On the link between Beamforming and Kernel-based Source Inversion

2020 ◽  
Author(s):  
Daniel Bowden ◽  
Korbinian Sager ◽  
Andreas Fichtner ◽  
Małgorzata Chmiel
Keyword(s):  
Prior Information ◽  
Time Window ◽  
Great Promise ◽  
Source Inversion ◽  
Finite Frequency ◽  
Noise Sources ◽  
Data Driven Approach ◽  
Gradient Based ◽  
Cross Correlations ◽  
A Current

<p>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 (i.e., Tromp et al., 2010, Ermert et al. 2017, Sager et al. 2018). These kernel-based approaches show great promise, both in mathematical rigour and in results, but may remain difficult to understand or implement for the wider community. Here we show that these two different classes of methods, beamforming and kernel-based inversion, are achieving exactly the same result in certain circumstances. This means existing beamforming and backprojection methods can also incorporate prior information in a mathematically correct manner.</p><p>We start with a description of a relatively simple beamforming or backprojection algorithm, based on time-domain shifting and measurement of waveform coherence. Only by changing the order of steps, we begin to resemble the kernel-based approaches. By adding a physical model for the distribution of noise sources, and therefore synthetic correlation functions, we can extend backprojection to an iterative, gradient-based inversion scheme. Adjoint methods and a direct simulation of correlation wavefields can later be used to increase computational efficiency, but we stress that these are not needed to understand the approach.</p><p>Given the equivalence of these approaches between these two communities, both sides can benefit from bridging the gap. For example, for kernel-based inversion schemes, a current challenge lies in defining the misfit and time window over which a correlation will be scored; a windowing function based on beamform images offers a more intuitive way to identify significant contributions in the noise wavefield, exploiting more than just the direct surface-wave arrivals.</p>

scholarly journals Connecting beamforming and kernel-based noise source inversion

2020 ◽  
Vol 224 (3) ◽  
pp. 1607-1620
Author(s):  
Daniel C Bowden ◽  
Korbinian Sager ◽  
Andreas Fichtner ◽  
Małgorzata Chmiel
Keyword(s):  
Waveform Inversion ◽  
Great Promise ◽  
Source Inversion ◽  
Noise Sources ◽  
Full Waveform ◽  
Data Driven Approach ◽  
Cross Correlations ◽  
Selection For ◽  
Window Selection ◽  
Synthetic Models

SUMMARY 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.

Rapid Global Finite-Frequency Ambient Noise Source Inversion

2020 ◽  
Author(s):  
Jonas Igel ◽  
Laura Ermert ◽  
Andreas Fichtner
Keyword(s):  
Ambient Noise ◽  
Noise Source ◽  
Computational Cost ◽  
User Interaction ◽  
Real Data ◽  
Source Distribution ◽  
Source Inversion ◽  
Finite Frequency ◽  
Noise Sources ◽  
Cross Correlations

<p>Common assumptions in ambient noise seismology such as Green’s function retrieval and equipartitioned wavefields are often not met in the Earth. Full waveform ambient noise tomography methods are free of such assumptions, as they implement knowledge of the time- and space-dependent ambient noise source distribution, whilst also taking finite-frequency effects into account. Such methods would greatly simplify near real-time monitoring of the sub-surface. Additionally, the distribution of the secondary microseisms could act as a new observable of the ocean state since its mechanism is well understood (e.g. Ardhuin et al., 2011).</p><p>To efficiently forward-model global noise cross-correlations we implement (1) pre-computed high-frequency wavefields obtained using, for example, AxiSEM (Nissen-Meyer et al., 2014), and (2) spatially variable grids, both of which greatly reduce the computational cost. Global cross-correlations for any source distribution can be computed within a few seconds in the microseismic frequency range (up to 0.2 Hz). Similarly, we can compute the finite-frequency sensitivity kernels which are then used to perform a gradient-based iterative inversion of the power-spectral density of the noise source distribution. We take a windowed logarithmic energy ratio of the causal and acausal branches of the cross-correlations as measurement, which is largely insensitive to unknown 3D Earth structures.</p><p>Due to its parallelisation on a cluster, our inversion tool is able to rapidly invert for the global microseismic noise source distribution with minimal required user interaction. Synthetic and real data inversions show promising results for noise sources in the North Atlantic with the structure and spatial distribution resolved at scales of a few hundred kilometres. Finally, daily noise sources maps could be computed by combining our inversion tool with a daily data download and processing toolkit.</p>

Heterogeneous cloud-supercomputing framework for daily seismic noise source inversion

