Critical network infrastructure analysis: interdiction and system flow

Effective management of critical network infrastructure requires the assessment of potential interdiction scenarios. Optimization approaches have been essential for identifying and evaluating such scenarios in networked systems. Although a primary function of any network is the distribution of flow between origins and destinations, the complexity and difficulty of mathematically abstracting interdiction impacts on connectivity or flow has been a challenge for researchers. This paper presents an optimization approach for identifying interdiction bounds with respect to connectivity and/or flow associated with a system of origins and destinations. Application results for telecommunications flow are presented, illustrating the capabilities of this approach.

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Holocene Avulsions of the Euphrates River in the Najaf Area of Western Mesopotamia: Impacts on Human Settlement Patterns

We present a study that reconstructs the ancient courses of the Euphrates in part of the Mesopotamian floodplain west and southwest of the ancient site of Babylon. The focus is on tracing paleochannel courses, determining when these paleochannels were active, and understanding the patterns of avulsion. The research was carried out using a combination of geological, geomorphological, remote sensing, historical, and archaeological approaches. Fieldwork included “ground truthing” of the remote sensing work, manually drilling boreholes (up to 7 m in depth), sedimentary and geomorphological documentation and sample collection for radiocarbon dating. As a result, five main courses of the Euphrates in five different periods have been mapped in this area, including four previously unidentified and/or unlocated migrations that linked the different periods. The main courses are the Purattum Course (before 3100–1000 BC), the Arahtum Course (1000–125 BC), the Sura Course (125 BC to AD 1258), the Hilla Course (13th to 19th century AD), and the Hindiya Course (19th to 20th century AD). There has been an overall migration of the main channel from east to west across the study area over time. The location of avulsion nodes changed along the length of the river, mainly downstream over time, but with a cluster of avulsion events near Babylon and a notable man‐made interference in the 20th century at the Hindiya Barrage.     

Quasi-shear wave ray tracing by wavefront construction in 3-D, anisotropic media

Wavefront construction (WFC) methods provide robust tools for computing ray theoretical traveltimes and amplitudes for multivalued wavefields. They simulate a wavefront propagating through a model using a mesh that is refined adaptively to ensure accuracy as rays diverge during propagation. However, an implementation for quasi-shear (qS) waves in anisotropic media can be very difficult, since the two qS slowness surfaces and wavefronts often intersect at shear-wave singularities. This complicates the task of creating the initial wavefront meshes, as a particular wavefront will be the faster qS-wave in some directions, but slower in others. Analogous problems arise during interpolation as the wavefront propagates, when an existing mesh cell that crosses a singularity on the wavefront is subdivided. Particle motion vectors provide the key information for correctly generating and interpolating wavefront meshes, as they will normally change slowly along a wavefront. Our implementation tests particle motion vectors to ensure correct initialization and propagation of the mesh for the chosen wave type and to confirm that the vectors change gradually along the wavefront. With this approach, the method provides a robust and efficient algorithm for modeling shear-wave propagation in a 3-D, anisotropic medium. We have successfully tested the qS-wave WFC in transversely isotropic models that include line singularities and kiss singularities. Results from a VTI model with a strong vertical gradient in velocity also show the accuracy of the implementation. In addition, we demonstrate that the WFC method can model a wavefront with a triplication caused by intrinsic anisotropy and that its multivalued traveltimes are mapped accurately. Finally, qS-wave synthetic seismograms are validated against an independent, full-waveform solution.     

Characterizing uncertainties for quantifying bathymetry change between time-separated multibeam echo-sounder surveys

Changes of bathymetry derived from multibeam sonars are useful for quantifying the effects of many sedimentary, tectonic and volcanic processes, but depth changes also require an assessment of their uncertainty. Here, we outline and illustrate a simple technique that aims both to quantify uncertainties and to help reveal the spatial character of errors. An area of immobile seafloor is mapped in each survey, providing a common ‘benchmark’. Each survey dataset over the benchmark is filtered with a simple moving-averaging window and depth differences between the two surveys are collated to derive a difference histogram. The procedure is repeated using different length-scales of filtering. By plotting the variability of the differences versus the length-scale of the filter, the different effects of spatially uncorrelated and correlated noise can be deduced. The former causes variability to decrease systematically as predicted by the Central Limit Theorem, whereas the remaining variability not predicted by the Central Limit Theorem then represents the effect of spatially correlated noise. Calculations made separately for different beams can reveal whether problems are due to heave, roll, etc., which affect inner and outer beams differently. We show how the results can be applied to create a map of uncertainties, which can be used to remove insignificant data from the bathymetric change map. We illustrate the technique by characterizing changes in nearshore bed morphology over one annual cycle using data from a subtidal bay, bedrock headland and a banner sand bank in the Bristol Channel UK.     

The role of site effects in the comparison between code provisions and the near field strong motion of the Emilia 2012 earthquakes

A damaging seismic sequence hit a wide area mainly located in the Emilia-Romagna region (Northern Italy) during 2012 with several events of local magnitude \(\hbox {M}_\mathrm{l} \ge 5\) , among which the \(\hbox {M}_\mathrm{l}\) 5.9 May 20 and the \(\hbox {M}_\mathrm{l}\) 5.8 May 29 were the main events. Thanks to the presence of a permanent accelerometric station very close to the epicentre and to the temporary installations performed in the aftermath of the first shock, a large number of strong motion recordings are available, on the basis of which, we compared the recorded signals with the values provided by the current Italian seismic regulations, and we observed several differences with respect to horizontal components when the simplified approach for site conditions (based on Vs30 classes) is used. On the contrary, when using the more accurate approach based on the local seismic response, we generally obtain a much better agreement, at least in the frequency range corresponding to a quarter wavelength comparable with the depth of the available subsoil data. Some unresolved questions still remain, such as the low frequency behaviour ( \(<\) 1 Hz) that could be due either to complex propagation at depth larger than the one presently investigated or to near source effects, and the behaviour of vertical spectra whose recorded/code difference is too large to be explained with the information currently available.