Everyday Applied Geophysics 1

Electrical Methods

Nicolas Florsch, Frederic Muhlach

• Publisher: ISTE Press - Elsevier
• Year: 2017
• Print ISBN: 9781785481994
• eBook ISBN: 9780081019672
• Topic: Physical Sciences
• Subtopic: Geophysics

Everyday Applied Geophysics 1 covers the physical methods permitting the environmental exploration of the sub-surface in 1, 2, 3 or 4 dimensions (the last is for time-lapse in terms of physical environmental state and geometry). The ground is transparent to electrical currents, electromagnetic induction, magnetic fields and seismic (acoustic) waves. All extend our senses by using the propagation of these phenomena through underground materials. The book specifically addresses the methods feasible, accessible and affordable to all users, and provides simple apparatus electronic diagrams. The book also features open-source and free software links for data interpretation.- Covers physical methods permitting the environmental exploration of the sub-surface in 1, 2, 3 or 4 dimensions- Addresses the methods feasible, accessible and affordable to all users- Provides simple apparatus electronic diagrams, as well as open-source and free software links for data interpretation

1

Introduction and General Points

Abstract

This book is dedicated to curious individuals and explorers of knowledge and technologies that have not been awarded a Nobel prize (yet). It has been written for tinkerers who want to employ tools to analyze the near subsoil (less than 20 m) with what is at hand. It has been designed for all those who want to carry out what we call “applied” geophysics (or “subsurface geophysics” as well as “environmental geophysics”, etc.). The authors have mentioned it with individuals from modest backgrounds in mind, as geophysical materials are expensive (naturally, as they are not produced in the thousands).

Keywords

Apparent resistivity; Georeferencing; Hypothesis elimination; Measuring protocols; Method of least squares; Point-by-point; Walking or drones

1.1 Introduction: the audience of this book

This book is dedicated to curious individuals and explorers of knowledge and technologies that have not been awarded a Nobel prize (yet). It has been written for tinkerers who want to employ tools to analyze the near subsoil (less than 20 m) with what is at hand. It has been designed for all those who want to carry out what we call “applied” geophysics (or “subsurface geophysics” as well as “environmental geophysics”, etc.). The authors have mentioned it with individuals from modest backgrounds in mind, as geophysical materials are expensive (naturally, as they are not produced in the thousands).

If we had to point out a prerequisite, it would be a certain familiarity with the laws of electricity and the difference between tension (or difference in potential as well as voltage, in volt) and intensity (in amperes). Thus, neophytes and amateurs of all kinds are welcome.

Sometimes, more complex calculations are shown in gray boxes for those math brains with a more developed interest, but they do not constitute the bulk of the work and may be disregarded without any serious consequence.

This book assumes that the reader will rummage on the Internet, looking for the all possible additional information at his or her level: figures, documents of all kinds, we can even find some videos dedicated to geophysics on YouTube. The content of this work can hardly be found in or is completely absent from specialized books, which besides are very expensive, and/or Websites dedicated to applied geophysics and its applications. We propose opening new avenues and offering some practice. The authors rely on the reader’s curiosity and are convinced that increasing one’s personal experience through practice is a fulfilling activity.

One of the goals is that the reader, after reading this book, will be able to analyze the subsurface and carry out electrical surveys to find out the depth of an aquifer or electrical mapping to find out the clay portions of the soil.

There is relatively little space in this first volume for applications: a choice had to be made. However, several examples of applications can naturally be found on the Internet.

1.1.1 What is applied (or subsurface) geophysics?

Applied geophysics is the use of concepts, quantities, devices and physical sciences to explore the near subsurface. Think about it… with our own eyes we can see the Andromeda Galaxy (on a moonless and cloudless night, without any interference lights from cities and provided that we are in the northern hemisphere; in the southern hemisphere, we will see the Magellanic Clouds, i.e. galaxies orbiting the Milky Way). Andromeda is 2 million light years away, i.e. around 24 trillion km. However, in our garden, there is no way of seeing what goes on below the first millimeter of soil.

There is still time to close this book and move on to astronomy.

Our “remote senses”, such as sight, require physical quantities (in this case light), which do not cross the soil. If only the soil were as transparent as the crystalline water of a lagoon.

Applied geophysics precisely complements our senses, simultaneously replacing our organs with instruments and quantities with other quantities to which these instruments are sensitive and which are above all able to penetrate the soil. The goal of geophysics is exactly to make the soil transparent. Our eyes do not see these physical fields¹, such as the electric field or the magnetic field. The idea here is to transform these fields, measured with instruments, into graphs and maps that this time will be interpretable. To begin with, let us consider three examples. First, let us consider electric current. It crosses the materials that form the subsoil fairly easily – otherwise, where would lightning go and what is the purpose of earthed sockets? However, these materials can be more or less conducting. A clay soil, for example, is easily a thousand times more conducting than a granite soil. Second, magnetic “fields”, the very ones that orient our compass: they cross the soil fairly well, unless they find a metallic barrier (for example heavily reinforced concrete). Third, acoustic waves; it is better to refer to seismic waves, but it is the same kind of physical process: a mechanic perturbation propagates in space.

Applied geophysics aims to see what goes on underground, let us say from a few centimeters to a few dozen meters, unless we focus on oil exploration, which goes deeper. Beyond this, the means must increase, and we will refer instead to global geophysics.

“Seeing” matter is something that medical imaging can also perform with X-rays and a scanner, magnetic resonance imaging, and several other methods.

Just like the brain interprets the signals that trigger our senses, an (applied) geophysicist must then interpret (by reasoning, but also by relying on their cumulative skills and experience) “data”, “observations” and “measurements” to deduce something, if possible a map of the subsoil.

To deduce what exactly? We must be realistic. In Jurassic Park, paleontologists “image” a small dinosaur skeleton on a screen. However, there are some physical limitations, which we will come back to later, preventing us from obtaining the same resolution as our eyes.

In terms of demand, in any case, applied geophysics helps us find and describe aquifers. It can map soil parameters whose subsoil nature and geometry may be determined, using some supplementary information (for example the geological context), in the sector explored. To a certain extent, it can determine the position of a fault or the thickness of a clay layer, as well as many other things, for example it can mark out polluted areas, find buried metallic objects or outline the foundations of former settlements.

Applied geophysics is in no sense a universal solution. It is more precise and exact to say that it spells out what the subsoil is not rather than what it is. It proceeds by refuting and eliminating. This is due to the fact that physical laws are inflexible and do not agree on their own with our thirst for knowledge. Those who do not understand this will hit a dead end. In the end, in the face of the great variety of geological and environmental situations, geophysics only provides some physical parameters, such as resistivity or the speed of seismic waves. An infinitely rich world in the subsurface is met by a very limited geoph...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Foreword
• 1: Introduction and General Points
• 2: Direct Current Electrical Methods
• 3: The Spontaneous Polarization Method
• 4: The Induced Polarization (IP) Method
• 5: Equipment
• 6: An Acquisition System Designed for the Electrical Prospection of Soil
• Appendix: Practical Links, Programs and Material
• Bibliography
• Index