Using a simple resistance law to estimate friction velocity from sodar measurements

Sodar measurements are used to study the layer-averaged winds in the unstable Planetary Boundary Layer (PBL). The field data are from the öresund experiment which was carried out over öresund, the strait between Denmark and Sweden during the period May 15 to June 14 1984. A simple resistance law proposed by Garratt et al. (1982) is used to derive friction velocity from sodar measurements of the mean wind. This was found to be in very good agreement with observations.The author is currently doing his military service in the Greek army.     

Present climate trend analysis of the Etesian winds in the Aegean Sea

Wildfires are a common experience in Alaska where, on average, 3,775?km² burn annually. More than 90% of the area consumed occurs in Interior Alaska, where the summers are relatively warm and dry, and the vegetation consists predominantly of spruce, birch, and cottonwood. Summers with above normal temperatures generate an increased amount of convection, resulting in more thunderstorm development and an amplified number of lightning strikes. The resulting dry conditions facilitate the spread of wildfires started by the lightning. Working with a 55-year dataset of wildfires for Alaska, an increase in the annual area burned was observed. Due to climate change, the last three decades have shown to be warmer than the previous decades. Hence, in the first 28?years of the data, two fires were observed with an area burned greater than 10,000?km², while there were four in the last 27?years. Correlations between the Palmer Drought Severity Index and the Canadian Drought Code, against both the number of wildfires and the area burned, gave relatively low but in some cases significant correlation values. Special emphasis is given to the fire season of 2004, in which a record of 27,200?km² burned. These widespread fires were due in large part to the unusual weather situation. Owing to the anticyclonic conditions of the summer of 2004, the composite anomaly of the 500?mb geopotential height showed above normal values. The dominance of a ridge pattern during summer resulted in generally clear skies, high temperatures, and below normal precipitation. Surface observations confirmed this; the summer of 2004 was the warmest and third driest for Interior Alaska in a century of climate observations. The fires lasted throughout the summer and only the snowfalls in September terminated them (at least one regenerated in spring 2005). Smoke from the forest fires affected the air quality. This could be demonstrated by measurements of visibility, fine particle matter, transmissivity of the atmosphere, and CO concentration.     

Adaptive filtering of GOCE-derived gravity gradients of the disturbing potential in the context of the space-wise approach

Filtering and signal processing techniques have been widely used in the processing of satellite gravity observations to reduce measurement noise and correlation errors. The parameters and types of filters used depend on the statistical and spectral properties of the signal under investigation. Filtering is usually applied in a non-real-time environment. The present work focuses on the implementation of an adaptive filtering technique to process satellite gravity gradiometry data for gravity field modeling. Adaptive filtering algorithms are commonly used in communication systems, noise and echo cancellation, and biomedical applications. Two independent studies have been performed to introduce adaptive signal processing techniques and test the performance of the least mean-squared (LMS) adaptive algorithm for filtering satellite measurements obtained by the gravity field and steady-state ocean circulation explorer (GOCE) mission. In the first study, a Monte Carlo simulation is performed in order to gain insights about the implementation of the LMS algorithm on data with spectral behavior close to that of real GOCE data. In the second study, the LMS algorithm is implemented on real GOCE data. Experiments are also performed to determine suitable filtering parameters. Only the four accurate components of the full GOCE gravity gradient tensor of the disturbing potential are used. The characteristics of the filtered gravity gradients are examined in the time and spectral domain. The obtained filtered GOCE gravity gradients show an agreement of 63–84 mEötvös (depending on the gravity gradient component), in terms of RMS error, when compared to the gravity gradients derived from the EGM2008 geopotential model. Spectral-domain analysis of the filtered gradients shows that the adaptive filters slightly suppress frequencies in the bandwidth of approximately 10–30 mHz. The limitations of the adaptive LMS algorithm are also discussed. The tested filtering algorithm can be connected to and employed in the first computational steps of the space-wise approach, where a time-wise Wiener filter is applied at the first stage of GOCE gravity gradient filtering. The results of this work can be extended to using other adaptive filtering algorithms, such as the recursive least-squares and recursive least-squares lattice filters.     

