Fractal generation of surface area of porous media

Many natural porous geological rock formations, as well as engineered porous structures, have fractal properties, i.e., they are self-similar over several length scales. While there have been many experimental and theoretical studies on how to quantify a fractal porous medium and on how to determine its fractal dimension, the numerical generation of a fractal pore structure with predefined statistical and scaling properties is somewhat scarcer. In the present paper a new numerical method for generating a three-dimensional porous medium with any desired probability density function (PDF) and autocorrelation function (ACF) is presented. The well-known Turning Bands Method (TBM) is modified to generate three-dimensional synthetic isotropic and anisotropic porous media with a Gaussian PDF and exponential-decay ACF. Porous media with other PDF's and ACF's are constructed with a nonlinear, iterative PDF and ACF transformation, whereby the arbitrary PDF is converted to an equivalent Gaussian PDF which is then simulated with the classical TBM. Employing a new method for the estimation of the surface area for a given porosity, the fractal dimensions of the surface area of the synthetic porous media generated in this way are then measured by classical fractal perimeter/area relationships. Different 3D porous media are simulated by varying the porosity and the correlation structure of the random field. The performance of the simulations is evaluated by checking the ensemble statistics, the mean, variance and ACF of the simulated random field. For a porous medium with Gaussian PDF, an average fractal dimension of approximately 2.76 is obtained which is in the range of values of actually measured fractal dimensions of molecular surfaces. For a porous medium with a non-Gaussian quadratic PDF the calculated fractal dimension appears to be consistently higher and averages 2.82. The results also show that the fractal dimension is neither strongly dependent of the porosity nor of the degree of anisotropy assumed.     

Geochronology of high-pressure rocks on Sifnos (Cyclades,Greece)

Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M ₁ phengite.K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M ₁+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon.K-Ar and Rb-Sr ages on 2M ₁ phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.     

Ein Algizidtest unter Laboratoriumsbedingungen

Vorgestellt wird cine Algizidtestvariante, die es erlaubt, unter laboratoriumsmäßig standardisierten Bedingungen ein repräsentatives Sortiment von Vertretern aus den Gruppen der Cyanophyceen, Diatomeen sowie monadaler, kokkaler und ulotrichaler Chlorophyceen zur Überprüfung der Wirksamkeit neu entwiekelter Substanzen einzusetzen. Die Testergebnisse liegen bereits nach 10 bzw. 24 h vor. Die elf Algenreinkulturstämme werden asynchron in Suspensionen bei 30 °C unter Dauerlicht kultiviert. Die Überprüfung der Wirksamkeit der Herbizide Amitrol, Paraquat, Atrazin, Fenuron und 2,4-D-Säure sowie der Algizide Quartolan, Cumasina ,,aktiv“ und Dimanin A zeigte, daß sich der als Bezugsorganismus eingesetzte Reinkulturstamm der Species Chlorella vulgaris (A 6) in seiner Empfindlichkeit gegenüber algiziden Substanzen als repräsentativ für einen großen Teil der übrigen Testorganismen erwies. Abweichungen in der Algizidempfindlichkeit einzelner Vertreter haben ihre Ursache vorwiegend in physiologischen und anatomisehen Besonderheiten bzw. im jeweiligen Volumen-Oberflächenverhältnis und der Bildung von Coenobien. Deshalb kann ein Test mit den ausgewählten Organismen in kurzer Zeit eine umfassende Aussage über den Wirkungsgrad algizider Substanzen ermöglichen.     

The influence of sea waves on the wind profile

From wind profile and wave measurements performed during the JONSWAP II experiment, relations between the dimensionless profile slope and the significant wave height are derived. It is shown that the wind profile is distorted by the waves especially in the vicinity of the water surface. The wave influence on the profile seems to be restricted to heights below about three wave heights. Above this level, the dimensionless profile slope is an approximately constant value corresponding to a drag coefficient of about 1.15 × 10^–3.     

