Joint AVO inversion, wavelet estimation and noise-level estimation using a spatially coupled hierarchical Bayesian model

The main objective of the AVO inversion is to obtain posterior distributions for P-wave velocity, S-wave velocity and density from specified prior distributions, seismic data and well-log data. The inversion problem also involves estimation of a seismic wavelet and the seismic-noise level. The noise model is represented by a zero mean Gaussian distribution specified by a covariance matrix. A method for joint AVO inversion, wavelet estimation and estimation of the noise level is developed in a Bayesian framework. The stochastic model includes uncertainty of both the elastic parameters, the wavelet, and the seismic and well-log data. The posterior distribution is explored by Markov-chain Monte-Carlo simulation using the Gibbs' sampler algorithm. The inversion algorithm has been tested on a seismic line from the Heidrun Field with two wells located on the line. The use of a coloured seismic-noise model resulted in about 10% lower uncertainties for the P-wave velocity, S-wave velocity and density compared with a white-noise model. The uncertainty of the estimated wavelet is low. In the Heidrun example, the effect of including uncertainty of the wavelet and the noise level was marginal with respect to the AVO inversion results.     

Surface water changes in the norwegian sea during last deglacial and holocene times

Stable carbon and oxygen isotopes of the polar planktic foraminifera Neogloboquadrina pachyderma sinistral from sediment cores of the Norwegian Sea reveal several anomalous ¹³C and δ¹⁸O depletions in the surface water during the last glacial to interglacial transition and during the later Holocene. The depletions that are observed between the Last Glacial Maximum (LGM) and the end of the main deglacial phase were caused by massive releases of freshwater from thawing icebergs, which consequently resulted in a stratification of the uppermost surface water layer and a non-equilibrium between the water below and the atmosphere. At ~8.5 ka (¹⁴C BP) this strong iceberg melting activity ceased as defined by the cessation of the deposition of ice-rafted detritus. After this time, the dominant polar and subpolar planktic foraminiferal species rapidly increased in numbers. However, this post-deglacial evolution towards a modern-type oceanographic environment was interupted by a hitherto undescribed isotopic event (~7–8 ka) which, on a regional scale, is only identified in eastern Norwegian Sea surface water. This event may be associated with the final pulse of glacier meltwater release from Fennoscandia, which affected the onset of intensified coastal surface water circulation off Norway during a time of regional sea-level rise. All these data indicate that surface water changes are an integral part of deglacial processes in general. Yet, the youngest observed change noted around 3 ka gives evidence that such events with similar effects occur even during the later Holocene when from a climatic point of view relativelystable conditions prevailed.     

Hydrothermal Cu-Ni-Au-Ag mineralization in a granodiorite sill north of Cradock, Republic of South Africa

An undulating sheet-like granodiorite body is intrusive into sedimentary rocks and dolerite of the Karoo Supergroup north of Cradock. The top of this sheet, where in contact with dolerite, is characterised by the presence of graphite containing nodules of sedimentary origin, and by sulphide-arsenide mineralization. REE and other geochemical evidence indicate that the granodiorite and the ore-bearing horizon are genetically related, but the latter is more altered by hydrothermal fluids. Similarities in REE distribution patterns also imply very little assimilation of sedimentary material. The granodiorite displays plume-like features signifying that the material was plastic or semi-molten when segregation of the plume-like features took place. The Ni-mineralisation in the ore-bearing horizon is not linked to the MgO or Fe₂O₃ of the granodiorite or of the ore-bearing zone, and is regarded as a separate event of hydrothermal origin. Received: 26 July 1996 / Accepted: 17 March 1997     

Mineralogical and chemical variations in hypogene and supergene kaolin deposits in a mobile fold belt the Central Andes of northwestern Peru

