Moment Tensor Inversion in Strongly Heterogeneous Media at Pyhasalmi Ore Mine, Finland Václav Vavryčuk (Academy of Sciences of the CR) Daniela Kühn (NORSAR)

Overview Introduction o Pyhäsalmi ore mine, Finland o P-wave polarity pattern Waveform modelling o 2-D modelling o 3-D modelling Homogeneous vs heterogeneous model o P-wave polarities o focal mechanisms Amplitude vs waveform inversion o selected data o comparison of results o waveform fit Summary Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

microseismic monitoring:  since January 2003  safety of the underground personnel  optimisation of mining process network:  12 1-C geophones 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 1D/3D models Moment tensor inversion Summary

- Down + Up Earthquake source mechanisms

Focal solution: shear fracture Moment tensor: + volume change Full moment tensor mathematical description: pure double-couple mathematical description: nine force couples isotropic deviatoric + best double couple CLVD +

Complex polarity pattern of P-wave first onset Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Waveform modelling

620 m E3D: viscoelastic 3-D FD code (Larsen and Grieger, 1998) strong interaction with mining cavities: reflection, scattering, conversion Waveform modelling: 2D Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Waveform modelling: 3D Introduction Waveform modelling 1D/3D models 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 modelling synthetic seismograms Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Homogeneous & heterogeneous models

Geophone network (artificial). source location source mechanisms Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Comparison 1-D/3-D Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Observed amplitudes Retrieved source mechanism Synthetic source mechanism Moment tensor inversion for a homogeneous model ISO = 23 % DC = 37 % CLVD = 40 % ISO = 0 % DC = 100 % CLVD = 0 % Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Moment tensor inversion: amplitudes versus waveforms

Representation theorem: Moment tensor inversion: point source Amplitude inversion Space and time factorization matrix notation generalized linear inversion Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

first maximum amplitude amplitude of the direct wave Amplitude picking I direct wave scattered wave Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

first maximum amplitude is not always the amplitude of the direct wave Amplitude picking II ? direct wave waveform complexity (head wave?) scattered wave Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Representation theorem: point source frequency domain Waveform inversion in principle, the same inversion algorithm as for the amplitude inversion, but run repeatedly for many frequencies time domain Moment tensor inversion: matrix notation generalized linear inversion Time factorization: time-independent moment tensor Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Amplitude inversion: homogeneous model of the medium Green’s functions calculated using ray theory inversion of P-wave amplitudes (20-30 amplitudes) frequencies: Hz Waveform inversion: 3-D heterogeneous model of the medium Green’s functions calculated using the FD code inversion of full waveforms (15-20 waveforms) frequencies: Hz Amplitude vs. waveform inversion Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Events near cavity: no. 5 no. 17 Events near ore body /host rock transition: no. 12 no. 26 Selected events Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

ch 22 ch 21 ch 15 ch 14 ch 8 ch 7 ch 3 ch 2 ch 30 Complex waveforms, strong reflections, difficulty to identify the S wave (in some cases) Event 12: data Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

amplitude inversionP-wave inversionfull wave inversionevent no OREBODYOREBODY CAVITYCAVITY Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

amplitude inversionP-wave inversionevent no. OREBODYOREBODY CAVITYCAVITY full wave inversion Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

amplitude inversionP-wave inversionevent no. OREBODYOREBODY CAVITYCAVITY full wave inversion Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

first rupturesecond rupture strike = 105º dip = 91º rake = -75º DC = 38% CLVD = -14% ISO = -48% strike =90º dip = 87º rake = -121º DC = 24% CLVD = -42% ISO = -34% Event 17: two ruptures P-wave inversion: Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Event 17: waveform fit GOOD FIT! ch 7 ch 12 ch 20 ch 21 ch 22 ch 30 ch 11 ch 29 ch 28 Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Event 17: waveform fit Amplitude misfit ch 5 ch 4 ch 3 ch 10 ch 8 ch 9 ch 16 ch 15 ch 24 ch 14 Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Event 17: waveform fit Phase misfit ch 2 ch 13 ch 23 Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

structural model in mines usually is very complex large and abrupt changes in velocity at cavities the model varies in time Summary I earthquake source is complex (single forces, non-DC components, complex source history) Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

inversion in a homogeneous model may lead to: incorrect mechanism spurious non-DC Summary II Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary radiated wave field is complex (reflected, converted, scattered waves, head waves)

Amplitude inversion: simple approach limited applicability (simple Green’s functions are not adequate) no control on frequency bands amplitudes can be wrongly interpreted Full waveform inversion: 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 model frequency band of inverted waves can be easily controlled inversion from P-wave only seems to be more reliable than from the whole seismogram (due to multiple scattering) promising but computationally demanding and laborious Summary III Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary

Thank you!

Complexity of velocity model Introduction Waveform modelling 1D/3D models Moment tensor inversion Summary