Regional conductance map of Andaman and Nicobar region

To image the electrical conductivity distribution, fluxgate magnetometers are operated at five sites in Andaman and Nicobar region. Transfer functions are estimated for the period range 8–128 min, from nighttime transient geomagnetic variations, using robust regression analysis. The observed induction arrows in Andaman Islands are found to point towards east despite deep sea located towards its west. This indicates that fore-arc basin (Andaman–Nicobar deep) is more conducting than the region of outer non-volcanic Island arc.Thin sheet model requires the conductance of 10,000–35,000 S (with increase conductivity towards the south) for explaining the observed induction pattern. The observed induction pattern at Andaman–Nicobar stations can be explained in terms of high conducting Cretaceous–Tertiary sediments filling the Andaman–Nicobar deep. High conductivity over Invisible bank has been attributed to the partial melts/volatile fluids derived from the subducting Indian plate that are intruding into the eastern margin of fore-arc basin through the West Andaman Fault (WAF).The induction pattern at Great Nicobar station (Campbell Bay) may be related to the highly conducting sediments filling the Mergui basin along with mafic intrusions. Also crustal transition occurs below the Mergui Terrace at the Malayan coast contributing to the enhanced conductivity anomaly.     

Delineation of groundwater prospective zones from a delta region of India,using geoelectrical and water quality approach

Geoelectrical survey was carried out in the western delta region of River Vasista Godavari, Andhra Pradesh, India, for delineation of groundwater prospective zones due to acute shortage of water supply for various purposes. Forty-six vertical electrical soundings (VES) were done, employing the Schlumberger configuration with a maximum AB/2 of 160 m. The interpreted results of VES show four to five layers with variable thicknesses, such as topsoil zone (1.5–3 m), clay zone (0.84–32 m), finer sand zone (2–72 m), medium to coarse sand zone (4 to 28.8 m) and clay zone (1.2–∞ m), indicating a multi-aquifer system. These results are corroborated with the known lithologs of the study area. Further, the resistivity is also compared with electrical conductivity (EC) of groundwater observed nearby shallow wells representing buried channel (BC), flood plain (FP) and coastal (C) zones, which indicate slightly brackish to brackish water (EC: 1470–6010 µS/cm), whereas the groundwater observed from deep wells shows the fresh (EC: 726–1380 µS/cm), fresh to brackish (EC: 1010 to 3250 µS/cm), and brackish water (EC: 3020 to 4170 µS/cm) located in BC, FP and C zones, respectively. This survey reveals the prospective aquifer zones with potable water at VES locations of 4–6, 8, 10, 11, 14, 16–28, 33–36, 39 and 42–44, where the resistivity values vary from 10 to 40 Ω m. The slightly brackish and brackish water zones are also observed from the resistivity of less than 10 Ω m at shallow depth in BC (VES-22, 37, 38 and 46), FP (VES-1, 2, 7, 29, 30 and 40) and C (VES-3, 4, 9, 12, 13, 15, 31, 32, 41 and 45) zones. As a result, the present investigation has delineated the freshwater zones at shallow (<?12 m) and also at deeper depths (30–45 m) as prospective areas, where BC zone occurs. Freshwater pockets also identified in FP (VES-8 and 39) and C (VES-11, 14 and 15) zones. Thus, this study helps to solve the drinking and irrigation water problems.     

Alkaline rocks of the deccan traps

Recent investigations on the Deccan Traps reveal many new findings of alkaline rocks more commonly occurring as minor intrusions than as lava flows. In comparison to the vast extent of the Deccan Traps, the alkaline rocks are negligible in their volume and are confined to tectonic belts in parts of Western India. The rocks exhibit no systematic variation in their petrographical and chemical characters thereby suggesting that they were not derived from a primary alkali olivine-basalt magma. The possibility of derivation of alkaline magma locally along the rift zones is proposed. Some of the alkaline rocks are shown to have been formed due to the effective role of volatiles in bringing dissociation of feldspar in certain cases, and alkali metasomatism in others. The syntexis of the pre-Deccan Trap carbonate rocks along the Narmada rift zone is also responsible for some occurrences.     

