MATHEMATICAL MODEL AND ANALYTICAL SOLUTION FOR THE VIBRATION OF INCLINED FLUID-TRANSPORTING SUBMARINE PIPELINES

Due to the complexity of submarine environments, the nature of the dynamic response of free-spanning submarine pipelines, particularly inclined pipelines, is unclear. This paper aims to theoretically analyze the vibration behaviors of inclined fluid-transporting free-spanning submarine pipelines in the deepwater area. The mathematical model for the vibration of inclined fluid-transporting pipelines is established considering the influence of gravity on vibration response, and a non-linear wake oscillator is employed to model the vortex shedding behind the pipeline free span. The partial differential equation system is solved through the generalized integral transform technique (GITT), which is an analytical or semi-analytical method. Parametric analysis are carried out to investigate the effects of the inclination on the dynamic response of fluid-transporting pipelines. It is found that the inclination of the free- spanning pipeline will radically alter the natural frequency of the structure, and consequently the VIV lock-in region. In addition, the slope of the seabed will cause a more significant internal flow effect. The thorough theoretical understanding of inclined fluid-transporting pipelines helps increase the design accuracy for pipelines installed on a seabed with a highly irregular topography.

Morphology and Late Pleistocene-Holocene Sedimentation of the Strait of İstanbul (Bosphorus): A review

The Bosphorus (Istanbul Strait) is natural strait that connects the Black Sea with the Aegean Sea via the Sea of Marmara and Dardanelles Strait. It is a 31 km long and 3.5 km wide winding channel, with an irregular bottom morphology. It has depressions up to -110 m deep, and two sills with depths of -35 and -58 m in the south and north, respectively.Presently, a two-layer water exchange exists through the strait, with the Mediterranean and Black Sea waters forming the lower and upper layers, respectively. The Bosphorus channel extends as shelf valleys on the Black Sea and Sea of Marmara shelves. However, it operated as a river valley or an estuary during the stadial low-stand periods.The infill sedimentary succession of the Bosphorus channel is up to ∼100 m thick above the Palaeozoic-Cretaceous basement with an irregular topography. The oldest sediments are sandy to muddy fluvial-lacustrine facies of late Pleistocene age, which are preserved only in up to -160 m-deep scoured depressions of the basement. They are overlain by mid-late Holocene estuarine-marine shelly sandy to muddy sediments with patches of bioherms and shelly lag deposits.The Bosphorus outlet areas of the Black Sea and Sea of Marmara are characterized by a submarine fan and a shelf valley, respectively. The fan system in the Black Sea started depositing ∼900 yr after the initial vigorous marine water incursion at ∼8.4 14C kyr BP. On the Marmara shelf, extension of the Bosphorus channel is a sinuous shelf valley with a channel-leveé complex, which was deposited by the Black Sea outflow during the 11-10 14C kyr BP. Catastrophic floodings of the Sea of Marmara by torrential Black Sea outflows during the Greenland Interstadial melt water pulses, as well as the strong Mediterranean current towards the Black Sea during the interglacial periods, were responsible for carving the Bosphorus channel and the shelf valleys, as well as removing the sediments belonging to the earlier periods.

The relative importance of wind-driven and chimney effect cave ventilation: Observations in Postojna Cave (Slovenia)

Density-driven chimney effect airflow is the most common form of cave ventilation, allowing gas exchange between the outside and the karst subsurface. However, cave ventilation can also be driven by other mechanisms, such as barometric changes or pressure differences induced by the outside winds. We discuss the mechanism and dynamics of wind-driven ventilation using observations in Postojna Cave, Slovenia. We show how seasonal airflow patterns driven by the chimney effect are substantially modified by outside winds. Wind flow over irregular topography forms near-surface air pressure variations and thus pressure differences between cave entrances at different locations. These pressure differences depend on wind speed and direction and their relationship to surface topography and the location of cave entrances. Winds can act in the same or opposite direction as the chimney effect and can either enhance, diminish or even reverse the direction of the density-driven airflows. To examine the possibility of wind-driven flow, we used a computational fluid dynamics model to calculate the wind pressure field over Postojna Cave and the pressure differences between selected points for different configurations of wind speed and direction. We compared these values with those obtained from airflow measurements in the cave and from simple theoretical considerations. Despite the simplicity of the approach and the complexity of the cave system, the comparisons showed satisfactory agreement. This allowed a more general assessment of the relative importance of wind pressure for subsurface ventilation. We are certain that this example is not unique and that the wind-driven effect needs to be considered elsewhere to provide better insights into the dynamics of cave climate, air composition or dripwater geochemistry.

