Internal tide generation from isolated seamounts and continental shelves

We model linear, inviscid non-hydrostatic internal tides generated by the interaction of a barotropic tide with variable topography in two dimensions. We first derive an asymptotic solution for the nonuniform barotropic flow over the topography that serves as forcing for the baroclinic equations. The resulting internal-tide generation problem is reformulated as a Coupled-Mode System (CMS) by means of a series decomposition of the baroclinic stream function in terms of vertical basis functions. We solve this CMS numerically and also provide a method for estimating the sea-surface signature of internal tides. We consider several seamounts and shelf profiles and perform calculations for a wide range of (topographic) heights and slopes. For subcritical topographies, the energy flux as a function of height exhibits local maxima, separated by cases of weakly- or even non-radiating topographies. For supercritical topographies, the energy flux generally increases with height and criticality. Our calculations agree with the Weak Topography Approximation only for very small heights. Perhaps more surprisingly, they agree with the Knife Edge model only for moderately supercritical topographies. We also compare the effect of the adjusted barotropic tide on the energy flux and the local properties of the baroclinic field with other semi-analytical methods based on a uniform barotropic tide. We observe significant differences in the flow field near the topographies only

Unrolled three-operator splitting for parameter-map learning in Low Dose X-ray CT reconstruction

We propose a method for fast and automatic estimation of spatially dependent regularization maps for total variation-based (TV) tomography reconstruction. The estimation is based on two distinct sub-networks, with the first sub-network estimating the regularization parameter-map from the input data while the second one unrolling T iterations of the Primal-Dual Three-Operator Splitting (PD3O) algorithm. The latter approximately solves the corresponding TV-minimization problem incorporating the previously estimated regularization parameter-map. The overall network is then trained end-to-end in a supervised learning fashion using pairs of clean-corrupted data but crucially without the need of having access to labels for the optimal regularization parameter-maps

Learning Regularization Parameter-Maps for Variational Image Reconstruction Using Deep Neural Networks and Algorithm Unrolling

We introduce a method for the fast estimation of data-adapted, spatially and temporally dependent regularization parameter-maps for variational image reconstruction, focusing on total variation (TV) minimization. The proposed approach is inspired by recent developments in algorithm unrolling using deep neural networks (NNs) and relies on two distinct subnetworks. The first subnetwork estimates the regularization parameter-map from the input data. The second subnetwork unrolls iterations of an iterative algorithm which approximately solves the corresponding TV-minimization problem incorporating the previously estimated regularization parameter-map. The overall network is then trained end-to-end in a supervised learning fashion using pairs of clean and corrupted data but crucially without the need for access to labels for the optimal regularization parameter-maps. We first prove consistency of the unrolled scheme by showing that the unrolled minimizing energy functional used for the supervised learning -converges, as tends to infinity, to the corresponding functional that incorporates the exact solution map of the TV-minimization problem. Then, we apply and evaluate the proposed method on a variety of large-scale and dynamic imaging problems with retrospectively simulated measurement data for which the automatic computation of such regularization parameters has been so far challenging using the state-of-the-art methods: a 2D dynamic cardiac magnetic resonance imaging (MRI) reconstruction problem, a quantitative brain MRI reconstruction problem, a low-dose computed tomography problem, and a dynamic image denoising problem. The proposed method consistently improves the TV reconstructions using scalar regularization parameters, and the obtained regularization parameter-maps adapt well to imaging problems and data by leading to the preservation of detailed features. Although the choice of the regularization parameter-maps is data-driven and based on NNs, the subsequent reconstruction algorithm is interpretable since it inherits the properties (e.g., convergence guarantees) of the iterative reconstruction method from which the network is implicitly defined

Motion estimation and correction for simultaneous PET/MR using SIRF and CIL

SIRF is a powerful PET/MR image reconstruction research tool for processing data and developing new algorithms. In this research, new developments to SIRF are presented, with focus on motion estimation and correction. SIRF's recent inclusion of the adjoint of the resampling operator allows gradient propagation through resampling, enabling the MCIR technique. Another enhancement enabled registering and resampling of complex images, suitable for MRI. Furthermore, SIRF's integration with the optimization library CIL enables the use of novel algorithms. Finally, SPM is now supported, in addition to NiftyReg, for registration. Results of MR and PET MCIR reconstructions are presented, using FISTA and PDHG, respectively. These demonstrate the advantages of incorporating motion correction and variational and structural priors. This article is part of the theme issue 'Synergistic tomographic image reconstruction: part 2'

Mass flows, turbidity currents and other hydrodynamic consequences of small and moderate earthquakes in the Sea of Marmara

