Numerical Calculation of Equivalent Cell Permeability Tensors for General Quadrilateral Control Volumes

A new method for upscaling fine scale permeability fields to general quadrilateral-shaped coarse cells is presented. The procedure, referred to as the conforming scale up method, applies a triangle-based finite element technique, capable of accurately resolving both the coarse cell geometry and the subgrid heterogeneity, to the solution of the local fine scale problem. An appropriate averaging of this solution provides the equivalent permeability tensor for the coarse scale quadrilateral cell. The general level of accuracy of the technique is demonstrated through application to a number of flow problems. The real strength of the conforming scale up method is demonstrated when the method is applied in conjunction with a flow-based gridding technique. In this case, the approach is shown to provide results that are significantly more accurate than those obtained using standard techniques.     

地震前兆时空非均匀性指标Cv值的实验检验

在双向加载条件下，对两种构造和介质不同的岩石标本进行变形实验，测量了标本表面不同部位的应变．在此基础上，利用王晓青和陈学忠等提出的描述地震前兆非均匀分布的参量————Cv值，分析了标本变形失稳过程中应变异常的分布，以期从实验角度检验Cv值方法并探讨其物理意义．研究表明，Cv值的变化与岩石变形特征的变化有关，是描述前兆分布非均匀性的一种有效指标；Cv值在失稳发生前先异常上升，然后下降，并在Cv值恢复过程中或恢复后伴随着失稳事件的发生．因此，Cv值是一种有效的预报指标．      

Seismic scattering at the top of the mantle Transition Zone

We document strong seismic scattering from around the top of the mantle Transition Zone in all available high resolution explosion seismic profiles from Siberia and North America. This seismic reflectivity from around the 410 km discontinuity indicates the presence of pronounced heterogeneity in the depth interval between 320 and 450 km in the Earth’s mantle. We model the seismic observations by heterogeneity in the form of random seismic scatterers with typical scale lengths of kilometre size (10-40 km by 2-10 km) in a 100-140 km thick depth interval. The observed heterogeneity may be explained by changes in the depths to the α-β-γ spinel transformations caused by an unexpectedly high iron content at the top of the mantle Transition Zone. The phase transformation of pyroxenes into the garnet mineral majorite probably also contributes to the reflectivity, mainly below a depth of 400 km, whereas we find it unlikely that the presence of water or partial melt is the main cause of the observed strong seismic reflectivity. Subducted oceanic slabs that equilibrated at the top of the Transition Zone may also contribute to the observed reflectivity. If this is the main cause of the reflectivity, a substantial amount of young oceanic lithosphere has been subducted under Siberia and North America during their geologic evolution. Subducted slabs may have initiated metamorphic reactions in the original mantle rocks.     

Flat mantle reflectors in Eastern China: possible evidence of lithospheric thinning

Recent 24 s deep seismic reflection records revealed five flat reflectors in the lithospheric mantle in Eastern China. With increasing depth, they are named M1 to M5 and can be seen on both field single-shot and stacked records. Reflector M1 corresponds to the Moho discontinuity, whereas M5 may be the reflection from the bottom of the current lithosphere, which is about 78 km deep according to geothermal measurements. The other three reflectors seem peculiar and might result from interactions between the lithosphere and deeper mantle. Based on lithological and geochemical data, it is suggested that the lithosphere has been thinned from about 150 km to about 60 km in the Late Mesozoic, and then has been thickened to about 78 km during the Cenozoic. The thinning process produced a granulite layer in the old lower crust caused by magmatic underplating, whereas an eclogite layer formed beneath owing to the subduction of the Paleo-Tethys and Yangtze Craton during the Permian and Early Mesozoic. Reflector M2 at about 12 s two-way traveltime (TWT) might result from the Paleozoic Moho, which represents the boundary between the previous granulite and eclogite facies. Reflector M3 at about 14 s might correspond to the bottom of the eclogite layer, beneath which the old lithospheric mantle remained. The old and the newly developed mantle may have different compositions, resulting in reflector M4. The multi-layered mantle reflectors demonstrate a mantle structure that possibly correlates with the lithospheric thinning process that occurred in Eastern China during the Late Mesozoic. The discovery of multi-layered mantle reflectors in the studied areas indicates a high heterogeneity of the upper mantle. Reflection seismology with improved technology, together with velocity and resistivity imaging and rock-physics measurements, can provide more details of the heterogeneity and related dynamic processes that occurred in the lithospheric mantle.     

Crossover from Nonlinearity Controlled to Heterogeneity Controlled Mixing in Two-Phase Porous Media Flows

We use high resolution Monte Carlo simulations to study the dispersive mixing in two-phase, immiscible, porous media flow that results from the interaction of the nonlinearities in the flow equations with geologic heterogeneity. Our numerical experiments show that distinct dispersive regimes occur depending on the relative strength of nonlinearity and heterogeneity. In particular, for a given degree of multiscale heterogeneity, controlled by the Hurst exponent which characterizes the underlying stochastic model for the heterogeneity, linear and nonlinear flows are essentially identical in their degree of dispersion, if the heterogeneity is strong enough. As the heterogeneity weakens, the dispersion rates cross over from those of linear heterogeneous flows to those typical of nonlinear homogeneous flows.     

