Contents of APRIL 2005 - Vol. XXVI No.2

ESTIMATION OF POROSITY USING PRINCIPAL COMPONENT AND STEPWISE MULTIPLE REGRESSION ANALYSIS OF SEISMIC ATTRIBUTES - A CASE STUDY
- Y.P.Singh, R.K.Pathak and K.K.Nath

HIGH RESOLUTION SEISMIC SURVEYS - GULF OF CAMBAY
- S.Kathiroli, D.V.Rao, P.S.Rama Chandra Murthy

ARCHEAN-PROTEROZOIC COLLISION TECTONICS ACROSS CHOTA NAGPUR GRANITE GNEISSIC COMPLEX AND SINGHBHUM CRATON : BASED ON GRAVITY STUDIES INTEGRATED WITH GEOLOGICAL INFORMATION
 - R.P. Rajasekhar and D.C.Mishra

INTEGRATION OF SATELLITE IMAGERY, GEOPHYSICAL AND GEOCHEMICAL DATA SETS FOR GOLD MINERALISATION IN NORTHERN PART OF HUTTI-MASKI GREENSTONE BELT, KARNATAKA, INDIA
- R.Ananda Reddy

A CONSTRAINED NONLINEAR INVERSION APPROACH TO QUANTITATIVE INTERPRETATION OF SELF-POTENTIAL ANOMALIES CAUSED BY CYLINDER AND SPHERE-LIKE STRUCTURES
- J.Asfahani and M.Tlas

CRUSTAL IMAGING OF THE MAHANADI DELTA AND ITS TECTONIC SIGNIFICANCE
- Laxmidhar Behera

ESTIMATION OF POROSITY USING PRINCIPAL COMPONENT AND STEPWISE MULTIPLE REGRESSION ANALYSIS OF SEISMIC ATTRIBUTES - A CASE STUDY

Y.P.Singh, R.K.Pathak and K.K.Nath

Director (Geophysics) Oil India Limited, Duliajan, Assam

Abstract

A scheme for estimation of porosity from 3-D seismic attributes and well data is described. The methodology aims to find best statistical relationship for estimation of porosity from available well data and seismic attributes extracted from 3-D seismic volume. Principal Component Analysis (PCA) of extracted seismic attributes creates a new series of independent attributes (PC attributes). Forward stepwise multiple regression analysis derives optimum set of attributes from larger set of attributes, which also justify the efficacy of seismic and PC attribute sets. The reliability of the derived relationship is estimated using cross validation criteria. The validation error, which is the average error for all the hidden wells, is used as a measure of the likely prediction error when the relationship is applied over the area away from wells. The scheme has been tested on the real data set from one of the oil fields in Upper Assam India. The prediction/ validation error observed with PC attributes is lower than that of the same derived from seismic attributes. For nonlinear transformed sets of PC and seismic attributes, the prediction validation error reduces significantly from one observed with linear sets of PC and seismic attributes respectively. The prediction / validation error was lowest in case of nonlinear transformed set of PC attributes. This establishes that the porosity map for entire study area calculated by using nonlinear transformed PC attributes would be most accurate. The data reduction and other computational advantages associated with PC attributes make it a favorable solution for accurate porosity estimation.

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HIGH RESOLUTION SEISMIC SURVEYS - GULF OF CAMBAY

S.Kathiroli, D.V.Rao, R.S.Rama Chandra Murthy

National Institute of Ocean Technology, Chennai

Abstract

The preliminary subbottom and side scan surveys carried out in the Gulf of Cambay recorded a few N-S structural trends. For obtaining better definition of these structural trends, high resolution seismic surveys were carried out along 23 E-W trending lines across the Gulf. While a lot of literature is available on the tectonic movements and sea level changes in the basin during Tertiary, not much is understood on the Quanternary tectonics/neotectonics of the gulf. The present study has thrown some light on the neotectonics of the region in the recent past with the seismic sections depicting the area to have undergone intense folding and faulting. The layers are sheared and conspicuously absent with several features of faulting at surface and subsurface. In all, 8 major lineaments are demarcated with majority of them trending NW-SE direction. Several focal centers for the fault mechanism have been identified. Due to active neotectonism associated with these focal centers, continuity of the subsurface layers is affected and often discontinuities in the eastern side. Several vertical faults, often having less than a meter displacement, are identified.

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ARCHEAN - PROTEROZOIC COLLISSION TECTONICS ACROSS CHOTO NAGPUR GRANITE GNEISSIC COMPLEX AND SINGHBHUM CRATON: BASED ON GRAVITY STUDIES INTEGRATED WITH GEOLOGICAL INFORMATION

