Everyday Applied Geophysics 2

Magnetics and Electromagnetism

Nicolas Florsch,Frederic Muhlach,Michel Kammenthaler

• Publisher: ISTE Press - Elsevier
• Year: 2018
• ISBN:
  9780081025970
• Topic: Physical Sciences
• Subtopic: Geophysics

Everyday Applied Geophysics 2: Electromagnetics and Magnetics covers the physical methods permitting the environmental exploration of the sub-surface in 1, 2, 3 or 4 dimensions (the latter for time-lapse in terms of physical environmental state and geometry). The book specifically addresses the feasible methods that are accessible and affordable to all users, providing a simple apparatus of electronic diagrams and free, Internet open-source software links for data interpretation.

• Proposes a practical, accessible and affordable applied geophysics resource for sub-surface environmental exploration
• Explains the topic's application to groundwater, raw material and resources, agriculture, buried pollutions and archaeology

1

Magnetic Methods ¹

Abstracts

Records suggest that the compass was invented in China. But the moment when the “magnet stone” (magnetite), with chemical formula Fe₃O₄, began 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.

Keywords

Demagnetizing field; Fluxgate sensor; Hysteresis cycle; Lorentz force; Magnetic anomaly; Magnetic dipole; Magnetism; Proton magnetometers; Pseudo-vertical gradient; Signal filtering

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 Fe₃O₄, began 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 .

The effect of the Earth’s field on the needle is characterized by its intensity (torque) and the direction in which the needle points. The Earth field thus has an intensity and a direction: we will represent this by a vector ³ . The magnetic field vector has an application point (the compass, for example), a direction (horizontal, toward “magnetic” north) and an intensity (which could be characterized by measuring the torque acting on the needle, for example with a torsion balance ⁴ ).

Thus, the compass needle is subjected to the Earth’s magnetic field. This field exists everywhere on Earth. Near the magnetic poles, if we really left the needle free to move, it would point toward the ground, and would do so at an angle of inclination of about 60° in France.

1.1.1 To introduce the topic: an example of magnetic prospecting or mapping

1.1.1.1 Main principles

The basic idea that will be discussed in the chapters of this book is that the Earth’s field, which is essentially generated in the depths of the Earth’s core, comprises localized anomalies, which are determined by the presence of structures in the subsoil. It is these anomalies, in other words deviations from normal, that are mapped. By “normal”, we mean the value of the field that would exist if the structures of the near-subsoil did not exist. There is always an a priori of scale that depends on the exploration goal, which is why this distinction between “normal field” and “anomalous field” is, by definition, subjective. At a geological scale of thousands of km², an anomaly may have a multikilometer spatial dimension. This is, for example, the effect of a dike (even if it is not cropped out).

A well-known historical example of regional-scale prospecti...