A Statistical Approach to Geological Mapping

A geological map is the representation, on a two-dimensional plane, of the disposition of three-dimensional rock bodies exposed on the earth's surface. The problem of mapping is essentially that of dividing an area into homogeneous subregions on the basis of the exposed rock types. Automatic Bayesian methods of model selection using default Bayes factors have been employed to solve the problem of choosing a set of boundaries between homogeneous subregions, assuming no complication excepting low-angle tilting affected rock bodies. The method is tested on two data sets. A sampling scheme for optimum allocation of observation points is also presented.     

Reservoir induced seismicity—a new model

Deficiencies of different existing models on ‘Reservoir Induced Seismicity’ have been discussed and a new mathematical model, which enhances a better understanding of triggering mechanism in terms of changes in effective stresses, in situ stresses and water level variations, has been discussed in this paper. In the model fractured rock is simulated by a fluid-filled elastic material subject to Mohr–Coulomb failure criterion. The model has been found to be capable of responding effectively to site specific attributes. It can recognize and explain the phenomenon of time lag observed in several actual cases. It is also capable of simulating stabilization of rock–reservoir system after a period of activities that follow the initial stage of filling. One dimensional and two dimensional, isotropic and anisotropic cases have been analysed and the model predictions have been found to agree qualitatively with the field observations.     

A method of calculating 0–20 Å solar X-ray flux and its spectral distribution using 9.1 cm spectroheliograms

1–8 Å, 2–12 Å and 8–20 Å non-flare X-ray flux data and 9.1 cm spectroheliograms for 1237 days during the period July 1966 to June 1970 have been studied to derive physical models of λ < 20 Å X-ray emitting regions on the Sun under quiescent (non-flare) conditions. The preferred regions of emission below 20 Å which coincide with the coronal active regions characterised by enhanced 9.1 cm microwave emission are found to have temperature lying between 1.8 and 3 × 10⁶ K, emission measure 10⁴⁹–10⁵⁰ and electron density 10⁹-10¹⁰ per cc. The average area of an active region is 10²⁰ cm². A slow gradient of temperature and electron density is seen to exist around a region of peak activity, both temperature and electron density decreasing outwards. Based on the derived physical model of the emitting regions a new method is presented for calculating X-ray flux and spectral energy distribution in this wave length region using daily 9.1 cm solar spectroheliograms. The calculated values are in good agreement with the observed values.     

Isotopic and related studies of Antarctic ice samples

The ice samples obtained from Dakshin Gangotri, Antarctica show the presence of nuclear debris, attributed mainly to French nuclear explosions. Cosmogenic⁷Be occurs at levels of 30 dpm/L. The vertical profile ofδD in 6 m long drill core ranges between ?130 and ?180‰ compared to Standard Mean Ocean Water (SMOW). No systematic change with depth is seen. Small amounts of dust obtained by filtering melt water show presence of metallic spherules. Absence of elements characteristic of meteoritic or cometary debris suggests that most of them are of volcanic or industrial origin.     

Folding of boudinaged layers

The boudinage structure in the Väddö area in Sweden developed during an early isoclinal folding. During a second deformation which deformed the axial surfaces and the limbs of the isoclinal folds, the shapes of boudins were variously modified. The resulting structures include folding of the individual boudins into half-waves, a tile-like piling up of straight or gently folded boudins and arrangement of straight boudins in a herring-bone pattern. Similar structures have been produced experimentally in soft models containing ‘boudins’ of different competencies. The relative importance of the different physical parameters which contribute towards the development of the various structures is examined. In particular, the experiments showed that when the competence contrast between the boudin and the host material was rather small, the shape of the boudins was modified by a second deformation into trapezoidal forms often with flame-shaped projections at the edges. From an analysis of the experimental and natural structures, a set of criteria has been formulated which can be used to identify structures formed by superposed deformations and to give an approximate idea of the competence contrast between the boudin and the host rock during the second deformation.     

Onlamb's problem in a magneto-elastic initially stressed conducting medium

Summary The object of the present paper is to investigate the so-called planeLamb's problem in an initially stressed magneto-elastic semi-space bounded by a plane. The results obtained in this investigation are in agreement with the corresponding results of the classical planeLamb's problem when the semi-space is supposed to be initially unstressed and non-magnetic.     

