Introduction

The study of the earth's magnetism is the oldest branch of geophysics. It is generally believed that the Chinese were the first to make use of the north-seeking property of ‘lodestone’ (a magnetite-rich rock piece). However, the idea that the earth itself acts as a magnet was realized much later. It was in the year 1600, when William Gilbert published his book De Magnete, that the concept of the earth's magnetic field and its directional behavior was put on a scientific footing.

The first systematic studies of local anomalies in the direction of the earth's field were made in Sweden for iron-ore prospecting probably as early as 1640 and regularly by the end of that century. However, it was not until the late 1870s that special instruments were developed by Thalén and Tiberg for routine use in prospecting surveys. Until about the 1940s the magnetic field instruments were tedious to operate and most of the field operations were restricted to small-scale land surveys. The development of the fluxgate magnetometer (during World War II) and the proton magnetometer in the mid 1950s brought about a revolutionary change in the speed of operations that made large-scale surveys possible by using airborne and ship-towed magnetometers.