Looking into the Earth An Introduction to Geological Geophysics

Most electromagnetic (e-m) methods of surveying are used for targets similar to those of resistivity surveys, because both respond to variations in the resistivity (or conductivity) of the subsurface. The main difference is that e-m methods ‘induce’ current flows in the subsurface, usually without using electrodes. Many e-m methods can therefore be used in aerial as well as ground surveys.

E-m methods are particularly useful for ground surveys where the surface layer has a very high resistivity – such as dry sand or frozen ground – which prevents resistivity electrodes making electrical connection with more conductive layers below; conversely, a very conductive surface layer limits penetration more severely for e-m methods than it does for resistivity ones. A further limitation of e-m surveying is that generally it maps the subsurface less precisely than resistivity surveying. Smaller e-m instruments are quick to use on the ground because there are no electrodes and wires to set out.

Magnetotelluric, MT, surveying relies on naturally induced currents and can investigate down to tens, or even hundreds, of kilometres. Ground-penetrating radar, GPR, operates quite differently, by reflecting radar waves from subhorizontal interfaces, and so has similarities with seismic reflection, except that the discontinuities are of electrical rather than seismic properties. Like seismic reflection, it can provide high-resolution sections, but penetration is limited to a few metres, which limits its use to shallow targets, which include engineering, hydrogeological, and archaeological as well as some geological ones.