Time-domain computation of scattering from a heterogeneous layer

Adopting Born and ray approximations, time-domain synthetic seismograms for P-P and P-S scattering from a plane wave incident on a thin, laterally heterogeneous layer are presented in this paper. The time-domain P coda is a convolution between a structure function and the second-order derivative of the time function of the incident P wave. Examples of synthetic seismograms are given using a time function from a computed short-period seismogram for a point explosive source in a half-space. These show that it is impossible, with realistic values of the parameters involved, to generate significant codas when only single scattering is involved.     

Flexible spectral methods for the generation of random fields with power-law semivariograms

Random field generators serve as a tool to model heterogeneous media for applications in hydrocarbon recovery and groundwater flow. Random fields with a power-law variogram structure, also termed fractional Brownian motion (fBm) fields, are of interest to study scale dependent heterogeneity effects on one-phase and two-phase flow. We show that such fields generated by the spectral method and the Inverse Fast Fourier Transform (IFFT) have an incorrect variogram structure and variance. To illustrate this we derive the prefactor of the fBm spectral density function, which is required to generate the fBm fields. We propose a new method to generate fBm fields that introduces weighting functions into the spectral method. It leads to a flexible and efficient algorithm. The flexibility permits an optimal choice of summation points (that is points in frequency space at which the weighting function is calculated) specific for the autocovariance structure of the field. As an illustration of the method, comparisons between estimated and expected statistics of fields with an exponential variogram and of fBm fields are presented. For power-law semivariograms, the proposed spectral method with a cylindrical distribution of the summation points gives optimal results.     

Methodical approach to distribution of epilithic and drifting algae communities in a French subalpine river: Inferences on water quality assessment

In a sub-alpine river, the Asse, with an unpredictable discharge regime, chlorophyll, density and taxonomic diversity of both drifting and periphytic communities were studied from a methodical approach. The investigations based on stamping two close cross-sections to know their spatial distributions in a heterogeneous bed substrate showed a heterogeneous pattern of colonization mainly influenced by hydrological variables as the current velocity, the size of the substrata (gravel-pebble-boulder). For the drifting algae, mainly composed with benthic species temporarily unhooked to the substrata, the heterogeneity of the distribution was reduced in fast flow which induced a mixing of the whole mass of water. For the periphyton, some results indicated that the greatest diversity and the highest chlorophyll concentration were found on pebble substratum where the current velocity was moderate. These results have also shown that the classical methods used to day for investigations were inadequate for most Mediterranean rivers. It was really difficult to estimate the water quality from classical index created for use in regular large plain rivers.     

Comparison of Aircraft and Ground-Based FluxMeasurements during OASIS95

Aircraft and ground-based measurements made during the1995 Australian OASIS field campaign are compared. The aircraft data were recorded during low-level flightsat 6 m above ground level and grid flights at altitudes of between 15 and 65 m, allin unstable atmospheric conditions. The low-level flights revealed an inadequate temperaturesensor response time, a correction for which was determined from subsequent work ina wind tunnel. Aircraft and ground-based measurements of mean wind speed, wind directionand air temperature agree to within 0.2 m s^-1, 4° and 0.9 °C respectively.Comparisons between aircraft and ground-based observations of the standarddeviations of vertical velocity, horizontal wind speed, air temperature and specifichumidity have slopes of 0.96, 0.97, 0.92 and 0.99 respectively but the observed scatter isroughly twice the random error expected due to the averaging length of the aircraft data andthe averaging period of the ground-based data. For the low-level flights, the ground-basedand aircraft measurements of sensible and latent heat flux show mean differences of 27 and-25 W m^-2 respectively, which reduce to 11 and -4 W m^-2 respectivelywhen analysis of aircraft data is limited to areas immediately adjacent to the fluxtowers. For the flights at 15 to 65 m above ground level, the mean differences between theground-based and aircraft measurements of sensible and latent heat flux are -22 and-1 W m^-2 respectively and these change to -1 and -7 W m^-2 respectively oncethe effect of surface heterogeneity is included. Aircraft and ground-based measurementsof net radiation agree to within 6% at one ground-based site but differ by 20% at a second.Aircraft measurements of friction velocity at 6 m above the ground agree well withground-based data, but those from flights between 15 and 65 m above ground level do not.This is because at these heights the aircraft measurements provide the local shear stress,not the surface shear stress. Overall, the level of agreement allows confidence in the aircraftdata provided due care is taken of instrument response times and differences in thesurfaces sampled by aircraft and ground-based systems.