Contents of October 2004 - Vol. XXV No.4

TURNKEY AIRBORNE GRAVITY - AN ACCESSIBLE AIRBORNE ACQUISITION TOOL FOR GEOPHYSICAL EXPLORATION
- J.M. Brown, E.E. Klingele and J.Verdun

INTEGRATED GEOLOGICAL AND GEOPHYSICAL STUDIES - EXAMPLES FROM INDIAN SHIELD
- V.Vijaya Rao, B.Rajendra Prasad and H.C.Tewari

MAGNETOTELLURIC INVESTIGATIONS IN DECCAN TRAP COVERED AREAS OF NAGPUR-WARDHA REGION, INDIA
S.V.S.Sarma, T.Harinarayana,G.Virupakshi, .Someswara Rao, Madhusudan Roa, Nandini Nagarajan, T.S.Sastry and S.Prabhakar, E.Rao

MAGNETOTELLURIC RESPONSE ON VERTICALLY INHOMOGENEOUS EARTH WITH HOMOGENEOUS TRANSITION MEDIUM
- J.A.Olowofela

INTEGRATION ISSUES FOR SINGLE POINT DATA
- J.Quigley and Michael Fleming

FINITE ELEMENT APPROACH FOR RESOLUTION OF REGIONAL-RESIDUAL GRAVITY ANOMALIES
Sangeetha Kannan and K.Mallick


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

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

The need for using different geophysical methods in deriving the subsurface structure of a region, in the light of their advantages and limitations is discussed. Integrated studies are essential in hydrocarbon exploration as well as in crustal studies, even though the data sets involved may be different. Results from the integrated geological, geophysical and geochemical / geochronological studies from the Aravalli-Delhi fold belt and the Central Indian Shear zone are presented. An integrated study provides the necessary link between surface geology and the subsurface through geophysics and effectively bridges the gaps between different disciplines of earth science. As multi-parametric and multi-disciplinary approach has proven advantageous and is most likely to succeed, the future of geosciences lies in integrated studies.

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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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FINITE ELEMENT APPROACH FOR RESOLUTION OF REGIONAL-RESIDUAL GRAVITY ANOMALIES

Sangeetha Kannan and K.Mallick

National Geophysical Research Institute, Hyderabad

Abstract

The regional – residual separation of Bouguer gravity anomalies in a Precambrian terrain in northwest Ontario, Canada by established analytical techniques like spectral factorization and upward continuation in a previous study did not yield results, suitable for quantitative gravity modeling. Similarly, in an oil field in Libya the interpreters could not obtain expected residual components by approximating the regional by trend surfaces. Therefore, the interpreters resorted to graphical smoothing to obtain the residual gravity map by taking into account the geology of the area and the density of the rock formations. In such a scenario, it is demonstrated that one analytical technique does exist to derive the regional gravity anomaly to satisfy the interpreter. A relatively new analytical technique, based on finite element analysis (FEA), that is mathematically simple and computationally efficient and does not require information on geology of the area and the density of the rock formations, has been successfully employed to compute the regional and the residual gravity anomalies of northwest Ontario region, Canada and Raguba oil fields in Libya. The FEA regional anomalies are what the interpreters were looking for in these two regions.

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