RESULTS OF GRAVITY SURVEY OVER RANIGANJ COALFIELD,INDIA*

Results of a gravity survey conducted over Raniganj coalfield, one of the Gondwana basins of Damodar Valley in north-eastern part of India, are presented. The gravity field was separated into regional and residual components. The residual Bouguer anomaly map shows that the coalfield is characterized by a gravity low of about—32 mGal associated with Gondwana sediments. The deepest part of basin is found to be located near Asansole (23° 40’N, 86° 55’E), where the maximum thickness of sediments is estimated to be about 1.3 miles (2.08 km). The faults along the northern as well as the southern boundaries are found to be normal. The Gondwana sediments appear to continue eastward beneath alluvium and laterite of Bengal basin as far as 87° 25’E.     

RESOLUTION OF RESPONSES DUE TO CONDUCTIVE OVERBURDEN AND ORE-BODY THROUGH TIME-DOMAIN ELECTROMAGNETIC MEASUREMENTS—A FIELD EXAMPLE*

A technique is suggested to separate the effect of conductive overburden from the recorded time-domain electromagnetic anomalies. Such a separation leads one to construct a relatively undistorted picture of the conductor underlying a conductive overburden. A parameter τ (with dimensions of time) is introduced to characterize various conductors. To illustrate the applicability of the technique, four ‘Input’ (Mark V) anomalies obtained over the Cuddapah District, A.P., India have been analyzed and different values of the time-parameter τ corresponding to the overburden and the mineralized zones are obtained.     

DETECTABILITY OF AN INTERMEDIATE LAYER BY PERPENDICULAR AND VERTICAL COPLANAR ELECTROMAGNETIC SOUNDING SYSTEMS EMPLOYING DIFFERENT PRIMARY EXCITATIONS*

The detectability of an intermediate layer in a three-layer earth model in the time domain has been investigated. The calculations were made for the perpendicular loop (designated system II) and vertical-coplanar (designated system III) electromagnetic (EM) sounding systems. The primary excitation employed is a train of half-sinusoidal and square waveforms of alternating polarity. The time-domain response has been determined by Fourier transformation of the matched complex mutual coupling ratios into the time domain and by linear digital filtering. Top and bottom layers have equal resistivity. EM responses have been computed for conductive and resistive intermediate layer with a wide range of thickness and for two values (500 m and 1000 m) of loop-separation. For the detectability analyses, the root mean square (rms) difference between three-layer and homogeneous-earth responses is adapted. The threshold value for detectability is defined as an rms difference of 10% and the measurement error is arbitrarily assumed to be of the order of 3%. It is observed that the perpendicular-loop system is better than the vertical-coplanar system in detecting thin intermediate layers (either conductive or resistive). For a loop separation of 1000 m and half-sinusoidal pulse excitation, the detectable thickness ratio (h₂/h₁) is 0.10 by system II for the conducting middle layers; for square pulse excitation the corresponding thickness ratios are 0.06 for system II and 0.12 for system III. For a loop separation of 1000 m and half-sinusoidal pulse excitation in detecting the resistive intermediate layers, the corresponding thickness ratios are 0.9 for system II and 2.25 for system III; while for square pulse excitation the thickness ratios are 0.55 for system II and 1.55 for system III. Results in the frequency domain and time domain (for half-sinusoidal and square pulsed field) have also been presented for systems II and III for detecting conducting layers by considering an earth model where p₁≠ p₃ and p₃ > p₁ (p is the resistivity). The loop separa- tion used is 1000 m. Direct comparisons between the frequency domain and time-domain results clearly demonstrate the superiority of frequency-domain systems for detecting con- ducting intermediate layers.     

EQUIVALENCE IN ELECTROMAGNETIC (FREQUENCY) SOUNDING*

An analysis of the equivalence phenomenon, analogous to that encountered in resistivity sounding, has been made for electromagnetic (frequency) sounding with the following systems: horizontal coplanar coils, perpendicular coils, vertical coplanar coils, and vertical coaxial coils. Cases analyzed are three-layer H and K type resistivity distributions. The theoretical responses for the EM sounding systems have been computed by the digital linear filter method using short filters. An analysis has been made of the resolution by the four EM systems considered f the equivalence effect. It is concluded that from the equivalence point of view electromagnetic (frequency) sounding has relatively better resolution compared to resistivity sounding for the H type three-layer cases. For the K type three-layer cases electromagnetic (frequency) sounding provides a serious problem in resolution similar to that encountered in resistivity sounding. Empirical relationships have been established for the H and K type cases respectively in order to numerically quantify the equivalence involved in electromagnetic (frequency) sounding.     

