Phase-array decomposition of a seismic section

A seismic re fraction/wide-angle reflection profile is analysed for the presence of correlated events ('phases'). The correlation problem is formulated in terms of temporally, spatially and frequency-local complex covariances. For robustness, the method concentrates on phase rather than amplitude information. This allows a computationally efficient algorithm that can make allowance for signal correlation length and can model curved wavefronts. A statistical test based on residual phase misfit across the analysed subarray is used to assess the probability that a detected event represents a real correlated signal.
With our chosen analysis parameters and confidence level (over 99.9 per cent). 1222 events were detected in the data. Using simple techniques based on 1-D earth models, detected events are associated with a small number of particular wave types. In this way, we have succeeded in classifying almost 95 per cent of the detected events. Those that remain describe those components of the data that are inconsistent with our simple ray paths in the 1-D assumption and with our prescribed tolerance. These include reverberations, near-surface guided waves and reflected waves from strongly laterally inhomogeneous structures. According to our modelling, about 25 per cent of the detected events are consistent with simple P -wave reflected energy, and these are to a very large extent (over 85 per cent) distinct from all the other wave-type models we have used. A direct mapping of the detected events into the offset-depth domain reveals dear internal and external consistencies among the detections for the various wave types. Estimated earth structure is consistent with models from previous analyses based on much larger data sets.
We have thus succeeded in extracting correlated events from the data and decomposing these, approximately but meaningfully, into distinct classes (ray paths)     

Interplanetary MIST

The London MIST meeting in November covered current research using satellites to investigate planetary environments across the solar system, including our own. Earth-bound instruments, GPS and modelling also came under discussion, as did a series of results from the Cluster mission. Neil Arnold reports.     

Morphology and distribution of fossil soils in the Permo-Pennsylvanian Wichita and Bowie Groups, north-central Texas, USA: implications for western equatorial Pangean palaeoclimate during icehouse–greenhouse transition

Analysis of stacked Permo‐Pennsylvanian palaeosols from north‐central Texas documents the influence of palaeolandscape position on pedogenesis in aggradational depositional settings. Palaeosols of the Eastern shelf of the Midland basin exhibit stratigraphic trends in the distribution of soil horizons, structure, rooting density, clay mineralogy and colour that record long‐term changes in soil‐forming conditions driven by both local processes and regional climate. Palaeosols similar to modern histosols, ultisols, vertisols, inceptisols and entisols, all bearing morphological, mineralogical and chemical characteristics consistent with a tropical, humid climate, represent the Late Pennsylvanian suite of palaeosol orders. Palaeosols similar to modern alfisols, vertisols, inceptisols, aridisols and entisols preserve characteristics indicative of a drier and seasonal tropical climate throughout the Lower Permian strata. The changes in palaeosol morphology are interpreted as being a result of an overall climatic trend from relatively humid and tropical, moist conditions characterized by high rainfall in the Late Pennsylvanian to progressively drier, semi‐arid to arid tropical climate characterized by seasonal rainfall in Early Permian time. Based on known Late Palaeozoic palaeogeography and current hypotheses for atmospheric circulation over western equatorial Pangea, the Pennsylvanian palaeosols in this study may be recording a climate that is the result of an orographic control over regional‐scale atmospheric circulation. The trend towards a drier climate interpreted from the Permian palaeosols may be recording the breakdown of this pre‐existing orographic effect and the onset of a monsoonal atmospheric circulation system over this region.     

The Gondwanan Carboniferous-Permian Boundary Revisited: New Data from Australia and Argentina

