Daniel Bowden

Last Name

Bowden

First Name

Daniel

ORCID

0000-0003-3332-5146

Organisational unit

03971 - Fichtner, Andreas / Fichtner, Andreas

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Publications 1 - 10 of 30

Earthquake Ground Motion Amplification for Surface Waves
Bowden, Daniel; Tsai, Victor C. (2017)
Geophysical Research Letters
Surface waves from earthquakes are known to cause strong damage, especially for larger structures such as skyscrapers and bridges. However, common practice in characterizing seismic hazard at a specific site considers the effect of near‐surface geology on only vertically propagating body waves. Here we show that surface waves have a unique and different frequency‐dependent response to known geologic structure and that this amplification can be analytically calculated in a manner similar to current hazard practices. Applying this framework to amplification in the Los Angeles Basin, we find that peak ground accelerations for certain large regional earthquakes are underpredicted if surface waves are not properly accounted for and that the frequency of strongest ground motion amplification can be significantly different. Including surface‐wave amplification in hazards calculations is therefore essential for accurate predictions of strong ground motion for future San Andreas Fault ruptures.
Imaging noise sources: comparing and combining a matched-field processing technique with finite-frequency noise source inversions
Igel, Jonas K.H.; Bowden, Daniel; Sager, Korbinian; et al. (2021)
EGUsphere
Imaging the spatio-temporal variations of ambient seismic noise sources can provide important information to improve near real-time monitoring and noise tomography. Various methods have been developed to tackle this problem. For example, Matched-Field Processing (MFP) offers an efficient data-driven approach by testing different noise source locations and subsequently correlating and stacking. A more rigorous approach is treating it as a finite-frequency full-waveform inversion problem. In contrast to the MFP technique, an inversion framework allows for the incorporation of prior information and subsequent iterative updates of the noise source distribution by numerically modelling correlations and source sensitivity kernels. Bowden et al. (2020) discuss the similarities between these two methods and how one can be derived from the other. We aim to compare and contrast the two methods using real data from a regional to a global scale to locate the secondary microseismic sources in the ocean. Igel et al. (2021, in prep) use a logarithmic energy ratio as measurement for the sensitivity kernels, which is chosen due to its robustness with respect to unknown 3D Earth structures. However, some disadvantages of this type of measurement are not considering absolute amplitudes and discarding information outside of the expected surface wave arrival time window. By combining the two methods and first using MFP to create an initial model for the inversion, we are able to steer the inversion in the right direction, allowing us to use a more elaborate full-waveform measurement in the inversion and hence increasing the resolution and quality of the final model. Results for noise source inversions in the ocean on a daily basis using the combination of the two methods will be presented. This work paves the way for publicly available, daily, multi-scale ambient noise source maps.
Direct Observations of Surface‐Wave Eigenfunctions at the Homestake 3D Array
Meyers, Patrick; Bowden, Daniel; Prestegard, Tanner; et al. (2019)
Bulletin of the Seismological Society of America
Optimal processing for seismic noise correlations
Fichtner, Andreas; Bowden, Daniel; Ermert, Laura (2020)
Geophysical Journal International
A wide spectrum of processing schemes is commonly applied during the calculation of seismic noise correlations. This is intended to suppress large-amplitude transient and monochromatic signals, to accelerate convergence of the correlation process or to modify raw correlations into more plausible approximations of interstation Green’s functions. Many processing schemes, such as one-bit normalization or various other nonlinear normalizations, clearly break the linear physics of seismic wave propagation. This naturally raises the question: To what extent are the resulting noise correlations physically meaningful quantities? In this contribution, we demonstrate that commonly applied processing methods may indeed introduce an unphysical component into noise correlations. This affects not only noise correlation amplitudes but also, to a lesser extent, time-dependent phase information. The profound consequences are that most processed correlations cannot be entirely explained by any combination of Earth structure and noise sources, and that inversion results may thus be polluted. The positive component of our analysis is a new and easily applicable method that allows us to modify any existing processing such that it becomes optimal in the sense of (1) completely avoiding the unphysical component while (2) approximating the result of the original processing as closely as possible. The resulting optimal schemes can be derived purely on the basis of observed noise, without any knowledge of or assumptions on the nature of noise sources. In addition to the theoretical analysis, we present illustrative real-data examples from the Irish National Seismic Network and the Lost Hills array in Central California. We anticipate that optimal processing schemes may be most useful in applications that exploit complete correlation waveforms, amplitudes and weak arrivals, or small (time-dependent) phase shifts.
Offshore Southern California lithospheric velocity structure from noise cross-correlation functions
Bowden, Daniel; Kohler, Monica D.; Tsai, Victor C.; et al. (2016)
Journal of Geophysical Research: Solid Earth
Connecting beamforming and kernel-based noise source inversion
Bowden, Daniel; Sager, Korbinian; Fichtner, Andreas; et al. (2021)
Geophysical Journal International
Beamforming and backprojection methods offer a data-driven approach to image noise sources, but provide no opportunity to account for prior information or iterate through an inversion framework. In contrast, recent methods have been developed to locate ambient noise sources based on cross-correlations between stations and the construction of finite-frequency kernels, allowing for inversions over multiple iterations. These kernel-based approaches show great promise, both in mathematical rigour and in results, but are less physically intuitive and interpretable. Here we show that these apparently two different classes of methods, beamforming and kernel-based inversion, are achieving exactly the same result in certain circumstances. This paper begins with a description of a relatively simple beamforming or backprojection algorithm, and walks through a series of modifications or enhancements. By including a rigorously defined physical model for the distribution of noise sources and therefore synthetic correlation functions, we come to a framework resembling the kernel-based iterative approaches. Given the equivalence of these approaches, both communities can benefit from bridging the gap. For example, inversion frameworks can benefit from the numerous image enhancement tools developed by the beamforming community. Additionally, full-waveform inversion schemes that require a window selection for the comparisons of misfits can more effectively target particular sources through a windowing in a beamform slowness domain, or might directly use beamform heatmaps for the calculation of misfits. We discuss a number of such possibilities for the enhancement of both classes of methods, testing with synthetic models where possible.
