Everyday Applied Geophysics 2: Magnetics and Electromagnetism PDF
Preview Everyday Applied Geophysics 2: Magnetics and Electromagnetism
Everyday Applied Geophysics 2 Series Editor André Mariotti Everyday Applied Geophysics 2 Magnetics and Electromagnetism Nicolas Florsch Frédéric Muhlach Michel Kammenthaler First published 2018 in Great Britain and the United States by ISTE Press Ltd and Elsevier Ltd Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address: ISTE Press Ltd Elsevier Ltd 27-37 St George’s Road The Boulevard, Langford Lane London SW19 4EU Kidlington, Oxford, OX5 1GB UK UK www.iste.co.uk www.elsevier.com Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ® MATLAB is a trademark of The MathWorks, Inc. and is used with permission. The MathWorks does not ® warrant the accuracy of the text or exercises in this book. This book’s use or discussion of MATLAB software or related products does not constitute endorsement or sponsorship by The MathWorks of a particular pedagogical approach or particular use of the MATLAB® software. For information on all our publications visit our website at http://store.elsevier.com/ © ISTE Press Ltd 2018 The rights of Nicolas Florsch, Michel Kammenthaler and Frédéric Muhlach to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. British Library Cataloguing-in-Publication Data A CIP record for this book is available from the British Library Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress ISBN 978-1-78548-280-9 Printed and bound in the UK and US Foreword The scientific books published by ISTE Press include a multidisciplinary series called Earth Systems – Environmental Sciences, and it is in this context that today I present a work dedicated to geophysical prospecting and its applications, coordinated by Professor Nicolas Florsch. Its title, Everyday Applied Geophysics, deserves to be explained in more detail. First, we should recall the important role played in some scientific fields by the so-called “amateurs”. This is especially the case for astronomy, a field where a socioepistemology of amateur practices, whose main points can be summed up here, has been established. These amateurs are not organized to compete with professionals, as they evidently lack the skills and the necessary resources. However, this is not a case of popular science: their practices, beyond the understanding of the sky, stars and the universe, are active and mobilized by the desire to make discoveries. Astronomy is a science where amateurs can obtain significant observation data, which are very useful for scientists. On a smaller scale, some amateurs, for example, are quite involved in electronics and radio communication. However, so far this has not been the case for Everyday Applied Geophysics, a domain that has potentially numerous applications associated with the exploration of the near subsoil: looking for water, archeological remains, geological peculiarities, etc. x Everyday Applied Geophysics 2 Moreover, making Everyday Applied Geophysics available for researchers based in developing countries is a challenge of the utmost importance. The goal is to open this field and allow everyone to employ the tools and methods used for exploring the near subsoil in order to highlight reservoirs or flow paths, locate holes, define geological stratifications, follow pollution plumes, search for archeological remains, etc. If curious and exploring amateurs may be involved, the main objective of the scientific community of these countries, which needs financially and technologically affordable tools, is to implement cheap and unsophisticated methods and techniques that, nonetheless, will produce plenty of essential data. Let us provide an example to illustrate this point. Some geophyscal devices cost up to tens of thousands of dollars (or euros) on the market; in this work, we will discover that with a few hundred euros, or even less, we can implement a system that, despite being naturally lower in terms of data acquisition, allows everyone to carry out actual and effective subsurface geophysical prospection. This work will also focus on the issue of self-learning. The existing literature does not tackle practical aspects either in terms of material implementation or basic interpretation concepts (actual resolution of the methods, sensitivity, etc.). This work is also very useful insofar as it can solve the problem of signal acquisition: it provides open-source “Arduino” solutions, supported by a downloadable program, for data acquisition in the field. Thus, this work, which is unique in its genre and accessible to everyone (with a few more technical and/or mathematical boxed passages), bridges a double gap in the existing scientific literature by: – providing accessible tools for the exploration of the near subsoil: from tools to acquisition systems (the latter being available with the use of computers) including a guide of free programs; – providing practical information for implementation that cannot be found in other works, such