Diffraction imaging by prestack reverse‐time migration in the dip‐angle domain

Prestack image volumes may be decomposed into specular and non‐specular parts by filters defined in the dip‐angle domain. For space‐shift extended image volumes, the dip‐angle decomposition is derived via local Radon transform in depth and midpoint coordinates, followed by an averaging over space‐shifts. We propose to employ prestack space‐shift extended reverse‐time migration and dip‐angle decomposition for imaging small‐scale structural elements, considered as seismic diffractors, in models with arbitrary complexity. A suitable design of a specularity filter in the dip‐angle domain rejects the dominant reflectors and enhances diffractors and other non‐specular image content. The filter exploits a clear discrimination in dip between specular reflections and diffractions. The former are stationary at the specular dip, whereas the latter are non‐stationary without a preferred dip direction. While the filtered image volume features other than the diffractor images (for example, noise and truncation artefacts are also present), synthetic and field data examples suggest that diffractors tend to dominate and are readily recognisable. Averaging over space‐shifts in the filter construction makes the reflectors? rejection robust against migration velocity errors. Another consequence of the space‐shift extension and its angle‐domain transforms is the possibility of exploring the image in a multiple set of common‐image gathers. The filtered diffractions may be analysed simultaneously in space‐shift, scattering‐angle, and dip‐angle image gathers by means of a single migration job. The deliverables of our method obviously enrich the processed material on the interpreter's desk. We expect them to further supplement our understanding of the Earth's interior.     

Migration velocity analysis and waveform inversion

Least‐squares inversion of seismic reflection waveform data can reconstruct remarkably detailed models of subsurface structure and take into account essentially any physics of seismic wave propagation that can be modelled. However, the waveform inversion objective has many spurious local minima, hence convergence of descent methods (mandatory because of problem size) to useful Earth models requires accurate initial estimates of long‐scale velocity structure. Migration velocity analysis, on the other hand, is capable of correcting substantially erroneous initial estimates of velocity at long scales. Migration velocity analysis is based on prestack depth migration, which is in turn based on linearized acoustic modelling (Born or single‐scattering approximation). Two major variants of prestack depth migration, using binning of surface data and Claerbout's survey‐sinking concept respectively, are in widespread use. Each type of prestack migration produces an image volume depending on redundant parameters and supplies a condition on the image volume, which expresses consistency between data and velocity model and is hence a basis for velocity analysis. The survey‐sinking (depth‐oriented) approach to prestack migration is less subject to kinematic artefacts than is the binning‐based (surface‐oriented) approach. Because kinematic artefacts strongly violate the consistency or semblance conditions, this observation suggests that velocity analysis based on depth‐oriented prestack migration may be more appropriate in kinematically complex areas. Appropriate choice of objective (differential semblance) turns either form of migration velocity analysis into an optimization problem, for which Newton‐like methods exhibit little tendency to stagnate at nonglobal minima. The extended modelling concept links migration velocity analysis to the apparently unrelated waveform inversion approach to estimation of Earth structure: from this point of view, migration velocity analysis is a solution method for the linearized waveform inversion problem. Extended modelling also provides a basis for a nonlinear generalization of migration velocity analysis. Preliminary numerical evidence suggests a new approach to nonlinear waveform inversion, which may combine the global convergence of velocity analysis with the physical fidelity of model‐based data fitting.     

Fisheries policy, research and the social sciences in Europe: Challenges for the 21st century

Despite evidence of a broadening of the science base for European fisheries policy with the incorporation of an ecosystem approach and increasing use of economic modelling, the contribution of the social sciences to policy related research remains less conspicuous. Progress has occurred in the understanding of institutional structures and the theory of fisheries governance, but analysis of EU funded research in the 6th Framework Programme (2002–2006) points to the absence of social science except in multi-disciplinary projects. The diasporic nature of fisheries social science and the absence of clearly articulated social objectives from fisheries policy are among the more plausible explanations for this unconformity. Prospects for reform of the CFP in 2012—including a redistribution of responsibilities between central and regional institutions—offer enhanced opportunities for the social sciences in interdisciplinary and specialist areas of policy related research. Responding to the challenge will necessitate the building of stronger networks within the family of social sciences and across disciplinary boundaries with the natural and economic sciences.     

