April, 2005

April, 2005   ||  Volume 9 No.2

Tectonics and Correlation of Upper Kaimur Group Sandstones by their Palaeomagnetic Study
PG.V.S. Poornachandra Rao, J.Mallikharjuna Rao, N.P.Rajendra Prasad, M.Venkateswarlu, B.Srinivasa Rao and S.Ravi Prakash
Palaeomagnetism Laboratory, National Geophysical Research Institute, Hyderabad – 500 007


The Upper Kaimur Group rocks of the Vindhyan Supergroup are represented by sandstones all over the Vindhyan basin. These have been quite widely sampled for palaeomagnetic studies from Baghain Sandstone (Panna), Dhandraul Sandstone (eastern Son valley), Diken Sandstone (Rajasthan) and Dudauni Sandstone (Guna-Shivpuri) etc. in order to understand their contemporaneity and tectonics associated with them. Palaeomagnetic signatures from the Dhandraul Sandstone and Dicken Sandstone Formations were recovered from oriented samples collected by using laboratory alternating field and thermal demagnetization studies. Results of these studies are presented in this paper in detail that helps identify the ChRM vectors in these formations. The Dhandraul Sandstone from the Son valley revealed both normal and reverse magnetic directions with upward and downward inclinations and the Diken Sandstone from Rajasthan revealed only normal magnetic direction with downward inclination. The remanent magnetic signatures in the Dhandraul Sandstone imply two reversals of the geomagnetic field and indicate shallow northern and southern palaeolatitudinal positions for the Indian subcontinent during Neoproterozoic period similar to that of the Baghain Sandstone of the Panna region. The Dicken Sandstone revealing similar ChRM directions as that of the Malani Rhyolite result in intermediate northern palaeolatitude. Thus the results of our palaeomagnetic studies on the Dhandraul Sandstone and Dicken Sandstone of the Upper Kaimur Group correlate well giving a clear tectonic movement of the Indian subcontinent from intermediate northern hemisphere to the shallow southern hemisphere position during the Neoproterozoic times.

Impact of Deforestation on Indian Monsoon –
A GCM Sensitivity Study
A.Gupta, R.K.Thapliyal, P.K.Pal and P.C.Joshi
Atmospheric Sciences Division, Meteorology and Oceanography Group

Space Applications Center, Ahmedabad – 380 015

E.mail: pcjoshi35@hotmail.com



The fact of rapid deforestation has been one of the major concerns since last decade. In India, the forest cover of the country has been declining since last several years. The impact of these changes on Indian monsoon is still an open question. In this work, an attempt has been made to address this question with the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3.6). The T42 version of CCM3 with a horizontal resolution of 2.8°x2.8° is used for this study. The influence of deforestation is modeled by the changes in the surface parameters such as land cover. The changes are made over the African Region and some parts of North-East Indian Region. The consequences of the changes in land cover are the changes in surface albedo, ground wetness, surface roughness etc. The standard T42 version of NCAR Community Climate Model (CCM3) which includes LSM as a coupled model is integrated for five years, for the control run. Further the impact of deforestation have been studied for three different scenarios. The model is integrated for five years with forest cover replaced by grassland in three different classes, viz. 100%, 50%, 25%. The model simulations (for JJA period), for 100% deforestation, are showing that there will be change in spatial distribution of rainrate in India i.e over Northern part of India, rainrate is expected to decrease upto 2mm/day whereas over Southern part of India, including Arabian Sea and Bay of Bengal, the rainrate will increase upto 5mm/day. Whereas in Africa and north-east India, where deforestation was done, rainrate will decrease up to 4mm/day. These changes in rainrate are due to changes in global circulation, caused due to large scale deforestation.

Water – where are we leading ourselves to?
National Geophysical Research Institute, Hyderabad – 500 007

Email: paravata@yahoo.com

Structure and variability of the Leeuwin Current in the south eastern Indian Ocean
Benny N. Peter, P.Sreeraj1 and K.G.Vimal Kumar2

Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka, 816-8580 Japan

1Cochin University of Science & TechnologyDept. of Physical Oceanography, Cochin – 682 0162National Institute of Oceanography, Regional Centre, CochinE.mail : benny@riam.kyushu-u.ac.jp, benny@gmail.com


