Contents of April, 2000 - Vol. XXI No.2

 

NEW AREAS FOR KIMBERLITE SEARCH BASED ON GRAVITY DATA IN ANDHRA PRADESH
Burra Subrahmanyam

DISCOVERY OF AN AURIFEROUS MAJOR QUARTZITE BODY, SOUTH OF KALLENAHALLI, CHITRADURGA SCHIST BELT, FROM INTEGRATION OF GEOLOGICAL MODELS FOR ARCHAEAN GOLD AND AIRBORNE RADIOMETRIC MAP
C. Ramachandran, Ramesh Acharya & V. Ramamurty

 

BASEMENT CONFIGURATION OF WESTERN OFFSHORE, KONKAN-KERALA BASIN
D. Rai & V. Ramaswamy

A MICROEARTHQUAKE SURVEY AT THE MBT ZONE : SIKKIM HIMALAYA
Reena De

IMPROVEMENT OF DETECTABILITY OF 3-D BODIES IN RESISTIVITY MEASUREMENTS USING SIMPLE SUBTRACTION METHOD
N. P. Singh & T. Lal

RESULTS OF WERNER DECONVOLUTION OF AEROMAGNETIC DATA ALONG THE DSS TRANSECTS ACROSS THE PROTEROZOIC CUDDAPAH BASIN, SOUTH INDIA
V. Babu Rao & T. Srinivasulu

 

BOUGUER GRAVITY IMAGE OF INDIA AND THE NEIGHBOURING ESCAP REGION : A STUDY OF TECTONICS AND NATURAL RESOURCES
Y.Sreedhar Murthy & U.Raval


NEW AREAS FOR KIMBERLITE SEARCH BASED ON GRAVITY DATA IN ANDHRA PRADESH

Burra Subrahmanyam

Retd. Director, GSI, Hyderabad 1-2-99/3, Street No.2, Habsiguda, Hyderabad - 500 007

Abstract

 

A conceptual geological model for the exploration of kimberlite was proposed based on the study of different scales of gravity maps in Wajrakarur, Andhra Pradesh (A.P.) to arrive at the final stage of detailed electrical investigation for actual location of kimberlite (Subramanyam & Puri, 1985 - 1986). The discoveries of kimberlite made during the past one and half decades stand testimony to the correctness of the model. Based on this, three new areas are suggested for search of kimberlites in A.P.

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DISCOVERY OF AN AURIFEROUS MAJOR QUARTZITE BODY, SOUTH OF KALLENAHALLI, CHITRADURGA SCHIST BELT, FROM INTEGRATION OF GEOLOGICAL MODELS FOR ARCHAEAN GOLD AND AIRBORNE RADIOMETRIC MAP

C. Ramachandran, Ramesh Acharya & V. Ramamurty

Geophysics Division, Geological Survey of India, Bandlaguda, Hyderabad – 500 068.

Abstract

We report the discovery of a gold bearing major quartzite body south of Kallenahalli, Chitradurga Schist belt, Karnataka, from an integration of structural controls defined by ore-deposit models for Archaean gold and airborne radiometric total count map. We interpret two contiguous N-S trending second-order shear zones from the airborne radiometric total count map, between west of M.N. Halli and Gonuru area. The contiguous faults, where their orientation changed from N-S to NW-SE south of Kallenahalli, show segmentation. The jog area between the fault segments – presently named ‘Kallenahalli fault jog’— is also the intersection of the N-S fault/shear zone with an E-W fault/shear zone. In view of the structural controls defined by ore deposit models for Archaean gold, the interpreted N-S second order contiguous fault/shear zones, the intervening area between them, and the Kallenahalli fault jog form potential areas for gold mineralization.

A ground check in the Kallenahalli fault jog showed a 700 meters long and about 20 meters wide major quartzite body. Because of the unique interpreted structural position of the quartzite body, four samples, collected at an interval of about hundred meters are analyzed for gold, and encouraging values are obtained in all samples (1.0 –1.4 g/t in three samples and 0.4 g/t in the fourth sample). The discovery of the auriferous major quartzite body show that quartzites also form important hosts for Archaean gold in Chitradurga schist belt and may be explored for besides other targets like sulphidic BIF and quartz veins. The studies show that the integration of airborne radiometric (total count) maps and ore deposit models, is a powerful tool to predict definitive exploration targets for Archaean gold. In addition, the studies brought out the use of radiometric maps for structural interpretation.

 

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BASEMENT CONFIGURATION OF WESTERN OFFSHORE, KONKAN- KERALA BASIN

D. Rai & V. Ramaswamy

Geophysics Divn., KDMIPE, ONGC, Dehradun

Abstract

Basement configuration of Konkan-Kerala basin of western offshore consisting of continental shelf and Alleppey platform is presented. Knowledge of basement configuration is very important for deciding exploration strategies in a basin. Shipborne magnetic data which is traditionally acquired along with seismic and gravity data has been utilized to bring out the basement configuration. Free-air and total intensity magnetic anomaly maps and subsurface information from drilled wells of the basin have been used for this purpose. Two computer base techniques have been used for quantitative analysis of magnetic data.

The most obvious signature on free-air gravity map is the regional steep gradient of the free air anomaly extended from SE to NW. Since the ocean-continental crustal boundary in the westcoast is generally accepted to exist much west of the Konkan-Kerala basin, the gravity gradient has to come from density inhomogenities caused by reasons other than the rise in the Moho level.

