Planar trajectory planning and tracking control design for underactuated AUVs

This paper addresses the combined problem of trajectory planning and tracking control for underactuated autonomous underwater vehicles (AUVs) on the horizontal plane. Given a smooth, inertial, 2D reference trajectory, the planning algorithm uses vehicle dynamics to compute the reference orientation and body-fixed velocities. Using these, the error dynamics are obtained. These are stabilized using backstepping techniques, forcing the tracking error to an arbitrarily small neighborhood of zero. Simulation results for a constant velocity trajectory, i.e. a circle, and a time-varying velocity one, i.e. a sinusoidal path, are presented. The parametric robustness is considered and it is shown that tracking remains satisfactory.     

Multi-angle backscatter classification and sub-bottom profiling for improved seafloor characterization

This study applies three classification methods exploiting the angular dependence of acoustic seafloor backscatter along with high resolution sub-bottom profiling for seafloor sediment characterization in the Eckernförde Bay, Baltic Sea Germany. This area is well suited for acoustic backscatter studies due to its shallowness, its smooth bathymetry and the presence of a wide range of sediment types. Backscatter data were acquired using a Seabeam1180 (180 kHz) multibeam echosounder and sub-bottom profiler data were recorded using a SES-2000 parametric sonar transmitting 6 and 12 kHz. The high density of seafloor soundings allowed extracting backscatter layers for five beam angles over a large part of the surveyed area. A Bayesian probability method was employed for sediment classification based on the backscatter variability at a single incidence angle, whereas Maximum Likelihood Classification (MLC) and Principal Components Analysis (PCA) were applied to the multi-angle layers. The Bayesian approach was used for identifying the optimum number of acoustic classes because cluster validation is carried out prior to class assignment and class outputs are ordinal categorical values. The method is based on the principle that backscatter values from a single incidence angle express a normal distribution for a particular sediment type. The resulting Bayesian classes were well correlated to median grain sizes and the percentage of coarse material. The MLC method uses angular response information from five layers of training areas extracted from the Bayesian classification map. The subsequent PCA analysis is based on the transformation of these five layers into two principal components that comprise most of the data variability. These principal components were clustered in five classes after running an external cluster validation test. In general both methods MLC and PCA, separated the various sediment types effectively, showing good agreement (kappa >0.7) with the Bayesian approach which also correlates well with ground truth data (r²?>?0.7). In addition, sub-bottom data were used in conjunction with the Bayesian classification results to characterize acoustic classes with respect to their geological and stratigraphic interpretation. The joined interpretation of seafloor and sub-seafloor data sets proved to be an efficient approach for a better understanding of seafloor backscatter patchiness and to discriminate acoustically similar classes in different geological/bathymetric settings.     

Correlation between the structural pattern and the development of the hydrographic network in a portion of the Western Thessaly Basin (Greece)

In the context of the present study the structural pattern in the Western Thessaly Basin (Greece) has been examined, based on structural data collected from the entire study area that were further correlated to the hydrographic network. The structural pattern of the area was revealed from tectonic analysis. Additionally, the topography, stratigraphy and sedimentology of the study area were taken into account. GIS techniques were used to map the spatial distribution of the geological and tectonic features on the topographic relief of the area. The oldest structures are contractional in nature, deformed by normal faulting related to the extensional episodes initiated in Serravallian times. It is inferred that the orientation of the stress field in the area has changed several times: the N-S stress field which was dominant during Late Serravallian times changed to NW-SE (Messinian-Zanclian) up to E-W in Zanclian and subsequently to roughly NNW-SSE (in late Piacencian). The NE-SW stress that was dominant in Pleistocene became N-S in later times. In addition, some changes in orientation are also indicated for the transitional periods of the pre-mentioned extensional episodes, possibly related to local events, or as a block-related deformation. The development of the 7^th order streams is probably related to the N-S extensional faulting initiated in Pliocene times, while the dominant direction of the 6^th, 5^th and 4^th order streams is possibly connected with the presence of the NNE-SSW and NW-SE extensional faults. Finally, the lower order streams are probably related to the most recent E-W striking normal faults.     

Testing optically stimulated luminescence dating on sand-sized quartz of deltaic deposits from the Sperchios delta plain,central Greece

This study reports on the first investigation into the potential of luminescence dating to establish a chronological framework for the depositional sequences of the Sperchios delta plain, central Greece. A series of three borehole cores(20 m deep) and two shallow cores(4 m deep), from across the delta plain, were extracted, and samples were collected for luminescence dating. The luminescence ages of sand-sized quartz grains were obtained from small aliquots of quartz, using the Single-Aliquot Regenerative-dose(SAR) protocol.The equivalent dose determination included a series of tests and the selection of the Minimum Age Model(MAM) as the most appropriate statistical model. This made it possible to confirm the applicability of quartz Optically Stimulated Luminescence(OSL) dating to establish absolute chronology for deltaic sediments from the Sperchios delta plain.Testing age results of the five cores showed that the deltaic sediments were deposited during the Holocene.A relatively rapid deposition is implied for the top ~14 m possibly as a result of the deceleration in the rate of the sea-level rise and the transition to terrestrial conditions, while on the deeper parts, the reduced sedimentation rate may indicate a lagoonal or coastal environment.     

