Physics‐Informed Neural Networks for Elliptical‐Anisotropy Eikonal Tomography: Application to Data From the Northeastern Tibetan Plateau

We develop a novel approach for multi-frequency, elliptical-anisotropic eikonal tomography based on physics-informed neural networks (pinnEAET). This approach simultaneously estimates the medium properties controlling anisotropic Rayleigh waves and reconstructs the traveltimes. The physics constraints built into pinnEAET's neural network enable high-resolution results with limited inputs by inferring physically plausible models between data points. Even with a single source, pinnEAET can achieve stable convergence on key features where traditional methods lack resolution. We apply pinnEAET to ambient noise data from a dense seismic array (ChinArray-Himalaya II) in the northeastern Tibetan Plateau with only 20 quasi-randomly distributed stations as sources. Anisotropic phase velocity maps for Rayleigh waves in the period range from 10–40 s are obtained by training on observed traveltimes. Despite using only about 3% of the total stations as sources, our results show low uncertainties, good resolution and are consistent with results from conventional tomography

Simulation of Interferometric Seismoelectric Green’s Function Recovery: For the SH-TE propagation mode

A recent novel technique known as seismic interferometry makes use of seismic ‘noise’ to reconstruct a Green’s function between two receivers by crosscorrelation. This technique can be applied for example in permanent subsurface monitoring using passive seismics or the creation of virtual sources on positions where only recordings were made. The aim of this thesis is to derive and understand equations for seismoelectric interferometry. The first part of this thesis focusses on a the calculation of SH-TE seismoelectrical responses in a 2D horizontally stratified earth. We decompose the two-way wave equation for SH-TE waves into upgoing and downgoing waves which we relate through a reflectivity formulation. The reflectivity formulation is based upon reflection matrices only, even though we can simulate both reflection and transmission experiments. We solve the SH-TE seismoelectric system in a 1D homogeneous world. In the second part of this thesis we derive interferometric Green’s fucntion representations from reciprocity theorems that relate two different states in one domain. Interferometric Green’s function representations express the Green’s function between two receivers as a function of crosscorrelations of responses of sources throughout a domain and on it’s boundary. We cast the seismoelectric system in a general diffusion, flow and wave equation and define a Green’s matrix for all different field and source types. Using this formulation we derive a source-receiver reciprocity relation for the Green’s matrix from the convolution type reciprocity theorem. The correlation type reciprocity theorem for the Green’s matrix is modified using source-receiver reciprocity to obtain the interferometric Green’s function representation. We study the SH-TE seismoelectrical interferometric representation 1D and 2D in homogeneous media. A seismoelectric interferometric representation was written to recover the causal response of the particle velocity at position B due to a electrical current source at position A, as a function of cross correlations of electric field recordings at A and particle velocity recordings at B. Provided there exists a dense coverage of sources in the domain and on it’s boundary, the representation was validated in both 1D and 2D. Approximations to the interferometric representation are investigated by studying the contributions of parts of the domain and boundary integrals. It was found that a dominant spurious event resides in the separate contributions of the domain and boundary integrals, that destructively interferes when both contributions are combined. The role of different source types in the interferometric representation was studied. In a homogeneous medium the measured events have propagated either as an electromagnetic wave or as a shear wave. The dominant contribution to the reconstruction of an electromagnetic event is by electromagnetic sources. Similarly, can the reconstructed shear wave event be attributed mainly due to seismic sources. In a medium with low electromagnetic and shear wave losses we could ignore the domain integral, this will result in amplitude errors and we will suffer from spurious events.Applied Earth Sciences/Applied GeophysicsGeoscience & EngineeringCivil Engineering and Geoscience

Electrokinetic fields and waves: Theory, experiments and numerical modeling

Geoscience & EngineeringCivil Engineering and Geoscience

Experimental validation of the electrokinetic theory and development of seismoelectric interferometry by cross-correlation

We experimentally validate a relatively recent electrokinetic formulation of the streaming potential (SP) coefficient as developed by Pride (1994). The start of our investigation focuses on the streaming potential coefficient, which gives rise to the coupling of mechanical and electromagnetic fields. It is found that the theoretical amplitude values of this dynamic SP coefficient are in good agreement with the normalized experimental results over a wide frequency range, assuming no frequency dependence of the bulk conductivity. By adopting the full set of electrokinetic equations, a full-waveform wave propagation model is formulated. We compare the model predictions, neglecting the interface response andmodeling only the coseismic fields, with laboratory measurements of a seismic wave of frequency 500 kHz that generates electromagnetic signals. Agreement is observed between measurement and electrokinetic theory regarding the coseismic electric field. The governing equations are subsequently adopted to study the applicability of seismoelectric interferometry. It is shown that seismic sources at a single boundary location are sufficient to retrieve the 1D seismoelectric responses, both for the coseismic and interface components, in a layered model.Geoscience & EngineeringCivil Engineering and Geoscience

Single- and Double-Sided Marchenko Imaging Conditions in Acoustic Media

Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ImPhys/Acoustical Wavefield ImagingApplied Geophysics and Petrophysic