ALBERT

Beschreibung: The German lake district "Mecklenburger Kleinseenplatte" between Berlin and Rostock shows a great variety of lakes having different trophical states. Remote sensing of water quality is investigated to help monitoring the lake waters and to guide the water sampling more carefully over space and time. Satellite data of the Indian Remote Sensing Satellite of summer 1996 are analysed for a general overview of the variety of lake water within the test area. To get more quantitative information hyperspectral scanner data of the Compact Airborne Spectrographic Imager (casi) are investigated. For these data a ratio of two bands pointing to chlorophyll absorption at 680 nm and a hull function over the absorption band have been applied. The results could be well related to the ground truth data taken by the Institute of Fresh Water Ecology in Brandenburg. They show differences in chlorophyll content even in December, a season of minimal bioproduction. Thus, further research in the summer season is promising to get good results in estimation of water quality parameters.

Beschreibung: Zusammenfasung: In den letzten 20 Jahren ist das globale Netz von Erdbebenstationen (GSN) modernisiert und erweitert worden. Es umfasst jetzt eine grosse Zahl von digitalen Stationen, die die seismischen Signale hochaufloesend ueber einen weiten Frequenzbereich registrieren. Durch diese internationalen Bemuehungen, an denen sich das GFZ im Rahmen des GEOFON-Programms beteiligt, hat sich nicht nur die Qualitaet der Erdbebenueberwachung deutlich verbessert, sondern es ist nun auch moeglich, die Feinstruktur des Erdinnerns mit hoeherer Praezision zu untersuchen. Letzteres wird hier an zwei Beispielen ueber die Tiefenerstreckung von kontinentalen und ozeanischen Strukturen sowie ueber die Feinstruktur der Uebergangszone zwischen oberem und unterem Mantel verdeutlicht. Durch die temporaere Verdichtung seismischer Netze mittels portabler Stationen ist es moeglich, spezifische Fragen zur Struktur der Lithosphaere und das gesamten Erdinnern zu untersuchen. Hierzu werden gewoehnlich Registrierungen von Fernbeben herangezogen, aus denen Strukturbilder in den Tiefen abgeleitet werden koennen, die mit explosionsseismischen Quellen wegen der geringeren abgestrahlten Energie nicht ausreichend durchstrahlt werden koennen. Als Beispiel hierzu zeigen wir Ergebnisse von einem Feldexperiment in Tibet. Abstract Starting about 20 years ago the global network of seismograph stations (GSN) has been upgraded and expanded to a large number of digital stations recording seismic signals with high resolution in a very broad frequency band. This coordinated international effort, with GFZ Potsdam contributing through its GEOFON program, has improved considerably the monitoring capabilities of seismic networks, and it provides the data that allow us to study Earth structure in unpredecedented detail. This is demonstrated for two examples dealing with the depth extent of continental and oceanic structure, and the transition zone between upper and lower mantle. In addition to permanent seismograph stations, portable seismograph networks have been used to temporarily increase the station density in areas of scientific interest, thus enabling detailed studies of both the structure of the lithosphere and the entire globe using data from distant earthquakes. The methods developed for the processing and interpretation of earthquake recorderings have resulted in improved structural images at greater depths that are difficult to probe by explosions because of their limited energy. This is demonstrated in an example of lithospheric and upper mantle studies in Tibet.

Beschreibung: Short-period three-component recordings of the seismic wave field of peaceful nuclear explosions (PNEs) along several long-range profiles in northern Eurasia are analyzed to study the structure of the upper mantle. We use the date to search for evidence of the disputed 520 km discontinuity in the mantle transition zone. Here we present short-period evidence for the existence of this proposed mantle discontinuity. On three long-range profiles in Siberia we observe in the distance range from 2400 to 2600 km a typical change of the apparent velocity of the first arrivals and the appearance of two characteristic travel time branches. We interpret these phenomena as being caused by the proposed upper mantel discontinuity. To determine the optimum one-dimensional P wave velocity model, synthetic seismograms were calculated with the reflectivity method. By comparing them with the observed data, we estimate the P wave velocity contrast across the 520 km discontinuity to be 0.25 +- 0.0005 km/s, while the velocity gradient below the boundary is 0.001 +- 0.0005 1/s. This result indicates that in the mantle transition zone both phase transitions and variations in chemical composition can occur.

