Die offenen sternhaufen M36, M38 und NGC 2477

The open star clusters M36, M38 and NGC 2477 were investigated by the strip method on charts of photographs taken with the Schmidt-Spiegel of the German Observatory Hoher List (M36/38) and the Curtis-Schmidt Telescope of the Cerro Tololo Interamerican Observatory (NGC 2477). The results are — Masses: 830, 2350, 4400 ; radii: 6.4, 10.0, 10.0 pc; central densities: 9.9, 15.3, 32.5 pc^–3; density laws: Gaussian (M36), generalized law of Schuster withn=3.00, 2.79; mean velocities of the stars 0.74, 1.01, 1.38 km s^–1.

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Mitteilungen Serie A.     

Periglacial evidence for a 1.91–1.89 Ga old glacial period at low latitude,Central Sweden

Cryoturbation and slump fold‐like sedimentary structures in ca. 1.9 Ga old dacitic metavolcanic sediments in West Bergslagen, Central Sweden, are recognized as a lowland periglacial environment. This type of environment is comparable with present day tundra in Siberia. Ice‐wedge casts and cryoturbation, together with polygonal frost patterns, are typical geomorphological structures above permafrost in this type of environment. The sedimentary environment could be interpreted as periglacial, broadly comparable to present day tundras. Intensive cryoturbation of the formation and close structural analogy with Quaternary ice‐wedges suggests a cold and humid environment. This discovery is corroborated by a previous report of glacial sediments and structures from NW Australia of ca. 1.8 Ga age. Both occurrences developed at low geographical latitudes, at locations far apart in the Late Palaeoproterozoic supercontinent Columbia. Either suggest the existence of a ca. 100 Ma long epoch of extreme, though possibly intermittent glaciations during the ca. 1.4 Ga long ‘Proterozoic gap’ (～2.2–0.77 Ga) from which no convincing glacial deposits were previously known.     

The Proterozoic (1.85 Ga) Älvestorp Conglomerate,Bergslagen, Central Sweden

The Älvestorp conglomerates, deposited in an alluvial fan setting, form part of the Svecofennian orogenic belt in west Bergslagen, south central Sweden and are estimated to be as old as c. 1.85 Ga. Reaching a thickness of one kilometre, their architecture and form suggest an alluvial origin. Along sections, massive conglomerates often grade into pebbly mudstones and greenschist facies slates, while pure slates with dolomite concretions and olistolites occur in dark slatey mudstones on the eastern shore of lake Brunnsjön. Inner fan trenches are filled with massive, clast‐supported conglomerates that contain more than 80 percent epiclastic tuffaceous material. The Älvestorp conglomerate is therefore classified as the product of a Proterozoic stream‐flow channel and debris flow, or alluvial fan. The Grythyttan Basin to the north originated by extension after the first of two orogenic stages of Bergslagen.     

Stratigraphie und Tektonik des Hochlandes von Costa-Rica

Ohne Zusammenfassung     

Spatial distribution and seasonal variation in 18O/16O of modern precipitation and river water across the conterminous USA

We report a quantitative analysis of regional differences in the the oxygen isotope composition of river water and precipitation across the USA because data are now available to undertake a more geographically and temporally extensive analysis than was formerly possible. Maps of modern, mean annual δ¹⁸O values for both precipitation (δ¹⁸O_PPT) and river water (δ¹⁸O_RIV) across the 48 contiguous states of the USA have been generated using latitude and elevation as the primary predictors of stable isotope composition while also incorporating regional and local deviations based on available isotopic data. The difference between these two maps was calculated to determine regions where δ¹⁸O_RIV is significantly offset from local δ¹⁸O_PPT. Additional maps depicting seasonal and extreme values for δ¹⁸O_RIV and δ¹⁸O_PPT were also constructed. This exercise confirms the presence of regions characterized by differences in δ¹⁸O_RIV and δ¹⁸O_PPT and specifically identifies the magnitude and regional extent of these offsets. In particular, the Great Plains has δ¹⁸O_RIV values that are more positive than precipitation, while much of the western USA is characterized by significantly lower δ¹⁸O_RIV values in comparison with local δ¹⁸O_PPT. The most salient feature that emerged from this comparison is the ‘catchment effect’ for the rivers. Because river water is largely derived from precipitation that fell upstream of the sample locality (i.e. at higher elevations) δ¹⁸O_RIV values are often lower than local δ¹⁸O_PPT values, particularly in catchments with high‐elevation gradients. Seasonal patterns in the isotopic data substantiate the generally accepted notion that amplitudes of δ¹⁸O variation are greatly dampened in river water relative to those of local precipitation. Copyright © 2005 John Wiley & Sons, Ltd.     