R.P.Rajasekhar and D.C.Mishra

National Geophysical Research Institute, Hyderabad

Abstract

Gravity data of Chota Nagpur Granite Gneissic Complex (CGGC) and Singhbhum craton in the eastern part of Satpura Mobile Belt (SMB) are analysed for crustal structure and their role in the evolution of this region. The two phases of lower crustal rocks in Chota Nagpur Granite Gneissic Complex (CGGC) related to 1.6 Ga and 1.0 Ga are associated with gravity highs. The gravity highs related to lower crustal rocks in this section are extensions of similar gravity highs from central India related to same rock types of pre and post sausar orogeny. The gravity lows of Central Indian Shear (CIS) in central past of SMB extends eastward as gravity lows of almost same order associated with South Purulia Shear Zone (SPSZ). The gravity modelling along a profile constrained from seismic section and gravity model along an adjoining profile across central part of SMB suggest high density rocks in upper crust associated with SPSZ as thrusted lower crustal rocks and crustal thickening under Singhbhum craton. The gravity highs of Singhbhum craton are related to high density mafic rocks of Chandil and Dalma-Dhanjori volcanics, which show affinity to island arc type magmatism. This tectonic setting suggest a Proterozoic collision between Bundelkhand craton to the north and Singhbhum craton to the south with South Purulia Shear Zone (SPSZ) as suture as has been suggested in case of CIS in central part of SMB. Thrusting and southward subduction during collision gave rise to lower crustal rocks of CGGC and subduction related magmatism of Chandil and Dalma -Dhanjori volcanics of Singhbhum craton.

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INTEGRATION OF SATELLITE IMAGERY, GEOPHYSICAL AND GEOCHEMICAL DATA SETS FOR GOLD MINERALISATION IN NORTHERN PART OF HUTTI-MASKI GREENSTONE BELT, KARNATAKA, INDIA

R.Ananda Reddy

Geological Survey of India, Hyderabad

Abstract

The utility of integrated satellite imagery, Bouguer gravity and magnetic maps along with geochemical data in deciphering the structural fabric of the northern part of Hutt-Maski supracrustal greenstone belt in relation to the occurrence of gold prospects, concealed geological features and identification of target areas are presented in this paper. Gravity map clearly brought out the disposition of the horse shoe shaped schist belt as a distinct gravity high and also the presence of internal granitoids associated with the shear / fracture / alteration zones as lows / low nosings. Residual gravity lows associated with known gold mineralisation correlate well with magnetic highs. Geochemical analysis of mineralized zones indicate intensive chemical changes associated with wall rock alteration in the region of residual gravity lows. Some of the lineaments identified from IRS imagery also corroborate well with the shear zones associated with gold mineralisation. A narrow corridor of gravity ‘high’ immediately south of Hutti mines indicate the extension of the schist belt, suggestive of continuity of Hutti lodes further south. Modelling of gravity data indicates that the schist belt has a depth of about 3km. Based on integrated analysis of the available data, new target areas for gold mineralisation are identified in the northern part of Hutti-Maski greenstone belt.

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A CONSTRINED NONLINEAR INVERSION APROACH TO QUANTITATIVE INTERPRETATION OF SELF POTENTIAL ANOMALIES CAUSED BY CYLINDER AND SPHERE-LIKE STRUCTURES

J.Asfahani and M.Tlas

Atomic Energy Commission, P. O. Box 6091, Damascus, Syria

Abstract

A new and simple technique based on constrained nonlinear programming concept has been proposed in this paper to interpret quantitatively the self-potential SP anomalies caused by vertical and horizontal cylinders, and sphere-like structures. A constrained nonlinear model has been formulated in an attempt to describe the geophysical problem related to the structures considered here. This model has been then solved by the Frank-Wolfe algorithm in order to estimate the geophysical parameters of the sphere and cylinder such as: the depth of the target z, the polarization angle q , and the electric dipole moment k. This algorithm is chosen for being robust and also its application to SP data converges rapidly towards the optimal estimation of parameters. This new constrained nonlinear programming technique has been first tested on theoretical models with random noise, where very close agreement has been obtained between assumed and evaluated parameters. The validity of this proposed new method is tested on a practical field example taken from Turkey, where available SP data existed that has been previously analyzed by different approaches. The agreement is good between the results obtained by the present study and those by other approaches. The relative advantages of the proposed technique are discussed and demonstrated.

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CRUSTAL IMAGING OF THE MAHANADI DELTA AND ITS TECTONIC SIGNIFICANCE

Laxmidhar Behera

National Geophysical Research Institute, Hyderabad

Abstract

Crustal imaging of the Mahanadi delta by modeling both wide-angle seismic and Bouguer gravity data indicates the presence of widespread volcanic activity occurred during Early Cretaceous along the rift zones. This volcanic activity corresponds to the breakup of greater India from east Gondwana (e.g., present Antarctica and Australia). The crustal model derived has velocities of 6.0, 6.5, 6.0, 7.0, and 7.5 km/s with estimated densities of 2.7, 2.8, 2.65, 2.9, and 3.05 g/cm3, respectively for different layers. The presence of mid-crustal low-velocity (6.0 km/s) and low-density (2.65 g/cm3) zone with Moho upwarping or crustal thinning and emplacement of ~10 km thick high-velocity (7.5 km/s) and high-density (3.05 g/cm3) material at the base of the crust strongly suggests basaltic underplating probably due to the Kerguelen hot spot activity and indicates a typical rift-related evolution of the delta. The rifting stages have a close correlation with the different sedimentation phases of the lower and upper Gondwana deposits in the Mahanadi delta. The volcanic activities are synchronous with ~117 Ma Rajmahal volcanism in India and the Lambert graben of East Antarctica and are closely associated with the Gondwana breakup in the India Antarctica sector.

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