Wave propagation in a general anisotropic poroelastic medium: Biot’s theories and homogenisation theory

Anisotropic wave propagation is studied in a fluid-saturated porous medium, using two different approaches. One is the dynamic approach of Biot’s theories. The other approach known as homogenisation theory, is based on the averaging process to derive macroscopic equations from the microscopic equations of motion. The medium considered is a general anisotropic poroelastic (APE) solid with a viscous fluid saturating its pores of anisotropic permeability. The wave propagation phenomenon in a saturated porous medium is explained through two relations. One defines modified Christoffel equations for the propagation of plane harmonic waves in the medium. The other defines a matrix to relate the relative displacement of fluid particles to the displacement of solid particles. The modified Christoffel equations are solved further to get a quartic equation whose roots represent complex velocities of the four attenuating quasi-waves in the medium. These complex velocities define the phase velocities of propagation and quality factors for attenuation of all the quasi-waves propagating along a given phase direction in three-dimensional space. The derivations in the mathematical models from different theories are compared in order to work out the equivalence between them. The variations of phase velocities and attenuation factors with the direction of phase propagation are computed, for a realistic numerical model. Differences between the velocities and attenuations of quasi-waves from the two approaches are exhibited numerically.     

Velocity of ground water percolation at Indaram, Godavari sub-basin, India

We present values of velocity of ground water percolation (Vg) over large depth intervals, varying from shallow to deeper depths in Indaram area of Godavari sub-basin. The velocities have been estimated using available measured geothermal data. Sub-surface temperatures were measured in seven boreholes. Terrestrial heat flow values are calculated using temperature data and measured values of thermal conductivity of core samples. The results show that Vg is ～3.4 ×10^?7 cm /sec in the top layers (70–150 m) and decreases to ～0.04×10^?7 cm/sec in the deeper levels around 350 m depth and becomes negligibly small thereafter, thereby, indicating that the overall permeability of the sub-surface layers, due to the occurrence of successions of permeable, semi-permeable layers gets reduced to more or less zero at depths around 350 m. The value of Thermal Peclet Number, which is the ratio of the heat transfer through convection to that through conduction, naturally becomes negligible around this depth in the area. The observed consistency of the magnitude of heat flow through various deep sections is a clear indicator that water percolation is practically reduced to zero at depths around 320–400 m and that conduction is the dominant mechanism of heat transfer below the inferred depth section, while the upper layers are dominated by recharge at various depths by near surface water from streams at Indaram.     

The behavior of natural and anthropogenic osmium in the Hudson River-Long Island Sound estuarine system

The extent to which riverine Os is trapped in a temperate estuary was the aim of this study. The behavior of Os through the Hudson River, East River and the Long Island Sound (LIS) system is addressed using both natural Os and anthropogenically mobilized Os. The Os concentration ([Os]) and isotopic composition (¹⁸⁷Os/¹⁸⁸Os) of the Mid-Atlantic Bight as inferred from the analysis of a water sample of 31‰ salinity (S) at Vineyard Sound, MA are 46 fM and 1.070, respectively. In comparison, the Hudson River at Newburgh, NY has [Os] = 68 fM and ¹⁸⁷Os/¹⁸⁸Os = 1.265. The Os concentration of the East River at the Whitestone Bridge is 51 fM and remains essentially constant proceeding eastward in the LIS despite the increase of salinity from 20‰ towards the higher value of the Mid-Atlantic Bight. The ¹⁸⁷Os/¹⁸⁸Os ratio of water at Whitestone Bridge is 0.945 and increases eastward through the Sound to 0.979 at 7 km and then to 1.019 at 39.6 km. The behavior of Os through LIS appears to be conservative at S > 20‰. On the basis of Os concentration and isotopic composition we infer that anthropogenic Os is being added to the East River through sewers with the likely isotopic ratio of ∼0.13 and that about 24% of riverine Os must be removed at S ? 20‰. There is a net transport of about 0.4-1 mole of anthropogenic Os per year from the East River into the LIS. The residence time of Os in the ocean at present must be about 39,000 years, unless an independent source of supply of Os can be identified.     

Electrical analogy for modelling thermal regime and moisture distribution in sandy soils

The soil mass is subjected to temperature variation due to several human activities (viz. tanks storing heated fluids, buried cables and pipelines, air-conditioning ducts, disposal of nuclear and thermal power plant wastes etc.), which result in heat-induced migration of the moisture in it. Though several studies have been conducted in the past to investigate the mechanism of heat migration through the soil mass, a methodology for ‘real-time measurement of the variations in temperature, flux and moving moisture front, in tandem, with respect to space' has rarely been attempted. In this context, extensive laboratory investigations were conducted to measure real-time flux and temperature variations in the sandy soils, and the validation of results has been done by employing an equivalent electrical circuit programme, LTspice. Subsequently, a mathematical model PHITMDS (i.e. Prediction of Heat-Induced Temperature and Moisture Distribution in Soil) has been developed and its utility and efficacy, for predicting the depth-wise temperature and heat-induced moisture migration, due to evaporation, in terms of position of moving moisture front in the sandy soil has been critically discussed and demonstrated.     