Sensitivity kernels for transmission fibre optics
Fichtner, Andreas; Bogris, Adonis; Bowden, Daniel; et al. (2022)
Geophysical Journal International
Fibre-optic sensing based on transmission offer an alternative to scattering-based distributed acoustic sensing (DAS). The ability to interrogate fibres that are thousands of kilometres long opens opportunities for studies of remote regions, including ocean basins. However, by averaging deformation along the fibre, transmission systems produce integrated instead of distributed measurements. They defy traditional interpretations in terms of simple seismic phases, thereby inherently requiring a full-waveform approach. For this, we develop a formalism to calculate sensitivity kernels of transmitted optical phase changes with respect to (Earth) structure using optical phase delay measurements. We demonstrate that transmission-based sensing can effectively provide distributed measurements when optical phase delays are analysed in different time windows. The extent to which a potentially useful sensitivity coverage can be achieved depends on the fibre geometry, and specifically on its local curvature. This work establishes a theoretical foundation for tomographic inversions and experimental design using transmission-based optical sensing.
Urban Shallow Subsurface Imaging and Hazard Assessment with Distributed Acoustic Sensing in Athens, Greece
Tian, Lu; Bowden, Daniel; Smolinski, Krystyna T.; et al. (2022)
AGU Fall Meeting Abstracts
In urban environments, existing telecommunication fiber-optic cables can enable geophysicists to characterize the shallow subsurface and conduct monitoring using Distributed Acoustic Sensing technology. Athens is a metropolitan city with a population of 3.5 million potentially exposed to severe seismic hazards. Here, we use data from a 23-km long “dark” fiber provided by the Hellenic Telecommunications Organization (OTE) in the northern suburbs of Athens. The data was acquired over 36 days in autumn 2021. We explore an ambient noise interferometry processing workflow for DAS, starting from a 1-km segment of the fiber and extending to other parts of the dataset. From the seismic noise correlations, we extract dominant Rayleigh wave dispersion measurements. We apply a trans-dimensional MCMc inversion method from surface wave dispersion to produce vertical 1D shear velocity models. We observe a low-velocity layer in the shallow (20 to 40 m) subsurface and show its lateral variations through the mapped 2D velocity models with high resolution (meter-scale). The velocity model is well-constrained in the top 100 meters, and it allows us to estimate site amplification effects that improve our understanding of urban hazards in Athens.
Extension of the Basin Rayleigh‐Wave Amplification Theory to Include Basin‐Edge Effects
Brissaud, Quentin; Bowden, Daniel; Tsai, Victor C. (2020)
Bulletin of the Seismological Society of America
The presence of sediments near the Earth’s surface can significantly amplify the strength of shaking during earthquakes. Such basin or site amplification effects have been well documented in numerous regions, yet the complex and often situational dependence of competing reasons for this amplification makes it hard to quantify in a general sense or to determine the most significant contributions. Simple 1D seismic profiles can be used to estimate the amplitude differences between a basin site and a hard‐rock reference site, but this ignores any reflections or conversions at the basin edge or a resonance effect depending on the basin’s geometry. In this article, we explore an analytic model based on coupling coefficients for surface Rayleigh waves to account for the lateral discontinuities at a basin’s edge (Datta 2018). We use this simple tool to explore the relationship between the basin’s Rayleigh‐wave amplification spectrum and various parameters such as basin depth, edge slope angle, and impedance contrast. The step‐by‐step construction of the model allows us to quantify the contributions from various wave propagation effects with the goal of identifying situations under which various basin‐edge effects must be considered in addition to purely 1D estimates. For the most velocity contrasts (less than a factor of 5), the error made by the 1D theory in predicting maximum Rayleigh‐wave basin amplification is under 35% for both the horizontal and the vertical components. For simple basins, the vertical amplification dominates at larger high frequencies and the horizontal at lower frequencies. Finally, we demonstrate from comparisons with spectral‐element wavefield simulations that realistic velocity structures can be reduced to a simpler “box” shape for the semi‐analytic formulation used here with reasonable results. For the purposes of estimating site‐amplification or microzonation, an improved model that accounts for basin‐edge effects can be implemented without high‐computational cost. © 2020 Seismological Society of America.
An illustrated guide to: Distributed and integrated fibre-optic sensing in seismology
Fichtner, Andreas; Walter, Fabian Thomas; Paitz, Patrick; et al. (2025)
Earthquake Science
The properties of laser signals are affected by deformation of the optical fibre through which they are transmitted. While this deformation dependence is undesirable in telecommunication, it can be exploited for the construction of novel seismic sensors that fill a niche in data acquisition where traditional seismometer arrays would be difficult to deploy. This includes densely populated urban centers, the oceans, volcanoes and the Earth’s polar regions. These notes complement a presentation on recent methodological developments and applications in fibre-optic seismology. The first part is focused on the use of distributed fibre-optic sensing in cryosphere research, and specifically the investigation of the internal structure and seismicity of glaciers and ice sheets. The second part is dedicated to recent advances in integrated fibre-optic sensing, with emphasis on novel measurement principles and sensitivity.

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Daniel Bowden

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