0SNR)
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Active Source Signal
SNR ~ 3
This is equivalent to about a 0.1% velocity change
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Original Signal
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Stacked Signal
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Relative Velocity Changes
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Relative Velocity Changes
Aim is to see how changes in velocities (directly related to changes in stress) can be correlated with mining.
Once we know this we can use future observations of velocity (stress) changes to help the mine with understanding the state of the rockmass.
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Questions
Questions?