The study reveals (i) a maximum basement depth of 6.5 km to the northwest of well CSP-1, (ii) a basement depth of about 6 km at Alleppey platform, (iii) almost a continuous increase in basement depth towards west across Miocene Hinge, (iv) a horst-graben morphology at basement level near Cochin and Alleppey and (v) a basement depth of about 3.5 km near well K-1-1.

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A MICROEARTHQUAKE SURVEY AT THE MBT ZONE : SIKKIM HIMALAYA

Reena De

Geological Survey of India, 4 Chowringhee Lane, Calcutta-700 016

Abstract

 

A microearthquake survey was carried out in the Sikkim Himalaya using a five-station temporary network. A total of about 200 earthquakes were recorded. 60 local earthquakes including microearthquakes which were recorded within 70 km of the network are located in this study. The microseismicity map shows that most of the earthquakes occur to the north of the MBT. There was no significant spatial or temporal variation of the seismic activity. The b-value, i.e. slope of the frequency-magnitude relation, is found to be less than the normal value 1.0. Hypocentral cross sections show that the earthquakes at shallow as well as at deeper crustal depths are correlatable with the MBT. From the preferred fault-plane solution of the deeper earthquakes it can be inferred that the MBT is seismogenic down to the Moho (Mohorovicic discontinuity) depth.

 

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IMPROVEMENT OF DETECTABILITY OF 3-D BODIES IN RESISTIVITY MEASUREMENTS USING SIMPLE SUBTRACTION METHOD

N. P. Singh & T. Lal

Department of Geophysics, Banaras Hindu University, Varanasi- 221 005, (UP)

Abstract

A simple processing method, based on subtraction of the response of one system of measurement from the other, is applied to the direct current resistivity profiling data to study its effectiveness in improving detectability and resolution of 3-D subsurface bodies. The method is applied to the set of synthetic profiling data obtained by Schlumberger and equatorial dipole-dipole configurations over various layered earth models containing 3-D subsurface bodies. The variation of processed anomalies referred as residual apparent resistivity are presented as a function of the position of Schlumberger array center, and are compared with those of the conventional Schlumberger apparent resistivity curves. Results depict that the residual apparent resistivity curves show more pronounced variation than those of the Schlumberger apparent resistivity curves. This pronouncement in the residual apparent resistivity curves results in an improved resolution and detection of the subsurface bodies.

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RESULTS OF WERNER DECONVOLUTION OF AEROMAGNETIC DATA ALONG THE DSS TRANSECTS ACROSS THE PROTEROZOIC CUDDAPAH BASIN, SOUTH INDIA

V. Babu Rao & T. Srinivasulu

National Geophysical Research Institute, Hyderabad-7, India

Abstract

Aeromagnetic coverage, Gravity surveys coupled with the two DSS transects across the basin makes the Proterozoic Cuddapah basin as one of the Geophysically well studied sedimentary basins of India. Magnetic surveys either ground or airborne are ideally suited to understand the basement configuration of sedimentary basins. Results of Werner deconvolution of the aeromagnetic profile data flown at an elevation of 3500 ft (1060 m) along the two DSS profiles across the Cuddapah basin are presented and discussed in the light of the Gravity and DSS studies.

Interpretation of the aeromagnetic data brought out block faulted nature of the basement beneath the Cuddapah basin. The edges of the several upthrown and downthrown basement blocks appear to agree well with the basement faults inferred from the DSS studies. The thrusted eastern margin and the contact of the Kurnools with the Nallamalais in the west central part are well brought out. The western margin is not seen as a fault contact from the magnetic interpretation. Instead, the arcuate line of sills and flows paralleling the western margin has come out as a major fault lineament along the Parnapalli- Kavali (DSS-1) profile. The Esawarakuppam dome is clearly seen along the Alampur-Koniki (DSS 2) profile. A basement trough with a 6.0 km thick sediments is inferred around Atmakuru and to the west of the Iswarakuppam dome (profile 2) while the maximum sediment thickness is of the order of 6.5 km in the SW part of the basin (profile 1). The sediment thickness along the thrusted eastern margin is of the order of about 3.0 km from magnetic interpretation, which is at variance with the DSS interpretation. The implications of the magnetic interpretation are discussed.

 

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BOUGUER GRAVITY IMAGE OF INDIA AND THE NEIGHBOURING ESCAP REGION : A STUDY OF TECTONICS AND NATURAL RESOURCES

 

Y.Sreedhar Murthy* & U.Raval**

* Centre of Exploration Geophysics, Osmania University, Hyderabad
**National Geophysical Research Institute, Hyderabad

 

Abstract

 

The 2-D gravity field map at 25 mGal interval of the region lying between latitudes 5º and 40º N and Longitudes 48º and 98º E and covering India, Burmah, Bangladesh, Bhutan, Nepal, Pakistan, Afghanistan, Iran, Qatar, Persian Gulf has been digitized to generate images. These images bring out very sharp gravity lineaments not inferred hitherto. It is seen that many of the geo-tectonic features like cratons, mobile belts, mineral/ hydrocarbon occurrences, epicentres/ high seismic belts, heat flow/ hotspring corridors get focussed exceptionally well. These images thus open up possibilities for delineating the causative factors behind these anomalies and their spatial patterns.

 

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