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801.
Melt density and the average composition of basalt 总被引:9,自引:0,他引:9
Densities of residual liquids produced by low pressure fractionation of olivine-rich melts pass through a minimum when pyroxene and plagioclase joint the crystallization sequence. The observation that erupted basalt compositions cluster around the degree of fractionation from picritic liquids corresponding to the density minimum in the liquid line of descent may thus suggest that the earth's crust imposes a density filter on the liquids that pass through it, favoring the eruption of the light liquids at the density minimum over the eruption of denser more fractionated and less fractionated liquids. 相似文献
802.
Twenty-seven colonies of wild scyphistomae ofChrysaora quinquecirrha were observed for a period of ten weeks during the summer. Asexual activities of individual organisms were recorded several times each week. Polyps most often proceeded to strobilate in mid-June and then strobilate a second time 18 days later. They then detached and changed position by means of stolons and then produced podocysts. Initial cysts first excysted and the resulting polyps then strobilated. A second strobilation was not generally observed. The developmental implications of these activities are discussed. 相似文献
803.
An extensive ancient archeologic site containing lithic artifacts and associated with mammoth remains was reported at Chinitna Bay, southern Alaska in 1943. The presence of such a site adjacent to the continental shelf at the base of the rugged Aleutian Range suggested that humans may have inhabited the inner shelf environment during the late Pleistocene at times of lowered sea level. Because of the site's potential significance, an interdisciplinary research team relocated and reinvestigated the area in 1978, but failed to find evidence of prehistoric human habitation. Geologic studies and radiocarbon dating indicate that the strata reported at the site are intertidal in origin, very late Holocene in age, and have undergone significant tectonic movement in the recent past. These observations indicate that the previously published observations of the Chinitna Bay site are probably invalid. 相似文献
804.
A 1950–1994 data set of major weather losses developed by the property insurance industry was examined to assess its potential utility in climate change research and use in assessing the relevance of recent extreme losses in the United States. A process for adjusting these historical storm losses to ever-changing factors including dollar values, amount of insurance coverage per area, and the sensitivity of society to damaging storms was developed by the industry. Analysis of the temporal frequency and losses of these adjusted weather catastrophes revealed differences according to the amount of loss. Temporal changes since 1975 in the catastrophes causing $35 to $100 million in loss were strongly related to changes in U.S. population, whereas catastrophes that created insured losses greater than $100 million appear related to both shifting weather conditions and to regional population changes. This evaluation revealed that the industry's catastrophe adjustment technique did not adequately allow for changes in various demographic and social factors affecting damage; however, results suggest use of population values for normalizing the adjusted catastrophe database to allow meaningful studies of their temporal variability. 相似文献
805.
Paul N. Swarztrauber David L. Williamson John B. Drake 《Dynamics of Atmospheres and Oceans》1998,27(1-4)
Cartesian coordinates are used to solve the nonlinear shallow-water equations on the sphere. The two-dimensional equations, in spherical coordinates, are first embedded in a three-dimensional system in a manner that preserves solutions of the two-dimensional system. That is, solutions of the three-dimensional system, with appropriate initial conditions, also solve the two-dimensional system on the surface of the sphere. The higher dimensional system is then transformed to Cartesian coordinates. Computations are limited to the surface of the sphere by projecting the equations, gradients, and solution onto the surface. The projected gradients are approximated by a weighted sum of function values on a neighboring stencil. The weights are determined by collocation using the spherical harmonics in trivariate polynomial form. That is, the weights are computed from the requirement that the projected gradients are near exact for a small set of spherical harmonics. The method is applicable to any distribution of points and two test cases are implemented on an icosahedral geodesic grid. The method is both vectorizable and parallelizable. 相似文献
806.