In western Peru kaolin-alunite deposits occur in Lower Cretaceous and Tertiary clastic, volcaniclastic and volcanic, mostly rhyolitic, rocks. Alunites from␣hypogene kaolin deposits yield  K /Ar ages of 11.5 ±␣0.7␣Ma and 13.3 ± 0.4 Ma. In addition to kaolin and alunite, the following minerals are present: white mica, smectite, barite, pyrophyllite, tridymite, cristobalite, α- and β-quartz, chamosite, gibbsite, and aluminum-phosphate-sulphate minerals (APS). APS mineralizations with REE-bearing svanbergite and florencite originate from supergene alteration. Woodhouseite, goyazite, crandallite and pure svanbergite develop in hypogene and supergene kaolin deposits. The distinction between hypogene and supergene kaolinization can be made using various element ratios in kaolin (P vs. S, Zr vs. Ti, Cr + Nb vs. Ti + Fe, and Ce + Y + La vs. Ba + Sr). S,␣Ba, and Sr are considerably enriched in kaolin during hydrothermal alteration, whereas Cr, Nb, Ti and lanthanide elements are concentrated mainly during weathering. Au and Ag become enriched during hypogene kaolinization (advanced argillitization). Kaolinization is associated with the evolution of the Central Andes as follows: (1) during the Lower Cretaceous kaolinization characterizes phases of relative tectonic quiescence during mountain building and took place in a miogeosynclinal back-arc basin. The kaolin-bearing sediments were laid down in flood plain to delta plain environments; (2) in the magmatic arc/back-arc basin (eugeosyncline) kaolinization was mainly associated with uplift and peneplanation; (3) in the magmatic arc proper, late Miocene kaolinization of volcanic and volcaniclastic rocks has many features in common with the high sulphidation epithermal Au deposits. Received: 11 August 1995 / Accepted: 8 May 1996     

Late Holocene variations in Arctic shelf hydrology and sea-ice regime: evidence from north of the Lena Delta

Diatom assemblage studies are used to interpret past changes in river runoff (salinity) and sea-ice regime in the vicinity of the vast Lena River delta, southern Laptev Sea shelf. On the basis of their distribution in surface sediments, the shelf region outside the strong influence of riverine waters is characterized by a dominance in sea-ice diatoms and other marine species. Their numbers increase steeply (>20%) within the area of drifting pack ice. In contrast, the marginal zone of the delta, where exceedingly low salinities prevail, is marked by freshwater diatoms showing values higher than 70%. Using the environmental information from the surface sediments, the downcore distribution patterns of the main ecological groups of diatoms were investigated on a sediment core that covers the past 2800 cal. years BP. Although the freshwater group indicates some temporal variations in salinities, the study site north of the Lena River delta remained under a dominantly riverine influence for most of the three recognized phases. In contrast, the relative abundance of sea-ice species gives evidence that pack-ice conditions were more severe during the oldest phase (older than ~2700 cal. years BP). The most significant changes are observed in the uppermost core section (younger than ~300 cal. years BP) when the relative abundance of freshwater diatoms decreases from 80% down to below 20%. This dramatic decrease is interpreted as a major shift from a more northward-directed to the modern, dominantly eastern outflow pattern. Because the dispersal and fate of riverine waters and its role on the ice regime as well as on water mass properties is a central issue in understanding short- and longer-term climatic changes in the Arctic and beyond, it needs to be tested using more cores if this most recent change in outflow pattern from the delta is connected to climate change or simply a result of channel migration within the delta.     

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Machine learning based identification of dominant controls on runoff dynamics

Hydrological models used for flood prediction in ungauged catchments are commonly fitted to regionally transferred data. The key issue of this procedure is to identify hydrologically similar catchments. Therefore, the dominant controls for the process of interest have to be known. In this study, we applied a new machine learning based approach to identify the catchment characteristics that can be used to identify the active processes controlling runoff dynamics. A random forest (RF) regressor has been trained to estimate the drainage velocity parameters of a geomorphologic instantaneous unit hydrograph (GIUH) in ungauged catchments, based on regionally available data. We analyzed the learning procedure of the algorithm and identified preferred donor catchments for each ungauged catchment. Based on the obtained machine learning results from catchment grouping, a classification scheme for drainage network characteristics has been derived. This classification scheme has been applied in a flood forecasting case study. The results demonstrate that the RF could be trained properly with the selected donor catchments to successfully estimate the required GIUH parameters. Moreover, our results showed that drainage network characteristics can be used to identify the influence of geomorphological dispersion on the dynamics of catchment response.     

Pleistozäne Salzstockbewegungen in Norddeutschland und ihre regionale Anordnung

Zusammenfassung Zahlreiche Salzstöcke in Nordwestdeutschland haben sich seit der letzten überdeckung mit eiszeitlichen Absätzen nachträglich bewegt. An der Oberfläche wurden dadurch einzelne Bereiche gehoben, andere senkten sich infolge Salzauslaugung. Stärkere Bewegungen sind auf Salzstrukturen beschränkt, die entlang tektonischer Schwächezonen aufgedrungen sind. Die jüngste Salztektonik setzt heute an der Oberfläche die gleichen exogenen Faktoren in Bewegung, welche Fazies und Lagerung im vorpleistozänen Deckgebirge der Strukturen beeinflu\t haben.