Contents of
July, 1998 - Vol. XIX No.3
CONTOURS TO IMAGES - PART 1: AN INNOVATIVE
METHODOLOGY
Y. Sreedhar Murthy, K. Govindarajan and V. Babu
Rao
SEISMIC ATTENUATION AS A HYDROCARBON INDICATOR - A
CASE STUDY
R.
Das Gupta and G. R. Saini
GEOPHYSICS IN THE EXPLORATION FOR GOLD IN C.K. HALLI
AREA, CHITRADURGA DISTRICT, KARNATAKA
C.
Ramachandran, H. V. Rao, V. Ramamurty and S. A. Khan
AN INTEGRATED APPROACH FOR
THE DELINEATION OF BURIED PALEOCHANNEL AT INDIRA SAGAR (GOSIKHURD) PROJECT,
MAHARASHTRA STATE, INDIA
J. M. Shirke, B. S. Shrotri, G. A. Panvalkar, M. Tripura and P.
V. Akut
A PARAMETRIC ANALYSIS OF EM
RESPONSE OF ELONGATED COVERED ORE DEPOSITS IN MULTIFREQUENCY MEASUREMENTS
D. Indira Nagubai and P. D. Saraf
K-T BOUNDARY ASTEROIDAL
IMPACT, GEODYNAMIC EVENTS AND CONTINENTAL BREAKUP ALONG THE WESTERN MARGIN OF
INDIA
P. K. AGRAWAL, O. P. Pandey and J. G. Negi
CONTOURS TO IMAGES - PART 1: AN INNOVATIVE METHODOLOGY
Y. Sreedhar Murthy*, K. Govindarajan** and V. Babu Rao***
*National Geophysical Research Institute,
Hyderabad - 500 007
**Dept. of Geophysics, Osmania University, Hyderabad - 500 007
***Association of Exploration Geophysicists, Hyderabad - 500 007
Abstract
The geophysical data is usually
presented as stacked profiles or as contour maps, which are essentially discrete
in nature. In contrast, the data in image form is continuous thereby enhancing
the image patterns.
We present here a methodology to
digitize contour maps using commonly available desktop tool like a personal
computer, scanner and a mouse. A simple program to auto tree and digitize
contour maps has been developed for the purpose. In this paper(part 1), we will
present the methodology, a flowchart of the program and examples of contour
maps digitized by this method. All image processing has been done using readily
available software.
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SEISMIC ATTENUATION AS
A HYDROCARBON INDICATOR - A CASE STUDY
R. Das Gupta and G. R. Saini
GDAP Department, Oil India Limited, Duliajan -
786 602, Assam.
Abstract
With the passage of time, as the
quality of decision making process is being expected/demanded to be as high as
possible, analysis of various attributes related to seismic has turned into an essentiality.
One such attribute, seismic quality factor, Q could play important role in
hydrocarbon exploration as it can provide significant information if put to
proper test in this regard. Recently, considerable amount of work has been
carried out on estimation of seismic attenuation or quality factor, Q. In this
paper, the results of Q estimations from VSP's and sonic log data of two wells
from a gas field located within OIL's PEL area in this state of Rajasthan,
using spectral ratio and seismic-sonic drift methods are presented. The
analysis from two wells adjacent to each other reveals that the Q estimates for
all the formations encountered in both wells are similar except for Khuiala
formation (of Eocene age), the Q values being 34±4 and 39±5 for well A and 21±2
and 24±3 for well B respectively. This significant difference in Q is due to
the fact that the formation in well B is hydrocarbon bearing whereas, in well
A, it is not. For future exploratory and development work this significant
observation can be used as an indicator for the presence of hydrocarbons in a
formation.
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GEOPHYSICS IN THE
EXPLORATION FOR GOLD IN C.K. HALLI AREA, CHITRADURGA DISTRICT, KARNATAKA
C. Ramachandran, H. V. Rao, V. Ramamurty and S. A. Khan*
Geophysics Division, Geological Survey of
India, Bandalaguda, Hyderabad - 500 068.
*Karnataka Circle, Geological Survey of India, Jayanagar, Bangalore - 560 082.
Abstract
Detailed geophysical surveys in C. K.
Halli - north block shows that giving importance to IP chargeability highs is
not always valid in the exploration for Archaean gold. If the geophysical
target is a thick sulphidic quartz vein emplaced within a more polarizable host
rock like argillites or at its contact, which is the case for some gold
prospects in Chitrdurga district, then the sulphidic quartz vein may show up in
a chargeability low zone over the higher background of argillites; or it may
not show any characteristic IP signature and would be located in the
chargeability gradient. In C. K. Halli - north block, the probable disposition
of a five meter wide sulphidic gold-quartz vein under thick soil cover is
inferred from a resistivity high, chargeability low and a total field magnetic
anomaly over a magnetic ferrodolomite associated with the quartz vein. The
inferred disposition of the mineralized quartz vein is proved in three
locations. The results show that the chargeability signature of gold-quartz
veins can be varied and depend on host rock polarization, besides other
factors.
