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Velocity-measurement bias of the ambient noise method due to source directivity: A case study for the Swedish National Seismic Network
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.ORCID iD: 0000-0002-4414-1046
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
(English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246XArticle in journal (Refereed) Epub ahead of print
Abstract [en]

The bias of velocity measurements from ambient-noise covariograms due to an anisotropic distribution of noise sources is studied assuming that the noise field consists of planar surface waves from large distance. First, general characteristics of the bias are described in terms of their dependence on wavelength, source-anomaly amplitude and width. Second, the expected bias of measurements in Sweden based on a noise-source model for the adjacent regions is analysed. The bias is conceptually explained and described in terms of two regimes, namely a high-frequency and a finite-frequency regime and their parameter domains quantified. Basic scaling laws are established for the bias. It is generally found to be small compared to lateral heterogeneity, except in the finite-frequency regime when inter-station distance is small compared to a wavelength and in regions of low levels of heterogeneity. The potential bias, i.e., its peak-to-peak variation, is generally higher for group-velocity than phase-velocity measurements. The strongly varying noise-source distribution as seen from Sweden results in predictions of relatively strong bias in the area at relevant frequencies and inter-station distances. Levels of heterogeneity in the Baltic shield are relatively low, rendering the potential bias significant. This highlights the need for detailed studies of source anisotropy before application of ambient-noise tomography, particularly in regions with weak velocity heterogeneity. Predicted bias only partially explains deviations of phase-velocity measurements from a regional average for individual station pairs. Restricting measurements to station pairs with inter-station distance exceeding five wavelengths limits the potential velocity bias in the area to within 1%. This rather dramatic restriction can be relaxed by directional analysis of the noise-source field and application of azimuthal restrictions to the selected station pairs for measurement.

National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-320163DOI: 10.1093/gji/ggx115OAI: oai:DiVA.org:uu-320163DiVA: diva2:1088838
Available from: 2017-04-16 Created: 2017-04-16 Last updated: 2017-04-17
In thesis
1. Analyses and Application of Ambient Seismic Noise in Sweden: Source, Interferometry, Tomography
Open this publication in new window or tab >>Analyses and Application of Ambient Seismic Noise in Sweden: Source, Interferometry, Tomography
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ambient seismic noise from generation to its application for determination of sub-surface velocity structures is analyzed using continuous data recordings from the Swedish National Seismic Network (SNSN). The fundamental aim of the thesis is to investigate the applicability of precise velocity measurements from ambient noise data. In the ambient noise method, a form of interferometry, the seismic signal is constructed from long-term cross correlation of a random noise field. Anisotropy of the source distribution causes apparent time shifts (velocity bias) in the interferometric signals. The velocity bias can be important for the study area (Sweden) which has relatively small velocity variations. This work explores the entire data path, from investigating the noise-source distribution to a tomographic study of southern Sweden.

A new method to invert for the azimuthal source distribution from cross-correlation envelopes is introduced. The method provides quantitative estimates of the azimuthal source distribution which can be used for detailed studies of source generation processes. An advantage of the method is that it uses few stations to constrain azimuthal source distributions. The results show that the source distribution is inhomogeneous, with sources concentrated along the western coast of Norway. This leads to an anisotropic noise field, especially for the secondary microseisms. The primary microseismic energy comes mainly from the northeast.

The deduced azimuthal source distributions are used to study the level of expected bias invelocity estimates within the SNSN. The results indicate that the phase-velocity bias is less than 1% for most station pairs but can be larger for small values of the ratio of inter-station distance over wavelength. In addition, the nature of velocity bias due to a heterogeneous source field is investigated in terms of high and finite-frequency regimes.

Graphical software for phase-velocity dispersion measurements based on new algorithms is presented and validated with synthetic data and by comparisons to other methods. The software is used for phase-velocity measurements, and deduced azimuthal source distributions are used for velocity-bias correction. Derived phase-velocity dispersion curves are used to construct two-dimensional velocity maps of southern Sweden at different periods based on travel-time tomography. The effect of the bias correction is investigated, and velocity maps are interpreted in comparison to previous geological and geophysical information.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 60 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1511
Keyword
Seismic interferometry, Ambient noise, Surface wave, Wave propagation, Inverse theory, Sweden
National Category
Geophysics
Research subject
Geophysics with specialization in Seismology
Identifiers
urn:nbn:se:uu:diva-320169 (URN)978-91-554-9906-8 (ISBN)
Public defence
2017-06-09, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2017-05-18 Created: 2017-04-17 Last updated: 2017-06-07

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Publisher's full texthttps://academic.oup.com/gji/article-abstract/doi/10.1093/gji/ggx115/3077204/Velocity-measurement-bias-of-the-ambient-noise?redirectedFrom=fulltext

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