Electromagnetic methods fall in two categories, frequency domain and time domain.
Frequency domain measures the amplitude and phase of an induced electromagnetic field. Time domain measures the decay time of an electromagnetic pulse induced by a transmitter. An EM-31 survey was conducted at a military base to define covered landfill cells. The geophysical expression of the trenches is sharp, although subtle, requiring a very small contour interval to expose the features. A number of EM methods are available for use, each with its own advantages and limitations.