2020 ◽  
Author(s):  
Alexey Gokhberg ◽  
Laura Ermert ◽  
Jonas Igel ◽  
Andreas Fichtner
Keyword(s):  
Real Time ◽  
Seismic Noise ◽  
Cross Correlation ◽  
Noise Source ◽  
Workflow Management ◽  
Source Distribution ◽  
Ambient Seismic Noise ◽  
Source Inversion ◽  
Finite Frequency ◽  
Noise Sources

<p>The study of ambient seismic noise sources and their time- and space-dependent distribution is becoming a crucial component of the real-time monitoring of various geosystems, including active fault zones and volcanoes, as well as geothermal and hydrocarbon reservoirs. In this context, we have previously implemented a combined cloud - HPC infrastructure for production of ambient source maps with high temporal resolution. It covers the entire European continent and the North Atlantic, and is based on seismic data provided by the ORFEUS infrastructure. The solution is based on the Application-as-a-Service concept and includes (1) acquisition of data from distributed ORFEUS data archives, (2) noise source mapping, (3) workflow management, and (4) front-end Web interface to end users.</p><p>We present the new results of this ongoing project conducted with support of the Swiss National Supercomputing Centre (CSCS). Our recent goal has been transitioning from mapping the seismic noise sources towards modeling them based on our new method for near real-time finite-frequency ambient seismic noise source inversion. To invert for the power spectral density of the noise source distribution of the secondary microseisms we efficiently forward model global cross-correlation wavefields for any noise distribution. Subsequently, a gradient-based iterative inversion method employing finite-frequency sensitivity kernels is implemented to reduce the misfit between synthetic and observed cross correlations.</p><p>During this research we encountered substantial challenges related to the large data volumes and high computational complexity of involved algorithms. We handle these problems by using the CSCS massively parallel heterogeneous supercomputer "Piz Daint". We also apply various specialized numeric techniques which include: (1) using precomputed Green's functions databases generated offline with Axisem and efficiently extracted with Instaseis package and (2) our previously developed high performance package for massive cross correlation of seismograms using GPU accelerators. Furthermore, due to the inherent restrictions of supercomputers, some crucial components of the processing pipeline including the data acquisition and workflow management are deployed on the OpenStack cloud environment. The resulting solution combines the specific advantages of the supercomputer and cloud platforms thus providing a viable distributed platform for the large-scale modeling of seismic noise sources.</p>

Imaging noise sources: comparing and combining a matched-field processing technique with finite-frequency noise source inversions

2021 ◽  
Author(s):  
Jonas Igel ◽  
Daniel Bowden ◽  
Korbinian Sager ◽  
Andreas Fichtner
Keyword(s):  
Noise Source ◽  
Time Window ◽  
Waveform Inversion ◽  
Temporal Variations ◽  
Source Distribution ◽  
Frequency Noise ◽  
Finite Frequency ◽  
Noise Sources ◽  
Matched Field Processing ◽  
Full Waveform

<p>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. </p><p>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. </p><p>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.</p>

scholarly journals Molecular Imprinting Strategies for Tissue Engineering Applications: A Review

Polymers ◽  
2021 ◽  
Vol 13 (4) ◽  
pp. 548
Author(s):  
Amedeo Franco Bonatti ◽  
Carmelo De Maria ◽  
Giovanni Vozzi
Keyword(s):  
Tissue Engineering ◽  
Molecularly Imprinted Polymers ◽  
Cell Interaction ◽  
Great Promise ◽  
Molecularly Imprinted ◽  
Proliferation And Differentiation ◽  
Promising Solution ◽  
Natural Tissue ◽  
A Current ◽  
Biophysical Cues

Tissue Engineering (TE) represents a promising solution to fabricate engineered constructs able to restore tissue damage after implantation. In the classic TE approach, biomaterials are used alongside growth factors to create a scaffolding structure that supports cells during the construct maturation. A current challenge in TE is the creation of engineered constructs able to mimic the complex microenvironment found in the natural tissue, so as to promote and guide cell migration, proliferation, and differentiation. In this context, the introduction inside the scaffold of molecularly imprinted polymers (MIPs)—synthetic receptors able to reversibly bind to biomolecules—holds great promise to enhance the scaffold-cell interaction. In this review, we analyze the main strategies that have been used for MIP design and fabrication with a particular focus on biomedical research. Furthermore, to highlight the potential of MIPs for scaffold-based TE, we present recent examples on how MIPs have been used in TE to introduce biophysical cues as well as for drug delivery and sequestering.

