Frequency Domain Electromagnetic Surveys
It is a method used when the electrical conductivity distribution of a large area wants to be known rapidly.
There are different sensors, depending on the depth of investigation. These depths vary between 1.5 and 60 meters. Once defined the
investigation depth, a mesh of readings is designed to cover the area under investigation. Following this procedure electrical conductivity maps are generated. These conductivity maps allow detecting the presence of buried structures and for that reason it is a method often used in archaeological surveys
Time Domain Electromagnetic Surveys (TDEM)
Contrary to the electromagnetic method based on the frequency domain that generates electrical conductivity map distributions, this methodology studies the variation of electrical resistivity with depth.
TDEM surveys are carried out with a transmitter and a receiver. An electrical (primary) current along the transmitter coil produces a primary magnetic field. A sudden drop of this (primary) current causes the interruption of the magnetic field, which produces the electromagnetic induction of a secondary electrical current in the ground which generates a transient secondary magnetic field. Changes of the secondary magnetic field with time induce a voltage in the receiver coil. The shape of the voltage decay curve contains information about the ground resistivity distribution.