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1.
Narcondam Island in the Andaman Sea represents a dacite–andesite dome volcano in the volcanic chain of the Burma–Java subduction complex. The pyroclasts of andesitic composition are restricted to the periphery of the dome predominantly in the form of block‐and‐ash deposits and minor base surge deposits. Besides pyroclastic deposits, andesitic lava occurs dominantly at the basal part of the dome whereas dacitic lava occupies the central part of the dome. The pyroclasts are represented by non‐vesiculated to poorly vesiculated blocks of andesite, lapilli, and ash. The hot debris derived from dome collapse was deposited initially as massive to reversely‐graded beds with the grain support at the lower part and matrix support at the upper part. This sequence is overlain by repetitive beds of lapilli breccia to tuff breccia. These deposits are recognized as a basal avalanche rather than lahar deposit. This basal avalanche was punctuated by an ash‐cloud surge deposit representing a sequence of thinly bedded units of normal graded unit to parallel laminated beds. 相似文献
2.
At the end of May 2012, the Po plain region in northern Italy was shaken by a long sequence of seismic events. The 2012 Northern Italy earthquake sequence counted two mainshocks, about 1,600 aftershocks and lasted for several weeks. Although the mainshocks, which occurred on May 20 and May 29, 2012, registered a moment magnitude of 5.9 and 5.8, respectively, these two events caused widespread soil liquefaction and substantial damages to the built environment. This paper reports lessons learnt from a field investigation conducted in the areas affected by the earthquake sequence. Based on the field observations, it was concluded that despite the relatively low magnitudes of the shocks, most of the damages occurred as a consequence of liquefaction phenomena and/or absence of retrofitting of historical structures. The latter comprise churches, tower bells, towers, castles and fortresses. It was found that the occurrence of liquefaction was mainly associated with the presence of saturated alluvial soil deposits which were characterised by high liquefaction susceptibility. It was noted that these highly liquefiable soils were mainly located in proximity of ancient river courses that were artificially diverted in the eighteenth century to mitigate flooding and other hydrological risks. 相似文献
3.
A case history is presented for the exploration of barite with the electrical resistivity method. Altogether eight locations were recommended out of which six correspond to barite bodies. In the present case the gravity method, which is commonly used for the location of barite because of its high density contrast with the host rocks, has failed, perhaps due to small size of the ore bodies. 相似文献
4.
The application of pulse EM and MAXMIN II systems in search of a kimberlite diatremes under conducting cover has been demonstrated through a field example. The effectiveness of the PEM system in the search for conductive zones of economic importance and in discrimination against geologic noise such as overburden has been investigated. The transient observation has been transformed to equivalent half-space resistivities at various times. The feasibility of mapping thin conducting overburden using the PEM system has been studied by inverting the transient measurement to the thin infinite horizontal sheet conductance. The PEM measurement is shown to be an economic and effective tool for shallow geoelectric mapping. 相似文献
5.
ASOKKUMAR BHATTACHARYA 《Sedimentology》1997,44(6):973-975
Flaser bedding of non-tidal origin has been described from point bar deposits of the Ajay river, NE India. Mud drapes are deposited during waning flood phases. The preferential accumulation of curled, desiccated mud laminae in ripple troughs during periods of exposure produces mud streaks resembling classical flaser bedding. This process of non-tidal flaser formation is unlike the common tidal origin related to flood and ebb cycles, and the less common mechanism of formation in fluvial systems by deposition of mud in ripple troughs after floods. 相似文献
6.
V. R. MARTHI J. N. CHENGALUR Y. GUPTA G. C. DEWANGAN D. BHATTACHARYA 《Journal of Astrophysics and Astronomy》2011,32(4):451-455
We describe the serendipitous discovery of a very steep-spectrum radio point source in low-frequency Giant Metrewave Radio
Telescope (GMRT) images of the supernova remnant (SNR) G76.9+1.0. The steep spectrum, as well as the location of the point
source near the centre of this SNR confirm that this indeed is the pulsar J2022+3842. Archival Chandra X-ray data shows a point source coincident with the radio point source. However, no pulsed radio emission was detected despite
deep searches at 610 MHz and 1160 MHz – which can be understood to be due to temporal broadening of the pulses. Weak pulsed
emission has indeed been seen at 2 GHz with the Green Bank Telescope (GBT), establishing the fact that scattering is responsible
for its non-detection at low radio frequencies. We underline the usefulness of low-frequency radio imaging as a good technique
to prospect for pulsar candidates. 相似文献
7.
