Contents
of October 2004 - Vol. XXV No.4
TURNKEY
AIRBORNE GRAVITY - AN ACCESSIBLE AIRBORNE ACQUISITION TOOL FOR GEOPHYSICAL
EXPLORATION
INTEGRATED
GEOLOGICAL AND GEOPHYSICAL STUDIES - EXAMPLES FROM INDIAN SHIELD
MAGNETOTELLURIC
INVESTIGATIONS IN DECCAN TRAP COVERED AREAS OF NAGPUR-WARDHA REGION, INDIA
MAGNETOTELLURIC
RESPONSE ON VERTICALLY INHOMOGENEOUS EARTH WITH HOMOGENEOUS TRANSITION
MEDIUM
TURNKEY
AIRBORNE GRAVITY - AN ACCESSIBLE AIRBORNE ACQUISITION TOOL FOR GEOPHYSICAL
EXPLORATION
J.M.Brown*, E.E.Klingele** and J.Verdun***
*
Lacoste and Romberg – Scintrex Inc., Boulder, Colorado, USA
** Institute of Geodesy, Geodesy and Geodynamics Laboratory, Zurich, Switzerland
*** University Montpellier II, Laboratory of Geophysics, Tectonics and
Sedimentary Geology, Montpellier, France
** Institute of Geodesy, Geodesy and Geodynamics Laboratory, Zurich, Switzerland
*** University Montpellier II, Laboratory of Geophysics, Tectonics and Sedimentary Geology, Montpellier, France
Abstract
Airborne
gravity systems have been a useful tool for geophysicists and scientists
for over two decades. The geophysical exploration method is ideal for
rapidly acquiring data over large-scale areas. With the advent of high
precision GPS, the technique became even more important for mapping medium
(10km to 100km) and high (1km to 10km) wavelengths of the earth’s gravitational
field. In the past, airborne gravity data acquisition systems have only
been available to a few select institutions and private companies. The
hardware technology was complex and difficult to install, and the necessary
software to compile, reduce and process the data was challenging to develop.
These difficulties have been significantly reduced during the last few
years. Many components necessary for airborne gravity data acquisition
are now available off-the-shelf. Software needed to perform the complex
task of merging GPS-derived vertical position information and gravity
sensor data can be acquired through agreements with agencies that have
placed considerable time and effort into research and development. As
important as the equipment and software, a large number of persons are
now familiar with the challenging aspects of airborne gravity acquisition
including both logistical and mathematical challenges. We present a brief
history of the development of airborne gravity methods and outline the
current state of the technology. We show that airborne gravity is at a
mature stage and can be offered as a turnkey system acquired and operated
by agencies and companies currently performing airborne geophysical surveys.
The availability of turnkey airborne gravity systems has significant implications
for the acquisition of world-wide medium and high frequency gravity field
wavelength data. Turnkey absolute gravity systems will make the method
more accessibleand will result in coverage acquired much more rapidly
than previously possible.
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INTEGRATED
GEOLOGICAL AND GEOPHYSICAL STUDIES - EXAMPLES FROM INDIAN SHIELD
V.Vijaya
Rao, B.Rajendra Prasad and H.C.Tewari
National Geophysical Research Institute, Hyderabad
Abstract
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MAGNETOTELLURIC
INVESTIGATIONS IN DECCAN TRAP COVERED AREAS OF NAGPUR-WARDHA REGION, INDIA
S.V.S.Sarma,
T.Harinarayana, G.Virupakshi, M.Someswara Rao, Madhusudan Rao, Nandini
Nagarajan, T.S.Sastry and S.Prabhakar, E.Rao
National
Geophysical Research Institute, Hyderabad
Abstract
In the context of the efforts for identification
and evaluation of hydrocarbon potential of different basins in the country,
Nagpur-Wardha is an important region in central India covered by Deccan
volcanics. New directions in the exploration strategies need to be evolved
to investigate the difficult and complex areas using judicious combination
of conventional as well as non-conventional techniques like Magnetotellurics
(MT) and other deep electromagnetic methods. With the support of Directorate
General of Hydrocarbons (DGH), National Geophysical Research Institute
(NGRI) has taken a lead in this direction to the application of MT, which
is one of the most effective techniques, particularly in investigating
areas covered by volcanics. This is because the target layer, a subtrappean
Gondwana sedimentary column, in general has a marked resistivity contrast
with the underlain basement as also to the overlying volcanic cover. The
results of MT studies have demarcated the area of interest from the viewpoint
of thick sediments, i.e.3-4km, in the region around Katol and Kondhali
areas. In the present study, the efficacy of the approach in the estimation
of the thickness of volcanic cover and subtrappean sediments and also
the regional tectonics of northwest extension of Pranahita-Godavari graben
are discussed.
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MAGNETOTELLURIC
RESPONSE ON VERTICALLY INHOMOGENEOUS EARTH WITH HOMOGENEOUS TRANSITION
MEDIUM
J.A.Olowofela
Department of Physics, University of Ibadan, Ibadan, Nigeria
Abstract
Magnetotelluric
response for vertically inhomegeneous Earth with transition homegeneous
zone is investigated. The impedance and appparent resistivities equations
are obtained and the variation of these with respect to the height of
te transition zone are presented graphically.
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INTEGRATED
ISSUES FOR SINGLE POINT DATA
J.Quigley
and Michael Fleming
Western Geco (Dubai)
Abstract
Conventional
onshore seismic systems typically use arrays of geophones to record data.
These arrays consist of multiple geophones positioned in a linear or areal
pattern. Simple summation of the geophone signals from such arrays does
not optimally remove noise and can attenuate the seismic signal, particularly
for non-vertical arrivals. For Vibroseis acquisition, a parallel situation
exists where multiple units are used together in a source array. In these
cases decomposition techniques may be used to recover point source data.
The concept of point data recording allows optimal data processing including
digital group forming (DGF), to be performed, resulting in more effective
noise attenuation and an optimally sampled wavefield as the starting point
for an integrated imaging, analysis and interpretation system. The application
of optimal data processing techniques to uncommitted field data is central
to the point data concept. Realizing the benefits of the method in an
efficient and effective manner requires the integration of expertise from
a range of disciplines and the deployment of suitable hardware and software
systems. Some key issues relating to the implementation of point data
acquisition and processing are discussed.
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