Beschreibung: Hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts. By analysing Nd, Pb and Be isotope profiles within crusts it is possible to reconstruct seawater tracer histories. Depth profiles of 10Be/9Be ratios in three Pacific ferromanganese crusts have been used to obtain growth rates which are between 1.4 and 3.8 mm/Ma. Nd and Pb isotopes provide intact records of isotopic variations in Pacific seawater over the last 20 Ma or more. There were only small changes in Pb isotope composition in the last 20 Ma. This indicates a constant Pb composition for the erosional sources and suggests further that erosional Nd inputs may have been uniform too. εNd values vary considerably with time and most probably reflect changes in ocean circulation. The Epsilon(tief:Nd) values of the crusts not only vary as a function of age but also as a function of water depth. From 25 to 0 Ma, crust VAl3/2 from 4.8 km water depth has a similar pattern of εNd variation to the two shallower crusts from 1.8 and 2.3 km, but about 1.0 to 1.5 units more negative. This suggests that εNd stratification in Pacific seawater, as demonstrated for the present day, has been maintained for at least 20 Ma. Each crust shows a decrease in εNd from 3-5 Ma to the present, which is interpreted in terms of an increase in the NADW component present in the Pacific. From 10 to 3-5 Ma ago the crusts show an increase in εNd. This suggests a decreasing role for a deep water source with εNd less than circum-Pacific sources. In this regard the Panamanian gateway restriction from ~ 10 Ma with final closure at 3-5 Ma may have played an important role in reducing access of Atlantic-derived Nd to the Pacific.

Beschreibung: The architecture and implementation of a rule-based fuzzy approach to the detection and classification of man made objects in satellite image data are presented. In a first processing step fuzzy clustering is used to obtain an initial coarse presegmentation of the data. Subsequently, a rule-based fuzzy system performs the classification. Its rules are generated both manually (based on common-sense or higher level expert knowledge) and automatically by a neural network evaluating image features which are difficult to extract or to evaluate directly. Some experimental results, a discussion of the potential of the approach and directions of possible future research conclude the paper.

Beschreibung: During the Kenya Rift International Seismic Project (KRISP 90) a 450-km east-west seismic profile was shot across the rift in the vicinity of the Equator. Reflectivity modelling of some of the P- and S-phases combined with results derived from the ray-trace forward model enable more information to be extracted from the data set. The Pg phase at the western end of the profile required a different velocity-depth gradient in the Archaean and the Proterozoic terranes crossed by the profile. This, combined with structural and seismic velocity differences derived from the forward model, provides the means for the two units to be distinguished throughout the whole crust. A phase in the PmP coda from shot points within the rift has been successfully modelled as a PmP multiple reflected from the base of or from within the rift infill. The clearest S-phase on the cross-rift profile, SmS, has been reproduced by reflectivity modelling and compared to the same phase from the KRISP 90 flank line: combining these observations with the results from the spectral analysis of the principal P- and S-phases observed from all the shot points on the cross-rift profile supports the contention that most of the seismic attenuation occurs immediately beneath the rift in the upper crust or in the rift infill, with little attenuation occurring in the lower crust.

