On the accuracy of the ground force estimated in vibroseis acquisition

For a linear elastic Earth the time derivative of the ground force is considered proportional to the far-field wavelet. Under the assumption that the baseplate is stiff and the bending forces of the baseplate are negligible, the ground force is also approximated by the sum of the accelerations of the baseplate and the reaction mass weighted by the respective masses. Combining these two assumptions, the time derivative of the weighted sum is considered proportional to the far-field wavelet. This result, often referred to as the far-field wavelet assumption, although convenient and most often employed is not always valid. We explore its validity using the spectral harmonic ratios of recorded data, which are used extensively in data filtering and analysis of vibratory data. We show that the far-field wavelet assumption fails particularly for harmonic components of even order. More compact soil after repeated shots further invalidates this assumption. Non-linear modelling of the ground under the vibrator point may provide a direction towards solving this discrepancy. Finally, we describe a method for the estimation of the harmonic spectral ratios.     

About applying linear structural method on ocean data: Adjustment of satellite wave data

Linear structural relations express the linear relationship between random variables when each of the variables under consideration is subjected to inherent variability and measurement error. The linear structural method is more general than the classical regression, which is often used incorrectly in calibration of altimeter data or validation of wave model results. In this work the linear structural method is elaborated and applied for the adjustment of TOPEX/Poseidon (T/P) data by means of in situ measured wave data in the Aegean Sea. In addition, a comparison of the buoy and T/P data with the WAM wave model predictions is performed revealing an underestimation trend of the model.     

Study of the N, P and Si fluxes between fish farm sediment and seawater. Results of simulation experiments employing a benthic chamber under various redox conditions

The rapidly expanding industry of marine cage fish farming of sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax) in the Mediterranean Sea over the last decade has often had damaging effects on the benthic aquatic environment near the fish farm installations. It has been observed that the food-remains, together with the pellets and metabolic products from fish, frequently form a “nepheloid” sediment layer covering large areas of the seabed. Under these conditions anoxia and/or hypoxia develop and affect benthic communities while the quality of the marine environment deteriorates for long periods, extending even beyond the life span of the fish farm itself. In most cases the affected areas act initially as sediment traps and in a second phase as secondary sources of organic carbon, nutrients and other substances.The aim of this paper is to explore the behaviour of such an affected area under different redox regimes by measuring the fluxes of ammonium, nitrite, nitrate, TDN, phosphate, TDP and silicate between the surface sediment and the overlying waters. To achieve this goal we constructed a prototype benthic chamber capable of sampling considerable quantities of affected undisturbed sediment along with its overlying water. The chamber was transported to the laboratory where simulation experiments reproducing the conditions occurring in nature, including the extreme ones, were carried out and studied carefully. The chamber allows the full and fine control of the dissolved oxygen concentration – and thus of the redox potential – as well as water temperature while subsamples of both water and sediment could be obtained and analysed for a series of chemical substances. The controlled laboratory chamber experiments reproduced four successional phases: 1) deoxygenation, 2) hypoxia, 3) reoxygenation and 4) anoxia.The results showed that even minor changes in the redox conditions at a relatively narrow zone near the water sediment interface have significant impacts on the concentrations of dissolved nitrogen, phosphorus and silicate compounds.With decreasing oxygen supply (phases 1, 2 and 4), the concentrations of ammonium, nitrite, TDN, phosphate, TDP and silicate rapidly increase, those of nitrate decrease. DON and DOP exhibit remarkable fluctuations.During reoxygenation (phase 3) the concentrations of ammonium stabilise, the nitrate concentration decreases while nitrite shows an increasing trend. Decreases in phosphate and silicate concentrations were also observed paralleled by TDP stabilization and DOP increasing trend. TDN shows a relatively small increase while the DON concentration fluctuates significantly.     

High resolution spatiotemporal analysis of erosion risk per land cover category in Korçe region,Albania

Some recent land use changes in Albania, such as deforestation, cropland abandonment, and urban sprawl, have caused serious increase of erosion risk. The main objective of this study was to map erosion risk in Korçe region and assess the degree at which every land use is concerned. The G2 erosion model was applied, which can provide erosion maps and statistical figures at month-time intervals using input from free European and global geodatabases. The mapping results in Korçe region were derived at a 30-m cell size, which is an innovation for G2. Autumn-winter months were found to be the most erosive, with average erosion rates reaching the maximum in November and December, i.e. 2.62 and 2.36 t/ha, respectively, while the annual rate was estimated at 10.25 t/ha/yr. Natural grasslands, shurblands, mixed forests, and vineyards showed to exhibit the highest mean erosion rates, while shrublands, broad-leaved forests and natural grasslands were found to be the most extended land covers risky for non-sustainable erosion rates (i.e. >10 t/ha/yr). A detailed examination of the detected hot spots is now necessary by the competent authorities, in order to apply appropriate, site-specific conservation measures. Notably, use of SPOT VGT data did not prevent the maps from having extended gaps due to cloudiness. Sentinel-2 time series, freely available by the European Space Agency (ESA), have the potential to improve spatiotemporal coverage of V-factor, thus further empowering the G2 model, in the near future.     