CORNEL OLARIU JANOK P. BHATTACHARYA MATTHEW I. LEYBOURNE STEPHEN K. BOSS ROBERT J. STERN 《Sedimentology》2012,59(2):704-728
Basin‐floor topography influences the flow path of hyperpycnal plumes and delta morphology during progradation of the Red River delta in Lake Texoma, USA. The Red River discharge is typically a hyperpycnal plume due to elevated total dissolved solids. Because the river plume is a bottom‐hugging hyperpycnal flow, lake bathymetry and topography strongly influence deposition and subsequent delta morphology. In addition to elevated total dissolved solid concentrations compared with Lake Texoma water, the density contrast of the Red River outflow is increased by high suspended‐sediment concentrations during high‐discharge events. Steep lateral slopes in the Lake Texoma basin deflect hyperpycnal river plumes and, subsequently, change the delta progradation direction before the delta reaches the opposite bank of the lake. Analysis of multi‐temporal aerial and satellite images indicates that the hyperpycnal delta follows the steepest lake‐bottom gradients, corresponding to the pre‐impoundment river thalweg (i.e. bypassing shallow parts of the lake). An analytical model for the hyperpycnal‐plume trajectory indicates plume deflection during low‐discharge or high‐discharge events, towards the deepest part of the basin. The magnitude of plume deflection is a function of river discharge and basin‐margin gradients. Plume deflection can vary between 10° and 80° from the channel axis towards the old river thalweg. The high deflection appears in the case of maximum basin side gradients of 12·8° and in conditions of low river discharge. During low‐discharge periods, the Red River delta builds a lobate shape with multiple terminal distributary channels whereas, during high‐discharge periods the Red River delta builds an elongate shape with a single large distributary channel. The elongate morphology of the delta is formed through the development of a single distributary channel and abandonment of the other distributaries. Therefore, the lobate shaped delta is expected to be preserved in the rock record. 相似文献
8.
Suresh R. Dash Subhamoy Bhattacharya Anthony Blakeborough 《Soil Dynamics and Earthquake Engineering》2010,30(1-2):32-39
This paper investigates the importance of bending–buckling interaction in seismic design of piles in liquefiable soils using numerical techniques. A pseudo-static analysis has been performed using a well documented case history, where the pile–soil interaction is modelled as a beam on nonlinear winkler foundation (BNWF). Six possible analytical methods, three force based and three displacement based, are performed in which the pile is subjected to both lateral and axial load. Three out of six analysis cases did not predict the failure of the piles when analysed only for bending (i.e., lateral loads only). The buckling analysis showed that the pile was also safe against pure buckling during full liquefaction. Further, two out of those three cases which did not predict failure in bending were reanalysed for bending–buckling interaction (i.e., lateral and axial loads acting simultaneously). These combined analyses showed a more realistic behaviour of pile response and did predict the pile failure. Hence, it can be concluded that if a pile is designed for bending and buckling criteria separately and safe for these individual design criteria, it may fail due to their combined effect. 相似文献
9.
A. BHATTACHARYA B. SPIERING S. K. SEN R. NATARAJAN A. C. MAZUMDAR 《Journal of Metamorphic Geology》1990,8(5):525-538
Manganese-rich and manganese-poor iron formations which occur as thin layers in the Halaguru-Satnuru area, south of Kabbaldurga, Karnataka, India are chemically intermediate between the ‘Algoma’and ‘Lake Superior’types, but higher in their MnO and TiO2 contents. The rocks are of four petrographic varieties: (a) quartz-magnetite-orthopyroxene-clinopyroxene, (b) quartz-magnetite-orthopyroxene-clinopyroxene-garnet, (c) quartz-magnetite-clinopyroxene-garnet, and (d) quartz-magnetite-clinopyroxene-garnet-plagioclase. In the orthopyroxene-clinopyroxene pairs, Mn-Mg and Mn-Fe exchange is ideal irrespective of the MnSiO3 contents of orthopyroxenes (0.6–1.8 mol. % in Mn-poor and 15–25 mol. % in Mn-rich compositions). Mg-Fe exchange in the same pair is however non-ideal. Mn-Fe exchange in orthopyroxene-garnet pairs is ideal. The distribution patterns in the other binaries are inconclusive regarding ideality of exchange. Orthopyroxene-garnet and clinopyroxene-garnet geothermometers, modified for high spessartine contents, give temperatures of 800 ± 30° C. A modified version of the Harley (1984) geothermometer registers 740 ± 60° C, in agreement with the consensus temperature value. The equilibrium log ffo2 values in the iron formations, as calculated from the reaction 6FeSiO3+ O2= 2Fe3O4+ 6SiO2 are in the range of ?14.2 to ?15.5. Algebraic analysis of variations of fo2 with composition of phases indicates buffering of O2 in the rocks. The absence of grunerite in these assemblages is compatible with XH2O being less than 0.3 in the ambient fluid. Computations from volatile equilibria in the C-O-H system, however, predict high XH2O values (>0.7) at ac= 1.0, implying that the activity of graphite must have been greatly reduced—this is in accordance with the absence of graphite in these rocks. 相似文献
10.
A nomogram has been devised for situations, in which the source of a self-potential anomaly can be approximated by an obliquely polarized sphere or horizontal cylinder embedded in a homogeneous half space. The nomogram can be used for rapid determination of three parameters of the target: (1) depth to the centre, (2) angle between the axis of polarization and the horizontal, (3) shift of the point vertically above the centre of the body from zero potential value. The nomogram has been tested and the parameters determined for SP results obtained over ore bodies Weiss and Süleymanköy in the Ergani Copper district, Turkey. The curves computed for the estimated parameters match the field curves well. 相似文献