Daily rainfall statistics of TRMM and CMORPH: A case for trans-boundary Gandak River basin

Satellite precipitation products offer an opportunity to evaluate extreme events (flood and drought) for areas where rainfall data are not available or rain gauge stations are sparse. In this study, daily precipitation amount and frequency of TRMM 3B42V.7 and CMORPH products have been validated against daily rain gauge precipitation for the monsoon months (June–September or JJAS) from 2005–2010 in the trans-boundary Gandak River basin. The analysis shows that the both TRMM and CMORPH can detect rain and no-rain events, but they fail to capture the intensity of rainfall.     

USE OF THE RESISTIVITY METHOD IN GEOLOGICAL MAPPING—CASE HISTORIES FROM RANIGANJ COALFIELD,INDIA*

The effectiveness of the electrical resistivity method has been studied using various configurations with different spacings over the Southern Boundary Fault in the northwestern part, and across a dolerite dyke named “Salma dyke” in the central part, of the Raniganj Coalfield, India. It has been observed that the delineation of the fault and the dyke was made possible under differential tropical weathering conditions. Geoelectric sections across the fault and the dyke have been prepared on the basis of Schlumberger sounding results. In profiling, Wenner, two-electrode, half-Schlumberger and part of Al-Chalabi's configurations were applied over the Southern Boundary Fault. Across the dyke, alpha-, beta-, and gamma-Wenner, Schlumberger, half-Schlumberger and two-electrode configurations were employed. Azi-muthal two-electrode sounding was also studied over the fault. The interpretation of the results of apparent resistivity profiles across the Southern Boundary Fault suggests that the Wenner and two-electrode configurations possess certain diagnostic features which help in mapping a single lithological contact, provided sufficient resistivity contrast exists. Although Schlumberger configuration seems to be quite suitable, other configurations may also be usefully employed over the dyke.     

Exsolution of dolomite and application of calcite–dolomite solvus geothermometry in high‐grade marbles: an example from Skallevikshalsen,East Antarctica

Calcite–dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium‐ to high‐grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite–dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite ± apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain ‘negative crystal’ inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850–870 °C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite–graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re‐equilibration of calcite that continued until Mg diffusion ceased at ～460 °C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub‐grain boundaries and formation of fine‐grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high‐grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions.     

Comments on: 'Petrogenesis of Proterozoic Lamproites and Kimberlites from the Cuddapah Basin and Dharwar Craton, Southern India'

Kimberlites intrude the late Archaean eastern Dharwar Cratonin two spatially separate fields (150 km apart, near Mahbubnagarin the north and Anantapur in the south) to the west of theProterozoic Cuddapah Basin in southern India. Two lamproitebodies intrude the thick Cuddapah Basin sediments near Chelimaand Zangamarajupalle, and a third occurs just outside the presentNE margin of the basin near Ramannapeta. Chalapathi Rao et al.(2004) have studied this spatially, and possibly temporally,close association of kimberlites and lamproites to provide insightsinto their genesis and genetic relationship. Although the mainemphasis of their paper is the petrochemical characterizationof these two suites of rocks, they also report Sr–Nd isotopedata for a small subset of samples, which they use to provideconstraints on the nature of the mantle source regions. Of some     

DIRECT INTERPRETATION OF TWO-ELECTRODE RESISTIVITY SOUNDINGS *

This paper describes the procedure for interpreting the apparent resistivity data measured with the two-electrode array directly with the help of kernel function. The calculation of kernel function from the observed resistivity curve is done by the method of decomposition. In the method of decomposition the resistivity curve is approximated by a sum of certain functions, whose choice is only restricted by the requirement that the contribution to the kernel function corresponding to them should be easily computable. A few such functions are classified. These, and the standard curves for corresponding kernel functions obtained by utilising an integral expression for two-electrode array expressing the kernel explicitly in terms of the apparent resistivity functions, are plotted on log-log scale. The determination of layer parameters, that is, the layer resistivities and thicknesses from the kernel function can be carried out by a method proposed by Pekeris (1940).