Everyday Applied Geophysics 1 - Electrical Methods

200 Pages·2018·51.49 MB·English

by  Nicolas Florsch

Everyday Applied Geophysics 1

Series Editor
André Mariotti
Everyday Applied
Geophysics 1

Electrical Methods

Nicolas Florsch
Frédéric Muhlach
First published 2018 in Great Britain and the United States by ISTE Press Ltd and Elsevier Ltd
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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
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the material herein.

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© ISTE Press Ltd 2018
The rights of Nicolas Florsch 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 978-1-78548-199-4

Printed and bound in the UK and US
Contents
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
Chapter 1. Introduction and General Points . . . . . . . . . . . . . . . . . 1
1.1. Introduction: the audience of this book . . . . . . . . . . . . . . . . . . . 1
1.1.1. What is applied (or subsurface) geophysics? . . . . . . . . . . . . . . 2
1.1.2. The spirit of this book: affordable methods on a
technological and financial level . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.3. An example to begin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.4. Drawing maps in isovalues . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2. Direct method, inverse method . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3. Sampling quantities on the surface, and which resolution
can be reached underground? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3.1. General points about measuring protocols . . . . . . . . . . . . . . . 16
1.3.2. The issue of the measuring grid . . . . . . . . . . . . . . . . . . . . . 18
1.3.3. The question of georeferencing . . . . . . . . . . . . . . . . . . . . . . 22
1.3.4. Mapping types of prospecting: point-by-point,
“walking” or drones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.4. Adapting the models to the targets . . . . . . . . . . . . . . . . . . . . . . 25
Chapter 2. Direct Current Electrical Methods . . . . . . . . . . . . . . . . 27
2.1. The electricity used in geophysics . . . . . . . . . . . . . . . . . . . . . . 27
2.1.1. The current flowing in the ground . . . . . . . . . . . . . . . . . . . . 27
2.1.2. Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.1.3. Separating volume conductivity from surface conductivity . . . . . 34
2.2. Ground resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.2.1. A first approach using hemispherical electrodes . . . . . . . . . . . . 35
vi Everyday Applied Geophysics 1
2.2.2. Realistic electrodes: metal posts . . . . . . . . . . . . . . . . . . . . . 37
2.3. The basic array for electrical prospecting . . . . . . . . . . . . . . . . . . 38
2.3.1. Stage 1: what is the potential at the distance (r) from
the electrode? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.3.2. Stage 2: describing “point” electrodes in more detail . . . . . . . . . 40
2.3.3. Stage 3: setting up all the electrodes: two transmitting
electrodes and two measuring electrodes . . . . . . . . . . . . . . . . . . . . 43
2.3.4. Stage 4: guarding against intruders . . . . . . . . . . . . . . . . . . . 48
2.4. Dangers involved in electrical prospecting and safety
regulations and rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
2.5. Apparent resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.5.1. A note about reciprocity . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.6. Arrays in electrical prospecting: depth of investigation
and sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2.6.1. Depth of investigation and sensitivity . . . . . . . . . . . . . . . . . . 58
2.7. Electrical resistivity tomography . . . . . . . . . . . . . . . . . . . . . . . 60
2.7.1. ERT arrays and sequences . . . . . . . . . . . . . . . . . . . . . . . . . 61
2.7.2. Using the programs RES2DMOD and RES2DINV . . . . . . . . . . 66
2.8. 3D tomography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
2.9. Direct current electrical resistivity mapping . . . . . . . . . . . . . . . . 76
2.10. Vertical electrical sounding (VES, or in short,
electrical sounding) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
2.10.1. VES and horizontally layered ground . . . . . . . . . . . . . . . . . 78
2.10.2. Difficulties and traps involved in electrical sounding . . . . . . . . 86
2.10.3. Carrying out vertical electrical sounding . . . . . . . . . . . . . . . 90