Searching for ancient gold mines in Filippoi area,Macedonia, Greece,using Worldview-2 satellite imagery

The study area is located near the town of Filippoi, north of the city of Kavala in northern Greece, known from ancient times for its rich gold mines, situated inside hydrothermal alteration zones (Fe–Mn oxide minerals). A Very High-Resolution (0.5 m pixel size) image of Worldview-2 satellite was digitally enhanced, yielding target areas of potential ore existence and lineaments. Ground-truth that followed digital image processing, revealed abandoned ancient mines, slags and ore occurrences. Also, a number of lineaments delineated on the satellite image were verified as faults.     

Conditional spectrum bidirectional record selection for risk assessment of 3D structures using scalar and vector IMs

Several proposals are explored for the hazard and intensity measure (IM) consistent selection of bidirectional ground motions to assess the performance of 3D structural models. Recent studies have shown the necessity of selecting records that thoroughly represent the seismicity at the site of interest, as well as the usefulness of efficient IMs capable of estimating the response of buildings with low scatter. However, the advances realized are mostly geared towards the structural analysis of 2D models. Few are the combined record, and IM selection approaches suggested expressly for nonlinear dynamic analysis of 3D structural models, especially when plan asymmetry and torsion sensitivity come into play. Conditional spectrum selection is leveraged and expanded here to offer a suite of approaches based on both scalar and vector IMs that convey information from two orthogonal horizontal components of the ground motion. Applications on multiple 3D building models highlight the importance of (a) employing the same IM for both record selection and response assessment and (b) maintaining hazard consistency in both horizontal components, when using either a scalar or a vector IM. All tested approaches that respect these conditions can be viable, yet the one based on the geometric mean of multiple spectral ordinates from both components over a period range seems to hold the most promise for general use.     

Shake table investigation on the seismic performance of hospital equipment supported on wheels/casters

This paper presents an experimental investigation on the seismic response of medical equipment supported on wheels and/or casters. Two pieces of equipment were tested: a large ultrasound machine and a cart carrying smaller medical equipment. In the first phase, the resistance of the wheels and casters of the equipment was characterized through a controlled‐displacement procedure on the shake table. In the second phase, an extensive shake table test program was carried out to investigate the seismic response of the equipment. The input signals for the shake table tests included floor motions of a four‐story steel braced‐frame hospital designed to satisfy seismic requirements of a site in the Los Angeles area. The results of 96 shake table tests reported in this study include the seismic performance of the equipment under both unlocked and locked conditions, located on various floor levels of the building. It was observed that engaging the casters' locking mechanism does not necessarily decrease the relative displacement. The displacement response was sensitive to the excitation intensity and the orientation of the equipment with respect to the input excitation. Based on the experimental observations, appropriate structural engineering demand parameters associated with the relative displacement and relative velocity demands of the equipment are proposed and used to develop conditional probability curves. Finally, in an effort to extend the results of this experimental study to similar equipment on wheels/casters, the performance of a simple numerical model in predicting the peak seismic demands is evaluated. Copyright © 2016 John Wiley & Sons, Ltd.     

Complex Site Effects in Thessaloniki (Greece): I. Soil Structure and Comparison of Observations with 1D Analysis

This paper presents a 2D model of the geological structure of Thessaloniki city and results of empirical and theoretical approaches for the evaluation of site response due to complex site effects. The construction of the 2D model is based on the available geophysical and geotechnical data in terms of the most important parameters needed to model site response. The well-known subsoil structure, despite the existence of some local uncertainties, gave the possibility to correlate the geometry and the dynamic properties of the 2D model with the results of site response determined from the analysis of one event in frequency and time domains and 1D numerical modelling. The study of site response shows the effect of the lateral variations on ground motion and suggests that the contribution of locally generated surface waves to the resonant peak may be important. In this case history, the limitations of the 1D approximation to simulate ground motion under complex soil conditions in both frequency and time domains are also shown. This paper lays the ground for a companion article dealing with 2D site effects.     

