ALBERT

Description: 〈span〉〈div〉ABSTRACT〈/div〉Using 3D broadband seismic data, we have investigated low-frequency seismic amplitude anomalies associated with and below various geologic formations in the Johan Sverdrup oil field situated in the central North Sea. Low-frequency anomalies are observed below the Intra-Draupne and the Heather and Hugin reservoirs, at the Svarte and Tor Fms, and below shallow channels. The Intra-Draupne Fm is the main oil reservoir of interest in the field, and it is relatively homogeneous, with observed low Q values of approximately 25–30. The Heather and Hugin reservoirs, which underlie the Intra-Draupne, are heterogeneous and oil bearing. We performed three-layer elastic modeling on a simple reservoir model based on the properties of the Intra-Draupne Fm, and the results suggest that as the thickness of the middle oil-bearing layer increases the dominant frequency decreases due to the tuning effect. The Svarte and Tor Formations are shallower in the section and are fractured. Low-frequency anomalies associated with these formations seem to indicate a clear correlation with zones of increased fracture density interpreted from high-resolution most-negative-curvature attribute maps. Low-frequency anomalies are also observed below shallow gas channels consisting of stratified sandy and shaly intervals with vertical variations in porosity and permeability. In addition, stacking tests using coarse and fine velocity analysis indicate no noticeable difference in the characteristics of the low-frequency anomalies, in general, at all levels. We conclude, therefore, that the observed low-frequency anomalies are unrelated to stacking issues.〈/span〉

Description: Our aim was to identify some of the characteristics of low-frequency anomalies. Specifically, we have looked, in 3D broadband data from the North Sea, for any offset dependence in these anomalies and any frequency-related change in normal moveout (NMO) velocity that could influence stacking power over different frequencies. After high-resolution spectral decomposition, two types of low-frequency anomaly have been identified associated with hydrocarbon-bearing reservoirs: (1) at the reservoir top and (2) below the reservoir, with a time delay of approximately 100–200 ms. Both types of anomalies indicate offset dependence. On the near-offset stacks, they are relatively strong, but they tend to be absent on the far-offset stacks. In addition, horizon velocity analysis, which was performed along the horizons picked at the tops of reservoir and nonreservoir intervals, has revealed frequency-dependent NMO velocity. For nonreservoir events, we found no significant difference between the NMO velocities for the low-frequency and high-frequency filtered common-midpoint gathers. However, along the anomalously low-frequency events observed at the tops of, and below, oil-bearing reservoirs, lower velocity is observed for low-frequency and higher velocity for high-frequency filtered gathers. If these properties turn out to be universally typical, increased understanding and inclusion of them could lead to improved workflows and help increase the reliability of low-frequency analysis as a hydrocarbon indicator.

Description: The 1300 km 3 tholeiitic lava flow field of the 14.98 Ma Roza Member of the Columbia River Basalt Province has the best-preserved vent system of any known continental flood basalt. Detailed geological mapping and logging of the exposed pyroclastic rocks along the 〉180-km-long vent system enable the reconstruction of pyroclastic edifices (partial cones) built around vents. The pyroclastic cones differ from those constructed during typical basaltic effusive eruptions and are characterized by low to moderate slope angles (〈20°), have minimum heights up to ~160 m, and are composed of dominantly coarse-grained, moderately to densely agglutinated and welded spatter and scoria that extend up to 750 m away from the vent. Thick, well-sorted fall deposits composed of moderately to highly vesicular scoria lapilli extend away from some vents and exhibit some characteristics comparable to the proximal deposits of violent Strombolian or subplinian eruptions. The recorded volcanic activity does not fit with common eruption styles of basaltic magmas, and the evidence indicates that the Roza eruption was punctuated by eruptive activity of unusually high intensity that was characterized by vigorous lava fountains. The extensive agglutinated deposits accumulated around the vents as a result of fallout from high (〉1 km) fountains enhanced by fallout from the lower parts of convective plumes that rose above the fountains.

Description: We assess bias in the record of kimberlite volcanism by using newly acquired size data on more than 900 kimberlite bodies from 12 kimberlite fields eroded to depths of between 0 m and 〉1200 m, and by a comparison with intraplate monogenetic basaltic volcanic fields. Eroded kimberlite fields are composed of pipes (or diatremes) and dikes, and within any one kimberlite field, regardless of erosion level, kimberlite bodies vary in area at Earth’s surface over 2–3 orders of magnitude. Typically 60%–70% of the bodies are 〈10% the area of the largest pipe in the field. The maximum size of a kimberlite pipe found in a field shows a relationship with estimated erosion levels, suggesting that the erosion level of a region could be used to predict the maximum potential size of a pipe where it intersects the surface. The data indicate that the selective removal of surface volcanic structures and deposits by erosion has distorted the geological record of kimberlite volcanism. Selective mining of preferentially large, diamondiferous kimberlite pipes and underreporting of small kimberlite pipes and dikes add further bias. A comparison of kimberlite volcanic fields with intraplate monogenetic basaltic volcanic fields indicates that both types of volcanism overlap in terms of field size, volcano number and size, and typical erupted volumes. Eroded monogenetic basaltic fields consist of dikes that fed effusive and weakly explosive surface eruptions, and diatremes (pipes) generated during phreatomagmatic eruptions, and they are structurally similar to eroded kimberlite fields. Reassessment of published data suggests that kimberlite magmas can erupt in a variety of ways and that most published data, taken from the largest kimberlite pipes, may not be representative of kimberlite volcanism as a whole. This refuels long-standing debates as to whether kimberlite pipes (diatremes) primarily result from phreatomagmatic eruptions (as in basaltic volcanism) or from volatile-driven magmatic eruptions.

