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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.
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.
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.
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.
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.
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.
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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