2.11. The rectangle method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
2.12. The mise-à-la-masse method . . . . . . . . . . . . . . . . . . . . . . . . . 97
2.12.1. Example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
2.13. Time lapse (or simply “monitoring”) in electrical methods . . . . . . . 102
2.14. A note about measuring ground resistance . . . . . . . . . . . . . . . . . 103
Chapter 3. The Spontaneous Polarization Method . . . . . . . . . . . . 105
3.1. The principle of SP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
3.2. The origin of the potentials in SP . . . . . . . . . . . . . . . . . . . . . . . 107
3.2.1. The electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
3.2.2. The sources of SP related to the Nernst equation . . . . . . . . . . . 111
3.2.3. The sources of SP related to the electrofiltration
equation and the application to aquifers and leakages . . . . . . . . . . . . 114
Contents vii
Chapter 4. The Induced Polarization (IP) Method . . . . . . . . . . . . . 121
4.1. The principle of induced polarization . . . . . . . . . . . . . . . . . . . . 121
4.2. Three types of measurements: temporal, frequency
and spectral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
4.2.1. Temporal IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
4.2.2. The so-called frequency IP . . . . . . . . . . . . . . . . . . . . . . . . 128
4.2.3. Phase measurement IP . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
4.2.4. Spectral induced polarization . . . . . . . . . . . . . . . . . . . . . . . 131
Chapter 5. Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5.1. Electrodes, wire bobbins and cables . . . . . . . . . . . . . . . . . . . . . 135
5.2. Equipment and methods used for spontaneous polarization . . . . . . . 136
5.2.1. Getting rid of disturbances and taking a good SP measurement . . 137
5.2.2. Measurement fluctuations . . . . . . . . . . . . . . . . . . . . . . . . . 138
5.3. Equipment and approaches used for direct current methods . . . . . . . 138
5.3.1. Adapting the impedance for the transmission of electricity . . . . . 139
5.3.2. Moving from direct to alternating current . . . . . . . . . . . . . . . 142
5.3.3. Alternating current transmitters . . . . . . . . . . . . . . . . . . . . . 145
5.3.4. “Hight voltage” transmitters . . . . . . . . . . . . . . . . . . . . . . . 152
5.4. Equipment for induced polarization . . . . . . . . . . . . . . . . . . . . . 155
Chapter 6. An Acquisition System Designed
for the Electrical Prospection of Soil . . . . . . . . . . . . . . . . . . . . . . 157
6.1. The presentation of the “open source” project . . . . . . . . . . . . . . . 157
6.2. The preliminary study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
6.2.1. The analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
6.3. Choosing the components . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
6.3.1. The microcontroller . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
6.3.2. The communication protocols of the microcontroller . . . . . . . . . 160
6.3.3. The analog–digital converter . . . . . . . . . . . . . . . . . . . . . . . 162
6.3.4. From the man–machine interface to data saving . . . . . . . . . . . . 165
6.3.5. The RTC module and data saving . . . . . . . . . . . . . . . . . . . . 166
6.4. Outline of the layout of components . . . . . . . . . . . . . . . . . . . . . 167
6.4.1. Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
6.4.2. List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
6.5. Preparing the microSD card . . . . . . . . . . . . . . . . . . . . . . . . . . 168
6.6. Running the program on the microcontroller . . . . . . . . . . . . . . . . 170
6.6.1. Additional configuration settings . . . . . . . . . . . . . . . . . . . . 173
6.7. The program and its menus . . . . . . . . . . . . . . . . . . . . . . . . . . 174
6.7.1. Initialization steps for the device . . . . . . . . . . . . . . . . . . . . . 174
6.7.2. Presentation of the menus . . . . . . . . . . . . . . . . . . . . . . . . . 176
viii Everyday Applied Geophysics 1
6.8. A practical example of how the device is used . . . . . . . . . . . . . . . 177
6.8.1. Preliminary configuration steps . . . . . . . . . . . . . . . . . . . . . 177
6.8.2. Taking the measurements . . . . . . . . . . . . . . . . . . . . . . . . . 178
6.8.3. The result file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
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.