The identification and correlation of the Carboniferous-Permian (Gzhelian-Asselian) boundary within the sedimentary sequences of Gondwana has always been a topic of debate. Type latest Carboniferous and earliest Permian marine sequences are characterised by warm tropical faunas and come from the Uralian Region of Russia and Kazakhstan. Faunas include conodonts and fusulinid foraminiferids which are prime tools for correlation. Such faunal groups are absent from most Gondwanan sequences where reliance for correlations must be placed primarily on brachiopods, bivalve molluscs and palynology. The Western Australian marine sequences, with their contained ammonoids, provide a pivotal link for the dating and correlating of Early Permian Gondwanan sequences with those of the type regions and their palynostratigraphical record is essential for trans-Australian correlations and correlations elsewhere throughout Gondwanaland.New data from the fully cored DM Tangorin DDHl bore hole, drilled in the Cranky Corner Basin, New South Wales, Australia, reveals a sequence of descending faunal zones. The stratigraphically highest zone with Eurydesrna cordaturn, encompasses the Late Sakmarian (Sterlitamakian). The middle zone with Torniopsis elongata, Sulciplica c r a m and Trigonotreta tangorini straddles the Sterlitamakian-Tastubian boundary, with the palynomorphs Pseudoreticulatispora pseudoreticulata high in the zone and Granulatisporites confluens low in the zone. An impoverished fauna with Trigonotreta nov., low in the Granulatisporites confluens Zone, is probably of latest Asselian or Tastubian age.Significant new data from Argentina has revealed marine faunas from below the occurrence of Granulatisporites confluens. These are considered to be of Asselian age. Outcrops of the Tupe Formation, with a marine fauna, at La Herradura Creek in the western Paganzo Basin, San Juan Province, are best regarded as being of mid to late Asselian age. The Tupe Fauna has been recognised as the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone. Previously, this fauna was considered to be of Late Carboniferous or Stephanian age. Three faunal associations are known from the Rio del Peii6n Formation, Rio Blanco Basin, La Rioja Province. The middle assemblage with Tivertonia, Costaturnulus, Kochiproductus and Trigonotreta, appears to correlate well with the Tupe Formation fauna. The lower assemblage, with Streptorhynchus, Etherilosia, Costaturnulus, Trigonotreta and a punctate spiriferid, as well as indeterminate productids, probably of Early Asselian age. The youngest assemblage includes a species ofRhynchopora that is close to Rhynchopora australasica from the latest Asselian-early Tastubian of Western Australia. The marine biostratigraphical data from Argentina has enabled a much greater understanding of the earliest Permian marine faunas to be achieved - a story that is apparently absent from the other cold and cool temperate regions of Gondwana.     

London MIST 2004

The annual one-day meeting of the MIST (Magnetosphere, Ionosphere and Solar–Terrestrial) community was held in November 2004 at Burlington House, with contributions on data and ideas ranging from Cassini to Cluster, with stops at many experiments in between.     

Sediment Distribution Around Glacially Abraded Bedrock Landforms (Whalebacks) at Lago Tranquilo, Chile

Whalebacks are convex landforms created by the smoothing of bedrock by glacial processes. Their formation is attributed to glacial abrasion either by bodies of subglacial sediment sliding over bedrock or by individual clasts contained within ice. This paper reports field measurements of sediment depth around two whaleback landforms in order to investigate the relationship between glacigenic deposits and whaleback formation. The study site, at Lago Tranquilo in Chilean Patagonia, is situated within the Last Glacial Maximum (LGM) ice limits. The two whalebacks are separated by intervening depressions in which sediment depths are generally 0.2 to 0.3 m. Two facies occur on and around the whalebacks. These facies are: (1) angular gravel found only on the surface of the whalebacks, interpreted as bedrock fracturing in response to unloading of the rock following pressure release after ice recession, and (2) sandy boulder‐gravel in the sediment‐filled depressions between the two whalebacks, interpreted as an ice‐marginal deposit, with a mixture of sediment types including basal glacial and glaciofluvial sediment. Since the whalebacks have heavily abraded and striated surfaces but are surrounded by only a patchy and discontinuous layer of sediment, the implication is that surface abrasion of the whalebacks was achieved primarily by clasts entrained in basal ice, not by subglacial till sliding.     

Uncertainty in adaptive capacity

The capacity to adapt is a critical element of the process of adaptation: it is the vector of resources that represent the asset base from which adaptation actions can be made. Adaptive capacity can in theory be identified and measured at various scales, from the individual to the nation. The assessment of uncertainty within such measures comes from the contested knowledge domain and theories surrounding the nature of the determinants of adaptive capacity and the human action of adaptation. While generic adaptive capacity at the national level, for example, is often postulated as being dependent on health, governance and political rights, and literacy, and economic well-being, the determinants of these variables at national levels are not widely understood. We outline the nature of this uncertainty for the major elements of adaptive capacity and illustrate these issues with the example of a social vulnerability index for countries in Africa. To cite this article: W.N. Adger, K. Vincent, C. R. Geoscience 337 (2005).