CONTENTS OF JULY 2013
Vol.XXXIV   No.3

 

Plate Tectonics Historical Perspective and Seismicity of Indian Plate.

D. C. Mishra

 

Geophysical Techniques for Uranium Exploration in Southwestern Margin of Caddapah Basin - A Case Study from Madyalabodu Area, Kadapa District, Andhra Pradesh, India.

Rajeev Kumar Meena, B. Srinivasa Rao, M. V.Satyanarayana, Viney Kumar, B.V. L. Kumar, R. L. Narasimha Rao and A. K. Chaturvedi

 

Close Grid Gravity Surveys at Varkala Beach, Kerala.

Dinesh Gupta, S.S.Ganguli and K.S. Sajinkumar

 

Multistoried Aquifer Studies with the help of Electrical Resistivity in Basaltic Terrain : A Case Study from Dhubdhubi Basin Solapur District, Maharashtra, India.

B. Narayanpethkar, S. M. Sabale, G. I. Patel and V. R. Ghodake

 

Site Response Studies in Pondicherry Region.

S. Trupti, P. Pavan Kishore, K.N.S.S.S. Srinivas and T. Seshunarayana

 

 

 

Plate Tectonics Historical Perspective and Seismicity of Indian Plate

D. C. Mishra
National Geophysical Research Institute, Hyderabad-500 007
(Council of Scientific and Industrial Research) India.
E-mail: dcm_ngri@yahoo.co.in

 

Abstract

   Plate tectonics has been one of the most fascinating theories developed during last century. Its importance lies in its explanation of most of the features and observations related to earth. This article describes the evolution of this theory at various stages and the scientists who pioneered it. In the end, the Indian plate and related seismicity is briefly discussed. Purpose of this article is to revisit plate tectonics and bring awareness among the present day of scientist about efforts that has gone into its discovery.

 

Key words: Continental drift, Sea floor spreading, magnetic anomalies, palaeomagnetism and plate tectonics.

 

 

 

Geophysical Techniques for Uranium Exploration in Southwestern Margin of Caddapah Basin - A Case Study from Madyalabodu Area, Kadapa District, Andhra Pradesh, India.

 

Rajeev Kumar Meena*, B. Srinivasa Rao**,
M. V.Satyanarayana***, Viney Kumar***, B.V. L. Kumar*,
R. L. Narasimha Rao* and A. K. Chaturvedi*

Atomic Minerals Directorate for Exploration and Research, Department of Atomic EnergyHyderabad*, Jaipur**,Nagpur***

 


Abstract

   Geophysics plays an important role in discovering the concealed atomic raw materials which are main ingredients for the generation of nuclear power. Reconnaissance and detailed geological, radiometric, hydro-geochemical surveys conducted in southwestern margin of Cuddapah basin has brought out a number of promising areas. In this part of the basin, uranium mineralization is mostly controlled by basement structural features faults/fractures, basement shears and unconformity related strataform/stratabound types.

 

   Locally uranium anomalies have been delineated along the E-W trending fault associated with lower part of Gulcheru formation and close to the unconformity contact with basement. Sparse exploratory drilling resulted in understanding the subsurface mineralization associated with brecciated chloritized zones adjacent to the basic dykes emplaced along the re-activated fault/fracture and close to the unconformity contact between lower part of the Gulcheru sediments and basement granite.

 

   Detailed geophysical surveys comprising magnetic and very low frequency electromagnetic (VLF-EM) surveys are conducted over an area of about 2.5 sq km for a strike length of 2 km in Madyalabodu area, southwestern margin of the basin. Magnetic surveys in the area recorded moderate to high amplitude linear magnetic anomalies representing the high susceptibility rocks. As the area is mostly traversed by non-magnetic sediments the magnetic anomalies reflects the linearly trending WNW-ESE and ENE-WSW subsurface intrusive basic dykes in strike direction. Interpretation of observed and processed magnetic image maps clearly brought out the structural fabric of the area faults/fractures, intersection of magnetic anomaly trends indicating shear zones and alteration zones. Location of basic dykes and its strike extension were clearly demarcated from the derived image maps. Analysis of radially averaged power spectrum has yielded average depths to the magnetization contrasts at three levels. The shallow depths, 30m and 50m, has given the depth to top of intrusive basic dykes and depth of 120m represent the deepest part of the basin in the survey area. VLF-EM in-phase and out-of-phase induced vertical magnetic field components, apparent resistivity and phase parameters are recorded to identify the fault/fracture and alteration zones. VLF-EM responses also supported the magnetic anomalies in identifying the structural features and alteration zones. The moderate magnetic anomaly amplitudes associated with low resistivity and cross overs on in-phase and out-of-phase anomalies are interpreted due to the altered/brecciated zones adjacent to the concealed basic dykes in Gulcheru formation. The mineralization intercepted in one of the boreholes drilled in the area is well correlatable with VLF-EM anomaly trends and moderate high magnetic anomalies. This characterization of geophysical signatures can be extended in the adjoining areas for further exploration by drilling.

