Fresnel volumes corresponding to PKP waves in the IASP91 model

Fresnel volumes provide a suitable tool for estimating the resolution limits or accuracy of tomographic methods based only on the arrival times of these seismic waves, which penetrate the deep interior of the Earth. The standard definition of the Fresnel volume was modified to include effects of multipathing, i.e., multivalued travel times. Computations were performed in the IASP91 model for three representative epicentral distances and two prevailing signal periods. Preliminary results have been obtained. They place a rather strong restriction on the resolution limits of tomographic methods, depending on the prevailing period, due to the relatively large fatness of PKP Fresnel volumes.     

The millennium eruption of Hekla in February 2000

The 18th historic eruption of Hekla started on 26 February, 2000. It was a short-lived but intense event, emitting basaltic andesitic (55.5 wt% SiO₂) pyroclastic fragments and lava. During the course of the eruption, monitoring was done by both instruments and direct observations, together providing unique insight into the current activity of Hekla. During the 12-day eruption, a total of 0.189 km³ DRE of magma was emitted. The eruptive fissure split into five segments. The segments at the highest altitude were active during the first hours, while the segments at lower altitude continued throughout the eruption. The eruption started in a highly explosive manner giving rise to a Subplinian eruptive column and consequent basaltic pyroclastic flows fed by column collapses. After the explosive phase reached its maximum, the eruption went through three more phases, namely fire-fountaining, Strombolian bursts and lava effusion. In this paper, we describe the course of events of the eruption of Hekla and the origin of its magma, and then show that the discharge rate can be linked to different style of eruptive activity, which are controlled by fissure geometry. We also show that the eruption phases observed at Hekla can be linked with inferred magma chamber overpressure prior to the eruption.     

Multi‐scale assessment of overflow‐driven lateral connectivity in floodplain and backwater channels using LiDAR imagery

Overflow‐driven lateral connectivity significantly influences the spatial distribution and diversity of floodplain habitats and biota. Proper understanding of lateral connectivity in floodplain and backwater channels is therefore critical for assessment of river quality and for targeting management or restoration actions. In this study, we present a methodological framework for spatial and temporal assessments of overflow‐driven lateral connectivity at two spatial scales: bypass reach and backwater channel. Firstly, we compute the relative elevations, as well as overflow discharge, duration, and frequency using a simple, raster‐based method that uses a LiDAR digital elevation model (DEM), rating curves, and streamflow time series. Subsequently, we analyse the accuracy of this approach with respect to the accuracy of a DEM and evaluate its further applications. Altogether, four 10‐km‐long bypass reaches and 11 backwater channels are analysed, located along the Rhône River corridor in France. The results proved the precision of the method to be affected by the LiDAR DEM accuracy, which was on average more precise in a typically homogeneous floodplain setting rather than for backwater channel plugs with pronounced topographic complexity and usually riparian forest canopy. Amongst the four studied reaches, Brégnier Cordon proved to have the greatest flooding dynamics, followed by Belley and Chautagne. The hydrological connectivity pattern of Pierre Bénite differed significantly. Three longitudinal patterns of hydrological connectivity of backwater channels displayed stepwise advancement of the water. The presented results can be used to assess ecological potential of floodplain habitats and their historic and prospective evolution through time. Copyright © 2014 John Wiley & Sons, Ltd.     

Normal modes and resonance in Ontario Lacus: a hydrocarbon lake of Titan

Ocean Dynamics - The natural modes of Ontario Lacus surface oscillations, the largest lake in Titan’s southern hemisphere, are simulated and analyzed as they are potentially of broad interest...     

Methane in the hyporheic zone of a small lowland stream (Sitka, Czech Republic)

Methane concentrations and selected chemical parameters in interstitial water were examined along subsurface flowpath in two subsystems (hyporheic and parafluvial sediments) in the Sitka stream, Czech Republic. Interstitial methane concentrations exhibited a distinct spatial pattern. In the hyporheic downwelling zone where the sediments are relatively well oxygenated due to high hydrologic exchange with the surface water, low interstitial methane concentrations, averaging 9.3 μg CH₄/l, were found. In contrast, upwelling sediments and parafluvial sediments (active channel sediments lateral to the wetted channel) had significantly higher methane concentrations (p < 0.05, and p < 0.01, respectively), averaging 43.2 μg CH₄/l and 160.5 μg CH₄/l, respectively. Dissolved oxygen was the highest where surface water entered hyporheic/parafluvial sediments and decreased with water residence time in the sediments (p < 0.01). Nitrate concentrations decreased along the flowpath and were significantly lower at downstream end of the riffle (p < 0.001). Sulfate concentrations also show a slight decline with the water residence time, but differences were not significant. Effect of both nitrate and sulfate on methanogenesis is also discussed. The interstitial methane concentration significantly increased with surface water temperature (p < 0.001) and was negatively correlated with redox potential (p < 0.01) and dissolved oxygen (p < 0.05).     

