Contents
of July, 1999 - Vol. XX No.3
EXPLORATION FOR GOLD IN A RESISTIVE
ENVIRONMENT - A CASE STUDY FROM DONA (EAST), JONNAGIRI SCHIST BELT, ANDHRA
PRADESH.
K. P. R. Vittal Rao, K. V. Satyanarayana, S. Srinivas and R. S. Acharya
WHERE AND HOW OF PROSPECTING FOR
DIAMONDS IN ANDHRA PRADESH, SOUTH INDIA.
V. Babu Rao and T. Srinivasulu
SEISMIC REFRACTIONS SURVEY OF THE
YESHWANTSAGAR RESERVOIR EMBANKMENTS FOR AUGMENTATION OF WATER SUPPLY TO INDORE,
M.P.
R. S. Ramteke, N. Ghosh, C. Subba Rao and M. S. Satpute
FINITE DIFFERENCE SIMULATION OF
2.5 D ACOUSTIC WAVE PROPAGATION
J. P. Narayan
S. K. Sarmah
EXPLORATION FOR GOLD IN A RESISTIVE ENVIRONMENT - A CASE STUDY
FROM DONA (EAST), JONNAGIRI SCHIST BELT, ANDHRA PRADESH.
K. P. R. Vittal Rao, K. V. Satyanarayana, S. Srinivas and R. S.
Acharya
Geological Survey of India
Abstract
Induced polarization methods are
generally successful in identifying sulphide associated auriferous zones when
the host rock and the surrounding rock units have a good contrast in
conductivity. Since I P phenomenon is basically related to electrochemical activity.
I P effects are dependent on the porosity of the rock units besides the two
types of conduction, electrolytic and electronic, normally present in any
mineralized rock. In Dona (east) block gold mineralization is associated with
highly sheared, altered granodiorite traversed by quartz veins with sparse
sulphide disseminations. As the country rock granodiorite and the host rock are
'resistive' reducing the porosity considerably, the I P signals are poor.
However, the alterations associated with the sheared rock enabled its
delineation recording high resistivities. The sulphide rich zones are reflected
as chargeability 'highs'. The S P response, though feeble, probably relates to
fractured zones. A few borehole sections alongwith geophysical profiles are
discussed to exemplify the above geological situation.
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WHERE AND HOW OF PROSPECTING
FOR DIAMONDS IN ANDHRA PRADESH, SOUTH INDIA.
V. Babu Rao and T. Srinivasulu
National Geophysical Research Institute,
Hyderabad - 500 007, India
Abstract
Exploration for Diamonds is synonymous
with the exploration for Kimberlite and Lamproites which were recognized as the
primary source rocks for the Diamonds. Andhra Pradesh is known as "Home of
Diamonds" since historic times as world famous Diamonds like Kohinoor,
Orloff and Pitt were won from the gravels of Krishna valley. The economic value
of the Diamonds, needs of the domestic industry and the spate of recent discoveries
of primary sources in Andhra Pradesh call for a relook into the probable areas
in Andhra Pradesh.
The exploration for kimberlites and
lamproites is dependent on the geological environment and requires
multi-disciplinary and integrated approach. The rarity and the smallness of the
kimberlite diatremes and dykes and the wide range of variation of their
physical properties make the exploration for kimberlites a challenging task.
The role of geophysics in diamond exploration is discussed in the light of the
past experience.
Based on the aeromagnetic survey over
parts of Cuddapah basin and the adjoining crystallines, a couple of dipole
anomalies located within the Cuddapah basin which could be due to suspected
kimberlites along with a few other prospective areas both within and outside
the Cuddapah basin are recommended for ground checks.
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SEISMIC REFRACTIONS SURVEY OF THE YESHWANTSAGAR RESERVOIR EMBANKMENTS
FOR AUGMENTATION OF WATER SUPPLY TO INDORE, M.P.
R. S. Ramteke, N. Ghosh, C. Subba Rao and M. S. Satpute
Central Water and Power Research Station,
Khadakwasla, Pune-24.
Abstract
Seismic refraction survey was carried
on the downstream side of the embankment of Yeshwantsagar reservoir near
Indore, M. P., to delineate the bedrock profile and to detect weak zones. If
any, for founding the structure of new spillway. Four continuous seismic
profiles of length varying from 105 m to 405 m were taken. The results of the
seismic survey revealed a three layer earth section. Seismic wave velocity
through the overburden ranged from 500 m/sec to 800 m/sec and it comprises
mainly soft clay. The second layer, comprising stiff clay/coarse sand with
gravel has a compressional wave velocity ranging from 1400 m/sec to 2500 m/sec.
The third layer, being compact basalt, has a compressional wave velocity
ranging from 4000 m/sec to 5200 m/sec indicating good quality of rock. Rock having
compressional wave velocity less than 4000 m/sec was considered to be weak,
weathered or fractured. The depth to the bedrock ranged between 6.7 m to 17.3 m
and was corroborative with borehole data.
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FINITE DIFFERENCE SIMULATION
OF 2.5 D ACOUSTIC WAVE PROPAGATION
J. P. Narayan
Department of Earthquake Engineering,
University of Roorkee, Roorkee - 247 667
Abstract
The purpose of the 2.5D acoustic wave
simulation is to find out the 3D wave propagation effects with the help of two
dimensional parameters. Linear (1991) developed a 2.5D acoustic wave equation
using 3D Green's function for a constant density medium. The 2.5D simulation is
of great importance because of the prevalence of the 2D survey in the seismic
exploration and the cost of the 3D seismic modelling. This paper presents the
development of 2.5D finite difference algorithm based on Liner's equation as well
as the simulated results and study of various numerical artifacts. Comparisons
of second as well as fourth order accurate 2.5D with 2D acoustic wave
simulation results have been given. The snapshot at different times and zero
off-set response of Sponge transmissive as well as Clayton and Engquist (1980)
absorbing boundary conditions have been studied. After the extensive analysis
it has been found that the stability condition and the requirement of number of
grid points per - wavelength to avoid grid dispersion is the same as for the 2D
acoustic cases.
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THE PROBABILITY OF OCCURRENCE
OF A HIGH MAGNITUDE EARTHQUAKE ON NORTHEAST INDIA.
S. K. Sarmah
Department of Environment Science, Gauhati
University
Present address: Arya Nagar, College Road, Guwahati - 781 016
Abstract
The north India is one of the
seismically active regions of the world. Two very high magnitude earthquakes
occurred in this region in 1897 (M=8.7) and 1950 (M=8.7) during last one
hundred years. History of the region indicates occurrence of destructive
earthquakes at the interval of about 50 years. Applications of b-value and
Gumbel's statistical methods to the earthquakes of magnitude 8 or greater. The
last big earthquake of magnitude 8.7 occurred in 1950. So, statistically the
region is ready for an earthquake of similar magnitude. Diminution of the
number of high magnitude earthquakes since 1960, diminution of ts/tp, ratio
with time and accumulation of strain in the region tendto indicate the
probability of occurrence of a high magnitude earthquake in the region in the
near future.
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