Non-linear stochastic models for seismic events with applications in event identification

Summary. A special class of p th-order autoregressive stochastic models is proposed for seismic records of explosions and earthquakes. These models are based on deterministic formulations proposed by Haskell, Von Seggern & Blandford and Aki. Statistical properties of the estimation procedure are derived and the model applied to seismic records of P waves. The method is shown to have applications in depth and reflection parameter estimation and in event discrimination.     

Spatial control of groundwater contamination,using principal component analysis

A study on the geochemistry of groundwater was carried out in a river basin of Andhra Pradesh to probe into the spatial controlling processes of groundwater contamination, using principal component analysis (PCA). The PCA transforms the chemical variables, pH, EC, Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO \(_3^- \) , Cl^?, SO \(_4^{2-} \) , NO \(_3^-\) and F^?, into two orthogonal principal components (PC1 and PC2), accounting for 75% of the total variance of the data matrix. PC1 has high positive loadings of EC, Na⁺, Cl^?, SO \(_4^{2-} \) , Mg²⁺ and Ca²⁺, representing a salinity controlled process of geogenic (mineral dissolution, ion exchange, and evaporation), anthropogenic (agricultural activities and domestic wastewaters), and marine (marine clay) origin. The PC2 loadings are highly positive for HCO \(_3^- \) , F^?, pH and NO \(_3^- \) , attributing to the alkalinity and pollution controlled processes of geogenic and anthropogenic origins. The PC scores reflect the change of groundwater quality of geogenic origin from upstream to downstream area with an increase in concentration of chemical variables, which is due to anthropogenic and marine origins with varying topography, soil type, depth of water levels, and water usage. Thus, the groundwater quality shows a variation of chemical facies from Na⁺ > Ca²⁺ > Mg²⁺ > K⁺: HCO \(_3^- \) > Cl^? > SO \(_4^{2-}>\) NO \(_3^- \) > F^?at high topography to Na⁺ > Mg²⁺ > Ca²⁺ > K⁺: Cl^? > HCO \(_3^- \) > SO \(_4^{2-}>\) NO \(_3^- \) > F^? at low topography. With PCA, an effective tool for the spatial controlling processes of groundwater contamination, a subset of explored wells is indexed for continuous monitoring to optimize the expensive effort.     

Effect of oblateness on triangular solutions at critical mass

At critical mass the triangular equilibria in the planar restricted three-body problem, when the more massive primary is an oblate spheroid with its equatorial plane coincident with the plane of motion, are in general unstable due to the presence of secular terms in the solutions of linearized equations of motion in the vicinity of these points. Existence of retrograde elliptic periodic orbits is established through suitable velocity components. The eccentricity of these orbits increases with the oblateness.     

Seismic bearing capacity of shallow strip footings

Seismic bearing capacity of shallow strip footings in soil has been obtained in the form of pseudo-static seismic bearing capacity factors N_cd, N_qd and N_d, denoting the cohesion, surcharge and unit weight components, respectively, by an extensive numerical iteration technique. Limit equilibrium method of analysis with composite failure surface is assumed. The validity of the principle of superposition is examined. Effects of both the horizontal and vertical seismic acceleration coefficients have been found to always reduce the ultimate bearing capacity significantly. Results obtained by the present method of analysis are compared with the available results and are found to be the least in the seismic case.     

Geochemistry of carbonate precipitation from the groundwater in a coastal region

Groundwater samples were collected from a coastal region of Andhra Pradesh to assess the possible conditions of the formation of carbonates. The area experiences a semi-arid climate and is underlain by khondalites, over which the Quaternary sediments occur. The study of the geochemistry of groundwater indicates that groundwater is mostly of fresh, with alkaline nature. The study further suggest that the breakdown of feldspars as kaolinite during rock-water interaction, releases Ca²⁺. Soils/weathered products contribute high CO₂ under the open system. The Ca²⁺ and CO₂ are added to the groundwater through the infiltrating recharge water. They subsequently precipitate as fine-grained carbonates in the weathering profile due to evapotranspiration under a freshwater environment.