Drainage in finite-sized unsaturated zones

After the initiation of gravity drainage, water is often assumed to be either (a) draining under unit gradient, or (b) at capillary/gravity equilibrium. Both of these simplifications can be useful, but the regimes of validity of each assumption must be delineated. Water pressures are measured versus time and distance as water drains out of a 1.6 m long sand column to determine the relative effects of capillary and gravitational forces during drainage. For medium sized sands (0.15–0.3 mm in diameter), the capillary pressure is constant in space in a large region of the column for over 12 days, and the water continues to flow under unit gradient for relatively long time scales. Similar results are seen for finer sands, but with a much faster approach to equilibrium. Numerical simulations and analytical estimates are presented and compare favorably to the measurements. Together, the experimental, theoretical and analytical results are used to calculate when capillary/gravity equilibrium is reached as a function of porous media properties and length of the unsaturated zone. The ratio of the length of the unsaturated zone to the bubbling pressure is a key parameter in determining the drainage regime, and that even for relatively short unsaturated zones the equilibrium time scale can be on the order of years.     

Explosion seismic reflections from the Earth’s core

Recent seismological studies have presented evidence for the existence of a layer with ultra-low seismic velocities at the core-mantle boundary at ca. 2900 km depth. We report high-amplitude, high-frequency, and laterally coherent seismic arrivals from three nuclear explosions in Siberia. With recording station intervals of 15 km, the seismic phases are readily correlated and show the presence of a thin, ultra-low velocity zone in a region where it was not previously reported. The duration and complexity of the arrivals are inconsistent with a simple core-mantle boundary and require a hitherto unidentified, kilometre-scale, fine structure in the ultra-low velocity zone. The observations may be explained by a ca. 7 km thick, two-layer, ultra-low velocity zone with exceptional low velocities, which indicate the presence of high percentages of melt (>15%), in particular in the lower part of the zone. Waveform variation implies lateral change in the thickness and physical properties of the ultra-low velocity zone with a wavelength of less than 100 km.     

Application of DARLAM to Regional Haze Modeling

— The CSIRO Division of Atmospheric Research limited area model (DARLAM) is applied to atmospheric transport modeling of haze in southeast Asia. The 1998 haze episode is simulated using an emission inventory derived from hotspot information and adopting removal processes based on SO₂.¶Results show that the model is able to simulate the transport of haze in the region. The model images closely resemble the plumes of NASA Total Ozone Mapping Spectrometer and Meteorological Service Singapore haze maps. Despite the limitation of input data, particularly for haze emissions, the three-month average pattern correlation obtained for the whole episode is 0.61. The model has also been able to reproduce the general features of transboundary air pollution over a long period of time. Predicted total particulate matter concentration also agrees reasonably well with observation.¶The difference in the model results from the satellite images may be attributed to the large uncertainties of emission, simplification of haze deposition and transformation mechanisms and the relatively coarse horizontal and vertical resolution adopted for this particular simulation.     

Studies of Hydrochemical and Kinetic Characteristics of Small Water Bodies in the Context of Their Eutrophication

Comparison study data on the hydrochemical parameters, bacterio- and phytoplankton, and reduction–oxidation characteristics of two ponds in Krasnoyarsk are presented. These water bodies are of interest due to the fact that the ecosystems of these natural model objects respond to eutrophication in different ways. It is assumed that the reason for this difference is in the hydrophysical characteristics depending on the morphology of the basins of the water bodies rather than in the hydrochemical characteristics.     

Comment on: 'The 3D shear experiment over the Natih field in Oman: the effects of fracture-filling fluids on shear propagation' by C.M. van der Kolk, W.S. Guest and J.H.H.M. Potters

We find errors in theory and application in the paper by van der Kolk et al. (Geophysical Prospecting 49 , 179–197 (2001)) that invalidate their arguments for the cause of the reduction in the velocity of the vertically propagating slower split shear-wave in the gas cap over the Natih field in Oman. We suggest that existing theories are adequate and can explain the anomaly.     

Mantle plumes: heat-flow near Iceland

In the first of four pieces arising from Gill Foulger's challenge to the mantle plume hypothesis (last issue), Carol Stein and Seth Stein join the debate with some data and comment on heat-flow around Iceland.     

A Theoretical Comparison of Tsunamis from Dislocations and Landslides

— We use simple physical models to evaluate and compare the orders of magnitude of the energy generated into a tsunami wave by seismic dislocations and underwater slumps. We conclude that the two sources can generate tsunamis of comparable total energy. However, the slumping source is shown to be fundamentally dipolar in nature, which results in a low-frequency deficiency in the far-field. These simple conclusions corroborate the interpretation of the 1998 Papua New Guinea tsunami as being generated by an underwater slump.     

The use of inclined hemisphere projections for analyzing failure mechanisms in discontinuous rocks

This paper demonstrates the advantages of using inclined stereographic projections in kinematic analysis of rock blocks in discontinuous rock masses. Some examples of limiting cases are presented. The application of inclined projections is illustrated by its use in a mine slope in Brazil. It is clear from the discussion of these examples that inclined hemisphere projections provide better results than horizontal projections. It is also demonstrated that horizontal projections can lead to incorrect results in limiting cases.     

Determination of silica sand stiffness

Site investigation and evaluation of properties of soil or rock are important aspects of geotechnical design. Determination of the ground stiffness is one of the important parameters in geotechnical engineering. Since the measurement of shear modulus is very sensitive to soil disturbance, especially for sand, determination of the stiffness of soil in the field is more reliable than in laboratory tests on sampled specimens. Measurement of shear modulus is one of the most common applications of self-boring pressuremeter testing. As an in situ device, the pressuremeter provides a unique method for assessing directly the in situ shear modulus of a soil. This paper describes a laboratory study of silica sand stiffness, which varies with stress level and strain amplitude. The results show that the elastic shear modulus value is mainly dependent on the value of the mean effective stress and relative density.