- Time-Domain ElectroMagnetics (TDEM) method is based on EM induction underground generated by the turn-off of an artificial magnetic dipole source laying at the earth’s surface. The magnetic field variation induces eddy currents in the ground,diffusively propagating downward, with a velocity dependent on the subsoil resistivity. The secondary field measured at surface decays according to the resistivity of the passed through medium.
The magnetic dipole is created by means of a transmitter loop, even several hundreds meters wide. The loop is energised by currents usually ranging between 10 and 30 A. The secondary field is measured by a magnetic coil or a receiver loop, most of the times put at the centre of the Tx loop, but several different configurations are used.
Depending upon the acquisition settings and survey logisitics, it’s possible to acquire TDEM data at several locations a day, rapidly producing 1D and 2D resistivity models useful for preliminary assessment of shallow structures.
TDEM technique provides detailed modelling on shallow resistivity (up to few hundreds meters of depth), and can have many applications in engineering and environmental studies.
Furthermore, it is the more widely used tool for correction of “statically shifted” MT data, that occur for example, when small shallow heterogeneities are present at a site. For this reason, TDEM surveys are often carried out in geothermal prospects, where extreme resistivity contrasts are present.
|1D models profiling|
|Pseudo-2D resistivity section|
|Resistivity slices from 1D models|
|MT static shift factor compuertd by means of joint TDEM/MT inversion|