A THEORY OF ACOUSTIC DIFFRACTORS APPLIED TO 2-D MODELS*

Finite-offset seismic reflection modeling of acoustic waves, propagating in a two-dimensional depth section of arbitrary complexity, is discussed. The procedure developed employs the principles of simplified (far-field) diffractor theory and ray tracing. Each reflector is represented by a set of discrete secondary sources or diffractors and the wavefield associated with each diffractor is calculated directly in the time domain by ray tracing. Reflections and diffractions are subsequently built up by the numerical superposition of these wavefields. This superposition is nondispersive for all frequencies for which the Fresnel zones are large compared with the diffractor separation. All primary travel paths connecting the shot to diffractor and diffractor to geophone are accounted for together with phase changes induced by focal events. The method allows the modeling of arbitrary trace gathers for energy originating from selected reflectors. The nonsequential nature of the algorithm makes it suited to machines capable of carrying out many similar operations in parallel or concurrently. Diffractor theory also provides physical insight into wave scattering and focusing. In particular, the half-differential waveform associated with a line diffractor leads to an explanation of the 90° phase lead induced by a cylindrical focus and, similarly, the full differential waveform of a point diffractor can be used to explain the 180° phase shift induced by a point focus.     

WEIGHTED VERTICAL STACKING IN CRUSTAL SEISMIC REFLECTION STUDIES ON THE CANADIAN SHIELD*

Seismic reflection methods are being developed at the University of Manitoba to aid in determining fine crustal structure in the Precambrian of Manitoba and northwestern Ontario. Present-day environmental concern as well as mineshaft conditions necessitate the detonation of several smaller charges repeated, say, I times and followed by ‘vertical’ stacking. To obtain the familiar √I improvement in signal-to-noise (S:N) amplitude ratio applying the straight-sum (SS) method, one assumes, among other things, that both S:N ratio and signal variance are the same on all traces. Dropping these assumptions, as we must for our data, it becomes necessary to apply weighting coefficients to optimize the S:N ratio of the stacked trace. We still assume the signal shapes to be the same for repeated shots, so for the jth trace on the record of the ith shot we model the time series as: t_ij=a_i (s_j+n_ij); where a_i is a scaling factor. The proper weights w_i are then shown to be proportional to σ_si/σ²_ni where σ² is variance, or to γ_i/a_i where γ_i is S:N power ratio. Applying the weighted-stack (WS) method gives S:N amplitude ratios which are, on average, 55% of the optimal ratios expected from WS theory compared with only 24% for the SS method. The 45% shortfall in WS performance is ascribed mainly to trace-alignment (or time-delay) errors. Varying noise levels on individual traces, slight dissimilarity of signal shape, and correlated noise may also contribute to a lesser extent (in decreasing order of significance). This WS method appears to strike a good practical balance between S:N improvement and processing efficiency.     

A STRATIGRAPHIC CASE HISTORY USING THREE-DIMENSIONAL SEISMIC DATA IN THE GULF OF THAILAND *

A three-dimensional marine seismic survey was conducted in the Gulf of Thailand to aid in the development of a gas field indicated by three wildcat wells. The results and interpretation reported previously demonstrated improved fault resolution and better structural definition. Five successful appraisal wells have now been drilled, and these show that most of the sands have limited extent. Widespread character changes in the seismic data also support stratigraphic variations in many of the sands. Several new methods of 3D stratigraphic interpretation have been developed while investigating the depositional history of this area. Anomalous seismic amplitudes, tied to sands penetrated by wells and mapped from Seiscrop^TM horizontal sections in time and depth, have indicated the distribution of bars and channels. Horizon Seiscrop sections, each sliced through a single bed, have been used to delineate these depositional features directly. G-LOG^TM sections, displaying seismic logs derived by rigorous wave equation inversion, confirm the existence of these features. Sands greater than 10 m thick have proved mappable.     

