Bouguer Gravity Regional and Residual Separation: Application to Geology and Environment

Transcription

1 Bouguer Gravity Regional and Residual Separation: Application to Geology and Environment

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3 Bouguer Gravity Regional and Residual Separation: Application to Geology and Environment By K. Mallick Emeritus, National Geophysical Research Institute Hyderabad, India A. Vasanthi National Geophysical Research Institute Hyderabad, India K.K. Sharma Department of Applied Geology University of Madras, Chennai, India

4 iv Contents A C.I.P. Catalogue record for this book is available from the Library of Congress. ISBN ISBN (HB) (e-book) Copublished by Springer, P.O. Box 17, 3300 AA Dordrecht, The Netherlands with Capital Publishing Company, New Delhi, India. Sold and distributed in North, Central and South America by Springer, 233 Spring Street, New York 10013, USA. In all other countries, except SAARC countries Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan and Sri Lanka sold and distributed by Springer, Haberstrasse 7, D Heidelberg, Germany. In SAARC countries Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan and Sri Lanka sold and distributed by Capital Publishing Company, 7/28, Mahaveer Street, Ansari Road, Daryaganj, New Delhi, , India. Printed on acid-free paper All Rights Reserved 2012 Capital Publishing Company No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Printed in India.

5 Contents v Dedicated to the sacred memories of Dr Amalendu Roy and Dr Hari Narain

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7 Contents vii Foreword Bouguer Gravity Regional and Residual Separation Application to Geology and Environment addresses the age-old issue of identification and separation of the long and short wavelength components of information that are inherent in gravity anomalies, which are routinely used in geophysical techniques to decipher the subsurface structure of the Earth. Gravity observations on the surface of the Earth reflect the superimposed effect of the deeper and larger variations of masses as well as the shallower and more local variations, near the point of observation. Interpretation of Bouguer anomalies are often the first step of any geophysical investigation and the need to assess the long and short wavelength components separately is frequently crucial for the requirements of the investigation. While whole Earth large scale anomalies have been used by some experts to probe into mass/ density variations deep in the Earth s interior, most commonly the needs of a limited regional survey demands that the signals from large scale mass anomalies be discriminated from the local near-surface ones, which would enable these structures to be modelled. This technique is well known by the term regional-residual separation and traditionally it has been the onus of the analyst to make the distinction between the regional he would wish to remove and the residual he would analyse. While such an approach may sometimes be essential for preserving information pertaining to specific targets, nevertheless standardization of the regional-residual separation technique to eliminate subjectivity of the analyst has a lot in its favour and is highly desired by professionals in industries and those using commercial software packages, where biases introduced by different interpreters in different parts of a project may wreck a final interpretation of the region. Over the decades, this has led to the replacement of the graphical techniques of regional-residual separation with different analytical methods, which can be quantified and hence accounted for in the subsequent modelling exercises. This book presents a new analytical method, developed by the authors to define the regional component of the measured gravity field based on a finite element vii

8 viii Foreword extrapolation, where only eight or twelve nodal values are the boundary value. This procedure has the advantages of being uninfluenced by personal bias, independent of a priori information, as well as producing a regional which is smooth and unaffected by local variations of the field. The remaining residual would thus be inclusive of signatures from all immediate sources mass/density variations that are of interest in a given geophysical investigation. The authors provide a basic description of the gravity method as a tool of geophysical investigations in Chapter 1. Chapter 2 deals with current practices of regional-residual separation, their advantages and lacunae. Chapter 3 discusses the new technique at length, both theoretical and computational aspects. Chapters 4-9 are intended as demonstrations of the validity of the new method through detailed examples of applications in many different scenarios from geological and environmental problems to mineral exploration, studies of structure and isostasy, earthquake studies, engineering applications and investigations involving hunt for impact craters. In all these cases the authors show significant aspects of the anomaly of interest that is better illuminated by this approach compared to others normally used. All geophysical methods dealing with the unending complexities of the Mother Earth suffer from being ill-posed when it comes to mathematical treatment attempting quantifications of natural processes. Thus the solutions contain ubiquitous limitations of accuracy and sufficiency. Gravity methods have the added limitation of non-uniqueness predicted by the Green s Equivalence. The search for best possible interpretational approaches that reduce uncertainty of solution and enable their quality to be evaluated is thus an unending struggle for geophysicists and this Book makes a worthwhile contribution to this perennial endeavour. The authors have long years of expertise in dealing with both theoretical aspects of data treatment as well as experience of complexities of handling real field data. These are published in many refereed journals over the recent years. This book contains the distillation of all these experiences and brings to the readers a very thorough view of the topic, yet written in a very lucid and simplified format. Students, researchers and industry professionals shall find this book extremely useful in application of gravity methods for a better definition of the targets sought. Panikkar Professor & President, International Union of Geodesy and Geophysics National Geophysical Research Institute Hyderabad , India

