@article{IskenVasyuraBathkeDahmetal.2022, author = {Isken, Marius Paul and Vasyura-Bathke, Hannes and Dahm, Torsten and Heimann, Sebastian}, title = {De-noising distributed acoustic sensing data using an adaptive frequency-wavenumber filter}, series = {Geophysical journal international}, volume = {231}, journal = {Geophysical journal international}, number = {2}, publisher = {Oxford University Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggac229}, pages = {944 -- 949}, year = {2022}, abstract = {Data recorded by distributed acoustic sensing (DAS) along an optical fibre sample the spatial and temporal properties of seismic wavefields at high spatial density. Often leading to massive amount of data when collected for seismic monitoring along many kilometre long cables. The spatially coherent signals from weak seismic arrivals within the data are often obscured by incoherent noise. We present a flexible and computationally efficient filtering technique, which makes use of the dense spatial and temporal sampling of the data and that can handle the large amount of data. The presented adaptive frequency-wavenumber filter suppresses the incoherent seismic noise while amplifying the coherent wavefield. We analyse the response of the filter in time and spectral domain, and we demonstrate its performance on a noisy data set that was recorded in a vertical borehole observatory showing active and passive seismic phase arrivals. Lastly, we present a performant open-source software implementation enabling real-time filtering of large DAS data sets.}, language = {en} } @article{VasyuraBathkeDettmerDuttaetal.2021, author = {Vasyura-Bathke, Hannes and Dettmer, Jan and Dutta, Rishabh and Mai, Paul Martin and J{\´o}nsson, Sigurj{\´o}n}, title = {Accounting for theory errors with empirical Bayesian noise models in nonlinear centroid moment tensor estimation}, series = {Geophysical journal international / the Royal Astronomical Society, the Deutsche Geophysikalische Gesellschaft and the European Geophysical Society}, volume = {225}, journal = {Geophysical journal international / the Royal Astronomical Society, the Deutsche Geophysikalische Gesellschaft and the European Geophysical Society}, number = {2}, publisher = {Oxford University Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggab034}, pages = {1412 -- 1431}, year = {2021}, abstract = {Centroid moment tensor (CMT) parameters can be estimated from seismic waveforms. Since these data indirectly observe the deformation process, CMTs are inferred as solutions to inverse problems which are generally underdetermined and require significant assumptions, including assumptions about data noise. Broadly speaking, we consider noise to include both theory and measurement errors, where theory errors are due to assumptions in the inverse problem and measurement errors are caused by the measurement process. While data errors are routinely included in parameter estimation for full CMTs, less attention has been paid to theory errors related to velocity-model uncertainties and how these affect the resulting moment-tensor (MT) uncertainties. Therefore, rigorous uncertainty quantification for CMTs may require theory-error estimation which becomes a problem of specifying noise models. Various noise models have been proposed, and these rely on several assumptions. All approaches quantify theory errors by estimating the covariance matrix of data residuals. However, this estimation can be based on explicit modelling, empirical estimation and/or ignore or include covariances. We quantitatively compare several approaches by presenting parameter and uncertainty estimates in nonlinear full CMT estimation for several simulated data sets and regional field data of the M-1 4.4, 2015 June 13 Fox Creek, Canada, event. While our main focus is at regional distances, the tested approaches are general and implemented for arbitrary source model choice. These include known or unknown centroid locations, full MTs, deviatoric MTs and double-couple MTs. We demonstrate that velocity-model uncertainties can profoundly affect parameter estimation and that their inclusion leads to more realistic parameter uncertainty quantification. However, not all approaches perform equally well. Including theory errors by estimating non-stationary (non-Toeplitz) error covariance matrices via iterative schemes during Monte Carlo sampling performs best and is computationally most efficient. In general, including velocity-model uncertainties is most important in cases where velocity structure is poorly known.}, language = {en} } @article{HaendelOhrnbergerKrueger2018, author = {Haendel, Annabel and Ohrnberger, Matthias and Kr{\"u}ger, Frank}, title = {Frequency-dependent quality factors from the deconvolution of ambient noise recordings in a borehole in West Bohemia/Vogtland}, series = {Geophysical journal international}, volume = {216}, journal = {Geophysical journal international}, number = {1}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0956-540X}, doi = {10.1093/gji/ggy422}, pages = {251 -- 260}, year = {2018}, abstract = {The correct estimation of site-specific attenuation is crucial for the assessment of seismic hazard. Downhole instruments provide in this context valuable information to constrain attenuation directly from data. In this study, we apply an interferometric approach to this problem by deconvolving seismic motions recorded at depth with those recorded at the surface. In doing so, incident and surface-reflected waves can be separated. We apply this technique not only to earthquake data but also to recordings of ambient vibrations. We compute the transfer function between incident and surface-reflected waves in order to infer frequency-dependent quality factors for S waves. The method is applied to a 87m deep borehole sensor and a colocated surface instrument situated at a hard-rock site in West Bohemia/Vogtland, Germany. We show that the described method provides comparable attenuation estimates using either earthquake data or ambient noise for frequencies between 5 and 15 Hz. Moreover, a single hour of noise recordings seems to be sufficient to yield stable deconvolution traces and quality factors, thus, offering a fast and easy way to derive attenuation estimates from borehole recordings even in low- to mid-seismicity regions.}, language = {en} } @article{StrolloParolaiBindietal.2012, author = {Strollo, Angelo and Parolai, Stefano and Bindi, Dino and Chiauzzi, Leonardo and Pagliuca, Rossella and Mucciarelli, Marco and Zschau, Jochen}, title = {Microzonation of Potenza (Southern Italy) in terms of spectral intensity ratio using joint analysis of earthquakes and ambient noise}, series = {Bulletin of earthquake engineering : official publication of the European Association for Earthquake Engineering}, volume = {10}, journal = {Bulletin of earthquake engineering : official publication of the European Association for Earthquake Engineering}, number = {2}, publisher = {Springer}, address = {Dordrecht}, issn = {1570-761X}, doi = {10.1007/s10518-011-9256-4}, pages = {493 -- 516}, year = {2012}, abstract = {A temporary seismic network composed of 11 stations was installed in the city of Potenza (Southern Italy) to record local and regional seismicity within the context of a national project funded by the Italian Department of Civil Protection (DPC). Some stations were moved after a certain time in order to increase the number of measurement points, leading to a total of 14 sites within the city by the end of the experiment. Recordings from 26 local earthquakes (M-l 2.2-3.8 ) were analyzed to compute the site responses at the 14 sites by applying both reference and non-reference site techniques. Furthermore, the Spectral Intensity (SI) for each local earthquake, as well as their ratios with respect to the values obtained at a reference site, were also calculated. In addition, a field survey of 233 single station noise measurements within the city was carried out to increase the information available at localities different from the 14 monitoring sites. By using the results of the correlation analysis between the horizontal-to-vertical spectral ratios computed from noise recordings (NHV) at the 14 selected sites and those derived by the single station noise measurements within the town as a proxy, the spectral intensity correction factors for site amplification obtained from earthquake analysis were extended to the entire city area. This procedure allowed us to provide a microzonation map of the urban area that can be directly used when calculating risk scenarios for civil defence purposes. The amplification factors estimated following this approach show values increasing along the main valley toward east where the detrital and alluvial complexes reach their maximum thickness.}, language = {en} }