The present study analyses the structure and variability of the Leeuwin Current in the south Indian Ocean. Besides the historic hydrographic dataset various observations made during WOCE, TOGA and other experiments conducted in the study region are employed. The long-term average bimonthly dynamic topography with reference to 400db is prepared to infer the spatial and temporal current pattern. In addition to the temperature-salinity profiles, the temperature profiles are also included in the estimation of dynamic height employing the correlation between heat content and dynamic height. Sections of hydrographic properties are also present at different latitudes of the Leeuwin Current region. Intra-annual variability of the Leeuwin Current is clearly illustrated in the bimonthly maps. The Leeuwin Current shows strong spatial and temporal variations. It is strong in southern winter and weak during summer. The current is board and shallow in the north but becomes narrower and deeper in the south.

Relative Assessment of 3D – Analytic Signal Definitions – A Numerical Study

Rambhatla G.Sastry and Paras R.Pujari

Department of Earth Sciences, IIT Roorkee, Roorkee – 247 6671GEM Division, National Environmental Engineering Research Institute, Nagpur-440 020E-Mail: rgss1fes@iitr.ernet.in / rgssastry@yahoo.com

Two alternate definitions for 3D- analytic signal exist in geophysical literature and their relative performance needs an in-depth study for a proper option. So, a numerical study involving a conductive two-prism model is undertaken in this study. FDM generated secondary pole-pole potential data due to this model served as input to our stabilized analytic signal algorithm, RES3AS, which outputs several analytic signal parameters conforming to both the definitions of 3D-analytic signal. Analysis of analytic signal parameters has provided means of assessing position location, lateral widths and depths to top surfaces of both the prisms. Thus assessed model parameters for both options of 3D-analytic signal have suggested that the Nabighian’s (1984) definition for 3D-analytic signal is more effective than that due to Roest, Verhoef & Pilkington (1994) in the interpretation of secondary pole-pole data.

Analytical expression for hydraulic head distribution in a
homogeneous anisotropic aquifer with inclined bedding planes

Mathew K. Jose and Rambhatla G. Sastry1
National Institute of Hydrology, Jalvigyan Bhawan, Roorkee-247667, India
E-mail: mjose@nih.ernet.in; Fax: 91-1332-272123
1 Department of Earth Sciences, I.I.T, Roorkee-247667, India
E-mail: rgss1fes@iitr.ernet.in / rambhatla_gs@yahoo.com; Fax: 91-1332-273560

The existing analogy between dc current flow and ground water flow under steady state conditionsin earth medium, has allowed to extend the results from geoelectrical method in computing heads in a homogeneous anisotropic aquifer system with inclined bedding planes due to a surface water source. The results are presented as equipotential (hydraulic head) plots for different coefficients of anisotropy and orientation of bedding planes of soil strata.

Pre and post-excavation cross-hole seismic and geotomographic studies for a Nuclear Power Project
R.S.Wadhwa, N.Ghosh, M.S.Chaudhari, Ch.Subba Rao and Raja Mukhopadhyay
Central Water & Power Research Station, Pune – 411 024

The foundation site response evaluation to earthquake forces requires determination of both compressional and shear wave velocities. This information allows less conservative safety margins and thereby helps in reducing the cost of building construction. Cross-hole seismic studies in NX size (~80 mm dia) boreholes to evaluate Compressional (P-) and Shear (S-) wave velocities upto 51 m depth from the surface (EL 100 m) were carried out at Reactor Building (RB) RB-3 and RB-4 sites. It was found that at RB-3 site, the P-wave velocity was 5400 m/sec while the shear wave velocity with depth ranged between 2900 m/sec and 3200 m/sec.
The RB-3 site was then excavated upto EL 79.4 m and the rock was grouted by cement slurry. To study the effect of removal of overburden and blasting on the quality of rock, as also to decide the exact value of shear wave velocity to be adopted for designing the foundation of reactor building, cross-hole seismic studies upto EL 62.4 m were carried out.
In addition to calculating the average wave velocities, the post-excavation cross-hole data were also analysed by seismic ray geotomography to evaluate velocity field distribution with depth. The pre and post excavation P- and S- wave velocity values were similar from which it was inferred that blast energy was contained and extension of fractures was not inferred. Also post P- and S-wave velocity tomograms revealed that the velocities in horizontal and vertical directions were same indicating that the distribution of stresses in both directions was of the same order and inhomogeneities have no preferential direction of orientation.