Strong variation in weathering of layered rock maintains hillslope‐scale strength under high precipitation

The evolution of volcanic landscapes and their landslide potential are both dependent upon the weathering of layered volcanic rock sequences. We characterize critical zone structure using shallow seismic V_p and V_s profiles and vertical exposures of rock across a basaltic climosequence on Kohala peninsula, Hawai’i, and exploit the dramatic gradient in mean annual precipitation (MAP) across the peninsula as a proxy for weathering intensity. Seismic velocity increases rapidly with depth and the velocity–depth gradient is uniform across three sites with 500–600 mm/yr MAP, where the transition to unaltered bedrock occurs at a depth of 4 to 10 m. In contrast, velocity increases with depth less rapidly at wetter sites, but this gradient remains constant across increasing MAP from 1000 to 3000 mm/yr and the transition to unaltered bedrock is near the maximum depth of investigation (15–25 m). In detail, the profiles of seismic velocity and of weathering at wet sites are nowhere monotonic functions of depth. The uniform average velocity gradient and the greater depths of low velocities may be explained by the averaging of velocities over intercalated highly weathered sites with less weathered layers at sites where MAP > 1000 mm/yr. Hence, the main effect of climate is not the progressive deepening of a near‐surface altered layer, but rather the rapid weathering of high permeability zones within rock subjected to precipitation greater than ~1000 mm/yr. Although weathering suggests mechanical weakening, the nearly horizontal orientation of alternating weathered and unweathered horizons with respect to topography also plays a role in the slope stability of these heterogeneous rock masses. We speculate that where steep, rapidly evolving hillslopes exist, the sub‐horizontal orientation of weak/strong horizons allows such sites to remain nearly as strong as their less weathered counterparts at drier sites, as is exemplified by the 50°–60° slopes maintained in the amphitheater canyons on the northwest flank of the island. Copyright © 2017 John Wiley & Sons, Ltd.     

Determination of S-wave velocity structure using microtremors and spac method applied in Thessaloniki (Greece)

Array measurements of microtremors at 16 sites in the city of Thessaloniki were performed to estimate the V_s velocity of soil formations for site effect analysis. The spatial autocorrelation method was used to determine phase velocity dispersion curves in the frequency range from 0.8–1.5 to 6–7 Hz. A Rayleigh wave inversion technique (stochastic method) was subsequently applied to determine the Vs profiles at all the examined sites. The determination of V_s profiles reached a depth of 320 m. Comparisons with V_s values from cross-hole tests at the same sites proved the reliability of the SPAC method. The accuracy of the V_s profiles, the ability to reach large penetration depths in densely populated urban areas and its low cost compared to conventional geophysical prospecting, make Mictrotremor Exploration Method very attractive and useful for microzonation and site effects studies. An example of its application for the site characterization in Thessaloniki is presented herein.     

Pre-loading effect on dynamic soil properties: Seismic methods and their efficiency in geotechnical aspects

There are many publications on the investigation of soil properties using seismic prospecting. Among these properties, special attention has been given to shear wave velocity V_S, using more than two different methods for soil and site characterization. In this study, the in-hole, non-invasive refraction and surface wave inversion methods to evaluate soil improvement are investigated. The investigation was conducted on the new Egnatia highway (Northern Greece). Wave velocity profiles have been measured before and after preloading for the construction of an embankment at a soft soil site. The purpose is to quantify the dynamic properties and to evaluate the efficiency of the applied tools in detecting their variation. Among others, an emphasis was given to the observed improvement at particular layers of high sand content.