Description: The evolution of a tandem accelerator 14C dating system at Chalk River is recounted. Background problems and sources of instability are discussed and solutions are described. Details of sample chemistry and source preparation are presented.

Description: Canadian deuterium uranium (CANDU) pressurized heavy-water reactors produce 14C by neutron activation of trace quantities of nitrogen in annular gas and reactor components (14N(n,p)14C), and from 17O in the heavy water moderator by (17O(n,α)14C). The radiocarbon produced in the moderator is removed on ion exchange resins incorporated in the water purification systems; however, a much smaller gaseous portion is vented from reactor stacks at activity levels considerably below 1% of permissible derived emission limits. Early measurements of the carbon speciation indicated that 〉90% of the 14C emitted was in the form of CO2. We conducted surveys of the atmospheric dispersion of 14CO2 at the Chalk River Laboratories and at the Pickering Nuclear Generating Station. We analyzed air, vegetation, soils and tree rings to add to the historical record of 14C emissions at these sites, and to gain an understanding of the relative importance of the various carbon pools that act as sources/sinks within the total 14C budget. Better model parameters than those currently available for calculating the dose to the critical group can be obtained in this manner. Global dose estimates may require the development of techniques for estimating emissions occurring outside the growing season.

Description: The Chalk River Tandem Accelerator Mass Spectrometry System has reached a state of reliable measurement of 14C using 2 to 5mg elemental carbon prepared by Mg reduction of CO2. For two comparisons of a near-modern unknown with the NBS oxalic acid standard we obtain a total error of ∼±4.5%, consisting of a random system error of about ±3.5% combined with the statistical counting error. Measurements have been made on 70 samples in 30 days of running time during the past year. Samples included deep rock carbonates, cosmogenic 14C in meteorites, charcoal from earthquake fault zones, collagen of bone artifacts and fossil beetle-fragments.

Description: A solid sphere falling through a Bingham plastic moves in a small envelope of fluid with shape that depends on the yield stress. A finite-element/Newton method is presented for solving the free-boundary problem composed of the velocity and pressure fields and the yield surfaces for creeping flow. Besides the outer surface, solid occurs as caps at the front and back of the sphere because of the stagnation points in the flow. The accuracy of solutions is ascertained by mesh refinement and by calculation of the integrals corresponding to the maximum and minimum variational principles for the problem. Large differences from the Newtonian values in the flow pattern around the sphere and in the drag coefficient are predicted, depending on the dimensionless value of the critical yield stress Ygbelow which the material acts as a solid. The computed flow fields differ appreciably from Stokes’ solution. The sphere will fall only when Yg is below 0.143. For yield stresses near this value, a plastic boundary layer forms next to the sphere. Boundary-layer scalings give the correct forms of the dependence of the drag coefficient and mass-transfer coefficient on yield stress for values near the critical one. The Stokes limit of zero yield stress is singular in the sense that for any small value of Ygthere is a region of the flow away from the sphere where the plastic portion of the viscosity is at least as important as the Newtonian part. Calculations for the approach of the flow field to the Stokes result are in good agreement with the scalings derived from the matched asymptotic expansion valid in this limit. © 1985, Cambridge University Press. All rights reserved.

Description: Experimental results for a reactive non-buoyant plume of nitric oxide (NO) in a turbulent grid flow doped with ozone (O3) are presented. The Damköhler number (ND) for the experiment is of order unity indicating the turbulence and chemistry have similar timescales and both affect the chemical reaction rate. Continuous measurements of two components of velocity using hot-wire anemometry and the two reactants using chemiluminescent analysers have been made. A spatial resolution for the reactants of four Kolmogorov scales has been possible because of the novel design of the experiment. Measurements at this resolution for a reactive plume are not found in the literature. The experiment has been conducted relatively close to the grid in the region where self-similarity of the plume has not yet developed. Statistics of a conserved scalar, deduced from both reactive and non-reactive scalars by conserved scalar theory, are used to establish the mixing field of the plume, which is found to be consistent with theoretical considerations and with those found by other investigators in non-reactive flows. Where appropriate the reactive species means and higher moments, probability density functions, joint statistics and spectra are compared with their respective frozen, equilibrium and reaction-dominated limits deduced from conserved scalar theory. The theoretical limits bracket reactive scalar statistics where this should be so according to conserved scalar theory. Both reactants approach their equilibrium limits with greater distance downstream. In the region of measurement, the plume reactant behaves as the reactant not in excess and the ambient reactant behaves as the reactant in excess. The reactant covariance lies outside its frozen and equilibrium limits for this value of ND. The reaction rate closure of Toor (1969) is compared with the measured reaction rate. The gradient model is used to obtain turbulent diffusivities from turbulent fluxes. Diffusivity of a non-reactive scalar is found to be close to that measured in non-reactive flows by others.

Description: Canada and the USSR together possess most of the territory in the Northern Hemisphere underlain by permafrost or perennially frozen ground. As about one half of the land area of each country is affected, the permafrost region of the Soviet Union is 2½ times larger than that of Canada. Outside mountainous regions, permafrost extends southward in Canada to the southern tip of James Bay at lat 51° N (Brown, in press). Permafrost extends farther south in eastern Asia, however, and occurs in Outer Mongolia and Manchuria to about lat 47° N Fig 1) (Baranov, 1959; Nekrasov, 1962).