Seismic diffraction tomography technique using very fast simulated annealing method for delineating small subsurface features

Diffraction, once considered noise rather than signal in seismic tomography, has recently been successfully used for delineating smaller objects whose size is comparable to or less than the wavelength of the incident seismic wave. But Most of the seismic diffraction tomography works are based on Born's and Rytov's approximation of weak scattering to linearize the expression for the scattered field due to an object and this is the basis of Fourier Diffraction Theorem. However, In this paper, we used the amplitude of the total scattered wave field without considering any approximation of weak scattering to obtain the synthetic scattered field data for forward modeling. These synthetic data are then inversed by Very Fast Simulated Annealing method which is a nonlinear inversion technique to obtain the unknown model parameters. This algorithm is applicable to both weak as well as strong scatterers. The simulated results give fairly good estimate for unknown model parameters of the scattering object within acceptable error limits.     

Radiation condensation instability in a plasma with ionization and recombination

In this work, we consider radiation (thermal) instability in a weakly ionized plasma with continuous ionization and recombination. The situation can be visualized in the case of envelopes of planetary nebulae, which are envelopes of ionized plasmas surrounding red giant stars. Various observations report continuous photoionization of these plasmas by the highly energetic streams of photons emanating from the parent star. Recently, it has been shown that thermal instability can be a probable candidate in such plasmas for the existence of small scale structures (viz., striations) whose kinematic age is much smaller than that of the parent nebula. We therefore report a systematic study of these plasmas with photoionization and determine the instability domain. We have shown that the continuous ionization and recombination may lead to modification of the underlying instability, which may limit the size of the small structures that are believed to form from these instabilities, and may thus provide an explanation of the physical processes responsible for the existence of these structures. We further show that in many cases the system bifurcates to an ovserstable (growing wave) state from a condensation instability (monotonic) and vice versa.     

An assessment of using subsampling method in selection of a flood frequency distribution

In flood frequency analysis, a suitable probability distribution function is required in order to establish the flood magnitude-return period relationship. Goodness of fit (GOF) techniques are often employed to select a suitable distribution function in this context. But they have been often criticized for their inability to discriminate between statistical distributions for the same application. This paper investigates the potential utility of subsampling, a resampling technique with the aid of a GOF test to select the best distribution for frequency analysis. The performance of the methodology is assessed by applying the methodology to observed and simulated annual maximum (AM) discharge data series. Several AM series of different record lengths are used as case studies to determine the performance. Numerical analyses are carried out to assess the performance in terms of sample size, subsample size and statistical properties inherent in the AM data series. The proposed methodology is also compared with the standard Anderson–Darling (AD) test. It is found that the methodology is suitable for a longer data series. A better performance is obtained when the subsample size is taken around half of the underlying data sample. The methodology has also outperformed the standard AD test in terms of effectively discriminating between distributions. Overall, all results point that the subsampling technique can be a promising tool in discriminating between distributions.     

Comparison of mapping approaches for estimating extreme precipitation of any return period at ungauged locations

Stochastic Environmental Research and Risk Assessment - Reliable estimation of return period values of extreme precipitation at ungauged locations is considered to be a key exercise in...     

Dust ion acoustic (DIA) solitary waves in plasmas with weak relativistic effects in electrons and ions

In the new investigation of dust-ion acoustic (DIA) waves with negative dust charges and weakly relativistic ions and electrons in the plasma, compressive and rarefactive DIA solitons of interesting characters are established through the Korteweg-de Vries (KdV) equation. Eventually, the amplitudes of the compressive DIA solitons are found to be constant at some critical temperature ratio α _c (electron to ion temperature ratio) identifying some critical dust charge Z _dc . It is predicted, that the reception of dust charges by the plasma particles at the variation of temperature starts functioning to the growth of compressive soliton’s constant stage of amplitude after the state of critical α _c . The identification of critical dust charge (Z _dc ) which is found to be very great for solitons of constant amplitudes becomes feasible for very small dust to ion density ratio (σ). But it can be achieved, we observe, due to the relativistic increase in ion-density as in mass, which is also a salient feature of this investigation.     

