Electromagnetic induction (EM) methods respond to many of the same features as resistivity methods (conductivity is the inverse of resistivity) although they may be somewhat less sensitive. Both frequency domain and time domain instruments are available with the general discriminating factor that time domain (EM61) instruments respond primarily to ferrous and non-ferrous metal objects while frequency domain (EM31) instruments respond to both ferrous metal and subsurface variations in lithologic conductivity and moisture content.
The electromagnetic conductivity instrument operates by creating an alternating primary magnetic field at the transmitting coil. This primary field is produced by alternating current in the transmitting coil at an audio frequency. An electromagnetic field induced in the earth by the primary field causes secondary loops of electrical current to flow in the subsurface. This process is known as electromagnetic induction. An alternating secondary magnetic field results from this current flow in the earth .The receiving coil responds to an electromotive force generated by the resultant of the primary and secondary fields.
EM methods have proven to be quite reliable for the detection and mapping of buried structures such as building foundations, mapping lithologic boundaries caused by variations in soil type and moisture content, and for the detection of highly conductive metallic objects like steel drums, tanks, metallic utilities and other nondescript buried ferrous or non-ferrous metallic objects. EM methods can provide high quality information about soil types, moisture content, and conductivity of the pore space water. Under the proper conditions this allows the mapping of fill material, contaminant plumes, utility trenches, sinks and sinkholes, and overburden thickness.
Data is usually presented as a contour map of the in-phase or quadrature component of the conductivity field for frequency domain data. The quadrature-phase component is more responsive to lithology and moisture while the in-phase component responds primarily to ferrous metal objects. As in most geophysical surveys, test borings or well and cutting information are for correlating frequency domain measurements with on-site geology if refined analysis of the collected data is advantageous.
Time domain data is responsive to only ferrous and non-ferrous metal objects and provides little data on lithology other than magnetic susceptibility. Time-domain instruments typically record differential data over a number of time gates that allows determination of the depth and size of anomalies, and for that reason is the instrument of choice for locating buried munitions or ordnance (UXO). EM61 data is typically plotted in plan-view with isopleths of the millivolt response measured by the instrument.
The advantage of EM methods is that they do not require direct coupling with the earth, can be used in conditions unfavorable for resistivity surveys, and can be conducted much more quickly. A high density of readings can be taken with this equipment in conjunction with GPS location data. The resultant data set can be processed using various algorithms and contoured on a plan view of a Site showing important physical features. This integration of data provides continuity and facilitates data interpretation.