Thirty one sediment samples from different varieties viz. yellow, reddish brown, brick red and light yellow sands from red sediments of Bhimunipatnam, Andhra Pradesh were studied to understand the heavy mineral assemblage, their fractionwise distribution and concentration. The heavy mineral assemblage in red sediments is ilmenite, magnetite, sillimanite, garnet, zircon, rutile, kyanite, monazite etc. Total heavy minerals (THM) wt. % varies from 16.67 to 23.99% and their concentration is not uniform in all the sedimentary samples. The higher THM wt% in brick red sands (23.99%) followed by reddish brown sands (20.24%), light yellow sands (17.10%) and yellow sands (16.67%). The finer fractions have more concentration of THM wt% than coarse fraction. The vertical distribution of heavy minerals in each sedimentary unit indicates that these units are not formed in single phase of deposition. Less concentration of garnets in yellow and light yellow sedimentary units indicates that the garnets might be chemically altered into iron hydroxide–limonite which gives yellow colour to the sediments under slightly oxidizing environment. Low concentration of garnets in brick red and reddish brown sediments indicates that garnets might have been undergone chemical decomposition under acidic conditions leads to produce iron oxides (Hematite) causes for red colorization of these units. The heavy mineral assemblage in different sand units indicates that they are derived from Eastern Ghat Group of rocks (khondalites and charnockites). 相似文献
Cold-water (<3–11°C) carbonate is the predominant sediment on the Tasmanian shelf. Calcitic skeletal grains (bryozoa, foraminifera, echinoderms, etc.) predominate over aragonitic (gastropods, etc.) ones. Non-skeletal grains are mostly micritic intraclasts with some pellets.
Fibrous spherulitic and rhombohedral calcite submarine cements range up to 90% in the bryozoan sand. X-ray analyses show that the bryozoan sand is characterized by a spectrum of calcites (low to high magnesian) and some aragonite.
A uniform spread of Mg concentrations from 0.06 to 2.48 wt.% indicates <3–10°C ambient water temperatures. The Mn (10–360 ppm) and Fe (176–2499 ppm) concentrations increase with increasing Mg values due to the formation of impure CaCO3 phases. The Sr content in bryozoan sand (bryozoa = 3200 ppm Sr) decreases with increasing rhombohedral calcite cement, as low Mg-calcite precipitating from 3° C sea water would have 1350 ppm Sr. The bryozoan sand grains with fibrous spherulitic calcite cements have high Sr concentrations (4470–7000 ppm), in the same range as in aragonitic (detected only by X-ray analyses) bryozoan sand grains. The spherulitic calcite cements are either pseudomorphs after original aragonite cements or these calcite cements and aragonite were inverted from fibrous spherulitic vaterite, a predominant CaCO3 polymorph at temperatures <10°C. 相似文献
The stability of a uniformly sloped conventional rubble mound breakwater defenced by a seaward submerged reef is investigated using physical model studies. Regular waves of wide ranging heights and periods are used. Tests are carried out for different spacings between two rubble mound structures (X/d=2.5–13.33) and for different relative heights (h/d=0.625–0.833) and relative widths (B/d=0.25–1.33) of the reef. It is observed that a reef of width (B/d) of 0.6–0.75 constructed at a seaward distance (X/d) of 6.25–8.33 breaks all the incoming waves and dissipates energy and protects the breakwater optimally. 相似文献
During very intense geomagnetic storm of November 7–8, 2004 simultaneous presence of storm time disturbance dynamo and eastward and westward directed prompt penetration electric fields inferred from the ground based magnetometer data in the 75° E sector is presented. Magnetometer observations show that, on the whole, average ΔH variation on 8 November remains below the night time level compared to its quiet day variation. A number of upward and downward excursions have been observed between 0130 UT and 0800 UT in the ΔH variation on 8 November. These excursions in ΔH have been attributed to the episodes of eastward and westward prompt penetrating electric fields. Ionospheric response in the equatorial ionization anomaly region along 75° E has also been studied using the total electron content data recorded at five GPS stations, namely Udaipur, Bengaluru (IISC), Hyderabad (HYDE), Maldives (MALD) and Diego Garcia (DGAR). Observation of markedly suppressed EIA, in conjunction with ΔH variation which was m negative during the daytime on 8 November, indicates the presence of an external field of opposite polarity (the disturbance dynamo electric field) that either undermined, or overshadowed the daytime ambient (eastward) electric field to the extent that the equatorial plasma fountain could not become effective. 相似文献
Over-exploitation of groundwater results in decline of water levels, leading to intrusion of salt water along the coastal
region, which is a natural phenomenon. A groundwater quality survey has been carried out to assess such phenomena along the
coast of Visakhapatnam, Andhra Pradesh, India. Brackish groundwaters are observed in most of the wells. The rest of the wells
show a fresh water environment. The factors responsible for the brackish groundwater quality with respect to the influence
of seawater are assessed, using the standard ionic ratios, such as Ca2+:Mg2+, TA:TH and Cl−:HCO−3. Results suggest that the brackish nature in most of the groundwaters is not due to the seawater influence, but is caused
by the hydrogeochemical process. Some influence of seawater on the groundwater quality is observed along the rock fractures.
The combined effect of seawater and urban wastewaters is due to the inferior quality of groundwater in a few wells, where
they are at topographic lows close to the coast. 相似文献
Summary. ?Cyclone track predictions in the Indian seas (Bay of Bengal and Arabian Sea) with a quasi-Lagrangian model (QLM) have been
attempted. QLM has a horizontal resolution of 40 km and 16 sigma levels in the vertical. It is integrated in a domain of about
4400 × 4400 km2. A new initialization procedure to provide initial fields for running the model has been designed. The initialization procedure
consists of updating the global model forecasts, used as first guess, provided by the National Center for Medium Range Weather
Forecasting (NCMRWF), New Delhi. A new version of IMD’s operational optimum interpolation scheme has been created to suit
the QLM grid structure. Lateral boundary conditions are computed from the extended forecasts of NCMRWF. The track forecasts
in each case show a reasonable skill of the forecast model in predicting the direction of movement within acceptable limits
of forecast errors, which are comparable to some of the best models operated by advanced NWP centers of the world. Even the
recurving storms are well predicted. Evolution of the vertical motion fields are also studied which reveal some interesting
features, which are described in detail in the text. The composited vertical motion fields are projected against observed
rainfall distribution, which show a good spatial correspondence.
Received August 9, 2001; revised March 12, 2002; accepted June 17, 2002
Published online: May 8, 2003 相似文献