A Neotectonic based geomorphic analysis using remote sensing data to delineate potential areas of hydrocarbon exploration: Cachar area,Assam

TheAssamArakan fold thrust belt has highly deformed folded units of Tertiary sediments bounded by eastward dipping thrust slices with a convexity towards west. In the Tripura-Cachar region, this folded belt is characterized by the occurrence of wide synclines and narrow anticlines that hosts a number of hydrocarbon producing fields. In the Cachar area of Assam, most of these fields occur in the culmination of anticlinal structures. Other wells drilled in analogous structural settings are found to be dry. In this paper a neotectonic based geomorphic analysis is carried out to delineate a fault network and geomorphic highs in Cachar area as expressions of sub-surface structures which had subsequently been validated by available geophysical data. Of these geomorphic highs, those that are in the synclinal areas are believed to represent subtle sub-surface structural highs. Synclinal structures associated with NNE-SSW faults might be considered interesting for hydrocarbon exploration and are subsequently categorized following their degree of confidence for exploration of hydrocarbon. Additionally, a genetic model of the structures in the region is also proposed.     

Role of interannual equatorial forcing on the subsurface temperature dipole in the Bay of Bengal during IOD and ENSO events

Role of equatorial forcing on the thermocline variability in the Bay of Bengal (BoB) during positive and negative phases of the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) was investigated using the Regional Ocean Modeling System (ROMS) simulations during 1988 to 2015. Two numerical experiments were carried out for (i) the Indian Ocean Model (IOM) with interannual open boundary conditions and (ii) the BoB Model (BoBM) with climatological boundary conditions. The first mode of Sea Surface Height Anomalies (SSHA) variability showed a west-east dipole nature in both IOM and altimetry observations around 11°N, which was absent in the BoBM. The vertical section of temperature along the same latitude showed a sharp subsurface temperature dipole with a core at ~ 100 m depth. The positive (negative) subsurface temperature anomalies were observed over the whole northeastern BoB during NIOD (PIOD) and LN (EN) composites due to stronger (weaker) second downwelling Kelvin Waves. During the negative phases of IOD and ENSO, the cyclonic eddy on the southwestern BoB strengthened due to intensified southward coastal current along the western BoB and local wind stress. The subsurface temperature dipole was at its peak during October–December (OND) with 1-month lag from IOD and was evident from the Argo observations and other reanalysis datasets as well. A new BoB dipole index (BDI) was defined as the normalized difference of 100-m temperature anomaly and found to be closely related to the frequency of cyclones and the surface chlorophyll-a concentration in the BoB.

     

Ion acoustic solitons and double layers in electron–positron–ion plasmas with dust particulates

Propagation of small but finite amplitude ion acoustic solitons and double layers are investigated in electron–positron–ion plasmas in presence of highly negatively charged impurities or dust. The presence of negatively charged dust particulates can result in existence of two critical concentrations of ion–electron density ratio α. One of them α _D decides the existence of double layers, whereas the other one α _R decides the nature of the solitons and double layers. The system supports both compressive and rarefactive solitons as well as double layers. The parameter regimes of transitions from compressive to rarefactive solitons and double layers are also specified.     

Small Amplitude Nonlinear Dust Acoustic Wave Propagation in Saturn's F, G and E Rings

Nonlinear properties of small amplitude dust acoustic waves, incorporatingboth the ion inertial effect and dust drift effect have been studied.The effect of dust charge variation is also incorporated. It is seen thatdue to the dust charge variation, a Korteweg-de Vries (KdV) equationwith positive or negative damping term depending on the wave velocityand the ring parameters governes the nonlinear dust acoustic wave. It isseen that the damping or growth arises due to the assumption that dusthydrodynamical time scale is much smaller than that of the dust chargingscale. This assumption is valid only for planetary rings such as Saturn'sF, G and E rings. Numerical investigations reveal thatall the three rings in F, G and E, dust acoustic solitary wave admits both negative and positive potentials. Instability arises from the available freeenergy of drift motion of dust grains only for the wave with wave velocity 0, the drift velocity of the dust.     

Lateral variation in gravelly sediments and processes in an alluvial fan-fan-delta setting,north of Durgapur

Sedimentological study of the digitated lobes of gravel and associated sediments in the north of Durgapur results in identification of five major facies. Facies analysis indicates the sediments were deposited from an alluvial fan that has prograded in to an inferred shallow body of water. The analysis also suggests debris flow deposits dominate proximal part, modified grain flow and turbidity current the middle part and current generated channel and non-channel deposits in the distal part of the alluvial fan — fan-delta sequence.