Contents of April, July, October 2001 - Vol. XXII No.2, 3, 4.
Contents of January 2002 - Vol. XXIII No.1.

TWO DIMENSIONAL DIRECT CURRENT RESISTIVITY FORWARD MODELLING FINITE ELEMENT C++ SOURCE CODE
K.K. Roy and Priyank Jaiswal

COMPARISON OF DECONVOLUTION METHODS FOR SCALING REFLECTIVITY
Tommy Toverud*, Vijay P. Dimri** and Bjfrn Ursin*

ON GRAVITY AND MAGNETIC SIGNATURES OF SOME GREENSTONE BELTS OF EASTERN DHARWAR CRATON, SOUTH INDIA
V. Babu Rao

CROSSHOLE RESISTIVITY TOMOGRAPHY AND BOREHOLE-TO-SURFACE PROFILING FOR AQUIFER ELECTRICAL ANISOTROPHY STUDIES AT BOLIVER, AUSTRALIA
D.R. Pant*, Z. Baig** and S.A.Greenhalgh**

JOINT ANALYSIS OF AEROMAGNETIC AND GRAVITY MAPS IN THE WAVENUMBER DOMAIN: A CASE STUDY IN NORTHWEST, BANGLADESH
M. Muniruzzaman*, S. Enayet Ullah** and Idris Miah***

INTEGRATION OF AIRBORNE GEOPHYSICAL AND REMOTE SENSING DATA FOR TARGETING THE ZONES OF RARE EARTH MINERALS : A CASE STUDY OF GODHRA AREA, GUJARAT
R. Muralidharan*, K. Jagannadha Rao* and T.A.Dattanarayana**


TWO DIMENSIONAL DIRECT CURRENT RESISTIVITY FORWARD MODELLING FINITE ELEMENT C++ SOURCE CODE

K.K. Roy and Priyank Jaiswal

Department of Geology and Geophysics.
Indian Institute of Technology, Kharagpur.

Abstract

Finite Element source code is written in C++ for two dimensional direct current resistivity surface geophysics problem. Rayleigh-Ritz energy minimisation approach is followed for formulation of the problem. Three aspects of the problem are studied, viz. (i) Effect of matrix partitioning in reducing the storage and time for inversion of the sparse matrix. (ii) Effect of number of integration variables and their distribution for accurate computation of potentials (iii) effect of element size in implementation of Neumann’s boundary condition in the vicinity of the current electrode.

It is observed that matrix partitioning significantly reduces the computation time and space required for storage. Surface element in z direction should preferably be smaller for accurate implementation of Neumann’s boundary condition. Number of integration variable should be chosen according to (a) the accuracy demanded by the problem (b) the distances till which match between the analytical and computed curve is required (within the error limits) and (c) the computational facility available. C++ language has been used because the program utilises an object oriented concept in storage and inversion of the sparse matrix. Finite Element formulation and important source code modules are presented in Appendix A and B.

The model was tested showing the comparison of the variation of analytical and numerical potential values with distance from the source for different number of integration variables. Two electrode apparent resistivity profiles over an outcropping vertical dyke and an outcropping vertical contact are presented to show the nature of response and the level of accuracy achieved by working with 6264 three noded triangular elements on an AcerPower 6500 PC (Intel 233 MHz chip).

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COMPARISON OF DECONVOLUTION METHODS FOR SCALING REFLECTIVITY

Tommy Toverud*, Vijay P. Dimri** and Bjfrn Ursin*

*Dept. of Petroleum Tech. and Applied Geophysics, Norwegian University of Science and Technology, Norway.
** National Geophysical Research Institute, Uppal Road, Hyderabad, India.

Abstract

In conventional deconvolution, the seismic trace is assumed to consist of a white reflectivity series convolved with a minimum-delay wavelet plus white noise. Several investigations have shown, however, that the reflectivity series is scaling so that its power spectrum is proportional to frequency raised to some exponent.