Cross-borehole tomography with correlation delay times

Geophysics ◽  
2014 ◽  
Vol 79 (1) ◽  
pp. R1-R12 ◽  
Author(s):  
E. Diego Mercerat ◽  
Guust Nolet ◽  
Christophe Zaroli
Keyword(s):  
Time Window ◽  
Body Wave ◽  
Wave Dispersion ◽  
Spectral Element ◽  
Frequency Theory ◽  
P Wave ◽  
Checkerboard Pattern ◽  
Finite Frequency ◽  
Delay Times ◽  
Window Selection

We evaluated a comprehensive numerical experiment of finite-frequency tomography with ray-based (“banana-doughnut”) kernels that tested all aspects of this method, starting from the generation of seismograms in a 3D model, the window selection, and the crosscorrelation with seismograms predicted for a background model, to the final regularized inversion. In particular, we tested if the quasilinearity of crosscorrelation delays allowed us to forego multiple (linearized) iterations in the case of strong reverberations characterizing multiple scattering and the gain in resolution that can be obtained by observing body-wave dispersion. Contrary to onset times, traveltimes observed by crosscorrelation allowed us to exploit energy arriving later in the time window centered in the P-wave or any other indentifiable ray arrival, either scattered from, or diffracted around, lateral heterogeneities. We tested using seismograms calculated by the spectral element method in a cross-borehole experiment conducted in a 3D checkerboard cube. The use of multiple frequency bands allowed us to estimate body-wave dispersion caused by diffraction effects. The large velocity contrast (10%) and the regularity of the checkerboard pattern caused severe reverberations that arrived late in the crosscorrelation windows. Nevertheless, the model resulting from the inversion with a data fit with reduced [Formula: see text] resulted in an excellent correspondence with the input model and allowed for a complete validation of the linearizations that lay at the basis of the theory. The use of multiple frequencies led to a significant increase in resolution. Moreover, we evaluated a case in which the sign of the anomalies in the checkerboard was systematically reversed in the ray-theoretical solution, a clear demonstration of the reality of the “doughnut-hole” effect. The experiment validated finite-frequency theory and disqualified ray-theoretical inversions of crosscorrelation delay times.

Prior-Information-Based Finite-Frequency $\text{H}_{\infty}$ Control for Active Double Pantograph in High-Speed Railway

2017 ◽  
Vol 66 (10) ◽  
pp. 8723-8733 ◽  
Author(s):  
Xiaobing Lu ◽  
Zhigang Liu ◽  
Jing Zhang ◽  
Hongrui Wang ◽  
Yang Song ◽  
...  
Keyword(s):  
High Speed ◽  
Prior Information ◽  
Finite Frequency ◽  
High Speed Railway

Abstract 40: Xenon-Loaded Liposomes in Combination with IV tPA Extends the Time Window for Treatment of Thrombotic Stroke

Stroke ◽  
2013 ◽  
Vol 44 (suppl_1) ◽  
Author(s):  
Tao Peng ◽  
Yifeng Miao ◽  
George L Britton ◽  
Melvin E Klegerman ◽  
Susan T Laing ◽  
...  
Keyword(s):  
Infarct Size ◽  
Continuous Wave ◽  
Time Window ◽  
Infarct Volume ◽  
Neuroprotective Effect ◽  
Neuronal Cell ◽  
Great Promise ◽  
Tunel Staining ◽  
Stroke Treatment ◽  
Iv Tpa

BACKGROUND: Xenon (Xe) provides great promise for stroke treatment due to its unique neuroprotective effect. Building on previous work for Xe-loaded liposomes (Xe-ELIP) to effectively deliver Xe into the brain, this study investigates the effect of Xe-ELIP in combination with intravenously (IV) administered tissue plasminogen activator (tPA) to extend the time window of treatment for embolic stroke. METHODS: Thrombotic strokes were induced in rats by injecting a standardized blood clot into the middle cerebral artery. In the treatment groups, Xe-ELIP (20mg/kg) and tPA (10mg/kg) were administrated IV at 2 and 4 hours, respectively, after the stroke onset. Continuous wave ultrasound (1 MHz, 50% duty cycle, 1 W/cm 2 ) was applied over the common carotid artery during Xe-ELIP administration to trigger Xe release. Behavioral tests were conducted three days after stroke. Following sacrifice, brain sections were evaluated with triphenyltetrazolium chloride (TTC) and Tunel staining. Infarct size was presented as normalized infarct volume (%). RESULTS: Thrombotic stroke without treatment exhibited the largest infarct size (18.98±2%); tPA treatment reduced the infarct size to 6.1±1% (p<0.001 vs. no treatment). Xe-ELIP in combination with tPA treatment further reduced the infarct size to 1.8±0.4% (p=0.032 vs. tPA treatment; Fig 1a) with lower hemorrhagic adverse effects, improved neurological function and reduced apoptosis (Fig 1b). CONCLUSIONS: This study demonstrates that Xe-ELIP in combination with IV tPA provides improved therapeutic efficacy with reduced neuronal cell death and tPA-associated hemorrhagic side effects. These results have important implications for extending the time window of treatment of thrombotic stroke.