Origin of peraluminous minerals (corundum,spinel, and sapphirine) in a highly calcic anorthosite from the Sittampundi Layered Complex,Tamil Nadu,India

The highly calcic anorthosite (An_>95) from the Sittampundi Layered Complex (SLC) develops corundum, spinel and sapphirine that are hitherto not reported from any anorthositic rocks in the world. Petrological observations indicate the following sequence of mineral growth: plagioclase_matrix → corundum; clinopyroxene → amphibole; corundum + amphibole → plagioclase_corona + spinel; and spinel + corundum → coronitic sapphirine. Phase relations in the CaO–Na₂O–Al₂O₃–SiO₂–H₂O (CNASH) system suggest that corundum was presumably developed through vapour present incongruent melting of the highly calcic plagioclase during ultra-high temperature (UHT) metamorphism (T ≥ 1000 °C, P ≥ 9 kbar). Topological constraints in parts of the Na₂O–CaO–MgO–Al₂O₃–SiO₂–H₂O (NCMASH) system suggest that subsequent to the UHT metamorphism, aqueous fluid(s) permeated the rock and the assemblage corundum + amphibole + anorthite + clinozoisite was stabilized during high-pressure (HP) metamorphism (11 ± 2 kbar, 750 ± 50 °C). Constraints of the NCMASH topology and thermodynamic and textural modeling study suggest that coronitic plagioclase and spinel formed at the expense of corundum + amphibole during a steeply decompressive retrograde P–T path (7–8 kbar and 700–800 °C) in an open system. Textural modeling studies combined with chemical potential diagrams (μSiO₂–μMgO) in the MASH system support the view that sapphirine also formed from due to silica and Mg metasomatism of the precursor spinel ± corundum, on the steeply decompressive retrograde P–T path, prior to onset of significant cooling of the SLC. Extremely channelized fluid flow and large positive solid volume change of the stoichiometrically balanced sapphirine forming reaction explains the localized growth of sapphirine.     

Shake Table Tests and Numerical Modeling of Liquefaction of Kasai River Sand

As a part of the seismic safety evaluation of several bridges and other hydraulic structures located on Kasai River bed in India, the liquefaction potential of Kasai River sand is studied in 1-g shake table in laboratory and numerically using a commercial software FLAC 2D. The surface settlement, lateral spreading, predominant frequency, amplification of the ground motion and pore water pressure development in Kasai River sand in dry and liquefied states have been studied when subjected to sinusoidal motions of amplitude 0.35 g at a frequency of 2 Hz. The nonlinear curves used to represent shear strain dependency of stiffness and damping ratio of Kasai River sand are obtained from cyclic triaxial tests. Reasonably good agreement between the experimental and the numerical results is observed. It is found that the settlement and lateral spreading for the liquefied sand is 2.60 and 2.50 times than those of the sand in the dry state. The volumetric strain of the liquefied sand is found to be around 4%, which is significantly higher than 1.53% observed in the dry sand. It is observed that the amplification of the peak ground acceleration for the saturated sand is 1.08 and 1.32 times higher than that for the dry sand from theoretical and experimental results, respectively. The shear strain developed in the liquefied sand is 1.17 times more than that for dry sand. The fundamental and higher modal frequencies of dry sand are found to be 1.13, 1.117 and 1.119 times more than those for the saturated sand, respectively.     

Tectonics of the ophiolite belt from Naga Hills and Andaman Islands,India

The ophiolitic rocks of Naga Hills-Andaman belt occur as rootless slices, gently dipping over the Paleogene flyschoid sediments, the presence of blue-schists in ophiolite melange indicates an involvement of the subduction process. Subduction was initiated prior to mid-Eocene as proved by the contemporaneous lower age limit of ophiolite-derived cover sediment as against the accreted ophiolites and olistostromal trench sediment. During the late Oligocene terminal collision between the Indian and Sino-Burmese blocks, basement slivers from the Sino-Burmese block, accreted ophiolites and trench sediments from the subduction zone were thrust westward as nappe and emplaced over the down-going Indian plate. The geometry of the ophiolites and the presence of a narrow negative gravity anomaly flanking their map extent, run counter to the conventional view that the Naga-Andaman belt marks the location of the suture. The root-zone of the ophiolite nappe representing the suture is marked by a partially-exposed eastern ophiolite belt of the same age and gravity-high zone, passing through central Burma-Sumatra-Java. The ophiolites of the Andaman and Naga Hills are also conventionally linked with the subduction activity, west of Andaman islands. This activity began only in late Miocene, much later than onland emplacement of the ophiolites; it further developed west of the suture in its southern part. Post-collisional northward movement of the Indian plate subparallel to the suture, also developed leaky dextral transcurrent faults close to the suture and caused Neogene-Quatemary volcanism in central Burma and elsewhere.