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Lateral variations of scattering properties and implications for monitoring with coda wave sensitivity kernels

Abstract : Intrinsic absorption and scattering properties provide us with information about the physical state and heterogeneity of the Earth's crust. Moreover, they are important input parameters for spatial sensitivity kernels in coda wave monitoring applications.The scattering and intrinsic attenuation coefficients are usually obtained by observing the energy decay of earthquakes, leading to sparse spatial sampling and therefore average scattering values over a large region.In the first part of this thesis, we therefore use ambient noise recordings of a dense array to derive the scattering properties at small length scale.The study uses ambient noise cross-correlations to analyse the energy decay and scattering properties over a part of the North Anatolian Fault (NAF; Turkey) from ambient noise records in the frequency band 0.1 - 0.5 Hz.The target region has varying geological characteristics and is highly faulted around the northern strand of the NAF.In the noise correlation we measured the spatio-temporal energy decay of the coda waves.The local scattering and attenuation properties are obtained by global optimization of a 2-D solution of the radiative transfer equation for surface waves.We found that the mean free path and attenuation coefficient are considerably varying laterally with strong scattering observed in the region lying along the northern strand of NAF. Furthermore, a concentration of energy in the fault zone for a limited time was observed.We verified our observations with a phonon based Monte Carlo simulation, confirming the large contrast of heterogeneity between a narrow NAF and the surrounding crust.The findings of this first study are used in the second study of this thesis. The second part focuses on the implications of non-uniform scattering properties for monitoring with coda wave sensitivity kernels.For monitoring the temporal evolution of the sub-surface we need coda wave sensitivity kernels that linearly relate observed changes in recordings to physical medium changes.We computed travel-time, scattering and decorrelation kernels using a flexible Monte Carlo method, which is again based on the solution of the scalar version of the radiative transfer equation.In this work we showed that non-uniform scattering properties can have a profound but non-intuitive effect on coda wave sensitivity kernels.Moreover, a contrast in scattering may lead to additional energy transport channels that connect the source and receiver, depending on the mean free time versus lapse-time.Hence, it would be misleading to neglect the effect of the scattering distribution.Finally, we visually demonstrated the difference between travel-time and decorrelation kernels and consequently the importance of the angular dependency of the intensities.This study therefore emphasises that one needs to be careful with applying physical approximations to the wavefield, such as the diffusion approximation and and opens up new venues for imaging of spatio-temporal variations of crustal properties.
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Submitted on : Thursday, July 1, 2021 - 2:48:10 PM
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Chantal van Dinther. Lateral variations of scattering properties and implications for monitoring with coda wave sensitivity kernels. Earth Sciences. Université Grenoble Alpes [2020-..], 2021. English. ⟨NNT : 2021GRALU014⟩. ⟨tel-03275905⟩

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