An unusual diagenetic structure in the Precambrian banded iron formation (BIF) of Orissa,India, and its interpretation

An unusual type of late diagenetic tectonic and compaction structure simulating boudinage phenomena is described and documented from the Precambrian banded iron formation (BIF) of Orissa, India. The structure was seemingly initiated by the development of tension cracks in the hydroplastic stage followed by rotation and imbrication of the segments of the iron (magnetite) bands. The tension cracks were subsequently filled up by finely crystalline diagenetic quartz veins.     

Effects of joint orientation and rock mass quality on tunnel blasting

It is a well known fact that rock mass properties influence the process of fragmentation considerably. Model blasts and field investigations were carried out to find the effects of rock mass quality and joint orientation on tunnel blasting. Propagation of shock waves are partially restricted by joint planes. It was observed that the blast results (i.e., average fragment size and depth and cross-sectional area of the broken zone) were considerably influenced by joint orientation. Accordingly, it has been concluded that loading equipment with a larger capacity and deeper blast holes are required in formations with joint planes perpendicular to the tunnel axis. The number of blast holes, however, should be greater when joints are parallel to the tunnel axis. Furthermore, the powder factor (kg/m³) has been found to be directly related to rock mass quality (Q). Optimisation of pull, powder factor and overbreak is required in the case of weak formations with joints perpendicular to the tunnel axis. The use of contour blasting technique seems to be essential in poor and fair rock masses to minimise the overbreak, reduce the support cost and improve the stability of the opening.     

Air-sea exchange of CO2 in the Gulf of Kutch,northern Arabian Sea based on bomb-carbon in corals and tree rings

Radiocarbon analyses were carried out in the annual bands of a 40 year old coral collected from the Gulf of Kutch (22.6°N, 70°E) in the northern Arabian Sea and in the annual rings of a teak tree from Thane (19°14′N, 73°24′E) near Bombay. These measurements were made in order to obtain the rates of air-sea exchange of CO₂ and the advective mixing of water in the Gulf of Kutch. The Δ¹⁴C peak in the Thane tree occurs in the year 1964, with a value of ∼630‰, significantly lower than that of the mean atmospheric Δ¹⁴C of the northern hemisphere (∼ 1000‰). The radiocarbon time series of the coral was modelled considering the supply of carbon and radiocarbon to the gulf through air-sea exchange and advective water transport from the open Arabian Sea. A reasonable fit for the coral data was obtained with an air-sea CO₂ exchange rate of 11–12 mol m⁻² yr⁻¹, and an advective velocity of 28 m yr⁻¹ between the Arabian Sea and the Gulf of Kutch; this was based on a model generated time series for radiocarbon in the Arabian Sea. The deduced velocity (∼ 28 m yr⁻¹) of the advective transport of water between the gulf and the Arabian Sea is much lower than the surface tidal current velocity in this region, but can be understood in terms of net fluxes of carbon and radiocarbon to the gulf to match the observed coral Δ¹⁴C time series.     

Sea-floor classification using multibeam echo-sounding angular backscatter data: a real-time approach employing hybrid neural network architecture

The presently studied numerical model, e.g., composite roughness, is successful for the purpose of seafloor classification employing processed multibeam angular backscatter data from manganese-nodule-bearing locations of the Central Indian Ocean Basin. Hybrid artificial neural network (ANN) architecture, comprised of the self-organizing feature map and learning vector quantization (LVQ), has been implemented as an alternative technique for sea-floor roughness classification, giving comparative results with the aforesaid numerical model for processed multibeam angular backscatter data. However, the composite-roughness model approach is protracted due to the inherent need for processed data including system-gain corrections. In order to establish that tedious processing of raw backscatter values is unessential for efficient classification, hybrid ANN architecture has been attempted here due to its nonparametric approach. In this technical communication, successful employment of LVQ algorithm for unprocessed (raw) multibeam backscatter data indicates true real-time classification application.     

Use of a non-associated flow rule for determining the stability of a vertical circular excavation

An axisymmetric lower bound limit analysis technique in combination with the finite elements has been used to investigate the effect of considering a non-associated flow rule on the stability number (γH/c) for a vertical circular unsupported excavation in a general cohesive–frictional soil medium, where (1) H is the excavation height, (2) γ defines the unit weight of the soil mass, and (3) c indicates the cohesion of the soil mass. The results are derived for different magnitudes of dilative coefficient (η), friction angle (?), and normalized excavation height (H/b), where b = the radius of the excavation. The results clearly indicate the increase in γH/c with an increase in η value. It is expected that the charts provided in this note will be quite helpful for the practicing engineers.

