Electromagnetic (EM) Surveys
Electromagnetic (EM) surveys use the principle of induction to measure the electrical conductivity of the subsurface including soil, groundwater, rock and buried objects. EM meters respond strongly to metal. This can be an advantage when the target is metallic, but surveys can be affected by extraneous metallic objects. Electromagnetic surveys are useful for many different types of geological, engineering and environmental applications including:
Electro magnetic surveys are suitable for conductive materials such as clay and waterlogged ground, which are unsuitable for Ground Penetrating Radar techniques. Electromagnetic Theory
EM conductivity meters contain two sets of coils that are a set distance apart. The separation and orientation of the coils dictates the depth of penetration. One set of coils transmits a primary magnetic field, which generate eddy currents in the subsurface. These induce a secondary current in underground conductors which results in a alternating secondary magnetic field. The second coil senses these magnetic fields and logs the information to a data logger. A big advantage of electromagnetic surveys is that soil-to-instrument contact is not required. This allows much more rapid data acquisition over resistivity techniques (which utilise metal pins inserted into the ground) and overcomes coupling problems which can be associated with Ground Penetrating Radar methods. Consequently EM offer a rapid survey technique. Equipment
Reporting
Electromagnetic surveys are particularly useful for producing plan view maps, while resistivity methods are typically used to produce cross sections. Electromagnetic data is commonly presented as contour plots. The image on the right shows Electromagnetic (EM) survey data plotted as contour plot showing house remains below car park. |
Sandberg use Geonics conductivity meters for Electromagnetic surveys, most commonly the EM38 (shown on right) and EM31.
Investigating and solving structural and materials defects, are what we do best.