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 BOREHOLETOSURFACE 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. RayleighRitz 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).
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 minimumdelay 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 nonwhite part of the
reflectivity series before computing the deconvolution filter which is
the inverse of the minimumdelay wavelet.
In
a synthetic data example, using logs from a well offshore 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.
ON GRAVITY AND MAGNETIC SIGNATURES OF SOME GREENSTONE
BELTS OF EASTERN DHARWAR CRATON, SOUTH INDIA
V.
Babu Rao
Scientist
(Retd.), NGRI, H.No.1213381, St.No.18, Tarnaka, Hyderabad17.
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.
CROSSHOLE RESISTIVITY
TOMOGRAPHY AND BOREHOLETOSURFACE 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 boreholetosurface
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 boreholeto 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 2.5D
resistivity forward modelling to calculate the 2.5D Green’s function
for arbitrary media. It considers the source to be a point source and
the medium properties to be nonchanging in the strike (outofplane)
or ydirection for the 2.5D approximation in 3D situations. The forward
modeling and inversion with the 2.5D approximation is utilized in the
interpretation of the bipolebipole electodes configuration of data. The
basic computational quantity for inversion is imaging the target between
two boreholes using conjugate gradient solution (CGS) of a 2.5D Helmholtz
equation with the 2.5D Green’s function of the equation.
To study the electrical resisitivity
anisotropy of aquifers, the paper uses a novel method of boreholetosurface
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 boreholetoborehole resistivity
imaging of aquifers with bipolebipole electrodes configuration. Furthermore,
it is shown by the field examples with four electrodes boreholetosurface
radial profiling, results present the basic and important aspects of electrical
resistivity anisotropy variation inside the aquifer.
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 longwavelength magnetic power in a map of northwestern
Bangladesh is coherent with the corresponding gravity map. A pseudomagnetic
map can be computed from the latter by wavenumber domain filtering methods.
The coherence and response function linking the magnetic and psuedomagnetic
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 pseudogravity map identified
the source of the coherent anomalies as possibly the contrast in physical
properties between basement rocks and granitic intrusion.
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.