     

Finite difference modeling of seismic wave propagation in monoclinic media

Reported in the present paper are the results of the study of propagation of SH waves in the plane of mirror symmetry of a monoclinic multilayered medium with displacement normal to the plane. Dispersion equation has been obtained analytically ussing Haskell’s matrix method, while the finite-difference method has been employed to model the SH-wave propagation to study its phase and group velocities. The stability analysis has been carried out to minimize the exponential growth of the error of finite difference approximation in order to make the finite difference method stable and convergent. Further, variations of phase velocity with respect to both wave number and dispersion parameter for different stability ratios in monoclinic media have been examined and shown graphically. The effect of change of stability ratio on the group velocity of the wave propagation has been also investigated. Likewise, the effects of change of dispersion parameter on phase velocity and the variation of frequency with increase of wave number have been graphically represented and discussed.     

Nuclear Morphology and Lysosomal Stability of Molluskan Hemocytes as Possible Biomarkers of Arsenic Toxicity

The frequency of nuclear aberrations and neutral red retention time of hemocytes in the mollusk Lamellidens marginalis were recorded under exposure to sublethal concentrations of sodium arsenite in order to examine the sensitivity and effectiveness of these inexpensive assays for screening the toxicity of As³⁺ in a freshwater ecosystem. A dose and time dependent increase in the density of micronucleated and binucleated hemocytes and gill cells was indicative of the pronounced genotoxic effect of arsenic on this animal. The disruption of intrahemocyte homeostasis imposed by this natural toxicant was evident from a dose and time dependent reduction in the lysosomal stability of the hemocytes of the animal. The tested parameters are indicative of arsenic toxicity in L. marginalis in the freshwater systems of the arsenic affected geographical areas of West Bengal, India.     

Effect of intraband variability on stable isotope and density time series obtained from banded corals

Density, δ¹⁸O and δ¹³C were measured along two tracks, one close to the central growth axis and the other, ∼20ℴ off the axis, in a coral (Porites lutea) collected from the Stanley Reef, Central Great Barrier Reef, Australia. The δ¹⁸O variations in the coral are well correlated with sea surface temperature changes. The common variances between the two tracks were about 60% in the δ¹⁸O, δ{13}C, and the skeletal density variations. Part of the noise (40%) could be due to the difficulty of sampling exactly time contemporaneous parts of each band along the two tracks and part of it could be due to genuine intraband variability. In spite of the intraband variability, the time series obtained from the two tracks are similar, indicating that the dominant causative factor for the isotopic variations is external, i.e., the environmental conditions that prevail during the growth of the coral; density band formation does not appear to be directly controlled by the sea surface temperature.     

Divergent-rotational Nonlinear Energy Conversions in Wavenumber-frequency Domain During Summer Monsoon

--This work deals with computational modelling designed to understand the dynamical mechanism of low frequency monsoonal transients that results from nonlinear divergent-rotational (&gif1;) kinetic energy (KE) conversions due to the effects of Coriolis force, vorticity and divergence during the summer monsoon 1988 over the latitudinal belt 20°S-30°N at 850 hPa and 200 hPa. The results show two distinct spectral peaks spanning 30-45 days and 18-25 days in the energy conversions from the transient divergent motions to rotational motions. Due to the latitudinal variation of the earth's rotational effects, the conversion from the transient divergent to rotational motions, associated mainly with wavenumbers 1 and 2, tend to be more pronounced to the north of 15°N on the 30-45-day and 18-25-day time scales in the upper and lower tropospheres, respectively. The contribution of the stationary waves to maintenance of the low frequency rotational flow due to the effect of divergence through barotropic instability is significant at the upper troposphere. Divergent to rotational KE conversion by wave-wave interaction due to divergence is identified as an important mechanism for maintenance of low frequency oscillations in the lower troposphere. The upper tropospheric planetary scale divergent motions associated with 30-45-day oscillation gain substantial energy through nonlinear &gif1; interaction due to vorticity.     

Water resource accounting for an iron-ore mining area in India

A study has been conducted at the Bicholim iron-ore mine, North Goa, India to investigate the hydrogeological continuity between the mine's open pit and surrounding villages. Water resource accounting by demarcating different watersheds has also been carried out to analyse the major causes of groundwater scarcity in the region during dry periods. A resistivity survey and a ground penetrating radar survey were completed around the periphery of open pits, the surrounding village areas and Mayem Lake. The results of the two independent surveys and other related data confirm that there is no observable hydrogeological continuity between the pits, nearby villages and Mayem Lake. Hence, there may not be any significant effect on surrounding water resources due to mining or mine dewatering. Water resource accounting has revealed that only 15–20% of 3500 mm precipitation per annum becomes part of the groundwater resource. The remainder of the annual precipitation is lost as heavy runoff, evapotranspiration and subsurface capillary moisture. Closely spaced wells in the region have also aggravated the groundwater scarcity problem due to draw down effects. Therefore, an effective water resource conservation plan is required for the region to overcome the problem. Received: 18 January 1999 · Accepted: 17 May 1999