as the design of devices (from electrodes to current flow, for example to carry out an electrical survey), the protocol for the creation of geophysical maps, etc. Foreword xi We hope that this work reaches its audience and that the scientists that played a part in it may thus contribute to the removal of the ideological barrier between the world of basic research carried out in the academic world and applied research, as the markedly ideological gap that divides these two communities has not been entirely bridged yet. Besides, helping the development of the environmental field should be invaluable for a large number of countries. André MARIOTTI Emeritus Professor at the Université Pierre-et-Marie-Curie Honorary member of the Institut Universitaire de France Introduction Let us recall some introductory elements from Volume 1 in order to set the scene for this book. This is not a book on geophysics. If it were, it would be rather incomplete and well below the usual technical and mathematical standards for these kinds of works, which are vast (we provide a list at the end of this volume). It is not intended for experts in geophysics. Its aim is to make geophysical methods accessible to a wide audience, while simultaneously taking the digital landscape into account, which provides access to an ever-increasing and more accessible quantity of technical information. Thus, this series aims to complement what is already accessible, by attempting to define geophysical methods from a different perspective, and especially by bringing to light what cannot be (easily) found in the literature or online. As such, it is intended to be as practical as possible, and it must be used in conjunction with the curiosity of the reader, who should already have explored the field of geophysics a little but may be seeking something a little more hands-on. Some elements that were discussed in Volume 1 are universal and will not be repeated here (for example how to carry out a geophysical campaign and deploy measurements on the ground, prospection grid design, boustrophedon method). Similarly, some concepts will not be discussed here, such as the resistivity of rocks and the subsoil, which is alluded to in Chapter 2. The interested reader should be proactive and interactive; if a term or a concept is not familiar, the reader should refer themselves to a search engine. The authors have helped in this regard either with the symbol (#), which xiv Everyday Applied Geophysics 2 means “look up this term in a search engine”, or if they seek to point the user in a specific direction, they provide a direct link (so long as it is not too long to transcribe for those reading the paperback version of this book). Without the reader’s active participation, reading this book would be too fragmented. This volume focuses on two geophysical methods, as well as presenting tools for the graphic representation of geophysical maps. As far as methods are concerned, we propose instrumental solutions that are accessible to an amateur, from the do-it-yourselfers to more technical readers. A reader who does not yet have any equipment should find skilled persons in his or her entourage to help build devices for which we provide the electronic diagrams, or they can train themselves online: the resources are infinite and the driving forces to carry out a project are ageless – curiosity and desire to create. And perhaps a little patience too. 1 1 Magnetic Methods 1.1. Magnetism, the natural power for our compasses Records suggest that the compass was invented in China. But the moment when the “magnet stone” (magnetite), with chemical formula Fe O , began 3 4 to be used for navigation is unclear. As such, magnetite has been known since antiquity, since there is a mention by Pliny the Elder. “Magnetos” is actually the name of a Greek mountain that is rich in this mineral: as such, magnetite is not a rare mineral. From there to putting a piece of magnetite on a floating splint and seeing that it always oriented itself in the same direction, fate undoubtedly gave a hand. The needle of a modern compass is much more magnetic than a piece of magnetite. It is connected to a support structure that lets it rotate freely; by convention, its north pole is the one that points to geographic North. Upon nearing the poles, this direction becomes variable and no longer has anything to do with geographic North. They say that the needle panics... well, at least it is not the navigator. Let us not forget that it is not just by simple force that our compass points North, but rather it is the action of two opposing forces about an axis (here, we mean a forced axis because it is embodied by the axis of the compass), two forces that make the needle turn, and which we qualify as a single effect: “torque”2. 1 In addition to this chapter, there are other documents available: ftp://geom.geometrics.com/ pub/mag/Literature/AMPM-OPT.PDF. 2 https://en.wikipedia.org/wiki/Couple_(mechanics).
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