The geochemistry of Boron-rich groundwater of the Karlovassi Basin, Samos Island, Greece

The upper Miocene of Karlovassi Basin, Samos Island, Greece, contain continental evaporites such as colemanite, ulexite, celestite, gypsum and thenardite. These evaporites are related with volcanic tuffs, diagenetically altered in a saline-alkaline lake environment. The aim of the present paper is to: a) define the impact of the already known and possible buried borates and other evaporites to the geochemistry of the hydrogeological system of Karlovassi Basin, and; b) to assess the correlation between surface and underground evaporite deposits considering the spatial changes in the concentrations of the examined physicochemical parameters. Fieldwork, laboratory measurements and literature data revealed elevated boron values (2136–33012 ?/L) in the central part of Karlovassi Basin. In the same area, high amounts of strontium, sodium, lithium and sulfates also occur. It is proposed that these ions originate from the leaching of evaporites and authigenic minerals such as the Sr-rich clinoptilolite and the boron-bearing potassium feldspar. Boron values are abnormally high for freshwater aquifers, and are indicative of the presence of buried evaporites in the basin with unknown significance.     

Assessment of NO<Subscript>3</Subscript><Superscript>−</Superscript> contamination in a karstic aquifer,with the use of geochemical data and spatial analysis

Kopaida plain is a cultivated region of Eastern Greece, with specific characteristic related with the paleogeographic evolution and the changes in land use. The present article examines the contamination that derives from nitrates, in terms of contaminant levels, definition of sources and spatial distribution of contaminant plume. For this purpose, 50 water samples were collected from the karstic aquifer and analyzed for 15 parameters including major ions, trace elements, physicochemical parameters, and stable isotopes. The assessment of the above parameter values along with the notes derived by the statistical process revealed the existence of nitrate contamination which has been spatial defined with the aid of spatial interpolation techniques. The correlation of NO₃ ⁻ concentrations with the stable isotope values, defined the infiltration conditions and showed contaminant transport. Nitrate values revealed the potential environmental threat for local people, as 10% of the samples exceeded the parametric value of 50 ppm and 54% of them are above 25 ppm, indicating no optimal quality conditions. The origin of nitrate contamination seems to derive exclusively from the application of N-fertilizers, since the rest of potential sources were not verified by analytical data and field works.     

Effective Acceleration Model for the Arrival Time of Interplanetary Shocks driven by Coronal Mass Ejections

In a previous work (Paouris and Mavromichalaki in Solar Phys. 292, 30, 2017), we presented a total of 266 interplanetary coronal mass ejections (ICMEs) with as much information as possible. We developed a new empirical model for estimating the acceleration of these events in the interplanetary medium from this analysis. In this work, we present a new approach on the effective acceleration model (EAM) for predicting the arrival time of the shock that preceds a CME, using data of a total of 214 ICMEs. For the first time, the projection effects of the linear speed of CMEs are taken into account in this empirical model, which significantly improves the prediction of the arrival time of the shock. In particular, the mean value of the time difference between the observed time of the shock and the predicted time was equal to +3.03 hours with a mean absolute error (MAE) of 18.58 hours and a root mean squared error (RMSE) of 22.47 hours. After the improvement of this model, the mean value of the time difference is decreased to ?0.28 hours with an MAE of 17.65 hours and an RMSE of 21.55 hours. This improved version was applied to a set of three recent Earth-directed CMEs reported in May, June, and July of 2017, and we compare our results with the values predicted by other related models.     

An integrated macroelement formulation for the dynamic response of inelastic deformable rocking bodies

This paper presents a macroelement formulation for the prediction of the planar dynamic response of inelastic deformable rocking bodies. The formulation is based on a previous macroelement developed by the authors able to describe the cyclic response of inelastic rocking bodies, which takes into account the deformability both along the height of the member, as well as near the rocking end. Modifications of this formulation to account for other motion modes of rocking members during their dynamic response, namely, sliding and upthrow, as well as modifications to account for damping in a uniform manner during the whole motion, including impacts, are introduced. The dynamic response predicted by the macroelement for free-standing rigid and deformable rocking bodies is presented and compared with existing theoretical solutions, and the effect of deformability, damping, inelasticity, and friction on the response is discussed.     

A Solution to the Problem of Predicting the Suitability of Silty–clayey Materials for Cement-stabilization

Major geotechnical problems in construction involving silty–clayey soils are due to their low strength, durability and high compressibility of soft soils, and the swell–shrink nature of the overconsolidated swelling soils. Confronted with these problems, a suitable ground improvement technique is needed, for deep excavations in soft clays, for stability, durability and deformation control. Cement-stabilization is one of the alternatives. An increase in strength and durability, reduction in deformability are the main aims of this method. Conventional cement-stabilization methods are used mainly for surface treatment. However, the use of cement has recently been extended to a greater depth in which cement columns were installed to act as a type of soil reinforcement (deep cement–soil mixing and cement jet grouting). In situ engineering properties of these silty–clayey soils are often variable and difficult to predict. For this reason cement-stabilization methods have a basic target to control the aforementioned engineering properties of these clays so that the properties of a silty–clayey soil become more like the properties of a soft rock such as clayey shale or lightly cemented sandstone. So cement-stabilization of these soils is essential to control their engineering properties and to predict their engineering behaviour for construction. In an effort to predict, classify and study the suitability of silty–clayey soils for cement-stabilization both slaking and unconfined compressive strength tests were carried out on clayey–sand mixtures consisted of two types of clays, kaolin and bentonite. Finally diagrams were prepared to study the variation of slaking and strength due to compaction, curing time and cement percentage and also to predict areas of efficient cement-stabilization.