Beschreibung: The Middle Urals are characterized by a major virgation in the linear trend of the Urals orogen, and represent the most highly contracted part of the late Palaeozoic collisional belt. This part of the orogen is dominated by metamorphic complexes and major fault and shear zones. The Main Uralian Fault zone (MUF), the east-dipping suture of the orogen containing low-grade metamorphic rocks, separates the Sysert Complex in the east from the Ufaley Complex in the west. The Sysert Complex in the hanging wall of the MUF consists of intensely deformed gneisses, granitic intrusions and a metamorphosed mélange zone. Tectonic and isotopic investigations suggest the following stages for the evolution of the Sysert Complex: (a) pre-orogenic rifting and magmatism during Ordovician and Silurian times; (b) oceanic closure, island arc formation related to convergence and subduction during Devonian times; (c) major ductile deformation under amphibolite facies conditions related to NW-directed thrusting associated with crustal stacking during collision in Carboniferous times; (d) exhumation and contractional intracontinental tectonics during Permian times; and (e) closing of isotope systems related to cooling and the end of orogenic shortening through Triassic times. The Ufaley Complex, in the footwall of the MUF, is interpreted as an east-dipping crustal stack that records an amphibolite facies Uralian metamorphism. Lithologically the complex can be divided into pre-orogenic European basement (West Ufaley) and intensely deformed Palaeozoic metasediments and amphibolites (East Ufaley). High-pressure relics in the East Ufaley Complex are interpreted to be the result of subduction, whereas intense ductile deformation is related to overthrusting onto West Ufaley. The West Ufaley Complex is composed of gneisses, amphibolites, migmatites and granitic intrusions and has been thrust onto Devonian limestones along a major shear zone. In both Sysert and Ufaley Complexes, NW-trending stretching lineations and top-to-the-NW kinematic indicators suggest an oblique plate convergence with a significant sinistral component. The MUF is interpreted as a major normal fault that developed congruent with continental subduction and that compensated lithospheric thickening and the rapid exhumation of subducted crust in the footwall.

Beschreibung: The seismic refraction-wide-angle reflection experiments carried out in 1985 and 1990 in the Kenya rift (KRISP '85 and KRISP '90) show major crustal thickness variations both along and across the rift. Along the rift axis crustal thickness varies from 35 km in the south beneath the Kenya dome to 20 km in the north beneath the Turkana region. Due to the distribution of crustal thickness beneath the rift flanks, it can be stated that the major amount of variation in crustal thickness along the rift axis is due to the Tertiary rifting episode. The northwards decrease in crustal thickness can be correlated with changes in surface topography (northwards decrease), rift width (northwards increase), surface estimates of extension (5–10 km in the south and 35–40 km in the north) and Bouguer gravity, the regional northwards increase of which can be explained entirely by the change in crustal thickness. Below the 750 km long axial rift profile, uppermost mantle P(tief)n velocities are low, being 7.5–7.7 km/s. However, under the northern part of the rift two layers with velocities of 8.1 km/s and 8.3 km/s are embedded in the low-velocity mantle material at 40–45 km and 60–65 km depth, respectively. In contrast, the wide-angle data show that beneath the Kenya dome, in the southern part of the rift, low mantle velocities occur down to at least 65 km depth. This mantle velocity structure is indicative of the depth to the onset of melting being at least 65 km beneath the northern part of the rift and thus not being shallower than the depth (45–50 km) to the onset of melting under the Kenya dome to the south. A profile across the rift north of the Kenya dome at the latitude of Lake Baringo shows that the low uppermost mantle P(tief)n velocity of 7.5–7.7 km/s and crustal thinning of 5–10 km is confined to below the surface expression of the rift. An abrupt change in Moho depths and P(tief)n velocities occurs as the rift boundaries are crossed. Beneath the rift flanks, normal P(tief)n velocities of 8.0–8.2 km/s occur. The presence of hot mantle material beneath the Kenya dome since the onset of volcanism here at 15–20 Ma is still compatible with the abrupt change in mantle P-wave velocities as the rift boundaries are crossed. Petrological interpretation of the seismic velocities indicates a few (up to 5) percent basaltic melt in the mantle below the rift except in the two layers with velocities greater than 8.0 km/s under the northern part of the rift where some crystal orientation (anisotropy) is necessary. Below about 45–50 km depth beneath the southern part of the rift the magma could exist as in situ partial melt. The above results, taken together with results from teleseismic studies, petrology and surface geology, indicate anomalously hot mantle material appearing below the present site of the Kenya rift at about 20–30 Ma. The active uprising of this anomalously hot mantle material since this time has given rise to widespread volcanism along the whole length of the rift and has modified the crust beneath the rift by mafic igneous underplating and intrusion, especially into the basal crustal layer. Accompanying the uprise of the anomalously hot mantle material minor crustal extension (5–10 km) has occurred beneath the Kenya dome in the southern part of the rift where crustal thickness is large (35 km). Under the Turkana region in the northern part of the rift, a greater amount of extension (35–40 km) has taken place and the crustal thickness is small (20 km), although the depth to the onset of melting under the northern part of the rift is, if anything, greater than under the southern part of the rift.