MAXIMIZING THE VALUE OF PHOTOGRAMMETRIC SURVEYS IN THE CONSERVATION OF WALL PAINTINGS

In the field of wall painting conservation, photogrammetric surveys are an increasingly common documentation tool. The outputs from these surveys, including captured images, orthophotos, and three-dimensional models, are used by conservators to monitor and record condition phenomena and conservation treatments. To serve these conservation needs, the outputs should have visual fidelity, meaning they are a close representation of the subject based on their spatial resolution, overall focus, even lighting and color accuracy. However, wall paintings present unique technical challenges for photogrammetry considering their often large scale, irregular topography, range of surface attributes, and access issues. As a documentation technique more commonly applied to capture the geometry of buildings and objects, using photogrammetry to record both the topography and fine surface details of a wall painting requires thorough planning. This paper examines specific challenges in documenting wall paintings and how collaboration between conservators and heritage-recording specialists has improved practice and maximized the value of photogrammetric surveys for conservation.

The relative importance of wind-driven and chimney effect cave ventilation: Observations in Postojna Cave (Slovenia)

Density-driven chimney effect airflow is the most common form of cave ventilation, allowing gas exchange between the outside and the karst subsurface. However, cave ventilation can also be driven by other mechanisms, namely winds. We discuss the mechanism and dynamics of wind-driven ventilation using observations in Postojna Cave, Slovenia. We show how seasonal airflow patterns driven by the chimney effect are substantially modified by outside winds. Wind flow over irregular topography forms near-surface air pressure variations and thus pressure differences between cave entrances at different locations. These pressure differences depend on wind speed and direction and their relationship to surface topography and the location of cave entrances. Winds can act in the same or opposite direction as the chimney effect and can either enhance, diminish or even reverse the direction of the density-driven airflows. To examine the possibility of wind-driven flow, we used a computational fluid dynamics model to calculate the wind pressure field over Postojna Cave and the pressure differences between selected points for different configurations of wind speed and direction. We compared these values with those obtained from airflow measurements in the cave and from simple theoretical considerations. Despite the simplicity of the approach and the complexity of the cave system, the comparisons showed satisfactory agreement. This allowed a more general assessment of the relative importance of wind pressure for the subsurface ventilation. We are certain that this example is not unique and that the wind-driven effect needs to be considered elsewhere to provide better insights into the dynamics of cave climate, air composition or dripwater geochemistry.

Damped wave-equation-based first-arrival traveltime tomography using the embedded boundary method

First-arrival traveltime tomography (FATT) is used to delineate shallow velocity structures to identify static effects in oil exploration as well as to characterize the near surface for geotechnical purposes. Because FATT is generally used for land seismic data processing, it becomes necessary to consider irregular topography especially when performing wave-based tomography. However, the standard Cartesian finite-difference method cannot properly handle irregular topography. Hence, the embedded boundary method (EBM) is incorporated into the frequency-domain damped-wave equation in order to correctly describe irregular topography. The developed modeling algorithm is used to calculate first-arrival traveltimes and to perform FATT. The EBM-based modeling algorithm accurately describes the irregular surfaces of numerical velocity models on a regular mesh by exploiting the mirror image principle. The accuracy of the EBM-based traveltime calculation is validated by using two homogeneous velocity models with dipping and complex surfaces. The validation results demonstrate that the proposed algorithm is unaffected by the staircase approximation. The FATT is then applied to synthetic and real data to demonstrate the applicability of the developed algorithm to velocity models with complex topography. For the real data example, the inverted velocity model is used to apply static corrections. The processing results demonstrate an improvement in the continuity of seismic events, thus confirming the accuracy of the developed FATT method.

An iterative fast sweeping method for Eikonal equation in 2D anisotropic media on unstructured triangular meshes

Computation of traveltimes and ray paths is important for anisotropic tomography inversions. The Eikonal-equation-based method outperforms traditional ray methods by producing more accurate results. However, most existing Eikonal solvers are formulated on structured regular meshes, which are no longer accurate for models with the presence of irregular topography and subsurface interfaces. To solve Eikonal equation in vertically transversely isotropic (VTI) or tilted transversely isotropic (TTI) models with irregular geometry, we formulate a new iterative fast sweeping method on unstructured triangular meshes. The fixed-point iteration is implemented to capture the high-order nonlinear terms therein and a fast sweeping method on unstructured triangular meshes is implemented to solve the resulting elliptically anisotropic Eikonal equation at every iteration. We test the new algorithm for direct arrivals and reflected arrivals, and then use the calculated traveltimes to track the ray path in VTI/TTI media. Numerical tests demonstrate the validity and accuracy of the new method for models with rough topography and subsurface interface.

Mechanisms of slab avalanche release and impact in the Dyatlov Pass incident in 1959

The Dyatlov Pass incident is an intriguing unsolved mystery from the last century. In February 1959, a group of nine experienced Russian mountaineers perished during a difficult expedition in the northern Urals. A snow avalanche hypothesis was proposed, among other theories, but was found to be inconsistent with the evidence of a lower-than-usual slope angle, scarcity of avalanche signs, uncertainties about the trigger mechanism, and abnormal injuries of the victims. The challenge of explaining these observations has led us to a physical mechanism for a slab avalanche caused by progressive wind-blown snow accumulation on the slope above the hikers’ tent. Here we show how a combination of irregular topography, a cut made in the slope to install the tent and the subsequent deposition of snow induced by strong katabatic winds contributed after a suitable time to the slab release, which caused severe non-fatal injuries, in agreement with the autopsy results.