9 Preface The journey of exploration geophysics in the past seven decades or so has been both arduous and amazing, more so since 1980s. While sophisticated new equipments have helped to acquire accurate data and computer technology has revolutionized the data processing, the interpretation has remained the weak link. The real earth is far too complex to be simulated accurately for exact mathematical solutions. Therefore, the interpreters resort to oversimplify the geological targets by various assumptions with regard to structures and physical properties. To cite an example, in the case of the regional and residual separation in the potential field methods, that forms the topic of the present treatise, starting from simple graphical approach in 1930s and 1940s, the success of which mainly depended on the interpreter s judgement, the analytical techniques were developed in 1950s and 1960s to make the methods more and more objective. These new techniques and their application found favour with the interpreters. However, as more and more interpreters started using these techniques such as filtering, trend surface, polynomial fitting and continuation etc., the shortcomings of these techniques surfaced. As a result, the graphical smoothing finds its place till this day. Most of the analytical techniques failed to be completely objective, since there was a need to choose the appropriate frequency range in frequency filtering, the degree of polynomial in polynomial fitting and the choice of height in upward continuation. Once again there was a need for direct involvement of the interpreters. The other important factor that came as a handicap in all these techniques was to consider all the data in the survey, inclusive of the anomalies. With this backdrop the finite element technique that is described here with case studies in Chapters 4 through 9 has achieved dramatic success in various fields like mineral and hydrocarbon exploration, structural studies, earthquake and engineering geophysics, geodetic studies and delineating meteoritic impact structures. This has been possible because its theory is simple, data from the anomalous zones for regional computation have been excluded, assumptions with regard to model parameters and their physical properties are kept minimum to make it more objective and the weighting coefficients or shape functions are ix

10 x Contents Preface defined without ambiguity. This space-domain technique is not computerintensive, robust and appears to be more objective compared to any other regional-residual separation technique. Besides three of us, our team had active, though part-time, support of Mr. Ch. Venkateswarlu and Ms Sangeetha Kannan. Chapter 9 that deals with meteoritic impact structures is primarily the work of Ms Sangeetha Kannan. The first recognition of our work came from Professor V.K. Gaur, Chief Editor of The Journal of Earth and Planetary Sciences of Indian Academy of Science. While going through a short paper on the application of the finite element techniques to compute regional gravity anomaly, Professor Gaur, former Director of NGRI and Secretary of Dept. of Ocean development, asked, Would you like our journal to carry these new results? The manuscript was quickly processed and published. The encouraging comments of the reviewers and the publication served as shots in our arms. Next, we looked for a wider horizon and sent a manuscript to Geophysics, the ultimate destination of a paper dealing with exploration. However, this manuscript was pre-reviewed by Professor Amalendu Roy, a high-profile potential field expert, who had in his heydays extensively published in Geophysics. He gave a green signal, the second turning point in our journey. David Chapin and Bob Pawlowski, the well-known experts, were generous in their reviews was the reaction of O. Hansen, Associate Editor. We slowly and steadily gained grounds. David Chapin s final rating Outstanding and Bob s Good put us on cloud nine. This is the third turning point for us. The computation of residual anomaly, equivalent to isostatic anomaly, without explicitly invoking any isostatic model, Airy-Heiskanen, Pratt-Hayford or Vening Meinsz, was our great achievement. The finite element results compared well or were better than those obtained by model-based direct approach of Jachens and Griscom of USGS, Klingele and Kissling of Switzerland, and Pilkington et al. of Canadian Geological Survey. This indeed thrilled us. Further, we were delighted when the finite element residual anomalies corroborated with the uplift of Swiss Alps far better than the modelbased isostatic anomalies obtained by Klingele and Kissling of Switzerland. George Everest, Surveyor General, Survey of India, had discovered the concept of isostasy in 19th century. Many techniques since then have been put forward to compute isostatic anomaly, the latest and the widely used one being the model-based approach. However, the shortcomings of this technique are oversimplification of Airy-Heiskanen model by assuming one density for the crustal rocks and one crustal depth over a large area. The finite element approach overcomes these obvious shortcomings. Therefore, our claim, though it may sound tall, is no exaggeration, that Isostasy was discovered in India and a sound technique to compute its effect came also from India. We have taken help of many of our colleagues and friends in India and abroad by way of stimulating discussions. Prof. V.K. Gaur, Dr H.K. Gupta and Dr V. Dimri, former directors of NGRI were very supportive of our activities.