Beschreibung: The architecture of the European Variscides has been subdivided by Kossmat (1927) into paleogeographically coherent units which are presently interpreted as former plate fragments. The Mid-German Crystalline Rise (MGCR) separates two fragments (Rhenohercynian and Saxothuringian belts) at the site of an inferred plate boundary and reequilibrated orogenic root. The commonly favoured model interprets the MGCR as the magmatic arc on Saxothuringian crust above a south-dipping subduction zone in Upper Devonian and Carboniferous times. Data from the MGCR, the kinematic evolution of the Mid-European Variscides, and first order volume balancing suggest a reinterpretation of this unit which challenges classical views on the MGCR as well as on the subdivision of Variscan architecture. The MGCR is composed of two rock groups with different tectonic identity. A Lower Carboniferous low pressure-high temperature magmatic arc association on Lower Paleozoic basement rests tectonically on a stack of medium pressuremedium temperature rocks of inferred Rhenohercynian origin. The latter were tectonically accreted to the base of the overriding plate by tectonic underplating. The entire process was controlled by oblique convergence. This led to regional partitioning of the plate kinematic vector into contractional domains (lower Rhenohercynian plate and back-arc area of the upper Saxothuringian plate), bulk heterogeneous plate margin parallel extensional domains (MGCR), and plate margin parallel wrench domains (MGCR boundaries). During this process material was continually transferred from the lower plate to the upper plate, uplifted and exhumed by net crustal extension. The concomitant removal of parts of the former arc and the entire orogenic root necessitates a reappraisal of Variscan architecture and evolution.

Beschreibung: Additional keywords: compensated sonic, National Energy Authority Orkustofnun, seismicity, gesteinsphysikalische Parameter, Wiederholungsmessungen, spezifischer Widerstand, Wellen-Geschwindigkeiten, Porositaet, Island, gesteinsphysikalische Parameter, spezifischer Widerstand, Wellen-Geschwindigkeiten, Porositaet

Beschreibung: Combined on- and offshore wide-angle seismic data were collected in southern Costa Rica along a profile that extends from the Middle America Trench and crosses the Pacific continental margin, the isthmus, and reaches to the Caribbean coast. Closely spaced marine airgun sources were used in the Pacific, coincident with the preexisting seismic reflection line P-1600 recorded by SHELL Int. Four large explosive charges were detonated in boreholdes onshore. The combined interpretation of the near-vertical and wide-angle data provides a detailed crustal model. The profile is located on the northern flank of Cocos Ridge, where we have determined that the subducting oceanic plate has a thickness of 11 to 12 km. It dips down with an average angle of 17° and can be imaged to a depth of at least 35 km with the available data. Erosion of the margin wedge is suggested by a thick low velocity zone overlying the downgoing slab. Like elsewhere along the Middle America Trench, the body of rock underlying forearc and slope sediment has high velocities (〉4.0 km/s). At deeper levels (〉5 km), the crustal rocks along the profile generally show high velocities (6.0 to 6.6 km/s). To the southwest, these high velocities can be followed to the mid-shelf area. The inverted Térraba Basin contains intermediate velocity sediment (3.0 km/s) with a total thickness of about 2 km. The Limón Basin has up to 6 km of Tertiary and Quaternary sediment. An intracrustal discontinuity is found at a depth of about 18 km, but cannot be determined in detail due to the limited offset of the available data.

Beschreibung: Additional keywords: Exkursionen