Magnetic surveys

Introduction

This large learning resource concentrates on background about using Earth's magnetic field to learn about its subsurface. Practicalities of interpreting maps, profiles, or inversion models are not discussed.

Magnetic surveys

Geophysical magnetic surveying makes use of the fact that Earth's magnetic field causes, or induces, subsurface materials to become magnetized. Referring to the following three-component outline, all applied geophysics problems can be discussed in terms of a source of energy that is put into the ground, the effects on that energy due to subsurface variations in the relevant physical property, and the measurements that detect those changes to the input energy. Signals are interpreted in terms of the subsurface distribution of the physical property, which in the case of magnetic surveys is magnetic susceptibility.

Using the same colour scheme as the figure above, Figures 2a - 2e illustrate how this concept applies to magnetic surveys. In this case, the energy source is Earth's global magnetic field (Figure 2a) which has a strength and direction at every location on the Earth (Figure 2b). Subsurface materials (Figure 2c) become magnetized by this field (Figure 2d), and the data (Figure 2e) will involve measurements of the magnetic field at the Earth's surface, in the air, in space, or within boreholes. The measured magnetic field will be a superposition of Earth's field and the induced secondary fields caused by magnetization of buried materials.

2a. Earth's magnetic field. 2b. It has strength and direction everywhere.

2c. No incident magnetic field. 2d. Earth's field causes material to become magnetized.

2e. Data are a superposition of Earth's field and resulting induced fields.

The physical property - susceptibility

Earth materials contain magnetic particles. Generally these are oriented in random directions and they produce no overall magnetic field (Figure 3a). However, when subject to an inducing field such as Earth's natural magnetic field, H₀, these particles will align themselves and the material will become magnetized (Figure 3b). The strength of that induced magnetization, M, depends upon the magnetic susceptibility, K, of the material. In fact, the strength of this induced magnetization field is M=KH₀. Note that M and H₀ are vector quantities and K is a scalar value - the physical property of the material.

3a.

3b.

In the field, the superposition of natural and induced fields is measured because they exist together. In general, after a magnetic survey is completed, the natural and induced fields are separated; then the residual induced (or anomalous) magnetic field is interpreted in terms of the magnitude and distribution of susceptible material under the ground. The resulting model of subsurface susceptibility must then be interpreted in terms of useful geologic and geotechnical parameters (rock types, structures, buried objects, etc.).

Some materials retain a natural permanent or "remanent" magnetization. This is a third component of measurable magnetic fields which complicates the interpretation of magnetic surveys because there is no way to separate the induced and remanent components. All content in this resource assumes remanent magnetization is zero, but this is usually not the case.

More details about the magnetic susceptibility of geological materials (and remanent magnetization) are given in a separate AGLO resource about magnetic susceptibility.

Typical problems where magnetics is useful

• Geologic mapping using ground or airborne magnetic data.
• Ore body characterization (location, depth, volume, mineral composition).
• Geotechnical (finding and mapping utilities and geologic materials or structures).
• Archeological object and feature mapping.
• Mapping continental scale geologic structure.
• Planetary scale investigations from satallite platforms (Earth, Mars, etc.).
• Paleomagnetics (sea floor spreading and rock dating).