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41.
Image gathers as a function of subsurface offset are an important tool for the inference of rock properties and velocity analysis in areas of complex geology. Traditionally, these gathers are thought of as multidimensional correlations of the source and receiver wavefields. The bottleneck in computing these gathers lies in the fact that one needs to store, compute, and correlate these wavefields for all shots in order to obtain the desired image gathers. Therefore, the image gathers are typically only computed for a limited number of subsurface points and for a limited range of subsurface offsets, which may cause problems in complex geological areas with large geologic dips. We overcome increasing computational and storage costs of extended image volumes by introducing a formulation that avoids explicit storage and removes the customary and expensive loop over shots found in conventional extended imaging. As a result, we end up with a matrix–vector formulation from which different image gathers can be formed and with which amplitude‐versus‐angle and wave‐equation migration velocity analysis can be performed without requiring prior information on the geologic dips. Aside from demonstrating the formation of two‐way extended image gathers for different purposes and at greatly reduced costs, we also present a new approach to conduct automatic wave‐equation‐based migration‐velocity analysis. Instead of focusing in particular offset directions and preselected subsets of subsurface points, our method focuses every subsurface point for all subsurface offset directions using a randomized probing technique. As a consequence, we obtain good velocity models at low cost for complex models without the need to provide information on the geologic dips. 相似文献
42.
G. Muraleedharan Mourani Sinha A. D. Rao N. Unnikrishnan Nair P. G. Kurup 《Natural Hazards》2009,49(2):165-186
We used field and model wave data to investigate that zero crossing and average wave period distribution follow Gamma distribution.
Since Gamma CDF is an infinite power series, further mathematical treatment is difficult. Hence its shape parameter is approximated
to the nearest integer to arrive at Erlang distribution. An expression is derived from Erlang distribution to estimate various
mean wave periods and significant wave period and validated by relative root mean square (RRMS) error criteria. It is shown
by mathematical logic that the significant wave period distribution follows Erlang (or Gamma) distribution and is validated.
The average of one-third and one-tenth highest wave periods (T
s
) estimated from Erlang distribution are in accordance with the values computed from recorded buoy and numerical coastal wave
model wave period data. The predicted T
s
values from coastal wave model wave period data underestimates the values from buoy wave period data. 相似文献
43.
The abundance of baroduric bacteria from nine sediment cores (0–10 cm below sea floor) was examined for their response to simulated benthic disturbance in the Central Indian basin (CIB). While the total counts of bacteria decreased from 109 to 106 g?1 dry wt sediment, the average retrievable counts (CFU-colony forming units) improved by two orders of magnitude, i.e., from 102 104 g?1 dry wt sediment. The baroduric retrievable forms were dominated by Acinetobacter and Moraxella sp before the disturbance. After the disturbance the generic diversity was represented by Staphylococcus sp, Enterobacter sp, Micrococcus sp, Coryneforms sp, and Pseudomonas sp in addition to Acinetobacter. These observations were corroborated by changes in enzymatic activities of the retrievable bacteria, which could lead to changes in the biochemical characteristics of the sediment. Thus the simulated disturbance brought about an increase in culturable abundance, taxonomic and functional diversity of deep-sea sediment of the CIB. 相似文献
44.
Uttarakhand state in India is well known for its mountainous ecosystems,traditional communities and a variety of ecotourism destinations.Among various tourism activities,River Rafting along the banks of the Ganges River has increased tremendously for two decades,and has had unprecedented impacts on the traditional communities and the bio-diversity.Therefore,it is meaningful to do a comprehensive study on the various impacts associated with river rafting so as to suggest the pathways to achieve the environmental sustainability in this region.In this study,we collected primary data from randomly selected population units across all stake holders such as local people(n = 100),and camp personnel(n = 22),through a pre-tested questionnaire survey between August 2009 and May 2010.The questionnaire contains issues on culture,social,economic,institutional and associated perceived impacts on pollution and biodiversity including views for sustainability.Secondary information was also collected from various sources and government records to supplement and strengthen the analysis.The impacts were analyzed qualitatively through a ranking mechanism to facilitate the decision making process.The perception of the interviewee about the various possible impacts of rafting was discussed with mitigating mechanism.The ranking analysis as percollected data reveals that economy and education of local community was improved significantly;however aquatic and terrestrial fauna,social cohesion and pollution(water,air,land) were significantly deteriorated.The results show that the existing practices are not sufficient to address the adverse impacts.Improvement in practices is necessary,mainly in the policy regime.Based on the analysis,some measures are recommended on how to protect community interest and environment with the development of river rafting as an ecotourism opportunity. 相似文献
45.
