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A Two-Step Time-Frequency Moment Tensor Inversion: Application to Mining Data Václav Vavryčuk 1, Daniela Kühn 2 1 Institute of Geophysics, Prague 2 NORSAR, Kjeller

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Motivation To be able to invert for focal mechanisms and moment tensors: accurate robust and stable Difficulties: complex mining environment complex source-time function non-double-couple moment tensors Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Moment tensor inversions wave amplitudes (Vavryčuk et al. 2008; Fojtíková et al. 2010; Godano et al. 2011) amplitude ratios (Miller et al. 1998; Hardebeck & Shearer 2003; Jechumtálová & Šílený 2005) full waveforms (Šílený et al. 1992 Cesca et al. 2006; Cesca & Dahm 2008; Sokos & Zahradník 2009) applicable to simple media insensitive to amplifications non-linear applicable to simple media linear fast applicable to complex media linear more time consuming Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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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., installation of seismometer network by the ISS Int. Ltd. Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Strongly heterogeneous velocity model ore body: v p = 6.3 km/s host rock: v p = 6.0 km/s excavation area: v p = 0.3 km/s U D WE Velocity model Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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E3D: viscoelastic 3-D FD code (Larsen and Grieger, 1998) strong interaction with mining cavities: reflection, scattering, conversion Waveform modelling: 2D 620 m Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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- complex waveforms - long, strong coda - complex secondary arrivals - difficult to interpret P-wave polarities - difficult to identify S-wave arrivals observed seismograms Waveform modelling synthetic seismograms Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Moment tensor inversions wave amplitudes (Vavryčuk et al. 2008; Fojtíková et al. 2010; Godano et al. 2011) amplitude ratios (Miller et al. 1998; Hardebeck & Shearer 2003; Jechumtálová & Šílený 2005) full waveforms (Šílený et al. 1992 Cesca et al. 2006; Cesca & Dahm 2008; Sokos & Zahradník 2009) applicable to simple media insensitive to sensor amplifications non-linear applicable to simple media linear fast applicable to complex media linear more time consuming Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Full waveform MT inversions time-domain inversion frequency-domain inversion amplitude spectra (Cesca et al., 2006; Cesca & Dahm, 2008) complex spectra (Vavryčuk, 2011a,b) polarity of waves is neglected insensitive to time shifts non-linear complex source-time function polarity of waves is considered insensitive to time shifts linear simple source-time function polarity of waves is considered sensitive to time shifts non-linear complex source-time function simplified approach (Sokos & Zahradník 2009) Adamová et al. 2009) Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Goal of the study To develop a moment tensor inversion: combination of time and frequency approaches keeps advantages of all approaches (accurate, robust and stable) Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Moment tensor inversion: time-frequency approach Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Moment tensor inversion scheme Frequency-domain MTI using complex spectra Moment tensor Time-domain MTI Final moment tensor Source-time function + 1. step: 2. step: Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Full waveform MT inversions time-domain inversion polarity of waves is considered insensitive to time shifts linear simple source-time function polarity of waves is considered insensitive to time shifts linear complex source-time function time-frequency inversion Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Tests using synthetic data Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Synthetic tests source mechanism: DC and explosion source time function: noise: in amplitudes and in time shifts amplitude noise; 0-100% in 5% steps time shift noise: 0-0.01 s in steps of 0.005 s repeating inversions: 100 inversions two distinct maxima Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Double-couple source: ISO % Mean valueStandard deviation time-domain Inversion ISO = 3% frequency-domain Inversion ISO = 0% time-frequency Inversion ISO = 0% Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Explosive source: ISO % Mean valueStandard deviation time-domain Inversion ISO = 95% frequency-domain Inversion ISO = 100% time-frequency Inversion ISO = 100% Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Mining blast: ISO % Mean valueStandard deviation time-domain Inversion ISO = 66% frequency-domain Inversion ISO= 71% time-frequency Inversion ISO = 68% Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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P T P T P T time-domain inversion frequency-domain inversion time-frequency inversion Mining blast: DC, waveforms Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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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) radiated wave field is complex (reflected, converted, scattered waves, head waves) Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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Summary II the most promising approach: full waveform MTI simplified time-domain MTI is robust and stable two-step time-frequency MTI improves the performance by considering more complex source-time function inversion of blasts reveals some stable DC part Motivation Waveform modelling MTI strategy Summary Synthetic tests Application to real data

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LECTURE 6: SEISMIC MOMENT TENSORS

LECTURE 6: SEISMIC MOMENT TENSORS

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