Standard deconvolution is compared with two methods which use an estimate of the autocorrelation function (ACF) of the reflectivity series to design the deconvolution filter. In the first method the filter coefficients are found by solving a set of extended normal equations which depend on the ACF of the seismic trace and the ACF of the reflectivity series for lag zero and one. The second method uses the ACF of the reflectivity series to compute a whitening filter which removes the non-white part of the reflectivity series before computing the deconvolution filter which is the inverse of the minimum-delay wavelet.

In a synthetic data example, using logs from a well off-shore Norway, in which the power spectrum of the reflectivity series is proportional to frequency raised to the power of 0.31, both methods performed better than conventional deconvolution which is optimal only for a white noise reflectivity. The spectral shaping method performed slightly better than the method based on solving the extended normal equations.

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ON GRAVITY AND MAGNETIC SIGNATURES OF SOME GREENSTONE BELTS OF EASTERN DHARWAR CRATON, SOUTH INDIA

V. Babu Rao

Scientist (Retd.), NGRI, H.No.12-13-381, St.No.18, Tarnaka, Hyderabad-17.

Abstract

Archaean Greenstone belts are known repositories of precious and base metal mineralisation around the world and hence their recognition and delineation assume importance for mineral exploration. Among the geophysical methods, both gravity and aeromagnetics are capable of detecting them. The magnetic members of the Greenstone belts particularly, the Banded Iron formations (BIF) and the serpentinized ultramafics that are usually associated with them give rise to intense magnetic anomalies locally. Schistose lithologies are usually denser than the neighbouring Granites and Gneisses. Thus, intense magnetic anomalies and positive Bouguer gravity anomalies coupled with the regional strike of the Greenstone belts help to identify and delineate them even under soil and sediment cover. Detailed to semi detailed regional gravity surveys were done over the Jonnagiri schist belt, black cotton soil covered terrain to the west of the Ramgiri schist belt and to the south of the Ramgiri schist belt by the NGRI. The aeromagnetic and gravity signatures of some of the Greenstone belts over the Archaean terrain to the west of the proterozoic Cuddapah basin are presented and discussed. The regional gravity survey brought out the presence of a +12 milligal Bouguer gravity high over soil covered region to the west of Ramgiri schist belt which is inferred to be the southeastward continuation of the Sandur schist belt beneath the black cotton soil cover which runs as a parallel belt to the west of the outcropping Ramgiri schist belt. The depth extent of the Greenstone belts appear to vary from 2 to 8 km depending on the density contrast with the granites and gneisses. A comparison of the gravity and magnetic signatures suggest that gravity surveys are a better diagnostic tool than magnetics. Results of quantitative analysis over typical gravity profiles across the Jonnagiri and Ramgiri schist belts are presented and the possibility of southward extension of the Ramgiri schist belt is indicated.

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CROSSHOLE RESISTIVITY TOMOGRAPHY AND BOREHOLE-TO-SURFACE PROFILING FOR AQUIFER ELECTRICAL ANISOTROPHY STUDIES AT BOLIVER, AUSTRALIA

D.R. Pant*, Z. Baig** and S.A.Greenhalgh**

*Royal Nepal Academy of Science and Technology, Kathmandu, Nepal
** Dept. of Geology and Geophysics, The University of Adelaide, Australia

Abstract

Crosshole resistivity imaging and borehole-to-surface redial profiling are two very useful techniques used in geophysical exploration of aquifers. The merit of crosshole measurements lies in its ability to image targets between two boreholes while the borehole-to surface radial profiling has advantage of mapping azimuthally the major direction of favorable current flow inside conducting materials. These methods are employed both inside boreholes and in between two boreholes to map conductivity, electrical inhomogenities (geological anamolies) and anisotropy.

This paper uses a flexible and efficient finite element method(FEM) for 2.5-D resistivity forward modelling to calculate the 2.5-D Green’s function for arbitrary media. It considers the source to be a point source and the medium properties to be non-changing in the strike (out-of-plane) or y-direction for the 2.5-D approximation in 3-D situations. The forward modeling and inversion with the 2.5-D approximation is utilized in the interpretation of the bipole-bipole electodes configuration of data. The basic computational quantity for inversion is imaging the target between two boreholes using conjugate gradient solution (CGS) of a 2.5-D Helmholtz equation with the 2.5-D Green’s function of the equation.

