Electromagnetic methods (FDEM, TDEM) were developed for the purpose of measuring resistivity in the subsurface. The methods are used extensively around the world to provide solutions to engineering and environmental problems. These methods, which enable penetration to relatively great depths (up to hundreds of meters), allow to carry out deep measurements at a very high level of accuracy. In Israel, electromagnetic methods are mainly used for monitoring changes in groundwater salinity and for locating metals objects. The electromagnetic equipment used is mobile and easy to operate, and the measurements can be relatively quickly analyzed and interpreted.
detection of seawater intrusion into coastal aquifer
locating metal objects, pipes and buried utilities
mapping the electrical resistivity and the salinity of soils for the purpose of optimizing fertilization and improving crops
locating subsurface cavities, underground structures; mapping the boundaries of archeological sites
locating leaks from waste disposal sites and water reservoirs
mapping of shallow subsurface geological structures and detection of subsurface metallic minerals deposits
Electromagnetic methods (EM) are utilized for environmental, geological and archeological applications, and in recent years – for agricultural applications as well. These methods were developed for the purpose of measuring the electrical conductivity of the subsurface. Since electrical conductivity varies with changes in the medium's composition, for example – water and mineral content, voids, fissures and various other parameters, it is possible to obtain a map of the subsurface's electrical conductivity distribution.
EM methods are based on magnetic induction, and do not require direct contact with the ground – which facilitates field work. EM methods were first developed in the 1920s in Scandinavia, the USA and Canada for mapping metal deposits. Until the 1960s, EM devices were used to carry out measurements through continuous transmission and reception in one frequency. In later years, systems transmitting concurrently in a number of frequencies and using several sensors for reception in parallel were developed, in order to enable reconstructing a vertical section of the measured soil's properties. EM systems that transmit and receive concurrently, enabling to receive a vertical section through changes in the frequency used, are called Frequency Domain systems (FEM, FDEM). In the 1960s, initial attempts were made to construct systems that receive the soil's reaction during the turn-off times in transmission. This approach has become more widespread since the 1970s. EM systems that measure the variations in the measured value measured as a function of time are called Time Domain (TEM, TDEM). The most significant advantage of this method is that the received signal represents the ground's reaction only, without the interference of the initial electromagnetic field transmitted by the device.