S. Suresh Babu V. Sreekanth Vijayakumar S Nair S.K. Satheesh K. Krishna Moorthy 《Journal of Atmospheric and Solar》2010,72(11-12):876-882
Altitude profile of aerosol Single Scattering Albedo (SSA), derived from simultaneous in-situ airborne measurements of the coefficients of aerosol absorption and scattering off the west coast of India over the Arabian Sea (AS), during January 2009 is presented. While both the absorption and scattering coefficients decreased with altitude, their vertical structure differed significantly. Consequently, the derived SSA, with a surface value of 0.94, decreased with altitude, illustrating increasing relative dominance of aerosol absorption at higher altitudes. Altitude profile of SSA, when examined in conjunction with that of hemispheric backscatter fraction, revealed that the continental influence on the aerosol properties was higher at higher altitude, rather than the effect of marine environment. During an east–west transect across the peninsular India at an altitude of ~2500 m (free troposphere), it was found that the aerosol scattering coefficients remained nearly the same over both east and west coasts. 相似文献
46.
Ajith G. Nair D.S. Suresh Babu K.T. Damodaran R. Shankar C.N. Prabhu 《Journal of Asian Earth Sciences》2009,34(2):115-122
The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have been studied for mineralogical and chemical composition to assess the range of their physical and chemical variations with weathering. Chavara deposit represents a highly weathered and relatively homogenous concentration. Significant variation in composition has been documented with alteration. The most magnetic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 × 10?6 m3 kg?1, indicates the presence of haematite–ilmenite intergrowth. An iron-poor, titanium-rich component of the ilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit with an undesirably high Nb2O5 (0.28%), Cr2O3 (0.23%) and Th (149 ppm) contents. The ilmenite from Chavara is compared with that from the nearby Manavalakurichi deposit of similar geological setting and provenance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO2 (56.31–66.45%) contents highlight the advanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe3+/Fe2+ ratio compared to Manavalakurichi ilmenite. In fact, the ilmenite has significantly been converted to pseudorutile/leucoxene. 相似文献
47.
Rajani Nair Asha A. Juwarkar Tushar Wanjari S. K. Singh T. Chakrabarti 《Climatic change》2011,106(4):609-619
The micrometeorological technique of eddy covariance is a powerful tool for characterizing the carbon (C) budget of terrestrial
ecosystems. Eddy covariance method was used for estimating Net Ecosystem Exchange (NEE) of carbon dioxide between atmosphere
and revegetated manganese mine spoil dump at Gumgaon, India. In this paper, we analyzed the diel CO2 flux pattern and its response to various physical environmental conditions. The carbon balance of terrestrial ecosystems
is particularly sensitive to climatic changes. Study of diel pattern of CO2 flux showed that carbon uptake was dependent on sunlight. Effect of temperature and latent heat on the CO2 flux showed that rate of CO2 uptake increased proportionally, but later declined due to various factors like stomatal response, high evaporative demand,
circadian rhythm and/or a combination of all three. Net ecosystem production of revegetated land was found to be 28.196 KgC/ha/day
whereas average net carbon release by the ecosystem, through respiration was observed to be 5.433 KgC/ha/day. Thus, quantifying
net carbon (C) storage in degraded land is a necessary step in the validation of carbon sequestration estimates and in assessing
the possible role of these ecosystems in offsetting adverse impacts of fossil fuel emissions. 相似文献
48.
The south-west of Western Australia has experienced significant land-cover change as well as a decline in rainfall. Given
that most precipitation in the region results from frontal passages, the impact of land-cover change on the dynamics of cold
fronts is explored using the Regional Atmospheric Modeling System version 6.0. Frontal simulations are evaluated against high
resolution atmospheric soundings, station observations, and gridded rainfall analyses and shown to reproduce the qualitative
features of cold fronts. Land-cover change results in a decrease in total frontal precipitation through a decrease in boundary-layer
turbulent kinetic energy and vertically integrated moisture convergence, and an increase in wind speed within the lower boundary
layer. Such processes contribute to reduced convective rainfall under current vegetation cover. 相似文献
49.
50.
We have investigated the resonances in the earth-moon system around the sun including earth’s equatorial ellipticity. The resonance resulting from the commensurability between the mean motion of the moon and Γ (angle measured from the minor axis of the earth’s equatorial ellipse to the projection of the moon on the plane of the equator) is analyzed. The amplitude and the time period of the oscillation have been determined by using the procedure of Brown and Shook. We have shown the effects of Γ on the amplitude and the time period of the resonance oscillation using the data of the moon. It is observed that the amplitude decreases and the time period also decreases as Γ increases from 0° to 45°. 相似文献