Visco-Elastic Waves in Rock-Like Composites

The subject of this paper is the treatment of rocks - and, especially, fluid-saturated and partially saturated reservoir rocks, as composite visco-elastic media. By this we mean to study and partially answer the question of how the effective material (frequency-dependent and complex-valued stiffness/density) parameters can be estimated from a knowledge of the constituents of the rocks, their volume fractions, the statistical distribution of sizes, shapes, orientations and positions of the individual particles (minerals of quartz, clay, etc.) and cavities (pores, cracks, etc.); in addition to parameters related to the fluid and its ability to flow, at the scale of the microstructure as well as that of the wavelength (assumed to be long compared to the scale-size of the microstructure).Our approach is to develop and combine a theory of stochastic waves with established results for the micromechanics of defects in solids, as well as state-of-the-art models of wave-induced fluid flow. Specifically, we first derive an exact formal expression for the effective material parameters in terms of a dynamic T-matrix for the material, which satisfies a single integral equation of the Lippmann-Schwinger type (known from quantum scattering theory), but formulated in an abstract vector space, associated with the combination of the strain and velocity fields into a more general state vector . Inclusions-based models are developed on the basis of standard many-body techniques, known from the static T-matrix approach as well as nuclear collision theory. The t-matrix of a low-aspect-ratio spheroidal crack is expressed in terms of the familiar displacement discontinuity parameters of Hudson, via the so-called K-tensor, which is of interest in itself, for example, when connecting cracks to pores (in the presence of multiple solid constituents) on the basis of an expression for the t-matrix of a communicating cavity.The present theory can in principle be used beyond the Rayleigh limit, but explicit estimates of the effective material parameters have so far been derived only under the assumption that (scattering attenuation can be ignored) the wavelength is large compared to the scale-size of a representative volume element. Starting with the dynamic equations of motion, we show that the behaviour of the mean wave in the Rayleigh limit is indeed determined by the effective stiffness tensor associated with a static theory of composites, in conjunction with the spatially averaged density for the heterogeneous material as a whole. Thus, we have provided justification to the procedure we used in a series of related papers, where we started out with the static equilibrium condition and employed the elastic/visco-elastic correspondence principle. Numerical examples (dealing with the effects of randomly oriented cracks on the isotropic velocity and attenuation spectra of a dual porosity model of clay-sand mixtures, and the effects of spatial distribution on the anisotropic attenuation spectra of fully aligned cracks that are partially saturated with two different fluids) will be provided in order to complement those in our earlier papers.     

Expansion of an invasive coral species over Abrolhos Bank,Southwestern Atlantic

Invasive coral species of the genus Tubastraea have been increasingly recorded in Southwestern Atlantic waters since the 1980s. Their invasion and infestation are mainly related to port and oil exploration activities. For the first time the presence of Tubastraea tagusensis colonies is reported in Espírito Santo State, colonizing a port shore area, and incrusting oil/gas platform structures situated in the southern Abrolhos Bank, which is part of the most important coral reef system of the South Atlantic Ocean. Tubastraea colonies exhibit fast growth and high recruitment rates, and colonized 40% of the analyzed structures in just four years. The projection of port and oil/gas industry growth for the Espírito Santo State (more than 300%) highlights an alert to the dispersal of this alien species to natural areas.     

Environmental applications of geophysical and geochemical methods to map groundwater quality at Tuticorin, Tamilnadu, India

Coastal aquifers can become polluted due to natural and human activities, such as intrusion of saline water, discharge of effluents in industrial areas and chemical weathering of natural geological deposits. The present study is aimed mainly at understanding the geophysical and chemical characteristics of groundwater near Tuticorin, Tamilnadu, India by studying the electrical resistivity distribution of the subsurface groundwater by applying the Schlumberger vertical electrical sounding (VES) technique followed by chemical analysis of water samples. A total of 20 VES soundings were carried out to understand the resistivity distribution of the area and 21 water samples were collected to analyze the chemical quality. The interpretation and analysis of the results have identified different hydrogeologic behaviors, a highly saline coastal aquifer and freshwater locations. The results obtained from geophysical and geochemical sampling are in good agreement with each other. The approach shows the efficacy of the combination of geophysical and geochemical methods to map groundwater contamination zones in the study area.     

Local- and watershed-scale controls on the spatial variability of natural levee deposits in a large fine-grained floodplain: Lower Pánuco Basin, Mexico