WAVELENGTH SELECTION IN MULTICOMPONENT NEAR-INFRARED CALIBRATION

Modern scanning(near-)infrared reflectance/absorption(NIR)spectroscopes measure the absorptions orreflectances at a sequence of around 1000 wavelengths.Training data may consist of 10-100 carefullydesigned sample mixtures for which the true composition of the mixture is either known by formulationor accurately determined by wet chemistry.In future one wishes to predict the true composition fromthe spectrum.In this paper we compare a simple wavelength selection approach with methods whichretain all the wavelengths.It offers a powerful yet simple technique for choosing those wavelengths thatare specific to each pure component as against the other components(including the medium)for thevarying compositions.In the presence of a defined range of ingredients it thus chooses wavelengths whichare highly selective for each particular component.It has the added advantage of selecting wavelengthswhich are little effected by interaction effects and consequent non-linearities.The calibration data used consist of 125 observations of three sugars,each varying at five levels in afull 5~3 design.The validation set consists of 21 further samples specially selected to have compositionsoutside the range of the training sample.The selection methods perform much better on this predictionset than methods which retain all the wavelengths,700 in this case.The leave-one-out cross-validationinternal to the calibration data would point to the opposite finding and suggests that such cross-validations may be overly flattering to techniques such as partial least squares and may encourageoverfitting.After selection,simple straightforward least squares methods may be used,eschewing theneed for‘shrinkage’methods such as partial least squares or ridge regression.     

Atmospheric Effects of the Emerging Mainland Chinese Transportation System at and Beyond the Regional Scale

Local surface travel needs in the Peoples Republic of China (mainlandChina) have traditionally been met largely by nonpolluting bicycles. A majorautomobile manufacturing/importing effort has begun in the country over thelast decade, and planning documents indicate that the Chinese may strive toacquire more than 100 million vehicles early in the next century. By analogywith large automotive fleets already existing in the western world, bothregional and global scale pollution effects are to be expected from theincrease. The present work adopts the latest projections of Chinese automobilemanufacture and performs some quantitative assessments of the extent ofpollution generation.Focus for the investigation is placed upon the oxidant ozone. Emissions of theprecursor species nitrogen oxides and volatile organics are constructed basedon data for the current automotive sector in the eastern portion of the UnitedStates. Ozone production is first estimated from measured values forcontinental/oceanic scale yields relative to precursor oxidation. Theestimates are then corroborated through idealized two dimensional modeling ofthe photochemistry taking place in springtime air flow off the Asian land massand toward the Pacific Ocean. The projected fleet sizes could increase coastaland remote oceanic ozone concentrations by tens of parts per billion (ppb)in the lower troposphere. Influences on the tropospheric aerosol system andon the major greenhouse gas carbon dioxide are treated peripherally. Nitrogenoxides created during the vehicular internal combustion process willcontribute to nitrate pollution levels measured in the open Pacific. Thepotential for soot and fugitive dust increases should be considered as theautomotive infrastructure develops. Since the emerging Chinese automotivetransportation system will represent a substantial addition to the globalfleet and all the carbon in gasoline is eventually oxidized completely, asignificant rise in global carbon dioxide inputs will ensue as well.Some policy issues are treated preliminary. The assumption is made thatalterations to regional oxidant/aerosol systems and to terrestrial climate areconceivable. The likelihood that the Chinese can achieve the latest vehiclefleet goals is discussed, from the points of view of new production, positivepollution feedbacks from a growing automobile industry, and known petroleumreserves. Vehicular fuel and maintenance options lying before the Chinese areoutlines and compared. To provide some perspective on the magnitude of theenvironmental changes associated with an Asian automotive buildup, recentestimates of the effects of future air traffic over the Pacific Rim aredescribed.     

雅鲁藏布江缝合带的双陆内俯冲构造与部分熔融层特征──INDEPTH项目结果的初步综合

根据中、美两国合作开展的西藏深反射地震结果,提出雅鲁藏布江缝合带（YZS）的"双陆内俯冲"构造模式和缝合带南、北分别存在着不同特征的、规模不一的部分熔融层;提出YZS处断裂向深部延深有限,认为YZS处地壳增厚有4种机制：①地壳规模的大的俯冲增厚;②上部地壳内的俯冲和背冲增厚;③下地壳内的底部垫托增厚和挤压增厚;④深部熔融体的向上挤入而引起的地壳增厚.提出可能存在两条大的伸展性断裂,造成江孜南、北地块间出现了大升降.     

The Institutional Architecture of Extractive Reserves in Rondônia, Brazil

This paper examines the evolution and development of institutions involved in the establishment of extractive reserves. It focuses on the different actors, organizations and projects concerned with extractive reserves in Rondônia in western Amazonia, Brazil. Extractive reserves are widely seen as a means of integrating forest conservation and local socio-economic development. As such, they have gained the support of the international environmental movement, multilateral and state development organizations, and grassroots community and producer organizations. These institutions represent a vast array of diverse interests which range from conservation of biodiversity, economic and social cohesion, and regional and national development. Whilst the various actors and institutions claim mutual and complementary interests, they form shifting alliances and coalitions in order to further their interests. This paper maps these institutions and interests and describes how alliances have been formed and broken, and how this has contributed to the currently observed implementation of extractive reserves. The paper argues that extractive reserves are constrained by the dissonance or misfit in discourses and underlying worldviews, reflected by the aims and objectives, of the institutions which promote, oppose and lose and benefit from their implementation. This misfit is overcome, in part, by alliances formed between the different institutions, and in part by the creation of new institutions and projects.     

