Moment tensor inversion at Pyhasalmi ore mine:

accuracy test using explosions

Daniela Kühn (NORSAR)V. Vavrycuk (Academy of Sciences of the CR)

AIM 2nd annual meeting, 29-30 Sept 2011, Prague, Czech Republic

• microseismic monitoring: since January 2003 safety of the underground personnel optimisation of mining process

• network: 12 1-C geophones

+ 6 3-C geophones (ISS)

3-D geometry sampling rate: < 3000 Hz

• events: 1500 events /months (including blasting) -2 < Mw < 1.5

Pyhäsalmi ore mine, Finland

owned by Inmet Mining Co.

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Inconsistent polarities of P-wave first onset

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Moment tensor inversion:• homogeneous velocity model (as for locations)• amplitude inversion

Waveform modelling

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Complexity of velocity model

Introduction

Waveform modelling

Moment tensor

inversion

Summary

620 m

• E3D: viscoelastic 3-D FD code (Larsen and Schultz, 1995)• strong interaction with mining cavities: reflection,

scattering, conversion• healing of wavefronts

Waveform modelling: 2D section

Introduction

Waveform modelling

Moment tensor

inversion

Summary

- complex waveforms

- strong coda

- complex secondary arrivals

- scattering effects stronger on amplitudes than travel times, since size of heterogeneities (cavities, access tunnels) same order or smaller than wavelengths

arrival times computed by Eikonal solver still fit (wavefronts heal quickly after passing a cavitiy)

observed seismograms

Waveform modellingsynthetic seismograms

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Comparison 1-D/3-D

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Imaginary network!

Comparison 1-D/3-D

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Influence of proximity to cavity

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Real mine network!

Source depth → ray path

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Ray path → onset polarity

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Moment tensor inversion

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Amplitude picking

first maximum amplitude = amplitude of the direct wave

direct wave scattered wave

Introduction

Waveform modelling

Moment tensor

inversion

Summary

?

direct wave

energy diffractedaround cavity

scattered wave

first maximum amplitude is not always the amplitude of the direct wave

Amplitude inversion:

• homogeneous model of the medium

• Green’s functions calculated using ray theory

• inversion of P-wave amplitudes (20-30 amplitudes)

• frequencies: 250-500 Hz

cannot take into account distortion of rays on focal sphere

misinterpretation of amplitudes: which one is the direct wave?

Waveform inversion:

• 3-D heterogeneous model of the medium

• Green’s functions calculated using FD code

• inversion of full waveforms (15-20 waveforms)

• frequencies (at the moment): 25-100 Hz

inversion is performed in frequency domain

in principle same inversion algorithm as for amplitude inversion, but run repeatedly for every frequency band (0.5 Hz steps)

Amplitude vs. waveform inversion

Introduction

Waveform modelling

Moment tensor

inversion

Summary

expl 1

expl 3

expl 5

expl 1

expl 1

expl 3

expl 3

expl 5

expl 5

Explosions (coords in m):1) x=8306E y=2312N z=-1238

→ 26 m shift3) x=8218E y=2192N z=-1352

→ 68 m shift5) x=8214E y=2168N z=-1356

→ 59 m shift

Selected explosions: relocation

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Examples: good fit

Examples: amplitude misfit

Examples: phase misfit

Inversion results

Introduction

Waveform modelling

Moment tensor

inversion

Summary

explosion 1 explosion 3 explosion 5

Length of time windows

ISO percentage

Data duration [10 ms]

GF

dura

tion

[10

ms]

explosion 1

ISO percentage

Data duration [10 ms]

GF

dura

tion

[10

ms]

ISO percentage

Data duration [10 ms]

GF

dura

tion

[10

ms]

explosion 3 explosion 5

nearly all solutions have high isotropic percentage

best solutions near diagonal (length of GF time window = length of data time window)

best solutions P –wave + S-wave onset

Explosion 1

DC = 7%CLVD = -14%ISO = 79%

Introduction

Waveform modelling

Moment tensor

inversion

Summary

Amplitude inversion Waveform inversion

structural model in mines is very complex

large and abrupt changes in velocity at cavities

model varies in time

Summary: seismicity in mines

Introduction

Waveform modelling

Moment tensor

inversion

Summary

earthquake source is complex (single forces, non-DC components)

small changes in source position lead to large changes in ray propagation, rays can be strongly curved

radiated wave field is complex (reflected, converted, scattered waves, head waves)

In general:

• complex Green’s functions can be calculated by 3-D FD codes (accurate model needed!);

• sensitive to time shifts due to mislocation or due to inaccurate velocity model

• frequency band of inverted waves can be easily controlled => stability analysis

In particular:

• good network configuration => focal sphere nicely covered

• inversion algorithm:

• optimal with same window length for Green’s functions and data

• optimal with simultaneous inversion of P- and S-wave, but excluding S-wave coda

• yields high isotropic percentage, higher than amplitude inversion, almost

independently of window length

promising, but computationally demanding (especially the computation of Green’s

functions with sufficiently small grid point distances)

will be performed for selected events, not whole database

Summary: waveform inversion

Introduction

Waveform modelling

Moment tensor

inversion

Summary

http://commons.wikimedia.org/wiki/File:Preikestolen_Norge.jpg

Thank you

for your

attention!