Mean Profiles of Moisture Fluxes in Snow-Filled Boundary Layers 总被引:1,自引:0,他引:1
Profiles of moisture fluxes have been examined for convective boundary layers containing clouds and snow, using data derived from aircraft measurements taken on four dates during the 1983/1984 University of Chicago lake-effect snow project. Flux profiles were derived from vertical stacks of aircraft cross-wind flight legs taken at various heights over Lake Michigan near the downwind shore. It was found that, if ice processes are taken into account, profiles of potential temperature and water content were very similar to those presented in past studies of convective boundary layers strongly heated from below. Profiles of total water content and equivalent potential temperature adjusted for ice were nearly invariant with height, except very near the top of the boundary layer, suggesting that internal boundary-layer mixing processes were rapid relative to the rates at which heat and vapour were transported into the boundary layer through entrainment and surface fluxes. Ice was found to play a significant, measurable role in boundary-layer moisture fluxes. It was estimated that 40 to 57% of the upward vapour flux was returned to the surface in the form of snow, converting about 45 to 64% of the surface latent heat flux into sensible heat in the snow-producing process. Assuming advective fluxes are relatively small (thought to be appropriate after the first few tens of km over the lake as suggested by past studies), the boundary layer was found to warm at a rate faster than could be explained by surface heat fluxes and latent heat releases alone, the remainder of the heating presumably coming from radiational processes and entrainment. Discussions of moisture phase change processes throughout the boundary layer and estimates of errors of these flux measurements are presented. 相似文献
807.
Vibroseis data recorded at short source–receiver offsets can be swamped by direct waves from the source. The signal-to-noise ratio, where primary reflections are the signal and correlation side lobes are the noise, decreases with time and late reflection events are overwhelmed. This leads to low seismic resolution on the vibroseis correlogram. A new precorrelation filtering approach is proposed to suppress correlation noise. It is the ‘squeeze-filter-unsqueeze’ (SFU) process, a combination of ‘squeeze’ and ‘unsqueeze’ (S and U) transformations, together with the application of either an optimum least-squares filter or a linear recursive notch filter. SFU processing provides excellent direct wave removal if the onset time of the direct wave is known precisely, but when the correlation recognition method used to search for the first arrival fails, the SFU filtering will also fail. If the tapers of the source sweeps are badly distorted, a harmonic distortion will be introduced into the SFU-filtered trace. SFU appears to be more suitable for low-noise vibroseis data, and more effective when we know the sweep tapers exactly. SFU requires uncorrelated data, and is thus cpu intensive, but since it is automatic, it is not labour intensive. With non-linear sweeps, there are two approaches to the S,U transformations in SFU. The first requires the non-linear analytical sweep formula, and the second is to search and pick the zero nodes on the recorded pilot trace and then carry out the S,U transformations directly without requiring the algorithm or formula by which the sweep was generated. The latter method is also valid for vibroseis data with a linear sweep. SFU may be applied to the removal of any undesired signal, as long as the exact onset time of the unwanted signal in the precorrelation domain is known or determinable. 相似文献
808.
809.
810.
Quantifying groundwater discharge to a small perennial stream in southern Ontario, Canada 总被引:3,自引:0,他引:3
Edwin E. Cey David L. Rudolph Gary W. Parkin Ramon Aravena 《Journal of Hydrology》1998,210(1-4):21-37
A study of the interaction between groundwater and surface water was undertaken within a small agricultural watershed in southern Ontario, Canada. Groundwater contributions to streamflow were measured along a section of stream during baseflow conditions and during rainfall events. Four techniques were used to estimate the contribution of groundwater to the stream along a 450 m reach (three during baseflow and one during stormflow conditions). Under baseflow conditions, streamflow measurements using the velocity–area technique indicated that the net groundwater flux to the stream during the summer months was 10 ml s−1 m−1. Hydrometric measurements (i.e. hydraulic gradient and hydraulic conductivity) taken using mini-piezometers installed in the sediments beneath the stream resulted in net groundwater flux estimates that were four to five times lower. Seepage meters failed to provide any measurements of water flux into or out of the stream. Therefore, based on these results, the velocity–area technique gives the best estimate of groundwater discharge. Hydrograph separations were conducted using
isotopic ratios and electrical conductivity on two large rainfall events with different antecedent moisture conditions in the catchment. Both events showed that pre-event water (generally considered groundwater) dominated streamflow and tile drain flow with 64%–80% of the total discharge contributed by pre-event water. High water table conditions within the catchment resulted in greater stream discharge and a greater contribution of event water in the streamflow than that observed under low water table conditions for similar intensity storm events. The results also showed that differences in riparian zone width, vegetation and surface saturation conditions between the upper and lower catchment can influence the relative magnitude of streamflow response from the two catchment areas. 相似文献