In this soil covered block southwest of
C. K. Halli, selected from airborne magnetic and radiometric maps, a few zones
with favourable indications for gold mineralization are identified from test geophysical
surveys. The studies also show that airborne geophysical maps when integrated
with geological exploration models, would provide definitive exploration target
areas for gold so that the expensive follow-up can be concentrated in selected
areas.
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AN INTEGRATED APPROACH FOR THE DELINEATION OF BURIED PALEOCHANNEL AT INDIRA SAGAR (GOSIKHURD) PROJECT, MAHARASHTRA STATE, INDIA
J. M. Shirke, B. S. Shrotri, G. A. Panvalkar, M. Tripura and P. V. Akut
Central Water and Power Research Station,
Khadakwasla, Pune, India.
Abstract
Buried paleochannels pose serious
problem to the river valley projects in view of reservoir competency.
Geophysical methods such as gravity, seismic refraction, etc. are useful
delineating these channels. These methods are quick and cheap as compared to
sub-surface exploration by drilling.
In the present paper, a case study of
Ghosikhurd dam is presented, Indira sagar (Ghosikhurd) project envisages
construction of a 11.35km long and 22.50m high dam across the wainganga river,
a tributary of the Godavari. Presence of buried paleochannel was confirmed by
ten exploratory drillholes between Ch. 1100m and Ch. 2310m along the dam axis.
Remote sensing technique was used to detect the surface manifestation. But for
delineating the exact path of the paleochannel from the dam axis to the present
river course gravity and seismic methods were applied. Gravity results obtained
from the profiles on the upstream of the dam axis have demarcated the width,
extent and depth of the buried paleochannel. Seismic results from two profiles
100m and 300m upstream have further confirmed the gravity studies.
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A PARAMETRIC ANALYSIS
OF EM RESPONSE OF ELONGATED COVERED ORE DEPOSITS IN MULTIFREQUENCY MEASUREMENTS
D. Indira Nagubai and P. D. Saraf
National Geophysical Research Institute,
Hyderabad - 500 007
Abstract
Several field situations have reported
elongated ore deposits possessing ring-shaped zones of different electrical
and/or magnetic properties. In the present study one such geological reality of
elongated multiple conductors has been approximated by a composite two-shelled
model. Various combinations of physical and/or geometrical parameters in the
composite system have been studied for multifrequency EM measurements.
Some significant findings are:
1) Effects due to undesirable
overlaying formations, covering conducting ore deposits, can be eliminated by using
multifrequency EM measurements.
2) Changes in the physical parameters
of the cover or target bodies are resolved distinctly in multifrequency
measurements and much significantly in imaginary - frequency patterns in
comparison to the corresponding other components of these response parameter.
3) Continuous view of all the
components of the composite system can be analyzed in different frequency
zones.
Some significant findings are: 1)
Effects due to undesirable overlaying formations, covering conducting ore
deposits, can be eliminated by using multifrequency EM measurements. 2) Changes
in the physical parameters of the cover or target bodies are resolved
distinctly in multifrequency measurements and much significantly in imaginary -
frequency patterns in comparison to the corresponding other components of these
response parameter. 3) Continuous view of all the components of the composite
system can be analyzed in different frequency zones.
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K-T BOUNDARY
ASTEROIDAL IMPACT, GEODYNAMIC EVENTS AND CONTINENTAL BREAKUP ALONG THE WESTERN
MARGIN OF INDIA
P. K. AGRAWAL, O. P. Pandey and J. G. Negi
Theoretical Geophysics Group, National
Geophysical Research Institute, Hyderabad - 500 007.
Abstract
The western margin and adjacent
offshore regions of the Indian subcontinent has been associated with several
catastrophic and geodynamic events since its dispersal from Gondwanaland in
early Cretaceous. The whole region is marked by a large flood basaltic province
and several prominent surface structural features. Geophysically, the area is
characterized by anomalous gravity field, high heat flow and high geothermal
gradients, suggesting heterogeneity and structural deformities within the
lithosphere. Analysis of geophysical data reveals that the crustal thickness
between latitude 13°N and 24°N varies from 18km to 35km and the lithosphere is
thinned to 40 km beneath certain segments.
In the present paper, we explain these
observations by a multiple cause hypothesis of: i) a possible large size bolide
impact at the K-T boundary (65Ma) near the Bombay offshore, ii) rapid northwind
motion of the Indian subcontinent between 80 and 53 Ma, and iii) ridge jump,
asthenospheric upwelling and consequent breakup of paleo-super Dharwar craton
of India around 80 Ma. This would explain the initiation of Carlsberg ridge and
Laxmi ridge in the Indian ocean, triggering of Deccan volcanism and detachment
of
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