Petrogenesis of Migmatites in Maine, USA: Possible Source of Peraluminous Leucogranite in Plutons?

In Maine, Siluro-Devonian turbidites were metamorphosed underhigh-T–low-P facies series conditions during deformationwithin a Devonian crustal-scale shear zone system, defined bykilometer-scale straight belts of apparent flattening strainthat anastomose around lozenges of apparent constrictional strain.At upper amphibolite facies grade, metapelites are partiallymelted, the onset of which is recorded by a migmatite front.The resulting migmatites are stromatic or heterogeneous, andsmaller-volume granites form sheets or cylinders according tothe structural zone in which they occur, suggesting that migmatitesand granites record syntectonic melt flow through the deformingcrust. Common leucogranite of the nearby coeval Phillips pluton,which was emplaced syntectonically, was sourced from crustalrocks with geochemical characteristics similar to those of thehost Siluro-Devonian succession. Migmatites have melt-depletedcompositions relative to metapelites. Leucosomes are peraluminousand represent the cumulate products of fractional crystallizationand variable loss of evolved fractionated liquid. Among theheterogeneous migmatites are schlieric granites, the geochemistryof which suggests melt accumulation before fractional crystallizationand loss of the evolved liquid. Smaller-volume granites areperaluminous with a range of chemistries that reflect variableentrainment of residual plagioclase and biotite, accumulationof products of fractional crystallization and loss of most ofthe evolved liquid. Common leucogranite of the Phillips plutonand larger granites in the migmatites have compositions thatsuggest crystallization of evolved liquids derived by fractionalcrystallization of primary muscovite dehydration melts. We inferthat the leucogranite represents the crystallized fugitive liquidfrom a migmatite source similar to that exposed nearby. Watertransported through the shear zone system dissolved in meltwas exsolved at the wet solidus to cause retrogression in sub-solidusrocks and retrograde muscovite growth in migmatites. KEY WORDS: anatexis of pelite; Maine; migmatite; peraluminous granite; plutons     

High-temperature 'clockwise'P-T paths and melting in the development of regional migmatites: an example from southern Brittany, France

The Southern Brittany Migmatite Belt (SBMB), which evolved through the metamorphic peak between c. 400 Ma and c. . 370 Ma ago, consists of a heterogeneous suite of high-grade gneisses and anatectic migmatites, both metatexites and diatexites. Rare garnet-cordierite gneiss layers record evidence of an early prograde P-T path. In these rocks, growth-zoned garnet cores and a sequence of included mineral assemblages in garnet, from core to rim, of Qtz + Ilm + Ky, Pl + Ky + St + Rt + Bt and Pl + Sil + St + Rt + Bt constrain a prograde evolution during which the reactions Ilm + Ky + Qtz→ Aim + Rt, Ms + Chl→ St + Bt + Qtz + V and St + Qtz→ Grt + Sil + V were crossed. Parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types. In all rock types, garnet has reverse zoned rims, and garnet replacement by cordierite and/or biotite and plagioclase suggests the following reactions have occurred: Grt + Sil + Qtz→ Crd → Hc ± Ilm, Bt + Sil + Qtz → Crd ± Hc → Ilm → Kfs + V and (Na + Ca + K + Ti) + Grt → Bt + Pl + Qtz. Microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained 'clockwise' P–T path for the SBMB. The high-temperature part of the path has a steep dT/dP slope characteristic of near isothermal decompression. It is proposed that the P-T path followed by the SBMB is the result of the inversion, by overthrusting, of a back-arc basin and that such a tectonic setting may be applicable to other high-temperature migmatite terranes. The near isothermal decompression is at least partly driven by the upward (diapiric) movement of the diatexite/anatectic granite core of the SBMB.     

Types of Igneous Cumulates

The term ‘cumulate’ is proposed as a group namefor igneous rocks formed by crystal accumulation. After accumulationof the crystal precipitate, various ways of completing the crystallizationprocess lead to differences in the rock finally produced. Thevarious types of cumulates which have so far been distinguishedare briefly described.