The basic principles of operation are based upon detecting the disturbance to the Earth’s magnetic field by buried magnetised materials. The method is entirely passive in that the instrumentation does not have to generate a field itself. The types of materials that affect the Earth’s magnetic field include basic igneous rocks, certain forms of mineralisation, and a wide variety of man-made materials and fabrications such as metal pipes, reinforcement bars, electric cables, types of furnace ash. Indeed any material that contains some form of iron oxide (e.g. a ferromagnetic substance) or other material that is susceptible to magnetisation records the strength of the Earth’s magnetic field at the location where the material acquires its magnetisation. For example, a volcanic lava flow acquires its magnetisation when the lava cools below the Curie temperature; this permanent magnetisation is known as Thermal Remanent Magnetisation. The same thing applies to bricks when they are fired in a kiln, or to metal pipes when they are extruded and then allowed to cool. Metal pipe segments represent individual bar magnets so that when a pipe line is constructed the magnetic response is akin to that arising from a line of bar magnets. Consequently, it is possible to determine the approximate length of individual segments of buried metal pipes from their magnetic signatures.
A single sensor is normally used to measure the magnetic field intensity (in units of nano-Tesla; nT), but two sensors mounted either vertically above or horizontally adjacent to each other can be used to derive the vertical or horizontal magnetic gradient. This is simply the difference in the magnetic field intensity as measured at each sensor corrected for the physical separation of the two sensors (in units of nano-Tesla per metre; nTm-1).