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***
*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 Seychelles block from the western margin.
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