Major Element, REE, and Other Trace Element Behavior in Amphibolite Weathering under Semiarid Conditions in Southern India

A body of komatiitic amphibolite, an enclave within the Archean high-grade orthogneisses in southern India, shows mild chemical weathering under semiarid conditions. Along fractures, chemical weathering has advanced (Chemical Index of Alteration &sqbl0;CIA&sqbr0;=53; CIA of fresh rock approximately 26) to the extent that secondary Mg-Fe-Al clay minerals have formed and the rock has turned brownish red, soft, and fine grained. The weathering process has resulted in the mobilization and redistribution of the so-called immobile elements Fe, Al, Ti, and REE effected by the nature of secondary mineral formation (talc vs. aluminous clay minerals) and also possibly by soil microbes. In the initial stages of secondary mineral formation, there is a small loss of Fe, Al, and REE (noticeably Eu). However, in the fracture zone as well as in the incipiently altered zone, there is significant REE enrichment, probably affected by a different precipitation mechanism. Mobilized REE may have come from a minor alteration of clinopyroxene.     

Mapping lithological variations in a river basin of West Bengal,India using electrical resistivity survey: implications for artificial recharge

Groundwater is a treasured earth’s resource and plays an important role in addressing water and environmental sustainability. However, its overexploitation and wide spatial variability within a basin and/or across regions are posing a serious challenge for groundwater sustainability. Some parts of southern West Bengal of India are problematic for groundwater occurrence despite of high rainfall in this region. Characterization of an aquifer in this area is very important for sustainable development of water supply and artificial recharge. Electrical resistivity surveys using 1-D and 2-D arrays were performed at a regular interval from Subarnarekha River at Bhasraghat (south) to Kharagpur (north) to map the lithological variations in this area. Resistivity sounding surveys were carried out at an interval of 2–3 km. Subsurface resistivity variation has been interpreted using very fast simulated annealing (VFSA) global optimization technique. The analysis of the field data indicated that the resistivity variation with depth is suitable in the southern part of the area and corresponds to clayey sand. Interpreted resistivity in the northern part of the area is relatively high and reveals impervious laterite layer. In the southern part of the area resistivity varies between 15 and 40 Ωm at a depth below 30 m. A 2-D resistivity imaging conducted at the most important location in the area is correlated well with the 1-D results. Based on the interpreted resistivity variation with depth at different locations different types of geologic units (laterite, clay, sand, etc.) are classified, and the zone of interests for aquifer has been demarcated. Study reveals that southern part of the area is better for artificial recharge than the northern part. The presence of laterite cover in the northern part of the area restricts the percolation of rainwater to recharge the aquifer at depth. To recharge the aquifer at depth in the northern part of the area, rainwater must be sent artificially at depth by puncturing laterite layers on the top. Such studies in challenging areas will help in understanding the problems and finding its solution.     

Uranium mineralisation associated with felsic volcanism at Mohar,Shivpuri district,Madhya Pradesh

Several small lensoidal bodies of felsic volcanics are exposed in a curvilinear pattern within the brecciated granitoids of Bundelkhand Gneissic Complex (BGC) at Mohar. Sub-surface data reveals extensive presence of these felsic volcanics below the sediment of Vindhyan Supergroup. It occurs like a sheet with thickness varying from 12 m to 134 m. Its lateral extent has been traced upto 4.8 km. Multiple flows of felsic magma are identified based on colour, granularity, cross cutting relations and cyclic distribution of multiple vesicular bands along the entire thickness of felsic magma. The felsic rock contains upto 13.21% K₂O. Chemical composition of these felsic volcanics varies across the column. Petrographically and chemically all these felsic volcanics are identified as rhyolite or rhyolite tuff.     

The role of fluids in the formation of talc deposits of Rema area,Kumaun Lesser Himalaya

Talc deposits of Rema area in the Kumaun Inner Lesser Himalaya are hosted within high magnesium carbonates of the Proterozoic Deoban Formation. These deposits occur as irregular patches or pockets mainly within magnesite bodies, along with impurities of magnesite, dolomite and clinochlore. Textures represent different phases of reactions between magnesite and silica to produce talc. Petrography, XRD and geochemistry reveal that the talc has primarily developed at the expense of magnesite and silica, leaving dolomite largely un-reacted. Early fluid inclusions in magnesite and dolomite associated with talc are filled with H₂O+NaCl+KCl ± MgCl₂ ± CaCl₂ fluids, which represent basin fluid system during diagenesis of carbonates. Their varied degree of re-equilibration was although not pervasive but points to increased burial, and hence requires careful interpretation. H₂O-CO₂ fluid with XCO₂ between 0.06 and 0.12 was equilibrated with talc formation. The reaction dolomite+quartz → talc was not extensive because T-X_CO2 was not favourable, and talc was developed principally after magnesite+quartz.     

Comparative study of MLT mean winds using MF radars located at 16.8°N and 8.7°N

The main aim of the paper is to describe mesosphere and lower thermosphere (MLT) wind field observed between June 2000 and May 2002 by medium frequency (MF) radars at two locations near the equatorial region and at tropical latitude. We have presented and compared observations of mean horizontal winds in the mesosphere and lower thermosphere (MLT) region over Kolhapur (16.8°N, 74.2°E) and Tirunelveli (8.7°N, 77.8°E). Our analysis reveals annual oscillation in the zonal wind and semi-annual oscillation in the meridional wind over Kolhapur. On the other hand, results over Tirunelveli reveal semi-annual oscillation (SAO) in the zonal wind and annual oscillation in the meridional wind. Also we have observed enhanced magnitude of wind speed in spring equinox period of 2002 exhibiting the signatures of the quasi-biennial oscillation (QBO) over Tirunelveli.