This study examines spatial variations in natural levee deposits within the lower reaches of a large coastal plain drainage system. The Pánuco basin (98,227 km²) drains east-central Mexico, and is an excellent setting to examine the influence of watershed and local controls on the morphology and sedimentology of natural levees. Although many fluvial systems in the U.S. Gulf Coastal Plain have been investigated, the rivers in the Mexican Gulf Coastal Plain have received comparatively little attention. Lateral and downstream characteristics of natural levee morphology and sediment texture are considered within the context of meandering river floodplain deposits. Data sources include total-stations surveying, sediment samples of surficial levee deposits, topographic maps (1:50,000), and aerial photographs (1:40,000). The slope of natural levees average 0.0049 m/m, whereas the texture (D₈₄) of levee deposits averages 0.12 mm. Natural levee characteristics vary due to local- and watershed-scale controls. The lateral reduction in levee height displays a curvilinear pattern that coincides with an abrupt change in sediment texture. The downstream pattern of natural levee texture exhibits the influence of local-scale perturbations superimposed upon a larger watershed-scale trend. Disruption to the fining trend, either by tributary inputs of sediment or reworking of Tertiary valley deposits, is retained for a limited distance. The influence of the channel planform geometry on levee morphology is examined by consideration of the radius of curvature (R_c) of meander bends, and is inversely related to natural levee width. This suggests that the planform geometry of river channels exerts a control on the dispersal of flood sediments, and is responsible for considerable local variability in the floodplain topography. The average width of natural levees increases with drainage area, from an average of 747 m in the Moctezuma to an average of 894 m in the Pánuco. However, in the lower reaches of the Pánuco valley the width of natural levees rapidly decreases, which is associated with fining of the suspended sediment load. Thus, the reduction in natural levee width signifies an abrupt change in the directionality of cause–effect relationships at the watershed-scale. Findings from this study elucidate linkages between meandering river channels and floodplains for a large lowland alluvial valley.     

Book reviews

The weather and climate of Australia and New Zealand

A.P. Sturman & N.J. Tapper, 1996

Oxford University Press, Melbourne

18 × 24 cm, xi + 476pp., ISBN 0–19–553393–3, A$55.00 (soft)

Australians and the environment

W. McLennan, 1996

Australian Bureau of Statistics (Catalogue No. 4601.0), Melbourne

21 × 30 cm, viii + 415pp., ISBN 0–642–20690–2, A$44.00 (soft)

The information age: economy, society and culture. Vol.1—The rise of the network society

M. Castells, 1996

Blackwell, Cambridge MA/Oxford

15 × 23 cm, xvii + 556 pp., ISBN 1–155786–617–1, US$25.95/£15.99 (soft)

Retailing, consumption and capital: towards the new retail geography

N. Wrigley & M. Lowe (eds), 1996

Longman, Harlow

16 × 23 cm, vi + 352 pp., ISBN 0–582–22824–7, £17.99 (soft)     

A reconnaissance study of stable isotope ratios in archaean rocks from the yilgarn block,Western Australia

Isotopic compositions of sulphur, carbon, and oxygen have been determined for constituents from a total of 103 samples of sedimentary rocks, mafic and ultramafic igneous rocks, nickel ores, and gold ores from the Archaean Yilgarn Block.

Sulphides in the bulk of the sedimentary rocks have δ³⁴S values close to 0‰ and appear to have precipitated from solutions which incorporated magmatic sulphur (either juvenile or derived from older rocks). There is no evidence for widespread sulphate reduction.

δ³⁴S values of sulphides in the nickel deposits and associated mafic/ultramafic igneous rocks are within the magmatic range. The small, high‐grade deposits of the Kambalda‐Nepean‐Scotia type have small positive δ³⁴S values, and the large, low‐to‐medium grade dunite‐associated deposits of the Mount Keith‐Perseverance type have small negative δ³⁴S values.

Sulphides in the Kalgoorlie gold ores are enriched in ³²S relative to those in their host dolerite, supporting an epigenetic origin for the gold, under moderately high fO₂ conditions.

The δ¹³C values do not provide unequivocal evidence for the source(s) of the reduced carbon (kerogen) in the sedimentary rocks. Whilst they are compatible with biogenic derivation, it is not possible to rule out contributions from pre‐biotic organic ‘soup’ or from hydrothermal solutions of deep‐seated origin.

Carbonate in the sedimentary rocks are predominantly in epigenetic, sulphide‐bearing veinlets. In many cases, their δ¹³C values suggest precipitation from hydro‐thermal solutions containing magmatically derived CO₂. In only two samples are the petrographic features and δ¹³C values compatible with marine carbonates. Talc‐carbonate altered ultramafic igneous rocks have δ¹³C values consistent with their incorporation of magmatically derived CO₂.

The ?δ¹³C (carbonate‐kerogen) values for most of the sedimentary rock studied fall in a narrow range around +10‰, suggesting isotopic exchange between oxidized and reduced carbon species at moderately high temperatures (>250°C).

δ¹⁸O values of carbonate from both sedimentary rocks and igneous rocks are mainly within the range +7.2‰ to +18.0‰. If the values are primary they are consistent with the formation of carbonate from hydrothermal solutions of magmatic and/or metamorphic origin. However, it is also possible the δ¹⁸O values are the result of post‐depositional equilibration with meteoric waters.     

A multiple-code simulation study of the long-term EDZ evolution of geological nuclear waste repositories

This simulation study shows how widely different model approaches can be adapted to model the evolution of the excavation disturbed zone (EDZ) around a heated nuclear waste emplacement drift in fractured rock. The study includes modeling of coupled thermal-hydrological-mechanical (THM) processes, with simplified consideration of chemical coupling in terms of time-dependent strength degradation or subcritical crack growth. The different model approaches applied in this study include boundary element, finite element, finite difference, particle mechanics, and elasto-plastic cellular automata methods. The simulation results indicate that thermally induced differential stresses near the top of the emplacement drift may cause progressive failure and permeability changes during the first 100 years (i.e., after emplacement and drift closure). Moreover, the results indicate that time-dependent mechanical changes may play only a small role during the first 100 years of increasing temperature and thermal stress, whereas such time-dependency is insignificant after peak temperature, because of decreasing thermal stress.