Geomorphological controls on seismic recordings in volcanic areas

2020 ◽  
Author(s):  
Simona Gabrielli ◽  
Luca De Siena ◽  
Matteo Spagnolo
Keyword(s):  
Time Window ◽  
Near Field ◽  
Debris Avalanche ◽  
Source Inversion ◽  
S Wave ◽  
Low Velocity ◽  
Lapse Time ◽  
Scattering Attenuation ◽  
Seismic Coda ◽  
Velocity Anomalies

&lt;p&gt;In volcanoes, topography, shallow heterogeneity, and even shallow morphology can substantially modify seismic coda signals. Coda waves are an essential tool to monitor eruption dynamics and model volcanic structures jointly and independently from velocity anomalies: it is thus fundamental to test their spatial sensitivity to seismic path effects. Here, we apply the Multiple Lapse Time Window Analysis (MLTWA) to measure the relative importance of scattering attenuation vs absorption at Mount St. Helens volcano (MSH) before its 2004 eruption. The results show the typical dominance of scattering attenuation in volcanoes at lower frequencies (3 - 6 Hz), while absorption is the primary attenuation mechanism at 12 Hz and 18 Hz. Still, the seismic albedo (measuring the ratio between seismic energy emitted and received from the area) is anomalously-high (0.95) at 3 Hz.&lt;/p&gt;&lt;p&gt;A radiative-transfer forward model of far- and near-field envelopes confirms this is due to strong near-receiver scattering enhancing anomalous phases in the intermediate and late coda across the 1980 debris avalanche and central crater. Only above this frequency and in the far-field, diffusion onsets at late lapse times.&amp;#160; We also implemented a layered model with a shallower layer with increased scattering properties to model late coda envelopes. While the broadening of late coda phases improves, this model cannot explain the phases of the intermediate coda with higher amplitude than the direct waves.&lt;/p&gt;&lt;p&gt;The scattering and absorption parameters derived from MLTWA are used as inputs to construct 2D frequency-dependent bulk sensitivity kernels for the S-wave coda in the multiple-scattering (using the Energy Transport Equations - ETE) and diffusive (AD, independent of MLTWA results) regimes. At 12 Hz, high coda-attenuation anomalies characterise the eastern side of the volcano using both kernels, in spatial correlation with low-velocity anomalies from literature. At 3 Hz, the anomalous albedo, the forward modelling, and the results of the tomographic imaging confirm that shallow heterogeneity beneath the extended 1980 debris-avalanche and crater enhance anomalous intermediate and late coda phases, mapping shallow geological contrasts.&lt;/p&gt;&lt;p&gt;The geomorphological map of MSH highlights extremely rough landforms (hummocky structures) of the already complex morphology of the debris avalanche. The comparison with the attenuation tomography reveals several matches, not only with the debris avalanche itself but also with other areas in the south flank of MSH, like the volcanoclastic plane, affected by intense eruptions in the past (e.g. Cougar stage, 28-18 ka).&lt;/p&gt;&lt;p&gt;We remark the effect those may have on coda-dependent source inversion and tomography, currently used across the world to image and monitor volcanoes.&lt;/p&gt;

3-D joint geodetic and strong-motion finite fault inversion of the 2008 May 12, Wenchuan, China Earthquake

2020 ◽  
Vol 222 (2) ◽  
pp. 1390-1404
Author(s):  
Leonardo Ramirez-Guzman ◽  
Stephen Hartzell
Keyword(s):  
Time Window ◽  
Joint Inversion ◽  
Strong Motion ◽  
Multiple Time ◽  
Source Inversion ◽  
Geodetic Inversion ◽  
Finite Fault Inversion ◽  
Finite Fault ◽  
China Earthquake ◽  
The Moment

SUMMARY We present a source inversion of the 2008 Wenchuan, China earthquake, using strong-motion waveforms and geodetic offsets together with 3-D synthetic ground motions. We applied the linear multiple time window technique considering geodetic and dynamic Green's functions computed with the finite-element method and the reciprocity and Strain Green's Tensor formalism. All ground motion estimates, valid up to 1 Hz, accounted for 3-D effects, including the topography and the geometry of the Beichuan and Pengguan faults. Our joint inversion has a higher moment (M0) than a purely geodetic inversion and the slip distribution presents differences when compared to 1-D model source inversions. The moment is estimated to be M0 = 1.2 × 1021 N·m, slightly larger than other works. Our results show that considering a complex 3-D structure reduces the size of large areas of 10 m slip or greater by distributing it in wider zones, with reduced slips, in the central portion of the Beichuan and the Pengguan faults. Finally, we compare our source with a relocated aftershock catalogue and conclude that the 4–5 m slip contours approximately bound the absence or presence of aftershocks.

Sign in / Sign up

Export Citation Format

Share Document