First results from the Gauribidanur Radio heliograph

Observations of the Sun at two frequencies (51 and 77 MHz) using the East-West arm of the Gauribidanur Radio heliograph are presented.     

The Mauritius Radiotelescope

The Mauritius Radiotelescope (MRT) is a T-shaped array of helical antennas with a 2048 m EW arm and a 890 m South arm. The primary objective of the telescope is to produce a sky survey in the declination zone -15° to -65° with a point source sensitivity of 200 mJy and an angular resolution of 4'×4.6'cosec(z) at 151.6 MHz, z being the zenith angle . This paper describes the telescope and the present status     

Observations of the Rosette Nebula using the Decameter Wave Radio Telescope at Gauribidanur

A map of Rosette Nebula in continuum absorption is made at 34.5 MHz using the Decameter Wave Radio Telescoe at Gauribidanur, India, with a resolution of 26×40, is presented. These observations are combined with the 2700 MHz measurements of Grahamet al. (1982) to derive the electron temperature distribution across the nebula. It is found that the temperatures in the southeastern parts of the nebula are around 5000 K and increase up to 8000 K towards the northwestern regions. It is suggested that the lower electron temperatures in the southeastern regions are due to the presence of more dust there compared to other regions in the nebula.     

2D stabilised analytic signal method in DC pole-pole potential data interpretation

Using analytic signal method, interpretation of pole-pole secondary electric potentials due to 2D conductive/resistive prisms is presented. The estimated parameters are the location, lateral extent or width and depth to top surface of the prism. Forward modelling is attempted by 2D-Finite Difference method. The proposed stabilised analytic signal algorithm (RES2AS) uses Tikhonov’s regularization scheme and FFT routines. The algorithm is tested on three theoretical examples and field data from the campus of Roorkee University. The stability of RES2AS is also tested on synthetic error prone secondary pole-pole potential data.     

Depth of detection of buried resistive targets with some electrode arrays in electrical prospecting

Assuming the minimum detectable anomaly to be 10%, depths of detection of a 2D vertical resistive sheet of thickness t are found to be 4.0t, 3.0t, 4.0t and 4.0t with Wenner, two-electrode, three-electrode and dipole-dipole (β-Wenner) arrays, respectively, when the array spread is in-line. On the other hand, the depths of detection obtained with a broadside spread of the arrays right over the sheet are much less and are correspondingly 2.5t, 2.0t, 2.5t and 2.5t. An increase in the depth extent W of the sheet from 10t to 20t does not increase its depth of detection with the arrays. The depths of detection of an infinitely resistive horizontal cylinder of radius R are respectively 1.5R, 1.8R, 2.0R and 2.0R with the above-listed arrays when the array spread is in-line. With broadside spread of any of the arrays, the depth of detection is seen to be 2.5R. In the case of a spherical target of radius R, the detection depths of any of the arrays are found to be small and to vary between 0.8R and 1.1R. Comparatively, the detection depths of resistive targets are much lower than those of conductive targets of the same size and shape, with any electrode array. Among all the arrays studied, the two-electrode array performs worst in the detection of resistive targets while it performs best in detecting conducting targets of limited lateral extent. In the case of a spherical target, either resistive or conductive, there is no distinct change in its detection depth with array.     

Occurrence of thorite and radioactive zircon in syenite pegmatite,North of Ashwaraopet,Khammam district,Andhra Pradesh,India

This paper reports the occurrence of thorite and confirms the earlier find of radioactive zircon by Subbarao, Murali and others from syenite pegmatite in the Vinayakapuram-Kunavaram alkaline belt in the Khammam district of Andhra Pradesh, India. Thorite has also been found as residual concentrations in soil overlying the pegmatite.     

Air-sea interaction over the tropical Indian Ocean during several contrasting monsoon seasons

Monthly mean anomaly fields of various parameters like sea surface temperature, air temperature, wind stress, effective radiation at the surface, heat gain over the ocean and the total heat loss between a good and bad monsoon composite and the evaporation rates over the Arabian Sea and southern hemisphere have been studied over the tropical Indian Ocean. The mean rates of evaporation on a seasonal scale over the Arabian Sea during a good and bad monsoon composites were equal (about 2·48 × 10¹⁰ tons/day). The evaporation rates over the southern hemisphere were greater during all the months. The mean evaporation rates over the southern hemisphere on a seasonal scale for the good and bad monsoon composites were 4·4 × 10¹⁰ and 4·6 × 10¹⁰ tons/day respectively. The maximum evaporation rates over the southern hemisphere were observed in August. The anomalies of wind stress, effective radiation at the surface and the heat gain over the ocean also exhibit large variations in August, as compared to other monsoon months.     

Summer cooling of the Arabian Sea—a review

The summer cooling of the Arabian Sea has been reviewed in relation to the dynamic and thermodynamic processes. The differences in the quantum of cooling in the coastal regions have been attributed to the variations in the strength of coastal upwelling along the respective coasts. In the central portions of the Arabian Sea, the thickness of the surface layer and the thermal structure appear to be predominantly governed by the turbulent mixing of cold waters entrained into the surface layer in association with the deepening of current shear zone. The surface heat losses during the passage of a cyclone over the east central Arabian Sea account for only 40% of the total heat change in the surface layer while the rest is lost into the interior. Also presented are climatological mean patterns of the forcing parameters and their variations during certain years.     

Downward continuation of (Vs)L profile in 1P (time-domain) with linear excitation using point electrodes. A short notet1

It is seen that the apparent chargeability (M_a)_L anomaly over a 2D graphite body splits into two distinct (V_s)_L anomalies which closely follow the apparent resistivity profile. This suggests that the electric field amplitude is distorted due to a superficial inhomogeneity creating a (V_s)_L anomaly, which bears no relation to the polarized body. The target depth obtained by continuation of such a profile is therefore, not acceptable.