 

 

 

Close Grid Gravity Surveys at Varkala Beach, Kerala.

 

Dinesh Gupta, S.S.Ganguli and K.S. Sajinkumar*
Geological Survey of India, SRO, Bandlaguda, Hyderabad-500068
*GSI, SU, Kerala, Trivandrum,
E-mail: dineshguptagsi@yahoo.co.in

 

Abstract

   Varkala beach, along the south west coast of Kerala, the present study area, is narrow with silver sands and is flanked by ~ 30 m cliff immediately to the east. The cliff section runs parallel to the coast line over a kilometer and exposes the litho units ranging from Tertiary to recent.

 

   Varkala and adjacent area forms a part of Kerala Khondalite Belt (KKB) of the Southern Granulite Terrain (SGT). The Precambrian crystalline of the area is unconformably overlain by Tertiary sequence of Warkalli formation of Mio-Pliocene age. The Warkalli formation comprises unconsolidated sands, variegated clays, white plastic clays, carbonaceous clay and lenses of lignite.

 

   At Varkala beach the basement is dipping towards east resulting in thickening of sedimentary rocks away from the beach. Gravity data indicates a fault along the contact between the beach and cliff with thick sequence of sediments in the down thrown block. In the second phase of tectonic event, this sediment deposited on the down thrown block was uplifted gaving rise to the present topography as a cliff.

 

   The thick clay layer present over the basement is the major geological factor causing wave undercutting as well over hang collapse.

 

Key words: Close grid gravity, cliff, South Granulite Terrain (SGT), Half-Graben, Varkala, Kerala.

 


 

 

Multistoried Aquifer Studies with the help of Electrical Resistivity in Basaltic Terrain: A Case Study from Dhubdhubi Basin Solapur District, Maharashtra, India.

 

B. Narayanpethkar, S. M. Sabale, G. I. Patel and V. R. Ghodake
Department of Applied Geology, School of Earth Sciences, Solapur University, Solapur.
E-mail: vyanky_g@yahoo.co.in

 

Abstract

   Electrical resistivity soundings have been taken in the basin of Dhubdhubi stream, a tributary of Bhima River in the Solapur District, Maharashtra, India. The results of the sounding data revealed two, three and multi layer earth sections. 2 D coverage of the soundings spread over the area facilitated to delineate subsurface interfaces of different layers and qualitative regional correlation between the subsurface geology and electrical resistivity. The electrical resistivity in turn has been used to map possible multistoried aquifers. It is interesting to note that the base of the top layer broadly resembles groundwater surface map, prepared from the observed pre-monsoon water levels in the field. Further, the flow system for second, third and fourth aquifers has also been visualized. It is interesting to note that for the low resistivity (less than 20 ohm-m), for the first aquifer the corresponding transmissivity shows lower values (between 20 to 60 m2 /day). For the resistivity range of 60 to 100 ohm-m, the transmissivity increases and the values are between 120 to 200 m2 / day. This is because of secondary porosity due to fractures. Therefore it is concluded that the secondary porosity plays an important role in the flow of groundwater in basaltic terrain. Thus, the resistivity studies are useful to: (1) delineate flow systems for multistoried aquifer, (2) Planning groundwater development programs and (3) to determine the depth to which artificial recharge is possible and therefore the type of recharge structures can be constructed.

 

Key words: Electrical resistivity, multistoried aquifer, Groundwater surface map, artificial recharge.


 

 

Site Response Studies in Pondicherry Region

 

S. Trupti, P. Pavan Kishore, K.N.S.S.S. Srinivas and T. Seshunarayana
CSIR- National Geophysical Research Institute, Uppal Road, Hyderabad- 500 007

 

Abstract

 

   Site response studies were conducted in Pondicherry region based on the ambient noise recording for short duration to determine the ground response frequencies and corresponding amplifications of soils at eighty three selected locations of Pondicherry. These studies were carried out using short duration data of ambient seismic noise and hammer source. Besides this, multichannel analysis of surface waves (MASW) technique was conducted at these locations to estimate the shear wave velocity to compute the predominant frequencies using frequency-shear wave velocity relationships. Both the studies reveal that the southern part of the study area with alluvial and thick soil cover exhibits low frequency in the range 1 to 3 Hz and the higher frequencies are observed in the northern part of Pondicherry which comprises sandstone/limestone.

 

Key words: Shear wave velocity, Frequency, Amplification, MASW.


 

 

 

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