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
THE PROBABILITY OF OCCURRENCE
OF A HIGH MAGNITUDE EARTHQUAKE ON NORTHEAST INDIA.
S. K. Sarmah
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.
Back to Contents of this issue
WHERE AND HOW OF
PROSPECTING FOR DIAMONDS IN ANDHRA PRADESH, SOUTH 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.
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.
FINITE DIFFERENCE
SIMULATION OF 2.5 D ACOUSTIC WAVE PROPAGATION
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.
THE PROBABILITY OF
OCCURRENCE OF A HIGH MAGNITUDE EARTHQUAKE ON
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.