Preliminary flood risk assessment: the case of Athens

Flood mapping, especially in urban areas, is a demanding task requiring substantial (and usually unavailable) data. However, with the recent introduction of the EU Floods Directive (2007/60/EC), the need for reliable, but cost effective, risk mapping at the regional scale is rising in the policy agenda. Methods are therefore required to allow for efficiently undertaking what the Directive terms “preliminary flood risk assessment,” in other words a screening of areas that could potentially be at risk of flooding and that consequently merit more detailed attention and analysis. Such methods cannot rely on modeling, as this would require more data and effort that is reasonable for this high-level, screening phase. This is especially true in urban areas, where modeling requires knowledge of the detailed urban terrain, the drainage networks, and their interactions. A GIS-based multicriteria flood risk assessment methodology was therefore developed and applied for the mapping of flood risk in urban areas. This approach quantifies the spatial distribution of flood risk and is able to deal with uncertainties in criteria values and to examine their influence on the overall flood risk assessment. It can further assess the spatially variable reliability of the resulting maps on the basis of the choice of method used to develop the maps. The approach is applied to the Greater Athens area and validated for its central and most urban part. A GIS database of economic, social, and environmental criteria contributing to flood risk was created. Three different multicriteria decision rules (Analytical Hierarchy Process, Weighted Linear Combination and Ordered Weighting Averaging) were applied, to produce the overall flood risk map of the area. To implement this methodology, the IDRISI Andes GIS software was customized and used. It is concluded that the results of the analysis are a reasonable representation of actual flood risk, on the basis of their comparison with historical flood events.     

Distribution, lithotypes and mineralogical study of newly formed thermogenic travertines in Northern Euboea and Eastern Central Greece

In the northwestern part of Euboea Island and the neighbouring part of the mainland in eastern central Greece, many hot springs exist. We collected and analysed the newly formed material around the hot springs. The samples were studied at the lab with X-Ray Diffraction, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). In all cases the studied materials were thermogenic travertine presenting many different lithotypes. The studied travertine deposits consist mainly of aragonite and calcite, but in some cases, as the main mineral phase, an amorphous hydrous ferric oxyhydroxide, probably ferrihydrite (creating a laminated iron-rich travertine deposit), was identified. The lithotypes that were identified were of great variety (spicular, shrubs, etc). Some of them (pisoliths, rafts and foam rock types) are quite rare and one of them (framework type) is described for the first time. Morphological data and field observations suggest possible inorganic and organic controls on carbonate precipitation. Similar lithotypes have been recorded at Mammoth hot springs, Yellowstone National Park in USA and at Rapolano Terme, Italy.     

Assessment of the sensitivity of the southern coast of the Gulf of Corinth (Peloponnese, Greece) to sea-level rise

The eustatic sea-level rise due to global warming is predicted to reach approximately 18?C59 cm by the year 2100, which necessitates the identification and protection of sensitive sections of coastline. In this study, the classification of the southern coast of the Gulf of Corinth according to the sensitivity to the anticipated future sealevel rise is attempted by applying the Coastal Sensitivity Index (CSI), with variable ranges specifically modified for the coastal environment of Greece, utilizing GIS technology. The studied coastline has a length of 148 km and is oriented along the WNW-ESE direction. CSI calculation involves the relation of the following physical variables, associated with the sensitivity to long-term sea-level rise, in a quantifiable manner: geomorphology, coastal slope, relative sea-level rise rate, shoreline erosion or accretion rate, mean tidal range and mean wave height. For each variable, a relative risk value is assigned according to the potential magnitude of its contribution to physical changes on the coast as the sea-level rises. Every section of the coastline is assigned a risk ranking based on each variable, and the CSI is calculated as the square root of the product of the ranked variables divided by the total number of variables. Subsequently, a CSI map is produced for the studied coastline. This map showed that an extensive length of the coast (57.0 km, corresponding to 38.7% of the entire coastline) is characterized as highly and very highly sensitive primarily due to the low topography, the presence of erosionsusceptible geological formations and landforms and fast relative sea-level rise rates. Areas of high and very high CSI values host socio-economically important land uses and activities.     

Determination of Factors Affecting Compressive Strength of Lean Rcc Mixtures: The experience of Filiatrinos Dam

In order to determine the properties of lean Roller compacted concrete mixture of Filiatrinos Dam, a number of trial mixtures were prepared and a trial section was constructed. The results of the above tests were supplemented with laboratory measurements and in situ tests that were performed during the quality control of the dam construction. Additionally, results of trial mixtures using flysch excavation material from the dam area and limestone aggregates with clay lumps from a nearby quarry were used. This paper examines the impact of the quality of aggregates and the cementitious materials’ content on compressive strength and also the influence of other factors, such as the percentage of compaction. Certain mathematical relations are suggested that can be used in the design phase of hardfill Dams as well as in the quality control during construction. Moreover, the influence of the cement content on the compressive strength and the percentage of the replacement of cement with flying ash are studied.