Recontextualising inshore fisheries: The changing face of British inshore fisheries management

Inshore fisheries are coming under increasing pressure to account for wider environmental impacts and relations with other users of marine space. However approaches to inshore fisheries management across Britain’s devolved governments are becoming even more strikingly divergent. While in England the century old local Sea Fisheries Committees are to be replaced with modernised structures, and in Scotland there are efforts to move to a locally driven management system, in Wales there has been a retreat from local co-management. Not only do the reforms pose ongoing challenges for good governance, not least in the handling of cross-scale interactions and user group participation, but they may also fall short in providing for systematic and full integration of fisheries and marine environmental management.     

Interface error analysis for numerical wave propagation

The numerical error associated with finite-difference simulation of wave propagation in discontinuous media consists of two components. The first component is a higher-order error that leads to grid dispersion; it can be controlled by higher-order methods. The second component results from misalignment between numerical grids and material interfaces. We provide an explicit estimate of the interface misalignment error for the second order in time and space staggered finite-difference scheme applied to the acoustic wave equation. Our analysis, confirmed by numerical experiments, demonstrates that the interface error results in a first-order time shift proportional to the distance between the interface and computational grids. A 2D experiment shows that the interface error cannot be suppressed by higher-order methods and indicates that our 1D analysis gives a good prediction about the behavior of the numerical solution in higher dimensions.      

Chemical and physical studies of type 3 chondrites XII: The metamorphic history of CV chondrites and their components

Abstract— The induced thermoluminescence (TL) properties of 16 CV and CV-related chondrites, four CK chondrites and Renazzo (CR2) have been measured in order to investigate their metamorphic history. The petrographic, mineralogical and bulk compositional differences among the CV chondrites indicate that the TL sensitivity of the ～130 °C TL peak is reflecting the abundance of ordered feldspar, especially in chondrule mesostasis, which in turn reflects parent-body metamorphism. The TL properties of 18 samples of homogenized Allende powder heated at a variety of times and temperatures, and cathodoluminescence mosaics of Axtell and Coolidge, showed results consistent with this conclusion. Five refractory inclusions from Allende, and separates from those inclusions, were also examined and yielded trends reflecting variations in mineralogy indicative of high peak temperatures (either metamorphic or igneous) and fairly rapid cooling. The CK chondrites are unique among metamorphosed chondrites in showing no detectable induced TL, which is consistent with literature data that suggests very unusual feldspar in these meteorites. Using TL sensitivity and several mineral systems and allowing for the differences in the oxidized and reduced subgroups, the CV and CV-related meteorites can be divided into petrologic types analogous to those of the ordinary and CO type 3 chondrites. Axtell, Kaba, Leoville, Bali, Arch and ALHA81003 are type 3.0–3.1, while ALH84018, Efremovka, Grosnaja, Allende and Vigarano are type 3.2–3.3 and Coolidge and Loongana 001 are type 3.8. Mokoia is probably a breccia with regions ranging in petrologic type from 3.0 to 3.2. Renazzo often plots at the end of the reduced and oxidized CV chondrite trends, even when those trends diverge, suggesting that in many respects it resembles the unmetamorphosed precursors of the CV chondrites. The low-petrographic types and low-TL peak temperatures of all samples, including the CV3.8 chondrites, indicates metamorphism in the stability field of low feldspar (i.e., <800 °C) and a metamorphic history similar to that of the CO chondrites but unlike that of the ordinary chondrites.     

The Common Fisheries Policy and UK quota management

Exclusive Economic Zones and the formulation of a Common Fisheries Policy have profoundly reshaped the political geography of the NE Atlantic fisheries. Outlining the characteristics of the new management framework, the paper draws attention to the use of Total Allowable Catches (TACs) as a mechanism for attempting to reconcile the need for stock conservation with demands for equitable allocation of catches between member states and the creation of an orderly market. Member states are free to manage their fisheries within the parameters set by TACs. For Britain, two systems have evolved: a centralised system for Area VII and a devolved system of ‘sectoral quota management’ for Areas IV and VI in which Producer Organisations can assume the role of quota managers on behalf of their members. The paper examines the latter system in detail and points to the inevitable conflicts between equity and efficiency.