Extending illumination using all multiples: application to 3D acquisition geometry analysis

Recent advances in survey design have led to conventional common‐midpoint‐based analysis being replaced by subsurface‐based seismic acquisition analysis, with emphasis on advanced techniques of illumination analysis. Among them is the so‐called focal beam method, which is a wave‐equation‐based seismic illumination analysis method. The objective of the focal beam method is to provide a quantitative insight into the combined influence of acquisition geometry, overburden structure, and migration operators on the resolution and angle‐dependent amplitude fidelity of the image. The method distinguishes between illumination and sensing capability of a particular acquisition geometry by computing the focal source beam and the focal detector beam, respectively. Sensing is related to the detection properties of a detector configuration, whereas illumination is related to the emission properties of a source configuration. The focal source beam analyses the incident wavefield at a specific subsurface grid point from all available sources, whereas the focal detector beam analyses the sensing wavefield reaching at the detector locations from the same subsurface grid point. In the past, this method could only address illumination by primary reflections. In this paper, we will extend the concept of the focal beam method to incorporate the illumination due to the surface and internal multiples. This in fact complies with the trend of including multiples in the imaging process. Multiple reflections can illuminate a target location from other angles compared with primary reflections, resulting in a higher resolution and an improved illumination. We demonstrate how an acquisition‐related footprint can be corrected using both the surface and the internal multiples.     

Deghosting by echo‐deblending

A marine source generates both a direct wavefield and a ghost wavefield. This is caused by the strong surface reflectivity, resulting in a blended source array, the blending process being natural. The two unblended response wavefields correspond to the real source at the actual location below the water level and to the ghost source at the mirrored location above the water level. As a consequence, deghosting becomes deblending (‘echo‐deblending’) and can be carried out with a deblending algorithm. In this paper we present source deghosting by an iterative deblending algorithm that properly includes the angle dependence of the ghost: It represents a closed‐loop, non‐causal solution. The proposed echo‐deblending algorithm is also applied to the detector deghosting problem. The detector cable may be slanted, and shot records may be generated by blended source arrays, the blending being created by simultaneous sources. Similar to surface‐related multiple elimination the method is independent of the complexity of the subsurface; only what happens at and near the surface is relevant. This means that the actual sea state may cause the reflection coefficient to become frequency dependent, and the water velocity may not be constant due to temporal and lateral variations in the pressure, temperature, and salinity. As a consequence, we propose that estimation of the actual ghost model should be part of the echo‐deblending algorithm. This is particularly true for source deghosting, where interaction of the source wavefield with the surface may be far from linear. The echo‐deblending theory also shows how multi‐level source acquisition and multi‐level streamer acquisition can be numerically simulated from standard acquisition data. The simulated multi‐level measurements increase the performance of the echo‐deblending process. The output of the echo‐deblending algorithm on the source side consists of two ghost‐free records: one generated by the real source at the actual location below the water level and one generated by the ghost source at the mirrored location above the water level. If we apply our algorithm at the detector side as well, we end up with four ghost‐free shot records. All these records are input to migration. Finally, we demonstrate that the proposed echo‐deblending algorithm is robust for background noise.     

From accessing to assessing forecasts: an end-to-end study of participatory climate forecast dissemination in Burkina Faso (West Africa)

This study compares responses to seasonal climate forecasts conducted by farmers of three agro-ecological zones of Burkina Faso, including some who had attended local level workshops and others who had not attended the workshops. While local inequalities and social tensions contributed to excluding some groups, about two-thirds of non-participants interviewed received the forecast from the participants or through various means deployed by the project. Interviews revealed that almost all those who received the forecasts by some mechanism (workshop or other) shared them with others. The data show that participants were more likely to understand the probabilistic aspect of the forecasts and their limitations, to use the information in making management decisions and by a wider range of responses. These differences are shown to be statistically significant. Farmers evaluated the forecasts as accurate and useful in terms of both material and non-material considerations. These findings support the hypothesis that participatory workshops can play a positive role in the provision of effective climate services to African rural producers. However, this role must be assessed in the context of local dynamics of power, which shape information flows and response options. Participation must also be understood beyond single events (such as workshops) and be grounded in sustained interaction and commitments among stakeholders. The conclusion of this study point to lessons learned and critical insights on the role of participation in climate-based decision support systems for rural African communities.