11 Contents xi Preface xi Mr D.N. Avasti, former Member, ONGC, Prof K.K. Roy, IIT, Kharagpur, Prof. B.N.P. Agarwal, ISM, Dhanbad, Prof. Y. Sreedhar Murthy, Osmania University, Dr M.N. Qureshy, Dr K.R. Gupta and Dr Ch. Shivaji, Dept of Science and Technology, New Delhi, Prof. John Hermance, Brown University, Rhode Island, Prof. Herald Lindner, University of Freiberg, Germany, and Prof. K. Hinze, Purdue University, USA took great interest in our techniques and gave very useful inputs. Our NGRI colleagues Dr S.K. Ghosh, Dr K. Prabhakara Rao, Dr D. Indira Nagubai, Dr V.K. Rao, Dr U. Raval, Dr D.C. Mishra, Dr R.N. Singh, Dr V. Chakravarthy, Dr B. Veeraiah, Mr. G. Ashok Babu, Dr Bijender Singh, Dr V.M. Tiwari, Dr D. Chandrasekhar, Dr Kirti Srivastava, Dr Ravi Srivastava, Dr M. Prashanti Laxmi and Dr K. Chandrakla have helped us in many possible ways. The numerous figures and illustrations have been prepared by M/s M. Jayarama Rao, M. Shankaraiah, B.S.P. Rana, Zaffar Ali, Md Nayeem, Sundara Rao, Kranti Kumar, O. Prasada Rao. Part of the manuscript and most of the research papers have been prepared by Mr G. Ramakrishna Rao. We sincerely thank all of them. We also thank Dr B.M. Khanna to lend full support to provide us books, journals and relevant literature every time we approached him. We are grateful to the Publishers for their co-operation and patience during the preparation of the manuscript that has crossed several deadlines. We have received unconditional support from our family members to carry out this work often beyond office hours. K. Mallick wishes to thank his wife, Vijaya and sons Shashank and Siddharth and daughters-in-law, Bindu and Vijaya; A. Vasanthi to her father A. Narayana Rao, husband Girish Purohit and sons Gautham and Shreyas; and K.K. Sharma to his wife Indira and son Ayush for their encouragement in all possible ways. K. Mallick A. Vasanthi K.K. Sharma

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13 Contents Foreword Preface vii ix 1. Introduction 1 2. Regional and Residual Gravity Anomalies: The Existing Issues 9 3. New Computational Schemes 19 Finite Element Approach 19 Theory 19 Accuracy 24 Computational Procedure 27 Comparison with Other Existing Techniques Applications to Geological and Environmental Problems: Hydrocarbon 36 Bouguer Gravity Anomalies of Paradox Basin, Utah, USA 36 Inner Moray Firth Basin, North Sea 41 Mahanadi Basin, Orissa, India 47 Kaladgi-Badami Basin, Karnataka Maharastra, India Applications to Geological and Environmental Problems: Minerals 68 Kimberlite Pipes 69 Gravity and Magnetic Studies over Wajrakarur Kimberlite Field 74 Gravity Studies over Narayanapet Kimberlite Field (NKF) 81 Gravity Studies over Cuddapah Diamond Field 96 Chromite 110 Chromite Deposits in Orissa, India 118 Gold Deposits in Chocolate Mountains, California and Arizona 121 Sulphur Deposits in West Texas, USA Gravity Method in Structural Studies 133 The Gangetic Plains 133 xiii

14 xiv Contents Cauvery Basin 141 Offshore Magnetic Anomalies 151 Liquefaction Zone in Bihar-Nepal Region Isostatic Studies and Vertical Crustal Movements 160 Isostasy 160 Gravity Anomalies of California, US 168 Vertical Crustal Movements Earthquake Studies and Engineering Applications 203 Gravity Field of NW Himalaya 203 Gravity Anomalies in Nepal Himalayas 215 Gravity Field of Nevada Test Site of US Gravity Studies on Impact Structures 232 Gravity Field of Manicougan Crater 234 Gravity Field of Sudbury Crater 247 Appendix I 259 Appendix II 262 References 267 Index 285