To study the electrical resisitivity anisotropy of aquifers, the paper uses a novel method of borehole-to-surface radial profiling with four electrodes configuration (ABMN) by attaching three electrodes (A and N inside the borehole and N on the earth’s surface) near the borehole. The fourth electrode(M) is moved azimuthally along the earth’s surface. Data acquisition is made with the fourth surface mobile electrode located along concentric circles with variation of radius from the borehole.

The real field examples at the Artificial Storage and Recovery (ASR) experiment site, Boliver, Australia show presented methods are suitable for borehole-to-borehole resistivity imaging of aquifers with bipole-bipole electrodes configuration. Furthermore, it is shown by the field examples with four electrodes borehole-to-surface radial profiling, results present the basic and important aspects of electrical resistivity anisotropy variation inside the aquifer.

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JOINT ANALYSIS OF AEROMAGNETIC AND GRAVITY MAPS IN THE WAVENUMBER DOMAIN: A CASE STUDY IN NORTHWEST, BANGLADESH

M. Muniruzzaman*, S. Enayet Ullah** and Idris Miah***

*Department of Physics, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh.
** Department of Applied Physics and Electronics, Rajshahi University, Rajshahi 6205, Bangladesh.
*** Geological Survey of Bangladesh, Segunbagicha, Dhaka, 1000, Bangladesh.

Abstract

A small fraction of the long-wavelength magnetic power in a map of northwestern Bangladesh is coherent with the corresponding gravity map. A pseudo-magnetic map can be computed from the latter by wavenumber domain filtering methods. The coherence and response function linking the magnetic and psuedo-magnetic maps depends on the assumed direction of magnetisation of the bodies that cause the common anomalies. Experience with modeling experiments shows that the correct direction can be selected by maximising the coherence, subject to the condition that the imaginary part of the response is zero. On this basis, the total magnetization vector of the body was found to have a declination of 620 and an inclination of 430. The direction of inferred magnetization is different from that of the present field, indicating that there must be a remanent component. The pseudo-gravity map identified the source of the coherent anomalies as possibly the contrast in physical properties between basement rocks and granitic intrusion.

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INTEGRATION OF AIRBORNE GEOPHYSICAL AND REMOTE SENSING DATA FOR TARGETING THE ZONES OF RARE EARTH MINERALS : A CASE STUDY OF GODHRA AREA, GUJARAT

R. Muralidharan*, K. Jagannadha Rao* and T.A.Dattanarayana**

Atomic Minerals Directorate for Exploration and Research Hyderabad*, Bangalore**

Abstract

Integrated study using airborne gamma ray spectrometric and remote sensing data over an area of 4,500 sq.km covering districts of Panchmahal, Kheda and Vadodara of Gujarat was carried out for delineating zones of Rare Minerals and Rare Earths (RMRE). The basic data obtained from Airborne Gamma Ray Spectrometric (AGRS) survey (eTh, K &eTh/K) and satellite outputs have helped in delineating Godhra granite, which is the known provenance of RMRE minerals. Zones of >14 ppm of eTh in Godhra granite terrain with drainage control (streams of >2nd order) were delineated. A total of 54 anomalies have been identified with anomalous values of eTh (>14to88ppm), K (<2.2 to 5.0%) and eTh/K (<0.0009 to 0.0024) extending over a distance of 1 to 7 km. Based on eTh, K and eTh/K values of the anomalies, these zones have been classified into three orders. Among these anomalies, ten have been reported earlier along the Vasva, Surya and Ujol Nadis. The Vasva and Surya Nadi anomalies, known for potential zones of xenotime, fall in I order where as the Ujol Nadi anomaly is more enriched in monazite than xenotime, falls in II order. These studies indicate that the remaining anomalous RMRE zones especially those of I order located along Jarawa, Ani, Heron and Mesri nadis hold promise for potential RMRE mineralization. Present study also indicates that this type of data integration is an ideal tool for RMRE mineral targeting.

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