Earthquake-induced submarine slope destabilization is known to cause mass wasting and turbidity currents, but the hydrodynamic processes associated with these events remain poorly understood. Instrumental records are rare, and this notably limits our ability to interpret marine paleoseismological sedimentary records. An instrumented frame comprising a pressure recorder and a Doppler recording current meter deployed at the seafloor in the Sea of Marmara Central Basin recorded the consequences of a Mw 5.8 earthquake occurring on 26 September 2019 and of a Mw 4.7 foreshock 2 d before. The smaller event caused sediment resuspension and weak current (&lt;4 cm s−1) in the water column. The larger event triggered a complex response involving a debris flow and turbidity currents with variable velocities and orientations, which may have resulted from multiple slope failures. A long delay of 10 h is observed between the earthquake and the passing of the strongest turbidity current. The distance traveled by the sediment particles during the event is estimated to have extended over several kilometers, which could account for a local deposit on a sediment fan at the outlet of a canyon (where the instrument was located), but the sedimentation event did not likely cover the whole basin floor. We show that after a moderate earthquake, delayed turbidity current initiation may occur, possibly by ignition of a cloud of resuspended sediment.</p

Νon linear water waves: Comparison of different variational methods

64 σ.Θεωρούμε το μη γραμμικό πρόβλημα διάδοσης υδάτινου κυματισμού πάνω από γενική βαθυμετρία. Παρουσιάζεται η εξαγωγή των εξισώσεων του προβλήματος από δύο μεταβολικές αρχές : την αρχή του Hamilton και την αρχή του Luke και δείχνουμε την σύνδεση τους. Στη συνέχεια χρησιμοποιούμε την τελευταία - ανευ συνδέσμων μεταβολική αρχή - σε συνδυασμό με την συνεπή, ακριβή, τοπική αναπαράσταση σε σειρά του κυματικού δυναμικού και εξάγουμε νέες εξισώσεις για το πρόβλημα ως προς τους άγνωστους συντελεστές της αναπαράστασης. Τέλος δείχνουμε την σύνδεση αυτών των εξισώσεων με τις εξισώσεις Ηamilton και δίνουμε μια νέα αναπαράσταση για τον τελεστή DtN δια μέσου της αναπαράστασης.We consider the non linear water wave problem over general bathymetry. The derivation of the evolution equations of the problem are derived via two different variational principles : Hamilton's principle and Luke's priniciple and we show their connection. In the sequel we use the latter - unconstrained - variational principle in conjunction with the consistent, accurate local-mode representation of the wave potential and derive new equations for the problem in terms of the unknown coefficients of the representation. Finally we show the connection of these equations with Hamilton's equations and give a new representation of the DtN operator in terms of the representation.Χρήστος Ε. Παπουτσέλλη

Seismic strengthening of the historic church of St. Nikolaos in Piraues

123 σ.Η εκκλησία του Αγίου Νικολάου βρίσκεται στο λιμάνι του Πειραιά Πρόκειται για έναν Ι.Ν. μνημείο που η θεμελίωση του έγινε 1887,ένω το τέλος των εργασιών και η σημερινή του εικόνα χρονολογείται από το 1902. Ο Ιερός Ναός καταπονήθηκε και παρουσίασε σημαντικές βλάβες από τους πρόσφατους σεισμούς συμπεριλαμβανομένου και του σεισμού των Αθηνών το 1999. Οι βλάβες αυτές οφείλονται κατά κύριο λόγο στο γεγονός ότι δεν υπάρχουν στοιχεία στο φορέα, για τη θωράκισή του σε περίπτωση σεισμού. Για την εκτίμηση της σεισμικής τρωτότητας του Ιερού Ναού, μια λεπτομερή μελέτη του κτιρίου διεξήχθει προκειμένου να καθοριστούν οι μηχανικές ιδιότητες, οι κατασκευαστικές λεπτομέρειες και η υφιστάμενη κατάσταση του κτιρίου καθώς και η σεισμική συμπεριφορά του μέσω εργαστηριακών και επιτόπου ελέγχων και μέσω της ανάλυσης με πεπερασμένα στοιχεία. Με βάση τα αποτελέσματα, προτείνονται μέτρα για την ενίσχυση και επισκευή του Ιερού Ναού, χρησιμοποιώντας ένα συνδυασμό επεμβάσεων όπως ομογενοποίηση της τοιχοποιίας με ενέματα, ινοπλισμένα πολυμερή και ελκυστήρες.The Church of St. Nikolaos is located at the port of Piraeus. The foundation was built in 1875 and the total temple was completed in 1902. A series of recent earthquakes, including the Athens 1999 earthquake, strained the temple and caused serious damages. These damages are mainly attributed to the absence of particular provisions to carry the earthquake loads. In order to evaluate the earthquake vulnerability of the church, a detailed study of the masonry structure was conducted in order to determine the mechanical properties, the building construction details and the current condition of the structure as well as its dynamic behavior through in situ and laboratory testing as well as through finite element analysis. Based on the results, strengthening and repair measures are proposed, using a combination of masonry consolidation techniques, fiber reinforced plastic materials (FRP) and steel tie rods.Σταύρος Ε. ΠαπουτσέλληςΕθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Δομοστατικός Σχεδιασμός και Ανάλυση των Κατασκευών