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 344 and 395 for well A and 212 and 243 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 13N and 24N 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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