USERS′ PERCEPTION OF KOWLOON PARK, HONG KONG： VISITING PATTERNS AND SCENIC ASPECTS

Hong Kong is a hyper-dense city with 7×10⁶ people living in an area of 1100km². One way to improve the livability of compacted and congested cities like Hong Kong is through the provision of urban parks, an aspect that has largely been under-researched. This study focuses on how users perceive and utilize various facilities in the Kowloon Park. The findings revealed that the Kowloon Park is one of the most preferred parks in Hong Kong for both local residents and tourists. Users were quite satisfied with the park’s facilities. Notably, the most important component of an urban park is its greenery. This is followed by water elements, seating places, and facilities for various recreational activities. The improvements users would like to see in urban parks include good design and management, meeting users’ needs, overcoming barriers to use, and providing a high quality and varied experience for different groups in the community. The findings of this study provide a good basis to address park management issues from the users’ perspective. In particular, parks should provide easy access, encourage optimum usage and enable complimentary improvements to the environment. Foundation item: Under the auspices of the Germany-Hong Kong Joint Research Scheme (No.GER/99-00/01) and the Faculty Research Grant of the Hong Kong Baptist University (No.FRG/00-01/I-38) Biography: WONG Koon-kwai (1950—), male, a native of Guangdong Province, Ph.D., associate professor, specialized in the analysis of human-environment interaction. E-mail: kenwong@hkbu.edu.hk     

Late Pleistocene–Holocene rise and collapse of Lake Suguta,northern Kenya Rift

The Late Pleistocene to Middle Holocene African Humid Period (AHP) was characterized by dramatic hydrologic fluctuations in the tropics. A better knowledge of the timing, spatial extent, and magnitude of these hydrological fluctuations is essential to decipher the climate-forcing mechanisms that controlled them. The Suguta Valley (2°N, northern Kenya Rift) has recorded extreme environmental changes during the AHP. Extensive outcrops of lacustrine sediments, ubiquitous wave-cut notches, shorelines, and broad terrace treads along the valley margins are the vestiges of Lake Suguta, which once filled an 80 km long and 20 km wide volcano–tectonic depression. Lake Suguta was deep between 16.5 and 8.5 cal ka BP. During its maximum highstand, it attained a water depth of ca 300 m, a surface area of ca 2150 km², and a volume of ca 390 km³. The spatial distribution of lake sediments, the elevation of palaeo-shorelines, and other geomorphic evidences suggest that palaeo-Lake Suguta had an overflow towards the Turkana basin to the north. After 8.5 cal ka BP, Lake Suguta abruptly disappeared. A comparison of the Lake Suguta water-level curve with other reconstructed water levels from the northern part of the East African Rift System shows that local insolation, which is dominated by precessional cycles, may have controlled the timing of lake highstands in this region. Our data show that changes of lake levels close to the Equator seem to be driven by fluctuations of spring insolation, while fluctuations north of the Equator are apparently related to variations in summer insolation. However, since these inferred timings of lake-level changes are mostly based on the radiocarbon dating of carbonate shells, which may have been affected by a local age reservoir, alternative dating methods are needed to support this regional synthesis. Between 12.7 and 11.8 cal ka BP, approximately during the Northern Hemisphere high-latitude Younger Dryas, the water level of Lake Suguta fell by ca 50 m, suggesting that remote influences also affected local hydrology.     

Wide-angle vibroseis test across the Rhine graben

Summary. As a joint operation of ECORS and DEKORP, a deep wide-angle seismic experiment using vibrators was carried out in the autumn of 1984. The object was to get information on the deep crust under the Rhine graben without crossing through sedimentary layers. Offsets were in the range of 50 to 89 km. In the first phase, two vibration points were executed in the Vosges mountains. A signal was received in the Black Forest from solely the farthest VP. In the second phase, fourteen VPs were executed in the Black Forest. No stacking or correlation was performed in the field in France. The quality of the results is good only if an equalization is applied before vertical stacking and correlation in the computing centre.