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Detecting temporal velocity changes using various methods
Haijiang Zhang
University of Science and Technology of China
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Outline
• Seismic tomography between different time periods
• Repeating earthquakes
• Coda wave interferometry
• Ambient noise
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Different ray distribution on velocity models
• Same theoretical models but with different ray distribution.
• Separately Inverted models are different
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Different ray distribution on velocity models
• Same theoretical models but with different ray distribution.
• Jointly Inverted models are different
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Regularization in time
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Scattering:
Scattering occurs when there are velocity heterogeneities in the medium with wavelengths on the order of λ of the wave.
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Snieder et al., Science, 2002
A simulation of 100 randomly-perturbed scatterers…
Moving the scatterers (here, 1/40th of the distance shown,so the perturbation is visible) mostly changes the shapesand amplitudes of the waveforms…
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Snieder et al., Science, 2002
Waveformsmeasured in agranite sampleat temperaturesof 45°C (blue)and 50°C (red).
Changing the matrix velocity, by contrast, introduces a shift(delay or advance) that increases with coda time.
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Near- and far-field effects of stress changes on cracks
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3-m separation between two wells
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Near- and far-field effects of stress changes on cracks
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Repeating events-with (almost) the same waveforms
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Seismic velocity imaging and monitoring of volcanic areas using
ambient seismic noise
F. Brenguier (2008)
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Noise sources
Shapiro et al. (2005)
sensor
Virtual seismic source reconstruction
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A case study: Piton de la Fournaise volcano, La Réunion island
S 21°
E 55°30
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Synthetic velocity decrease
Temporal changes of seismic velocities using cross-correlations of seismic noise
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Velocity changes on Piton de la Fournaise
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Brenguier et al., Nature Geoscience, 2008
Velocity drops preceding eruptions
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Seismic velocity changes as a proxy for magma forcing
O'Connell, R. J., and B. Budiansky (1974), Seismic Velocities in Dry and Saturated Cracked Solids, J. Geophys. Res., 79(35), 5412–5426
Seismic velocities are strongly sensitive to crack density.
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These velocity changes are linked to dilatation induced by stress changes
Magmatic intrusive complex
Link between temporal changes and structure