The seismotectonics of western Skagerrak

The seismically active Skagerrak region in the border area between Denmark and Norway has traditionally been associated with uncertain earthquake locations due to the limited station coverage in the region. A new seismic station in southern Norway and a recent update of the earthquake database of the Danish National Network have led to a much more complete and homogeneous data coverage of the Skagerrak area, giving the possibility of improved earthquake locations in the region. In this study, we relocate earthquakes in the Skagerrak area to obtain a more exact picture of the seismicity and investigate well-recorded events to determine the depth distribution. Hypocenter depths are found to be generally in the range 11–25 km. Furthermore, new composite focal mechanisms are determined for clusters of events with similar waveforms. Results indicate that the Skagerrak seismicity is associated with shallow, crustal faults oriented in the NS direction south of the Sorgenfrei–Tornquist Zone (STZ) as well as with the STZ itself. Mainly reverse faulting mechanisms along NE–SW oriented faults indicate maximum horizontal compression in the NW–SE direction. This is in agreement with World Stress Map generalizations, most likely associated with ridge push forces from the mid-Atlantic ridge, though modified probably by local crustal weaknesses.     

Palaeoecology, reference conditions and classification of ecological status: the EU Water Framework Directive in practice

The European Union's Water Framework Directive (WFD) requires that all Member States within the European Union determine reference conditions for aquatic ecosystems to provide a baseline against which to measure the effects of past and present activities. Reference conditions are subsequently used to classify the ecological status of European waters. The decisions regarding environmental status will be important future elements in the management of European coastal waters. We have developed a number of classification scenarios for total nitrogen (TN) in the overlying waters of the southern part of Roskilde Fjord, Denmark, taking as our basis a palaeoecological reconstruction of fluctuations in TN between 1850 and 1995. We present a provisional classification scheme for the ecological status of Roskilde Fjord, sensu the WFD. Decision(s) regarding the deviation from reference conditions will give a wide range of apparent ecological status from good, through moderate and poor, to bad depending upon the definition of an acceptable deviation from reference conditions. The determination of an acceptable deviation will ultimately be a political decision, and will result in a wide range in the protection of coastal waters in Europe. There is still, however, an urgent need for a sound scientific documentation of the various scenarios for the implementation of the WFD.     

Star and polyline glyphs in a grid plot and on a map display: which perform better?

Glyphs are small geometric shapes that in geovisualization are often used to represent multidimensional spatial data. The aim of this study is to investigate the effectiveness of their two types – star and polyline glyphs, as they can encode the same message and can provide similar functionality. Thus, if the two glyph types are similar and can be used for the same data, the question arises as to which of them better facilitates various user tasks. To address this question, an empirical study of 26 individual users is conducted to investigate differences in user performance for polyline and star glyphs shown either in a grid plot or on a map display. In this study, a task-based approach with eye-tracking is applied, as well as a subjective questionnaire and a psychological test of cognitive style. The finding is that polyline glyphs better facilitate tasks when datapoint values in glyphs are to be read, whereas star glyphs are better when a visual search among glyphs is to be done. Moreover, the results reveal that the map display works better than the grid plot. If star glyphs are to be used, the key (legend) needs to be better incorporated into a visual interface.     

Visualisation and personalisation of multi-representations city models

ABSTRACT

City models have a wide variety of uses that require different kind of data representation or data models. Having a dynamic model that enables picking the right representations (meshes, volumetric data, point cloud, etc.) can prove useful to adapt an application to each user's needs. In this paper, we present an original method to create personalised visualisations of 3D city models on the fly. By organising the server data in a hierarchy of tiles, we are able to generate personalised models based on the user's preferences. These preferences take the shape of a set of rules that apply to each tile or city object and allow the user to choose which representation of the object to use depending on its position or semantic information (classification, height, etc.). Our method is designed around existing standards, guaranteeing the interoperability of the produced models.     

Heat flow in the natural gas field of Hajduszoboszló

Summary The natural gas field of Hajduszoboszló in the Hungarian plain offered several virgin rock temperature measurements, further rock samples for measurement of conductivity. The Paleozoic bottom rock is covered by Mezozoic and Tertiary sediments inclusively flysch. One big and three small gas caps are found at a depth of about 1000 to 1200 in anticlyne structures. The average temperature gradient is 60° C/km. The heat flow is between 2.2 and 2.7 cal/cm² sec, which is consistent to other heat flow values measured in the Hungarian basin.